Internet DRAFT - draft-dekater-scion-pki

draft-dekater-scion-pki







Network Working Group                                        C. de Kater
Internet-Draft                                             N. Rustignoli
Intended status: Informational                         SCION Association
Expires: 5 September 2024                                        S. Hitz
                                                         Anapaya Systems
                                                            4 March 2024


                        SCION Control-Plane PKI
                       draft-dekater-scion-pki-05

Abstract

   This document presents the trust concept and design of the SCION
   _control-plane Public Key Infrastructure (PKI)_, SCION's public key
   infrastructure model.  SCION (Scalability, Control, and Isolation On
   Next-generation networks) is a path-aware, inter-domain network
   architecture.  The control-plane PKI, or short CP-PKI, handles
   cryptographic material and lays the foundation for the authentication
   procedures in SCION.  It is used by SCION's control plane to
   authenticate and verify path information, and builds the basis for
   SCION's special trust model based on so-called Isolation Domains.

   This document first introduces the trust model behind the SCION's
   control-plane PKI, as well as clarifications to the concepts used in
   it.  This is followed by specifications of the different types of
   certificates and the Trust Root Configuration.  The document then
   specifies how to deploy the whole infrastructure.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://scionassociation.github.io/scion-cppki_I-D/draft-dekater-
   scion-pki.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-dekater-scion-pki/.

   Source for this draft and an issue tracker can be found at
   https://github.com/scionassociation/scion-cppki_I-D.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   5
     1.3.  Trust Model . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Trust Relations within an Isolation Domain  . . . . . . .   7
       1.4.1.  Updates and Trust Resets  . . . . . . . . . . . . . .   8
       1.4.2.  Substitutes to Certificate Revocation . . . . . . . .   8
     1.5.  Overview of Certificates, Keys, and Roles . . . . . . . .   8
     1.6.  Trust as a Function . . . . . . . . . . . . . . . . . . .  10
       1.6.1.  Input . . . . . . . . . . . . . . . . . . . . . . . .  10
       1.6.2.  Output  . . . . . . . . . . . . . . . . . . . . . . .  11
   2.  Certificate Specification . . . . . . . . . . . . . . . . . .  11
     2.1.  SCION Control-Plane PKI Keys and Certificates -
           Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
       2.1.1.  Trust Hierarchy . . . . . . . . . . . . . . . . . . .  12
       2.1.2.  Control-Plane Root Certificate  . . . . . . . . . . .  12
       2.1.3.  Control-Plane CA Certificate  . . . . . . . . . . . .  13
       2.1.4.  Control-Plane AS Certificate  . . . . . . . . . . . .  13
       2.1.5.  Voting Certificates . . . . . . . . . . . . . . . . .  13
       2.1.6.  Certificates - Formal Overview  . . . . . . . . . . .  15
     2.2.  Certificate Specification . . . . . . . . . . . . . . . .  18
       2.2.1.  Basic Fields: SCION-Specific Constraints and
               Conditions  . . . . . . . . . . . . . . . . . . . . .  18



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       2.2.2.  Extensions  . . . . . . . . . . . . . . . . . . . . .  22
   3.  Trust Root Configuration Specification  . . . . . . . . . . .  29
     3.1.  TRC Specification . . . . . . . . . . . . . . . . . . . .  30
       3.1.1.   TRC Types and States . . . . . . . . . . . . . . . .  30
       3.1.2.   TRC Format . . . . . . . . . . . . . . . . . . . . .  33
       3.1.3.   TRC Signature Syntax . . . . . . . . . . . . . . . .  41
       3.1.4.   Certification Path - Trust Anchor Pool . . . . . . .  43
       3.1.5.   TRC Updates  . . . . . . . . . . . . . . . . . . . .  45
     3.2.  Initial TRC Signing Ceremony  . . . . . . . . . . . . . .  50
   4.  Deploying the CP PKI - Specifications . . . . . . . . . . . .  51
     4.1.  Deploying a TRC . . . . . . . . . . . . . . . . . . . . .  51
       4.1.1.   Base TRC . . . . . . . . . . . . . . . . . . . . . .  51
       4.1.2.   TRC Update . . . . . . . . . . . . . . . . . . . . .  51
     4.2.   Signing and Verifying Control-Plane Messages . . . . . .  52
       4.2.1.   Signing a Control-Plane Message  . . . . . . . . . .  52
       4.2.2.   Verifying a Control-Plane Message  . . . . . . . . .  53
     4.3.  Creating a New Control-Plane AS Certificate . . . . . . .  54
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  55
     5.1.  Dependency on Certificates  . . . . . . . . . . . . . . .  55
       5.1.1.  Compromise of an ISD  . . . . . . . . . . . . . . . .  56
       5.1.2.  Recovery from Compromise  . . . . . . . . . . . . . .  56
     5.2.  Denial of Service Attacks . . . . . . . . . . . . . . . .  57
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  58
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  58
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  58
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  59
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  59
   Appendix A.  Signing Ceremony Initial TRC . . . . . . . . . . . .  60
      Ceremony Participants  . . . . . . . . . . . . . . . . . . . .  60
     Ceremony Preparations . . . . . . . . . . . . . . . . . . . . .  61
        Location . . . . . . . . . . . . . . . . . . . . . . . . . .  61
        Devices  . . . . . . . . . . . . . . . . . . . . . . . . . .  61
       Preparation Steps . . . . . . . . . . . . . . . . . . . . . .  62
      Ceremony Process . . . . . . . . . . . . . . . . . . . . . . .  63
        Phase 1: Certificate Exchange  . . . . . . . . . . . . . . .  64
       Phase 2: Generation of the TRC Payload  . . . . . . . . . . .  65
       Phase 3: TRC Signing  . . . . . . . . . . . . . . . . . . . .  66
        Phase 4: TRC Validation  . . . . . . . . . . . . . . . . . .  66
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  66

1.  Introduction

   The control-plane PKI (CP-PKI) lays the foundation for the
   authentication procedures in SCION.  It handles all cryptographic
   material used in the public key infrastructure of SCION's control
   plane.  This section first introduces the key concepts of the SCION
   CP-PKI, including the trust model, its core elements (certificates,
   keys, and roles), and their relationships.  The sections after the



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   Introduction provide detailed specifications of the building blocks
   of the CP-PKI.

   *Note:* For extended information on the SCION next-generation inter-
   domain architecture, see [CHUAT22], especially Chapter 2, as well as
   the IETF Internet Drafts [I-D.scion-overview] and
   [I-D.scion-components].

1.1.  Terminology

   *Control-Plane PKI (CP-PKI)*: The control-plane PKI is the public-key
   infrastructure upon which SCION's control plane relies for the
   authentication of messages.  It is a set of policies, roles, and
   procedures that are used to manage trust root configurations (TRCs)
   and certificates.

   *Autonomous System (AS)*: An autonomous system is a network under a
   common administrative control.  For example, the network of an
   Internet service provider, company, or university can constitute an
   AS.  If an organization operates multiple networks that are not
   directly connected together, then the different networks are
   considered different ASes.

   *Isolation Domain (ISD)*: In SCION, autonomous systems (ASes) are
   organized into logical groups called isolation domains or ISDs.  Each
   ISD consists of ASes that span an area with a uniform trust
   environment (i.e., a common jurisdiction).  A possible model is for
   ISDs to be formed along national boundaries or federations of
   nations.

   *Core AS*: Each isolation domain (ISD) is administered by a set of
   distinguished autonomous systems (ASes) called core ASes, which are
   responsible for initiating the path-discovery and -construction
   process (in SCION called "beaconing").

   *Trust Root Configuration (TRC)*: A trust root configuration or TRC
   is a signed collection of certificates pertaining to an isolation
   domain (ISD).  TRCs also contain ISD-specific policies.

   *Authoritative AS*: Authoritative ASes are those ASes in an ISD that
   always have the latest TRCs of the ISD.  As a consequence,
   authoritative ASes also start the announcement of a TRC update.









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   *Base TRC*: A base TRC is a trust root configuration (TRC) that other
   parties trust axiomatically.  In other words, trust for a base TRC is
   assumed, not derived from another cryptographic object.  Each ISD
   must create and sign a base TRC when the ISD is established.  A base
   TRC is either the first TRC of the ISD or the result of a trust
   reset.

   *TRC Signing Ceremony*: The ceremony during which the very first base
   TRC of an ISD, called the initial TRC, is signed.  The initial TRC is
   a special case of the base TRC where the number of the ISD is
   assigned.

   *TRC Update*: A _regular_ TRC update is a periodic re-issuance of the
   TRC where the entities and policies listed in the TRC remain
   unchanged.  A _sensitive_ TRC update is an update that modifies
   critical aspects of the TRC, such as the set of core ASes.  In both
   cases, the base TRC remains unchanged.

   *Voting ASes*: Those ASes within an ISD that may sign TRC updates.
   The process of appending a signature to a new TRC is called "casting
   a vote".

   *Voting Quorum*: The voting quorum is a trust root configuration
   (TRC) field that indicates the number of votes (signatures) needed on
   a successor TRC for it to be verifiable.  A voting quorum greater
   than one will thus prevent a single entity from creating a malicious
   TRC update.

   *Grace Period*: The grace period is an interval during which the
   previous version of a trust root configuration (TRC) is still
   considered active after a new version has been published.

   *Trust Reset*: A trust reset is the action of announcing a new base
   TRC for an existing ISD.  A trust reset should only be triggered
   after a catastrophic event involving the loss or compromise of
   several important private keys.

1.2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.







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1.3.  Trust Model

   Given the diverse nature of the constituents in the current Internet,
   an important challenge is how to scale authentication of network
   elements (such as AS ownership, hop-by-hop routing information, name
   servers for DNS, and domains for TLS) to the global environment.  The
   roots of trust of currently prevalent public key infrastructure (PKI)
   models do not scale well to a global environment, because (1)
   mutually distrustful parties cannot agree on a single trust root
   (monopoly model), and because (2) the security of a plethora of roots
   of trust is only as strong as its weakest link (oligopoly model) -
   see also [BARRERA17].

   The monopoly model suffers from two main drawbacks: First, all
   parties must agree on a single root of trust.  Secondly, the single
   root of trust represents a single point of failure, the misuse of
   which enables the forging of certificates.  Also, its revocation can
   result in a kill-switch for all the entities it certifies.  The
   oligopoly model relies on several roots of trust, all equally and
   completely trusted.  However, this is not automatically better:
   Whereas the monopoly model has a single point of failure, the
   oligopoly model has the drawback of exposing more than one point of
   failure.

   Thus, there is a need for a trust architecture that supports
   meaningful trust roots in a global environment with inherently
   distrustful parties.  This new trust architecture should provide the
   following properties:

   *  Trust agility (see further below);

   *  Resilience to single root of trust compromise;

   *  Multilateral governance; and

   *  Support for policy versioning and updates.

   Ideally, the trust architecture allows parties that mutually trust
   each other to form their own trust "union" or "domain", and to freely
   decide whether to trust other trust unions (domains) outside their
   own trust bubble.

   To fulfill the above requirements, which in fact apply well to inter-
   domain networking, SCION introduces the concept of *Isolation
   Domains*. An Isolation Domain (ISD) is a building block for achieving
   high availability, scalability, and support for heterogeneous trust.
   It consists of a logical grouping of ASes that share a uniform trust
   environment (i.e., a common jurisdiction).  An ISD is administered by



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   one or multiple ASes, called the *voting ASes*. Furthermore, each ISD
   has a set of ASes that form the ISD core; these are the *core ASes*.
   The set of core and voting ASes can, but not necessarily have to,
   overlap.  It is governed by a policy called the *Trust Root
   Configuration* (TRC), which is negotiated by the ISD core.  The TRC
   defines the locally scoped roots of trust used to validate bindings
   between names and public keys.

   Authentication in SCION is based on digital certificates that bind
   identifiers to public keys and carry digital signatures that are
   verified by roots of trust.  SCION allows each ISD to define its own
   set of trust roots, along with the policy governing their use.  Such
   scoping of trust roots within an ISD improves security, as compromise
   of a private key associated with a trust root cannot be used to forge
   a certificate outside the ISD.  An ISD's trust roots and policy are
   encoded in the TRC, which has a version number, a list of public keys
   that serves as root of trust for various purposes, and policies
   governing the number of signatures required for performing different
   types of actions.  The TRC serves as a way to bootstrap all
   authentication within SCION.  Additionally, TRC versioning is used to
   efficiently revoke compromised roots of trust.

   The TRC also provides _trust agility_, that is, it enables users to
   select the trust roots used to initiate certificate validation.  This
   implies that users are free to choose an ISD they believe maintains a
   non-compromised set of trust roots.  ISD members can also decide
   whether to trust other ISDs and thus transparently define trust
   relationships between parts of the network.  The SCION trust model,
   therefore, differs from the one provided by other PKI architectures.

1.4.  Trust Relations within an Isolation Domain

   As already mentioned previously, the control-plane PKI, SCION's
   concept of trust, is organized on ISD-level.  Each ISD can
   independently specify its own Trust Root Configuration (TRC) and
   build its own verification chain.  Each TRC consists of a collection
   of signed root certificates, which are used to sign CA certificates,
   which are in turn used to sign AS certificates.  The TRC also
   includes ISD-policies that specify, for example, the TRC's usage,
   validity, and future updates.  A TRC is a fundamental component of an
   ISD's control-plane PKI.  The so-called *base TRC* constitutes the
   ISD's trust anchor.  It is signed during a signing ceremony by the
   voting ASes and then distributed throughout the ISD.








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1.4.1.  Updates and Trust Resets

   There are two types of TRC updates: regular and sensitive.  A
   *regular TRC update* is a periodic re-issuance of the TRC where the
   entities and policies listed in the TRC remain unchanged, whereas a
   *sensitive TRC update* is an update that modifies critical aspects of
   the TRC, such as the set of core ASes.  In both cases, the base TRC
   remains unchanged.  If the ISD's TRC has been compromised, it is
   necessary for an ISD to re-establish the trust root.  This is
   possible with a process called *trust reset* (if allowed by the ISD's
   trust policy).  In this case, a new base TRC is created.

1.4.2.  Substitutes to Certificate Revocation

   The CP-PKI does not explicitly support certificate revocation.
   Instead, it relies on the two mechanisms described above and on
   short-lived certificates.  This approach constitutes an attractive
   alternative to a revocation system for the following reasons:

   *  Both short-lived certificates and revocation lists must be signed
      by a CA.  Instead of periodically signing a new revocation list,
      the CA can simply re-issue all the non-revoked certificates.
      Although the overhead of signing multiple certificates is greater
      than that of signing a single revocation list, the overall
      complexity of the system is reduced.  In the CP-PKI the number of
      certificates that each CA must renew is manageable as it is
      limited to at most the number of ASes within an ISD.

   *  Even with a revocation system, a compromised key cannot be
      instantaneously revoked.  Through their validity period, both
      short-lived certificates and revocation lists implicitly define an
      attack window (i.e., a period during which an attacker who managed
      to compromise a key could use it before it becomes invalid).  In
      both cases, the CA must consider a tradeoff between efficiency and
      security when picking this validity period.

1.5.  Overview of Certificates, Keys, and Roles

   The base TRC constitutes the root of trust within an ISD.  Figure 1
   provides a first impression of the trust chain within an ISD, based
   on its TRC.  For detailed descriptions, please refer to Section 2 and
   Section 3.









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                                       TRC 2

                  +--------------------------------------+
                  |+------------------------------------+|
                  ||- Version       - Core ASes         ||
   +--------+     ||- ID            - Description       ||    +--------+
   | TRC 1  |     ||- Validity      - No Trust Reset    ||    | TRC 3  |
   | (Base  |---->||- Grace Period  - Voting Quorum     ||--->|        |
   |  TRC)  |     ||- ...                               ||    |        |
   +--------+     |+------------------------------------+|    +--------+
                  |+----------------+  +----------------+|
                  || Regular Voting |  |Sensitive Voting||
                  ||  Certificate   |  |  Certificate   ||
                  |+----------------+  +----------------+|
                  |+----------------+  +----------------+|
                  ||     Votes      |  |   Signatures   ||
                  |+----------------+  +----------------+|
                  |+------------------------------------+|
                  ||        CP Root Certificates        ||
                  |+----------+-------------+-----------+|
                  |           |             |            |
                  +-----------+-------------+------------+
                              |             |
                              |             |
                              v             v
                    +-----------+         +-----------+
                    |   CP CA   |         |   CP CA   |
                    |Certificate|         |Certificate|
                    +-+-------+-+         +-----+-----+
                      |       |                 |
                      |       |                 |
                      v       v                 v
             +-----------+ +-----------+      +-----------+
             |   CP AS   | |   CP AS   |      |   CP AS   |
             |Certificate| |Certificate|      |Certificate|
             +-----------+ +-----------+      +-----------+

                   Figure 1: Chain of trust within an ISD

   All certificates used in SCION's control-plane PKI are in X.509 v3
   format [RFC5280].  Additionally, the TRC contains self-signed
   certificates instead of plain public keys.  Self-signed certificates
   have the following advantages over plain public keys: (1) They make
   the binding between name and public key explicit; and (2) the binding
   is signed to prove possession of the corresponding private key.

   All ASes in SCION have the task to sign and verify control-plane
   messages.  However, certain ASes have additional roles:



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   *  *Core ASes*: Core ASes are a distinct set of ASes in the SCION
      control plane.  For each ISD, the core ASes are listed in the TRC.
      Each core AS in an ISD has links to other core ASes (in the same
      or in different ISDs).

   *  *Certification authorities (CAs)*: CAs are responsible for issuing
      AS certificates to other ASes and/or themselves.

   *  *Voting ASes*: Only certain ASes within an ISD may sign TRC
      updates.  The process of appending a signature to a new TRC is
      called "casting a vote"; the designated ASes that hold the private
      keys to sign a TRC update are "voting ASes".

   *  *Authoritative ASes*: Authoritative ASes are those ASes in an ISD
      that always have the latest TRCs of the ISD.  They start the
      announcement of a TRC update.

   All further details of the SCION control-plane PKI are specified in
   the following sections.

1.6.  Trust as a Function

   The SCION control-plane PKI can be seen as a function that transforms
   potential distrust among different parties into a mutually accepted
   trust contract including a trust update and reset policy as well as
   certificates used for authentication procedures in SCION's control
   plane.

   For the function to work, it is not necessary that the ASes of the
   ISD all trust each other.  However, all ASes MUST trust the ISD's
   core ASes, authoritative ASes, voting ASes, as well as its CA(s).
   These trusted parties negotiate the ISD trust contract in a
   "bootstrapping of trust" ceremony, where cryptographic material is
   exchanged, and the ISD's trust anchor (the initial Trust Root
   Configuration) is created and signed.

1.6.1.  Input

   Prior to the ceremony, the trusted parties must decide about the
   validity period of the TRC as well as the number of votes required to
   update a TRC.  They must also bring the required keys and
   certificates, the so-called root and voting keys/certificates.

   During the ceremony, the trusted parties decide about the number of
   the ISD.  This must be an integer in the inclusive range between 64
   and 4094.  The next table shows the current allocation of ISD numbers
   in SCION:




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        +========+================================================+
        | ISD    | Description                                    |
        +========+================================================+
        | 0      | The wildcard ISD.                              |
        +--------+------------------------------------------------+
        | 1 - 15 | Reserved for documentation and sample code     |
        |        | (analogous to [RFC5398].                       |
        +--------+------------------------------------------------+
        | 16 -   | Private use (analogous to [RFC6996]).  Can be  |
        | 63     | used for testing and private deployments.      |
        +--------+------------------------------------------------+
        | 64 -   | Public ISDs.  Should be allocated in ascending |
        | 4094   | order, without gaps and "vanity" numbers.      |
        +--------+------------------------------------------------+
        | 4095 - | Reserved for future use.                       |
        | 65535  |                                                |
        +--------+------------------------------------------------+

                      Table 1: ISD Number Allocations

1.6.2.  Output

   The output of the bootstrapping of trust ceremony, or the trust
   "function", are the ISD's initial Trust Root Configuration as well as
   mutually trusted and accepted CA and AS certificates--the latter are
   used to verify SCION's control-plane messages.  Together with the
   ISD's control-plane root certificates, the CA and AS certificates
   build the ISD's trust and verification chain.

2.  Certificate Specification

   This section provides a detailed specification of all certificates
   used in SCION's control-plane PKI.  It starts with an overview of the
   main keys and certificates.

2.1.  SCION Control-Plane PKI Keys and Certificates - Overview

   There are three types of control-plane (CP) certificates: root
   certificates, CA certificates, and AS certificates.  Together, they
   build a chain of trust that is anchored in the trust root
   configuration (TRC) file of the respective Isolation Domain (ISD).
   Additionally, there are regular and sensitive voting certificates,
   which define the keys to cast votes in a regular and a sensitive TRC
   update, respectively.

   All certificates in SCION's control-plane PKI are in X.509 v3 format
   [RFC5280].




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   The next section shows SCION's trust hierarchy.  This is followed by
   sections that describe the main certificates and corresponding key
   pairs of SCION's control-plane PKI as well as the voting certificates
   and keys.

2.1.1.  Trust Hierarchy

   The trust is anchored in the Trust Root Configuration (TRC) for each
   ISD.  The trust root is axiomatic: All trust derived from this anchor
   relies on all parties transitively trusting the TRC.

   The trust hierarchy looks like this:

   TRC
   ── Regular Voting Certificates
        └── TRC (next version, regular update)
   ── Sensitive Voting Certificates
        └── TRC (next version, sensitive update)
   ── CP Root Certificates
        └── CP CA Certificates
             └── CP AS Certificates

2.1.2.  Control-Plane Root Certificate

   The control-plane root private key is used to sign control-plane CA
   certificates.  Consequently, the control-plane root certificate with
   the control-plane root public key is used to verify control-plane CA
   certificates, i.e., root certificates determine which ASes act as CA
   in an ISD.

   In X.509 terms, CP root certificates are _self-signed_ CA
   certificates.  That is, issuer and subject of the certificate are the
   same entity, and the public key in the root certificate can be used
   to verify the root certificate's signature.  The CP root public key
   and proof of ownership of the private key are embedded in the Trust
   Root Configuration (TRC) of an Isolation Domain (ISD), via the self-
   signed CP root certificate.  This facilitates the bootstrapping of
   trust within an ISD, and marks the CP root certificates as the
   starting point of an ISD's certificate verification path.

   The recommended *maximum validity period* of a CP root certificate
   is: 1 year.

   *Note*: The TRC of each ISD contains a trusted set of control-plane
   root certificates.  This set builds the root of each ISD's
   verification path.  For more information on the selection of this
   trusted set of root certificates, see Section 3.




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2.1.3.  Control-Plane CA Certificate

   The control-plane CA private key is used to sign control-plane AS
   certificates.  Consequently, control-plane CA certificates holding
   the control-plane CA public key are used to verify control-plane AS
   certificates.

   The public key needed to verify the CA certificate is in a CP root
   certificate.  CA certificates do not bundle the root certificate
   needed to verify them.  In order to verify a CA certificate, a pool
   of root certificates must first be extracted from one or more active
   TRCs (as described in Section 4.2).

   The recommended *maximum validity period* of a CP CA certificate is:
   11 days.

2.1.4.  Control-Plane AS Certificate

   SCION ASes sign control-plane messages, such as Path Construction
   Beacons, with their AS private key.  Consequently, control-plane AS
   certificates holding the corresponding AS public key are required to
   verify control-plane messages.

   In X.509 terms, control-plane AS certificates are end-entity
   certificates.  That is, they cannot be used to verify other
   certificates.

   The recommended *maximum validity period* of a CP AS certificate is:
   3 days.

2.1.5.  Voting Certificates

   There are two types of voting certificates: the (1) regular voting
   certificates and the (2) sensitive voting certificates.  They contain
   the public keys associated with the private keys that are allowed to
   cast votes in the TRC update process.  Voting certificates are
   X.509-style certificates.

   Regular and sensitive voting certificates are used to verify regular
   and sensitive TRC updates, respectively, and are embedded in the TRC.











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2.1.5.1.  Regular Voting Certificate

   Regular voting certificates state which keys are allowed to cast
   votes in a regular update.  In X.509 terms, regular voting
   certificates are self-signed end-entity certificates.  This means
   that the issuer and subject of a regular voting certificate are the
   same entity, and the public key within the certificate can be used to
   verify the certificate's signature.  However, a regular voting
   certificate cannot be used to verify other certificates.

   The recommended *maximum validity period* of a regular voting
   certificate is: 1 year.

2.1.5.2.  Sensitive Voting Certificate

   Sensitive voting certificates specify which keys are allowed to cast
   votes in a sensitive update.  In X.509 terms, sensitive voting
   certificates are self-signed end-entity certificates.  This means
   that the issuer and subject of a sensitive voting certificate are the
   same entity, and the public key within the certificate can be used to
   verify the certificate's signature.  However, a sensitive voting
   certificate cannot be used to verify other certificates.

   The recommended *maximum validity period* of a sensitive voting
   certificate is: 5 years.

   *Note*:

      Both SCION's control-plane root certificates and control-plane CA
      certificates are in fact CA certificates.  That is, they can both
      be used to verify other certificates.

      One important difference between both certificate types lies in
      their validity period: A SCION control-plane root certificate has
      a recommended maximum validity period of one year, whereas the
      recommended maximum validity period of a SCION control-plane CA
      certificate is 11 days.  This is because a root certificate is
      part of the Trust Root Configuration of an ISD, which itself also
      has a recommended maximum validity period of one year (see Table 2
      below).  This ensures that the TRC must not be updated all the
      time and is thus relatively stable.

      The SCION root private key and public key/certificate are used to
      sign and verify the control-plane CA certificates, respectively.
      The control-plane CA certificates are explicitly NOT part of the
      TRC, for reasons of security.  The control-plane CA certificates
      are used to verify the control-plane AS certificates, which in
      turn are used to verify control-plane messages.  Routing is made



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      more secure if both the SCION control-plane CA and AS certificates
      can be renewed on a very regular basis.  Would the control-plane
      CA and AS certificates be part of the TRC, then the TRC would have
      to be updated constantly, which is undesirable.

2.1.6.  Certificates - Formal Overview

   Table 2 and Table 3 below provide a formal overview of the different
   types of key pairs and certificates in the control-plane PKI.

   +======================+==============+============================+
   | Name                 | Notation (1) | Used to verify/sign        |
   +======================+==============+============================+
   | Sensitive voting key | K_sens       | TRC updates (sensitive)    |
   +----------------------+--------------+----------------------------+
   | Regular voting key   | K_reg        | TRC updates (regular)      |
   +----------------------+--------------+----------------------------+
   | CP root key          | K_root       | CP CA certificates         |
   +----------------------+--------------+----------------------------+
   | CP CA key            | K_CA         | CP AS certificates         |
   +----------------------+--------------+----------------------------+
   | CP AS key            | K_AS         | CP messages, path segments |
   +----------------------+--------------+----------------------------+

                            Table 2: Key chain

   (1) K_x = PK_x + SK_x, where x = certificate type, PK_x = public key,
   and SK_x = private key























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   +==============+==========+============+=================+==========+
   | Name         | Notation | Signed     | Contains        | Validity |
   |              |          | with       |                 | (2)      |
   +==============+==========+============+=================+==========+
   | TRC (trust   | TRC      | SK_sens,   | C_root, C_sens, | 1 year   |
   | root conf.)  |          | SK_reg (1) | C_reg (1)       |          |
   +--------------+----------+------------+-----------------+----------+
   | Sensitive    | C_sens   | SK_sens    | PK_sens         | 5 years  |
   | voting       |          |            |                 |          |
   | cert.        |          |            |                 |          |
   +--------------+----------+------------+-----------------+----------+
   | Regular      | C_reg    | SK_reg     | PK_reg          | 1 year   |
   | voting       |          |            |                 |          |
   | cert.        |          |            |                 |          |
   +--------------+----------+------------+-----------------+----------+
   | CP root      | C_root   | SK_root    | PK_root         | 1 year   |
   | certificate  |          |            |                 |          |
   +--------------+----------+------------+-----------------+----------+
   | CP CA        | C_CA     | SK_root    | PK_CA           | 11 days  |
   | certificate  |          |            |                 | (3)      |
   +--------------+----------+------------+-----------------+----------+
   | CP AS        | C_AS     | SK_CA      | PK_AS           | 3 days   |
   | certificate  |          |            |                 |          |
   +--------------+----------+------------+-----------------+----------+

                           Table 3: Certificates

   (1) Multiple signatures and certificates of each type may be included
   in a TRC.
   (2) Recommended maximum validity period.
   (3) A validity of 11 days with 4 days overlap between two CA
   certificates is recommended to enable the best possible operational
   procedures when performing a CA certificate rollover.

   Figure 2 illustrates, at a high level, the relationship between a TRC
   and the five types of certificates.

   +--------------------+     +--------------------+          +--------------+     +---------------+
   |       TRC 1        +---->|       TRC 2       -+------>╳  |       TRC 3  +---->|       TRC 4   |
   |  (base, initial)   |     |  (regular update)  |          | (base, trust |     | (sensitive    |
+--+--------------------+     +--------------------+------+   |     reset)   |     |     update)   |
|                                                         |   +--------------+     +---------------+
|                                                         |
+--------------------------------------------+        +---+----------------------------------------+
|             TRC 1 (base, initial)          |        |             TRC 2 (regular update)         |
|+------------------------------------------+|        |+------------------------------------------+|
||- Version       - Core ASes               ||        ||- Version       - Core ASes               ||
||- ID            - Description             ||        ||- ID            - Description             ||



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||- Validity      - No Trust Reset          ||        ||- Validity      - No Trust Reset          ||
||- Grace Period  - Voting Quorum           ||        ||- Grace Period  - Voting Quorum           ||
||- ...                                     ||        ||- ...                                     ||
|+------------------------------------------+|        |+------------------------------------------+|
|+--------------------++--------------------+|        |+--------------------++--------------------+|
||Votes (cert.indices)||   Regular Voting   ||        ||Votes (cert.indices)||   Regular Voting   ||
||                    ||    Certificates    ||        ||                    ||    Certificates    ||
||    (empty)         ||                    ||        ||    (1),(2)...      ||                    ||
||                    ||+-----+ +-----+     ||        ||                    ||+-----+ +-----+     ||
||                    ||| (1) | | (2) |     ||        ||                    ||| (1) | | (2) |     ||
||                    |||C    | |C    | ... ||        ||                    |||C    | |C    | ... ||
||                    ||| reg | | reg |     ||        ||                    ||| reg | | reg |     ||
|+--------------------+|+--+--+ +--+--+     ||        |+--------------------+|+-----+ +-----+     ||
|+--------------------+|   |       |        ||        |+--------------------+|                    ||
||                    ||   |       +--------++-----+  ||                    ||                    ||
||                    ||   +----------------++-+   |  ||                    ||                    ||
||    Signatures      |+--------------------+| |   |  ||    Signatures      |+--------------------+|
||                    |+--------------------+| |   |  ||                    |+--------------------+|
||+------------------+|| Sensitive Voting   || |   |  ||+------------------+|| Sensitive Voting   ||
|||73 A9 4E AO 0D ...|||    Certificates    || |   +--+>|48 AE E4 80 DB ...|||    Certificates    ||
||+------------------+||+-----+ +-----+     || |      ||+------------------+||+-----+ +-----+     ||
||+------------------+||| (3) | | (4) |     || |      ||+------------------+||| (3) | | (4) |     ||
|||53 B7 7C 98 56 ...||||C    | |C    |     || +------+>|7E BC 75 98 25 ...||||C    | |C    |     ||
||+------------------+||| sens| | sens| ... ||        ||+------------------+||| sens| | sens| ... ||
||        ...         ||+-----+ +-----+     ||        ||        ...         ||+-----+ +-----+     ||
|+--------------------++--------------------+|        |+--------------------++--------------------+|
|+------------------------------------------+|        |+------------------------------------------+|
||          CP Root Certificates            ||        ||          CP Root Certificates            ||
||                                          ||        ||                                          ||
|| +-----+ +-----+ +-----+ +-----+          ||        || +-----+ +-----+ +-----+ +-----+          ||
|| | (5) | | (6) | | (7) | | (8) |          ||        || | (5) | | (6) | | (7) | | (8) |          ||
|| |C    | |C    | |C    | |C    |          ||        || |C    | |C    | |C    | |C    |          ||
|| | root| | root| | root| | root| .....    ||        || | root| | root| | root| | root| .....    ||
|| +-----+ +--+--+ +-----+ +--+--+          ||        || +-----+ +--+--+ +-----+ +--+--+          ||
|+------------+---------------+-------------+|        |+------------+---------------+-------------+|
+-------------+---------------+--------------+        +-------------+---------------+--------------+
              |               |                                     |               |
    +---------v-+           +-v---------+                 +---------v-+           +-v---------+
    |   CP CA   |           |   CP CA   |                 |   CP CA   |           |   CP CA   |
    |Certificate|           |Certificate|                 |Certificate|           |Certificate|
    +-----+-----+           +-----+-----+                 +-+-------+-+           +-----+-----+
          |                       |                         |       |                   |
          |                       |                         |       |                   |
          v                       v                         v       v                   v
    +-----------+           +-----------+          +-----------+ +-----------+        +-----------+
    |   CP AS   |           |   CP AS   |          |   CP AS   | |   CP AS   |        |   CP AS   |
    |Certificate|           |Certificate|          |Certificate| |Certificate|        |Certificate|
    +-----------+           +-----------+          +-----------+ +-----------+        +-----------+



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   Figure 2: TRC update chain and the different types of associated
   certificates.  Arrows show how signatures are verified; in other
  words, they indicate that a public key contained in a certificate
    or TRC can be used to verify the authenticity of another item.

2.2.  Certificate Specification

   All certificates used in the SCION control-plane PKI are X.509 v3
   certificates.  However, the SCION specification is in some places
   more restrictive.  This section defines these additional constraints
   and conditions compared to [RFC5280] for each type of SCION control-
   plane PKI certificate.

   *Note*: The settings for the SCION-specific constraints and
   conditions are based on the SCION open-source implementation
   scionproto (https://github.com/scionproto/scion/).  Adjusting these
   settings to the requirements of a customer implementation may be
   possible and is allowed.

2.2.1.  Basic Fields: SCION-Specific Constraints and Conditions

   This section briefly describes the fields of the SCION control-plane
   PKI certificates based on X.509.  These fields are relevant for each
   SCION certificate used in the control plane, regardless of the
   certificate type.  For detailed descriptions of the full generic
   format of X.509 v3 certificates, see [RFC5280] and X509
   (https://handle.itu.int/11.1002/1000/13031), clause 7.2.
   Additionally, the section lists the SCION-specific constraints and
   conditions compared to [RFC5280], per certificate field.

   TBSCertificate sequence: Contains information associated with the
   subject of the certificate and the CA that issued it.  It includes
   the following fields:

   *  version field: Describes the version of the encoded certificate.

      -  *SCION constraints*: "v1" and "v2" are not allowed.

      -  *Additional conditions and remarks*: MUST be set to "v3" (as
         extensions are used and mandatory in SCION).

   *  serialNumber field: A positive integer assigned by the CA to each
      certificate.  It MUST be unique for each certificate issued by a
      given CA.

   *  signature field: Contains the identifier for the algorithm used by
      the CA to sign the certificate.




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      -  *SCION constraints*: Currently, SCION only supports the ECDSA
         signature algorithm.  Find all details here: Section 2.2.1.1.

      -  *Additional conditions and remarks*: As a consequence, the
         parameters field in the AlgorithmIdentifier sequence MUST NOT
         be used.

   *  issuer field: Contains the distinguished name (DN) of the entity
      that has issued and signed the certificate (usually a CA).

      -  *SCION constraints*:

         o  This field MUST be non-empty.

         o  SCION implementations MUST ONLY use the “UTF8String” value
            type for all attributes (including the SCION-specific
            attribute ISD-AS number).

      -  *Additional conditions and remarks*: All SCION implementations
         MUST support the additional SCION-specific attribute ISD-AS
         number.  For details, see Section 2.2.1.2 and
         Section 2.2.1.2.1.

   *  validity field: Defines the validity period of the certificate.

      -  *SCION constraints*: All certificates used in SCION's control-
         plane PKI MUST have a well-defined expiration date.
         Certificates with a generalized time value are not valid and
         MUST be rejected.

      -  *Additional conditions and remarks*: SCION recommends a
         specific maximum validity period for each type of control-plane
         PKI certificate.  For details, see Section 2.1.6.  SCION
         implementations should adopt these values.

   *  subject field: Defines the entity that owns the certificate.

      -  *SCION constraints*:

         o  This field MUST be non-empty.

         o  SCION implementations MUST ONLY use the “UTF8String” value
            type for all attributes (including the SCION-specific
            attribute ISD-AS number).

      -  *Additional conditions and remarks*: The subject field is
         specified in the same way as the issuer field.  For details,
         see Section 2.2.1.2 and Section 2.2.1.2.1.



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   *  subjectPublicKeyInfo field: Carries the public key of the
      certificate's subject (the entity that owns the certificate, as
      defined in the subject field).  The subjectPublicKeyInfo field
      also identifies which algorithm to use with the key.

      -  *SCION constraints*: For constraints regarding the algorithm,
         see the signature field.

   *  issuerUniqueID field: If set, it enables reusing the issuer name
      over time.

      -  *SCION constraints*: This field is disallowed in SCION and MUST
         NOT be used.

   *  subjectUniqueID field: If set, it enables reusing the subject name
      over time.

      -  *SCION constraints*: This field is disallowed in SCION and MUST
         NOT be used.

   *  extensions sequence: Defines the extensions of the certificate.
      For a description of all extensions used in SCION, see
      Section 2.2.2.

2.2.1.1.  signature Field - Additional Information

   For security reasons, SCION uses a custom list of acceptable
   signature algorithms.  This list of acceptable signature algorithms
   is specified in the signature field.  The list currently only
   contains the ECDSA signature algorithm (defined in X962
   (https://webstore.ansi.org/standards/ascx9/ansix9621998)).  However,
   the list might be extended in the future.  The Object Identifiers
   (OIDs) for ECDSA are defined as ecdsa-with-SHA256, ecdsa-with-SHA384,
   and ecdsa-with-SHA512 in [RFC5758].

   *Important:* The accepted cryptographic algorithms listed in this
   document are the only currently accepted cryptographic algorithms.
   SCION implementations MUST reject cryptographic algorithms not found
   in the list.

   The only accepted curves for ECDSA are:

   *  NIST P-256 (NISTFIPS186-4, section D.1.2.3) (named secp256r1 in
      [RFC5480])

   *  NIST P-384 (NISTFIPS186-4, section D.1.2.4) (named secp384r1 in
      [RFC5480])




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   *  NIST P-521 (NISTFIPS186-4, section D.1.2.5) (named secp521r1 in
      [RFC5480])

   The OIDs for the above curves are specified in section 2.1.1.1 of
   [RFC5480].

   The appropriate hash size to use when producing a signature with an
   ECDSA key is:

   *  ECDSA with SHA-256, for a P-256 signing key

   *  ECDSA with SHA-384, for a P-384 signing key

   *  ECDSA with SHA-512, for a P-521 signing key

   *Important:* SCION implementations MUST include support for P-256,
   P-384, and P-521.

2.2.1.2.  issuer Field - Additional Information

   The issuer field contains the distinguished name (DN) of the CA that
   created the certificate.  [RFC5280], section 4.1.2.4, describes the
   field's syntax and attributes.  In addition to these attributes,
   SCION implementations MUST also support the SCION-specific attribute
   ISD-AS number.  This attribute is specified below.

2.2.1.2.1.  ISD-AS number Attribute

   The ISD-AS number attribute identifies the SCION ISD and AS.  In the
   SCION open source implementation, the attribute type is id-at-ia,
   defined as:
   id-at-ia AttributeType ::= {id-scion id-cppki(1) id-at(2) 1}

   where id-scion specifies the root SCION object identifier (OID).

   *Note*: The root SCION object identifier (OID) for the SCION open-
   source implementation is the IANA Private Enterprise Number '55324':
   id-scion ::= OBJECT IDENTIFIER {1 3 6 1 4 1 55324}

   The following points apply when setting the attribute value of the
   ISD-AS number attribute:

   *  The string representation MUST follow the canonical formatting
      defined in ISD and AS numbering
      (https://github.com/scionproto/scion/wiki/ISD-and-AS-numbering).

   *  The canonical string representation uses a dash separator between
      the ISD and AS numbers.



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   *  The ISD numbers are formatted as decimal.

   *  The canonical string formatting of AS numbers in the BGP AS range
      (0, 2^(32-1)) is the decimal form.  Larger AS numbers, i.e., from
      2^32 to 2^(48-1), use a 16-bit, colon-separated, lower-case, hex
      encoding with leading zeros omitted: 1:0:0 to ffff:ffff:ffff.

   *Example:* AS ff00:0:110 in ISD 1 is formatted as 1-ff00:0:110.

   The ISD-AS number attribute MUST be present exactly once in the
   distinguished name of the certificate issuer or owner, specified in
   the issuer or subject field, respectively.  Implementations MUST NOT
   create nor successfully verify certificates whose issuer and subject
   fields do not include the ISD-AS number at all, or include it more
   than once.

   *Note*: Voting certificates are not required to include the ISD-AS
   number attribute in their distinguished name.

2.2.2.  Extensions

   [RFC5280], section 4.2.1, defines the syntax of the Extensions
   sequence in a X.509 certificate.  Descriptions of each standard
   certificate extension can be found in [RFC5280], section 4.2.1.  The
   corresponding clauses in X509
   (https://handle.itu.int/11.1002/1000/13031) (10/2016) are clause 7.2
   and clause 9, respectively.

   Currently, the following extensions are relevant for SCION:

   *  authorityKeyIdentifier

   *  subjectKeyIdentifier

   *  keyUsage

   *  extKeyUsage

   *  basicConstraints

   The following sections describe the SCION-specifics in regard to
   these extensions.

2.2.2.1.  authorityKeyIdentifier Extension

   The authorityKeyIdentifier extension identifies the public key
   corresponding to the private key used to sign a certificate.




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   For the syntax and definition of the authorityKeyIdentifier
   extension, see [RFC5280], section 4.2.1.1, and X509
   (https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.1.

   The authorityKeyIdentifier extension provides three attributes to
   specify the public key:

   *  keyIdentifier

   *  authorityCertIssuer

   *  authorityCertSerialNumber

   In SCION, using the keyIdentifier attribute is the preferred way to
   specify the authorityKeyIdentifier extension.

   *Important:* SCION implementations may also support the use of the
   authorityCertIssuer and authorityCertSerialNumber attributes.
   However, if these attributes are set and support for them is missing,
   implementations should error out.

   This extension MUST always be non-critical.  However, SCION
   implementations MUST error out if the extension is not present AND
   the certificate is not self-signed.

2.2.2.2.  subjectKeyIdentifier Extension

   The subjectKeyIdentifier extension identifies certificates that
   contain a particular public key.  It can be used, for example, by
   control-plane messages to identify which certificate to use for
   verification.  The extension allows for overlapping control-plane CA
   keys, for example during updates.

   For the syntax and definition of the subjectKeyIdentifier extension,
   see [RFC5280], section 4.2.1.2, and X509
   (https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.2.

   This extension MUST always be non-critical.  However, SCION
   implementations MUST error out if the extension is not present.

2.2.2.3.  keyUsage Extension

   The keyUsage extension identifies the intended usage of the public
   key in the corresponding certificate.  For the syntax and definition
   of the keyUsage extension, see [RFC5280], section 4.2.1.3, and X509
   (https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.3.





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   The attributes of the keyUsage extension define possible ways of
   using the public key.  The attributes have the following meaning in
   SCION:

   *  digitalSignature: The public key can be used to verify the digital
      signature of a control-plane payload.

   *  contentCommitment: Not used.

   *  keyEncipherment: Not used.

   *  dataEncipherment: Not used.

   *  keyAgreement: Not used.

   *  keyCertSign: The public key can be used to verify the CA signature
      on a control-plane certificate.

   *  cRLSign: Not used.

   *  encipherOnly: Not used.

   *  decipherOnly: Not used.

   *Important:* If a certificate’s public key is used to verify the
   signature of a control-plane payload (digitalSignature attribute), it
   must be possible to trace back the private key used to sign the
   certificate.  This is done by referencing the ISD-AS and the subject
   key identifier (via the subjectKeyIdentifier extension).  For more
   information about the subjectKeyIdentifier extension, see
   Section 2.2.2.2.

   If present, the keyUsage extension should be marked as "critical".
   That is, the critical Boolean attribute of this extension must be set
   to TRUE (the default is FALSE).

   *Note*: If a certificate extension is marked "critical", the public
   key in the certificate should only be used for the purpose set in the
   critical extension.

   Each control-plane PKI certificate type uses the public key
   differently, and consequently also specifies the attributes of the
   keyUsage extension differently.  The next table shows the
   specifications per certificate type.







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    +==================+=======+=========+=========+==================+
    | Certificate Type |Root   | CA      | AS      | Voting (regular  |
    |                  |       |         |         | and sensitive)   |
    +==================+=======+=========+=========+==================+
    | _Attribute:_     |       |         |         |                  |
    +------------------+-------+---------+---------+------------------+
    | keyUsage         |MUST be| MUST be | MUST be | MAY be present   |
    | extension itself |present| present | present | (but is not      |
    |                  |       |         |         | required)        |
    +------------------+-------+---------+---------+------------------+
    | digitalSignature |MUST   | MUST    | MUST be | If the extension |
    |                  |NOT be | NOT be  | set     | is present, the  |
    |                  |set (1)| set (2) |         | digitalSignature |
    |                  |       |         |         | attribute MUST   |
    |                  |       |         |         | NOT be set       |
    +------------------+-------+---------+---------+------------------+
    | keyCertSign      |MUST be| MUST be | MUST    | If the extension |
    |                  |set    | set     | NOT be  | is present, the  |
    |                  |       |         | set     | keyCertSign      |
    |                  |       |         |         | attribute MUST   |
    |                  |       |         |         | NOT be set       |
    +------------------+-------+---------+---------+------------------+

     Table 4: keyUsage extension - Specifications per certificate type

   (1) The root certificate should not be used to verify control-plane
   messages.
   (2) The CA certificate should not be used to verify control-plane
   messages.

2.2.2.4.  extKeyUsage Extension

   The extKeyUsage extension specifies additional usages of the public
   key in the certificate.  For the syntax and definition of the
   extKeyUsage extension, see X509
   (https://handle.itu.int/11.1002/1000/13031), clause 9.2.2.4.

   SCION uses the following attributes of the Extended Key Usage
   extension, as defined in Section 4.2.1.12 of [RFC5280]:

   *  id-kp-serverAuth: If set, the public key can be used for SCION
      control-plane server authentication.

   *  id-kp-clientAuth: If set, the public key can be used for SCION
      control-plane client authentication.

   *  id-kp-timeStamping: If set, the public key can be used for the
      verification of timestamps.



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   Additionally, the Extended Key Usage extension sequence may include
   the SCION-specific attributes id-kp-root, id-kp-regular, and id-kp-
   sensitive.  These attributes are used in the Trust Root Configuration
   setup, to distinguish root certificates, regular voting certificates,
   and sensitive voting certificates from each other.  For more
   information, see Section 3.1.2.2.11.

   The specifications of the extKeyUsage extension differ per SCION
   control-plane PKI certificate type.  The next table provides an
   overview of the specifications per certificate type.

   +==============+===========+===========+=============+==============+
   | Certificate  | Root      | CA        | AS          | Voting       |
   | Type         |           |           |             | (regular     |
   |              |           |           |             | and          |
   |              |           |           |             | sensitive)   |
   +==============+===========+===========+=============+==============+
   | _Attribute:_ |           |           |             |              |
   +--------------+-----------+-----------+-------------+--------------+
   | extKeyUsage  | MUST be   | MAY be    | MUST be     | MUST be      |
   | extension    | present   | present   | present     | present      |
   | itself       |           | (not      |             |              |
   |              |           | required) |             |              |
   +--------------+-----------+-----------+-------------+--------------+
   | id-kp-       | MUST NOT  | MUST NOT  | MUST be     | MUST NOT be  |
   | serverAuth   | be        | be        | included,   | included     |
   |              | included  | included  | if the      |              |
   |              |           |           | certificate |              |
   |              |           |           | is used on  |              |
   |              |           |           | the server- |              |
   |              |           |           | side of a   |              |
   |              |           |           | control-    |              |
   |              |           |           | plane TLS   |              |
   |              |           |           | session.    |              |
   +--------------+-----------+-----------+-------------+--------------+
   | id-kp-       | MUST NOT  | MUST NOT  | MUST be     | MUST NOT be  |
   | clientAuth   | be        | be        | included,   | included     |
   |              | included  | included  | if the      |              |
   |              |           |           | certificate |              |
   |              |           |           | is used on  |              |
   |              |           |           | the client- |              |
   |              |           |           | side of a   |              |
   |              |           |           | control-    |              |
   |              |           |           | plane TLS   |              |
   |              |           |           | session.    |              |
   +--------------+-----------+-----------+-------------+--------------+
   | id-kp-       | MUST be   |           | MUST be     | MUST be      |
   | timeStamping | included  |           | included    | included     |



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   +--------------+-----------+-----------+-------------+--------------+
   | SCION-       | id-kp-    |           |             | Regular      |
   | specific     | root MUST |           |             | voting       |
   |              | be        |           |             | cert: id-    |
   |              | included. |           |             | kp-regular   |
   |              | For       |           |             | MUST be      |
   |              | details,  |           |             | included.    |
   |              | see       |           |             | For          |
   |              | Section   |           |             | details,     |
   |              | 2.2.2.4.1 |           |             | see Section  |
   |              |           |           |             | 2.2.2.4.1    |
   |              |           |           |             | Sensitive    |
   |              |           |           |             | voting       |
   |              |           |           |             | cert: id-    |
   |              |           |           |             | kp-          |
   |              |           |           |             | sensitive    |
   |              |           |           |             | MUST be      |
   |              |           |           |             | included.    |
   |              |           |           |             | For          |
   |              |           |           |             | details,     |
   |              |           |           |             | see Section  |
   |              |           |           |             | 2.2.2.4.1    |
   +--------------+-----------+-----------+-------------+--------------+

    Table 5: extKeyUsage extension - Specifications per certificate type

2.2.2.4.1.  SCION-Specific Attributes

   The id-kp-root, id-kp-regular, and id-kp-sensitive attributes must be
   specified as follows:

   *  Root certificate:
      id-kp-root AttributeType ::= {id-scion id-cppki(1) id-kp(3) 3}

   *  Regular voting certificate:
      id-kp-regular AttributeType ::= {id-scion id-cppki(1) id-kp(3) 2}

   *  Sensitive voting certificate:
      id-kp-sensitive AttributeType ::= {id-scion id-cppki(1) id-kp(3)
      1}

   where id-scion specifies the root SCION object identifier (OID).

   *Note*: The root SCION object identifier (OID) for the SCION open-
   source implementation is the IANA Private Enterprise Number '55324':
   id-scion ::= OBJECT IDENTIFIER {1 3 6 1 4 1 55324}





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2.2.2.5.  basicConstraints Extension

   The basicConstraints extension specifies whether the certificate
   subject may act as a CA.  For the syntax and definition of the
   basicConstraints extension, see X509
   (https://handle.itu.int/11.1002/1000/13031), clause 9.4.2.1.

   The basicConstraints extension includes the following attributes
   relevant for SCION:

   *  cA attribute: Specifies whether the certificate subject may act as
      a CA.  If yes, this attribute MUST be set to TRUE.

   *  pathLenConstraint attribute: This attribute is only relevant if
      the cA attribute is set to TRUE.  It specifies the maximum number
      of CA certificates that may follow this CA certificate in the
      certification chain.  Value "0" means that this CA may only issue
      end-entity certificates, but no CA certificates.  If the attribute
      is not set, there is no limit to the allowed length of the
      certification path.

   The settings of the basicConstraints extension differ for each SCION
   control-plane PKI certificate type.  The next table shows the
   specifications per certificate type.



























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   +===================+==========+============+=========+============+
   | Certificate Type  |Root      | CA         |AS       | Voting     |
   |                   |          |            |         | (regular   |
   |                   |          |            |         | and        |
   |                   |          |            |         | sensitive) |
   +===================+==========+============+=========+============+
   | _Attribute:_      |          |            |         |            |
   +-------------------+----------+------------+---------+------------+
   | basicConstraints  |MUST be   | MUST be    |SHOULD   | SHOULD NOT |
   | extension itself  |present   | present    |NOT be   | be present |
   |                   |          |            |present  |            |
   +-------------------+----------+------------+---------+------------+
   | cA                |MUST be   | MUST be    |If the   | If the     |
   |                   |set to    | set to     |extension| extension  |
   |                   |TRUE      | TRUE       |is       | is         |
   |                   |          |            |present, | present,   |
   |                   |          |            |this     | this       |
   |                   |          |            |attribute| attribute  |
   |                   |          |            |MUST be  | MUST be    |
   |                   |          |            |set to   | set to     |
   |                   |          |            |FALSE    | FALSE      |
   +-------------------+----------+------------+---------+------------+
   | pathLenConstraint |SHOULD be | SHOULD be  |If the   | If the     |
   |                   |set to    | set to "0" |extension| extension  |
   |                   |"1", MUST | (1), MUST  |is       | is         |
   |                   |be marked | be marked  |present, | present,   |
   |                   |as        | as         |this     | this       |
   |                   |"critical"| "critical" |attribute| attribute  |
   |                   |          |            |MUST be  | MUST be    |
   |                   |          |            |absent.  | absent.    |
   +-------------------+----------+------------+---------+------------+

         Table 6: basicConstraints extension - Specifications per
                             certificate type

   (1) Control-plane CAs can only issue end-entity certificates.

3.  Trust Root Configuration Specification

   This section provides an in-depth specification of the trust root
   configuration (TRC) file (see Section 3.1).  The TRC contains policy
   information about an ISD and acts as a distribution mechanism for the
   trust anchors of that ISD.  It enables securing the control-plane
   interactions, and is thus an integral part of the SCION
   infrastructure.






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   The initial TRC of an ISD is signed during a signing ceremony and
   then distributed throughout the ISD.  This signing ceremony follows
   specific rules; Section 3.2 describes these rules.

3.1.  TRC Specification

   The trust root configuration (TRC) is a signed collection of X.509
   (https://handle.itu.int/11.1002/1000/13031) v3 certificates.
   Additionally, the TRC contains ISD-specific policies encoded in a
   Cryptographic Message Syntax (CMS) [RFC5652] envelope.

   The TRC's certificates collection consists of a set of control-plane
   root certificates, which build the root of the certification chain
   for the AS certificates in an ISD.  The other certificates in the TRC
   are solely used for signing the next TRC, a process called "voting".
   The verification of a new TRC thus depends on the policies and voting
   certificates defined in the previous TRC.

   *Note:* See Section 2 for the general specifications of SCION's
   control-plane PKI certificates, as well as Section 2.1.2 and
   Section 2.1.5, for the specifications of the control-plane root
   certificates and voting certificates, respectively.

   This section provides a detailed specification of the TRC.  It
   presents the TRC format definitions and describes the TRC payload
   fields.  The section uses the ITU-T X.680
   (https://handle.itu.int/11.1002/1000/14468) syntax.

3.1.1.   TRC Types and States

   The following types of TRCs exist:

   *  Initial: The very first TRC of an ISD is the initial TRC of that
      ISD.  It is a special case of the base TRC, where the number of
      the ISD is specified.

   *  Base: A base TRC is either the initial TRC, or the first TRC after
      a trust reset (see Section 1.4.1).  Trust for a base TRC cannot be
      inferred by verifying a TRC update; base TRCs are trusted
      axiomatically, similarly to how root CA certificates are trusted
      by clients in the Web PKI.

   *  Update: All non-base TRCs are updated TRCs.  They are the product
      of either a regular or a sensitive update.

   A TRC can have the following states:





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   *  Valid: The validity period of a TRC is defined in the TRC itself,
      in the validity field (see Section 3.1.2.2.3).  A TRC is
      considered valid if the current time falls within its validity
      period.

   *  Active: An active TRC is a valid TRC that can be used for
      verifying certificate signatures.  This is either the latest TRC
      or the predecessor TRC, if it is still in its grace period (as
      defined in the gracePeriod field of the new TRC, see
      Section 3.1.2.2.4).  No more than two TRCs can be active at the
      same time for any ISD.

   Figure 3 shows the content of both a base/initial TRC and the first
   regularly-updated TRC based on the base TRC.  All elements of the
   shown TRCs are specified in detail in the following subsections.




































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+--------------------------------------------+        +--------------------------------------------+
|             TRC 1 (base, initial)          |        |             TRC 2 (regular update)         |
|+------------------------------------------+|        |+------------------------------------------+|
||- Version       - Core ASes               ||        ||- Version       - Core ASes               ||
||- ID            - Description             ||        ||- ID            - Description             ||
||- Validity      - No Trust Reset          ||        ||- Validity      - No Trust Reset          ||
||- Grace Period  - Voting Quorum           ||        ||- Grace Period  - Voting Quorum           ||
||- ...                                     ||        ||- ...                                     ||
|+------------------------------------------+|        |+------------------------------------------+|
|+--------------------++--------------------+|        |+--------------------++--------------------+|
||Votes (cert.indices)||   Regular Voting   ||        ||Votes (cert.indices)||   Regular Voting   ||
||                    ||    Certificates    ||        ||                    ||    Certificates    ||
||    (empty)         ||                    ||        ||    (1),(2)...      ||                    ||
||                    ||+-----+ +-----+     ||        ||                    ||+-----+ +-----+     ||
||                    ||| (1) | | (2) |     ||        ||                    ||| (1) | | (2) |     ||
||                    |||C    | |C    | ... ||        ||                    |||C    | |C    | ... ||
||                    ||| reg | | reg |     ||        ||                    ||| reg | | reg |     ||
|+--------------------+|+--+--+ +--+--+     ||        |+--------------------+|+-----+ +-----+     ||
|+--------------------+|   |       |        ||        |+--------------------+|                    ||
||                    ||   |       +--------++-----+  ||                    ||                    ||
||                    ||   +----------------++-+   |  ||                    ||                    ||
||    Signatures      |+--------------------+| |   |  ||    Signatures      |+--------------------+|
||                    |+--------------------+| |   |  ||                    |+--------------------+|
||+------------------+|| Sensitive Voting   || |   |  ||+------------------+|| Sensitive Voting   ||
|||73 A9 4E AO 0D ...|||    Certificates    || |   +--+>|48 AE E4 80 DB ...|||    Certificates    ||
||+------------------+||+-----+ +-----+     || |      ||+------------------+||+-----+ +-----+     ||
||+------------------+||| (3) | | (4) |     || |      ||+------------------+||| (3) | | (4) |     ||
|||53 B7 7C 98 56 ...||||C    | |C    |     || +------+>|7E BC 75 98 25 ...||||C    | |C    |     ||
||+------------------+||| sens| | sens| ... ||        ||+------------------+||| sens| | sens| ... ||
||        ...         ||+-----+ +-----+     ||        ||        ...         ||+-----+ +-----+     ||
|+--------------------++--------------------+|        |+--------------------++--------------------+|
|+------------------------------------------+|        |+------------------------------------------+|
||          CP Root Certificates            ||        ||          CP Root Certificates            ||
||                                          ||        ||                                          ||
|| +-----+ +-----+ +-----+ +-----+          ||        || +-----+ +-----+ +-----+ +-----+          ||
|| | (5) | | (6) | | (7) | | (8) |          ||        || | (5) | | (6) | | (7) | | (8) |          ||
|| |C    | |C    | |C    | |C    |          ||        || |C    | |C    | |C    | |C    |          ||
|| | root| | root| | root| | root| .....    ||        || | root| | root| | root| | root| .....    ||
|| +-----+ +-----+ +-----+ +-----+          ||        || +-----+ +-----+ +-----+ +-----+          ||
|+------------------------------------------+|        |+------------------------------------------+|
+--------------------------------------------+        +--------------------------------------------+

   Figure 3: The TRC on the left-hand side is the initial base TRC.
   The TRC on the right is the product of the first regular update
                           of the base TRC.






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3.1.2.   TRC Format

   The trust root configuration (TRC) of an ISD defines the roots of
   trust of the ISD, and builds the base of the ISD's control-plane PKI.
   It holds the root and voting certificates of the ISD and defines the
   ISD's trust policy.

3.1.2.1.   TRC Schema

   The following code block shows the format of a TRC specification file
   (the payload schema):

      TRCPayload  ::=  SEQUENCE {
          version   TRCFormatVersion,
          iD        TRCID,
          validity  Validity,

          gracePeriod   INTEGER,
          noTrustReset  BOOLEAN DEFAULT FALSE,
          votes         SEQUENCE OF INTEGER (SIZE (1..255)),

          votingQuorum  INTEGER (1..255),

          coreASes           SEQUENCE OF ASN,
          authoritativeASes  SEQUENCE OF ASN,
          description        UTF8String (SIZE (0..1024)),

          certificates       SEQUENCE OF Certificate }

      TRCFormatVersion  ::=  INTEGER { v1(0) }

      TRCID  ::=  SEQUENCE {
          iSD           ISD,
          serialNumber  INTEGER (1..MAX),
          baseNumber    INTEGER (1..MAX) }

      ISD  ::=  INTEGER (1..65535)

      Validity  ::=  SEQUENCE {
          notBefore  Time,
          notAfter   Time }

      ASN  ::=  INTEGER (1..281474976710655)

   The TRCPayload sequence contains the identifying information of a TRC
   as well as policy information for TRC updates.  Furthermore, it
   defines the list of certificates that build the trust anchor of the
   ISD.



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   For signature calculation, the data that is to be signed is encoded
   using ASN.1 distinguished encoding rules (DER) X.690
   (https://handle.itu.int/11.1002/1000/14472).  For more details, see
   Section 3.1.3.

3.1.2.2.   TRC Fields

   This section describes the syntax and semantics of all TRC payload
   fields.

3.1.2.2.1.   version Field

   The version field describes the version of the TRC format
   specification.

   Currently, the version MUST always be "v1".

3.1.2.2.2.   iD Field

   The iD field specifies the unique identifier of the TRC.

   The identifier is a unique sequence of

   *  ISD number (iSD attribute),

   *  base number (baseNumber attribute), and

   *  TRC serial number (serialNumber attribute).

   All numbers MUST be positive integers.

   *  The *ISD number* MUST be an integer in the inclusive range from 64
      to 4094 (i.e., the numbering range for public ISDs, see
      Section 1.6.1).

   *  The *base number* indicates the starting point of the current TRC
      update chain.  This starting point is either the ISD's initial TRC
      or the currently valid base TRC, if the valid base TRC differs
      from the initial TRC.  The latter MUST be the case after a trust
      reset.

   *  The *serial number* represents the current update cycle, counting
      from the initial TRC of a specific ISD.








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   A TRC where the base number is equal to the serial number is a base
   TRC.  The initial TRC is a special case of a base TRC.  An ISD's
   initial TRC MUST have a serial number of 1 and a base number of 1.
   With every TRC update, the serial number MUST be incremented by one.
   This facilitates uniquely identifying the predecessor and successor
   TRC in a TRC update chain.

   If a trust reset is necessary, a new base TRC is announced, in order
   to start a new and clean TRC update chain.  The base number of this
   new TRC update chain SHOULD be the number following the serial number
   of the latest TRC that was produced by a non-compromised TRC update
   for this ISD.

   *Example*
   The following simple example illustrates how to specify the ID of the
   TRCs in an TRC update chain for _ISD 74_. The IDs are given in a
   human-readable notation, where Bxx is the base number, and Sxx the
   serial number.

































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   +===========+=================+====================================+
   | Update    | TRC ID          | Remarks                            |
   +===========+=================+====================================+
   | Initial   | ISD74-B01-S01   |                                    |
   +-----------+-----------------+------------------------------------+
   | Regular   | ISD74-B01-S02   | Only the serial number is          |
   |           |                 | incremented.                       |
   +-----------+-----------------+------------------------------------+
   | Regular   | ISD74-B01-S03   | Only the serial number is          |
   |           |                 | incremented.                       |
   +-----------+-----------------+------------------------------------+
   | Sensitive | ISD74-B01-S04   | Only the serial number is          |
   |           |                 | incremented.                       |
   +-----------+-----------------+------------------------------------+
   | Trust     | ISD74-*B05*-S05 | A trust reset includes the         |
   | reset     |                 | creation of a new base TRC.  The   |
   |           |                 | new base number follows the serial |
   |           |                 | number "04" of the latest TRC      |
   |           |                 | resulting from a non-compromised   |
   |           |                 | TRC update for this ISD.           |
   +-----------+-----------------+------------------------------------+
   | Regular   | ISD74-B05-S06   | Only the serial number is          |
   |           |                 | incremented.                       |
   +-----------+-----------------+------------------------------------+
   | Regular   | ISD74-B05-S07   | Only the serial number is          |
   |           |                 | incremented.                       |
   +-----------+-----------------+------------------------------------+
   | And so on |                 |                                    |
   +-----------+-----------------+------------------------------------+

                 Table 7: ID of TRCs in TRC update chain

3.1.2.2.3.   validity Field

   The validity field defines the validity period of the TRC.  This is
   the period of time during which the TRC is in the "valid" state.  The
   notBefore and notAfter attributes of the validity field specify the
   lower and upper bound of the time interval during which a TRC can be
   active.

   *Note:* An active TRC is a valid TRC that can be used for verifying
   certificate signatures.  The time period during which a TRC is active
   can be shorter than the time period during which the TRC is valid.
   For more information, see Section 3.1.1.

   The validity field consists of a sequence of two dates, as defined in
   section 7.2. of X.509 (https://handle.itu.int/11.1002/1000/13031).




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   In addition to this standard definition, the following constraint
   applies to the validity field of the TRC used in SCION:

   *  All TRCs MUST have a well-defined expiration date.  SCION
      implementations MUST NOT create TRCs that use the
      "99991231235959Z" generalized time value, and verifiers MUST error
      out when encountering such a TRC.

3.1.2.2.4.   gracePeriod Field

   The gracePeriod field of a TRC specifies the period of time during
   which the predecessor TRC can still be considered active (the "grace
   period").  The grace period starts at the beginning of the validity
   period of the new TRC.

   The validity period of the predecessor TRC ends when

   *  the grace period has passed,

   *  the predecessor’s expiration time is reached, or

   *  the successor TRC of the new TRC has been announced.

   *Note:* The event that happens first marks the end of the
   predecessor's validity period.

   The gracePeriod field defines the grace period as a number of seconds
   (positive integer).

   The value of the gracePeriod field in a base TRC MUST be zero.  The
   value of the gracePeriod field in a non-base TRC SHOULD be non-zero.
   It should be long enough to provide sufficient overlap between the
   TRCs in order to facilitate interruption-free operations in the ISD.
   If the grace period is too short, some control-plane AS certificates
   might expire before the corresponding AS can fetch an updated version
   from its CA.

3.1.2.2.5.   noTrustReset Boolean

   The noTrustReset Boolean specifies whether a trust reset is forbidden
   by the ISD.  Within a TRC update chain, this value CANNOT be changed
   by a regular or sensitive update.  However, it is possible to change
   the noTrustReset value in the event of a trust reset, where a new
   base TRC is created.

   The noTrustReset field is optional and defaults to FALSE.





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   *Important:* Note that once the noTrustReset Boolean is set to TRUE
   and a trust reset is disallowed, this cannot be reversed.  Therefore,
   ISDs SHOULD always set this value to FALSE, unless they have
   sufficiently assessed the risks and implications of making a trust
   reset impossible.

   *Note:* A trust reset represents a special use case where a new base
   TRC is created.  It therefore differs from a TRC update (regular or
   sensitive), as the signatures in the new base TRC cannot be verified
   with the certificates contained in the predecessor TRC.  Instead, a
   trust reset base TRC must be axiomatically trusted, similarly to how
   the initial TRC is trusted.

3.1.2.2.6.  votes Field

   The votes field contains a sequence of indices that refer to the
   voting certificates in the predecessor TRC.  If index i is part of
   the votes field, then the voting certificate at position i in the
   certificates sequence of the predecessor TRC casted a vote on the
   successor TRC.  For more information on the certificates sequence,
   see Section 3.1.2.2.11.

   *Note:* In a base TRC, the votes sequence is empty.

   Every entry in the votes sequence MUST be unique.
   Further restrictions on votes are discussed in Section 3.1.5.

   *Note:* The votes sequence of indices is mandatory in order to
   prevent stripping voting signatures from the TRC.  Absence of the
   votes sequence makes it possible to transform a TRC with more voting
   signatures than the voting quorum into multiple verifiable TRCs with
   the same payload, but different voting signature sets.  This would
   violate the requirement of uniqueness of a TRC.

3.1.2.2.7.   votingQuorum Field

   The votingQuorum field defines the number of necessary votes on a
   successor TRC to make it verifiable.

   A voting quorum greater than one will prevent a single entity from
   creating a malicious TRC update.

3.1.2.2.8.   coreASes Field

   The coreASes field contains the AS numbers of the core ASes in this
   ISD.





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   Each core AS number MUST be unique in the sequence of core AS
   numbers.  That is, each AS number must appear only once in the
   coreASes field.

3.1.2.2.8.1.   Revoking or Assigning Core Status

   *  To revoke the core status of a given AS, remove the respective AS
      number from the sequence of AS numbers in the coreASes field.

   *  To assign the core status to a given AS, add the respective AS
      number to the sequence of AS numbers in the coreASes field.

   *Important:* Revoking or assigning the core status of/to an AS always
   requires a (sensitive) TRC update.

3.1.2.2.9.   authoritativeASes Field

   The authoritativeASes field contains the AS numbers of the
   authoritative ASes in this ISD.

   Authoritative ASes are those ASes in an ISD that always have the
   latest TRCs of the ISD.  As a consequence, authoritative ASes also
   start the announcement of a TRC update.

   *  Every authoritative AS MUST be a core AS and be listed in the
      coreASes field.

   *  Each authoritative AS number MUST be unique in the sequence of
      authoritative AS numbers.  That is, each AS number must appear
      only once in the authoritativeASes field.

3.1.2.2.9.1.   Revoking or Assigning Authoritative Status

   *  To revoke the authoritative status of a given AS, remove the
      respective AS number from the sequence of AS numbers in the
      authoritativeASes field.

   *  To assign the authoritative status to a given AS, add the
      respective AS number to the sequence of AS numbers in the
      authoritativeASes field.

   *Important:* Revoking or assigning the authoritative status of/to an
   AS always requires a (sensitive) TRC update.

3.1.2.2.10.   description Field

   The description field contains a UTF-8 encoded string that describes
   the ISD.



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   *  The description field SHOULD NOT be empty.

   *  The description of the ISD MUST be in English.  Additionally, the
      description field MAY contain information in other languages.

3.1.2.2.11.  certificates Field

   The voting ASes and the certification authorities (CAs) of an ISD are
   not specified explicitly in the ISD's TRC.  Instead, this information
   is defined by the list of voting and root certificates in the
   certificates field of the TRC payload.

   The certificates field is a sequence of self-signed X.509
   certificates.  Each certificate in the certificate sequence must be
   one of the following types:

   *  a sensitive voting certificate,

   *  a regular voting certificate, or

   *  a CP root certificate.

   A certificate that is no control-plane root or voting certificate
   MUST NOT be included in the sequence of certificates in the
   certificates field.

   *Note*: A certificate's type (voting or root) is specified in the
   extKeyUsage extension of the certificate, by means of the SCION-
   specific attributes id-kp-regular, id-kp-sensitive, and id-kp-root,
   respectively.  For more information, see Section 2.2.2.4.

   The following constraints MUST hold for each certificate in the
   certificates field of the TRC payload:

   *  Each certificate MUST be unique in the sequence of certificates.
      That is, each certificate must appear only once in the
      certificates field.

   *  The issuer / serialNumber pair for each certificate MUST be
      unique.

   *  If an ISD-AS number is present in the distinguished name of the
      certificate, this ISD number MUST be equal to the ISD number of
      the TRC (which is defined in the iD field (see Section 3.1.2.2.2).

   *  Every certificate MUST have a validity period that fully contains
      the validity period of this TRC.  That is, the notBefore date of
      this TRC's validity period MUST be equal to or later than the



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      certificate's notBefore date, and the notAfter date of this TRC's
      validity period MUST be before or equal to the certificate's
      notAfter date.

   *  Per certificate type, every certificate distinguished name MUST be
      unique.

   The following must hold for the entire sequence of certificates in
   the certificates field:

   *  votingQuorum <= count (sensitive voting certificates)
      That is, the quorum defined in the TRC's votingQuorum field
      (Section 3.1.2.2.7) must be smaller than or equal to the number of
      _sensitive_ voting certificates specified in the TRC's
      certificates field.

   *  votingQuorum <= count (regular voting certificates)
      That is, the quorum defined in the TRC's votingQuorum field
      (Section 3.1.2.2.7) must be smaller than or equal to the number of
      _regular_ voting certificates specified in the TRC's certificates
      field.

3.1.3.   TRC Signature Syntax

   A TRC contains policy information about an ISD and acts as a
   distribution mechanism for the trust anchors of that ISD.  Each TRC
   (payload) is digitally signed.  The syntax used to sign and
   encapsulate the TRC payload is the Cryptographic Message Syntax
   (CMS), as defined in [RFC5652].  The signed TRC payload is of the CMS
   signed-data content type, as defined in Section 5 of [RFC5652], and
   encapsulated in a CMS ContentInfo element, as defined in Section 3 of
   [RFC5652].  For detailed information on the general syntax
   definitions of the Cryptographic Message Syntax, see sections 3 and 5
   of [RFC5652].

3.1.3.1.  SCION-specific rules

   SCION implementations have to fulfil the following additional rules,
   on top of the general syntax rules from [RFC5652]:

   *  EncapsulatedContentInfo sequence:

      -  The eContentType field must be set to "id-data".

      -  The content of the eContent field must be the DER-encoded TRC
         payload.  This has the benefit that the format is backwards
         compatible with PKCS #7, as described in Section 5.2.1 of
         [RFC5652].



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   *  SignedData sequence:

      -  The certificates field MUST be left empty.  The certificate
         pool used to verify a TRC update is already specified in the
         certificates field of the predecessor TRC's payload (see also
         Section 3.1.2.2.11).

      -  The version field MUST be set to "1".  This is because SCION
         uses the "id-data" content type to encapsulate content info,
         and does not specify any certificate in the SignedData sequence
         (see also Section 5.1 of [RFC5652]).

   *  SignerIdentifier choice:

      -  The type of signer identifier chosen here MUST be
         IssuerAndSerialNumber.

   *  SignerInfo sequence:

      -  The version field MUST be set to "1".  This is because SCION
         uses the IssuerAndSerialNumber type of signer identifier (see
         also Section 5.3 of [RFC5652]).

      -  The algorithm specified in the signatureAlgorithm field MUST be
         one of the algorithms supported by SCION (for details, see
         signature Field - Additional Information (Section 2.2.1.1)).

      -  The digestAlgorithm is determined by the algorithm specified in
         the signatureAlgorithm field.

3.1.3.2.   TRC Equality

   The signer information in the signed TRC is part of an unordered set,
   per [RFC5652].  This implies that the signer information can be
   reordered without affecting verification.  Certain operations,
   however, require TRCs to be equal, according to the following
   equality definition:

   *Two TRCs are equal, if and only if their payloads are byte equal.*

   Two TRCs with byte equal payloads can be considered as equal, because
   the TRC payload exactly defines which signatures must be attached in
   the signed TRC:








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   *  The required signatures from voting certificates are explicitly
      mentioned in the votes field of the payload: If index "i" is part
      of the votes field, then the voting certificate at position i in
      the certificates sequence of the predecessor TRC casted a vote on
      the successor TRC.  See also Section 3.1.2.2.6.

   *  The required signatures for new certificates are implied by the
      currently valid TRC payload, and, in case of a TRC update, the
      predecessor payload.

3.1.4.   Certification Path - Trust Anchor Pool

   The certification path of a control-plane AS certificate starts in a
   control-plane root certificate.  The control-plane root certificates
   for a given ISD are distributed via the TRC.  However, AS
   certificates and the corresponding signing CA certificates are *not*
   part of the TRC, but bundled into certificate chains and distributed
   separately from the corresponding TRC.  This separation makes it
   possible to extend the validity period of the root certificate, and
   to update the corresponding TRC, without having to modify the
   certificate chain.  To be able to validate a certification path, each
   AS must build a collection of root certificates from the latest TRC
   of the relevant ISD.  The following section explains how to build a
   trust anchor pool.

   *Note:* Any entity sending information that is secured by the CP-PKI,
   such as control-plane messages, MUST be able to provide all the
   necessary trust material, such as certificates, to verify said
   information.  If any cryptographic material is missing in the
   process, the relying party MUST query the originator of the message
   for the missing material.  If it cannot be resolved, the verification
   process fails.  For more details, see 4.2 "Signing and Verifying
   Control-Plane Messages" Section 4.2.

3.1.4.1.   TRC Selection For Trust Anchor Pool

   The selection of the right set of TRCs to build the trust anchor pool
   depends on the time of verification.  The trust anchor pool is
   usually used to verify control-plane messages.  In this case, the
   time of verification is the current time.  However, if the trust
   anchor pool will be used for auditing, the time of verification is
   the point in time for which you want to check whether a given
   signature was verifiable.

   The selection algorithm for building the trust anchor pool is
   described in pseudo-python code below.





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       def select_trust_anchors(trcs: Dict[(int,int), TRC], verification_time: int) -> Set[RootCert]:
           """
           Args:
               trcs: The dictionary mapping (serial number, base number) to the TRC for a given ISD.
               verification_time: The time of verification.

           Returns:
               The set of CP Root certificates that act as trust anchors.
           """
           # Find highest base number that has a TRC with a validity period
           # starting before verification time.
           base_nr = 1
           for trc in trcs.values():
               if trc.id.base_nr > base_nr and trc.validity.not_before <= verification_time:
                   base_nr = trc.id.base_nr

           # Find TRC with highest serial number with the given base number and a
           # validity period starting before verification time.
           serial_nr = 1
           for trc in trcs[isd].values():
               if trc.id.base_nr != base_nr:
                   continue
               if trc.id.serial_nr > serial_nr and trc.validity.not_before <= verification_time:
                   serial_nr = trc.id.serial_nr

           candidate = trcs[(serial_nr, base_nr)]

           # If the verification time is not inside the validity period,
           # there is no valid set of trust anchors.
           if not candidate.validity.contains(verification_time):
               return set()

           # If the grace period has passed, only the certificates in that TRCs
           # may be used as trust anchors.
           if candidate.validity.not_before + candidate.grace_period < verification_time:
               return collect_trust_anchors(candidate)

           predecessor = trcs.get((serial_nr-1, base_nr))
           if not predecessor or predecessor.validity.not_after < verification_time:
               return collect_trust_anchors(candidate)

           return collect_trust_anchors(candidate) | collect_trust_anchors(predecessor)


       def collect_trust_anchors(trc: TRC) -> Set[RootCert]:
           """
           Args:
               trc: A TRC from which the CP Root Certificates shall be extracted.



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           Returns:
               The set of CP Root certificates that act as trust anchors.
           """
           roots = set()
           for cert in trc.certificates:
               if not cert.basic_constraints.ca:
                   continue
               roots.add(cert)
           return roots

3.1.5.   TRC Updates

   All non-base TRCs of an ISD are updates of the ISD's base TRC(s).
   The TRC update chain consists of regular and sensitive TRC updates.
   The type of update determines the (payload) information that changes
   in the updated TRC.  This section describes the rules that apply to
   updating a TRC in regard to the payload information contained in the
   TRC.  Some rules are valid for both update types, some only apply to
   a regular or a sensitive TRC update, respectively.  Based on the type
   of update, a different set of voters is necessary to create a
   verifiable TRC update - the section includes a description of the
   corresponding voting (signing) process.  To verify a TRC update, a
   relying party must perform a couple of checks.  These checks are
   listed at the end of the section.

3.1.5.1.   Changed or New Certificates

   In the context of a TRC update,

   *  A certificate is _changing_, if the certificate is part of the
      certificates sequence in the predecessor TRC, but no longer part
      of the certificates sequence in the updated TRC.  Instead, the
      certificates sequence of the updated TRC holds another certificate
      of the _same type_ and with the _same distinguished name_.

   *  A certificate is _new_, if there is *no* certificate of the same
      type and distinguished name at all in the certificates sequence of
      the predecessor TRC.

   *Note:* Every new sensitive or regular voting certificate in a TRC
   attaches a signature to the TRC.  This is done to ensure that the
   freshly included voting entity agrees with the contents of the TRC it
   is now part of.








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3.1.5.2.   Update Rules - Overview

   The following table gives an overview of the types of TRC update as
   well as the rules that must apply in regard to the updated TRC's
   payload information.
   The sections that follow provide more detailed descriptions of each
   rule.












































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   +===========+====================+==============+===================+
   | Type of   | Payload Updated    | Payload      | Payload Updated   |
   | Update    | TRC - Unchanged    | Updated TRC  | TRC: Other Rules  |
   |           | Elements           | - Required   | to Hold           |
   |           |                    | Changes      |                   |
   +===========+====================+==============+===================+
   | Both      | - iD field: iSD    | iD field:    | votes field:      |
   | Regular   | and baseNumber     | serialNumber | Number of votes   |
   | AND       | - noTrustReset     | MUST be      | (indices) =>      |
   | Sensitive | field              | incremented  | number set in the |
   | Updates   |                    | by 1         | votingQuorum      |
   |           |                    |              | field of the      |
   |           |                    |              | predecessor TRC   |
   +-----------+--------------------+--------------+-------------------+
   | Regular   | - Quorum in the    |              | votes field:      |
   | TRC       | votingQuorum       |              | - All votes must  |
   | Update    | field              |              | only refer to     |
   |           | - Core ASes in     |              | _regular_ voting  |
   |           | the coreASes       |              | certificates in   |
   |           | field              |              | the predecessor   |
   |           | - ASes in the      |              | TRC               |
   |           | authoritativeASes  |              | - Must include    |
   |           | field              |              | votes of each     |
   |           | - Nr. and          |              | changed regular   |
   |           | distinguished      |              | voting            |
   |           | names of root &    |              | certificate from  |
   |           | voting             |              | the predecessor   |
   |           | certificates in    |              | TRC               |
   |           | the certificates   |              | signatures field: |
   |           | field              |              | - Must include    |
   |           | - Set of           |              | signatures of     |
   |           | sensitive voting   |              | each changed root |
   |           | certificates in    |              | certificate from  |
   |           | the certificates   |              | the predecessor   |
   |           | field              |              | TRC               |
   +-----------+--------------------+--------------+-------------------+
   | Sensitive | If the update      |              | votes field:      |
   | TRC       | does not qualify   |              | - All votes must  |
   | Update    | as a regular       |              | only refer to     |
   |           | update, it is a    |              | _sensitive_       |
   |           | sensitive update   |              | voting            |
   |           |                    |              | certificates in   |
   |           |                    |              | the predecessor   |
   |           |                    |              | TRC               |
   +-----------+--------------------+--------------+-------------------+

       Table 8: Overview of the update types and corresponding rules




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3.1.5.3.   General Update Rules

   The following rules MUST hold for each updated TRC, independent of
   the update type:

   *  The iSD and baseNumber in the iD field MUST NOT change (see also
      Section 3.1.2.2.2).

   *  The serialNumber in the iD field MUST be incremented by one.

   *  The noTrustReset field MUST NOT change (see also
      Section 3.1.2.2.5).

   *  The votes sequence of the updated TRC MUST only contain indices
      that refer to sensitive or regular voting certificates in the
      predecessor TRC.  This guarantees that the updated TRC only
      contains valid votes authenticated by sensitive or regular voting
      certificates in the predecessor TRC.  For more information, see
      Section 3.1.2.2.6 and Section 3.1.2.2.11.

   *  The number of votes in the updated TRC MUST be greater than or
      equal to the number set in the votingQuorum field of the
      predecessor TRC (see Section 3.1.2.2.7).  The number of votes
      corresponds to the number of indices in the votes field of the
      updated TRC.

3.1.5.4.   Regular TRC Update

   A regular TRC update is a periodic re-issuance of the TRC where the
   entities and policies listed in the TRC remain unchanged.

   A TRC update qualifies as a regular update, if the following rules
   apply in regard to the TRC's payload information.

   *  The settings of the following fields in the updated TRC MUST
      remain the same compared to the predecessor TRC:

      -  The voting quorum set in the votingQuorum field.

      -  The core ASes specified in the coreASes field.

      -  The authoritative ASes specified in the authoritativeASes
         field.

      -  The number of sensitive and regular voting certificates as well
         as CP root certificates included in the certificates field, and
         their distinguished names.




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      -  The set of sensitive voting certificates specified in the
         certificates field.

   *  For every regular voting certificate that changes, the regular
      voting certificate in the predecessor TRC is part of the voters on
      the updated TRC.  That is, for each changed regular voting
      certificate, an index in the votes field of the updated TRC MUST
      refer to the changed regular voting certificate in the predecessor
      TRC.

   *  For every CP root certificate that changes, the updated TRC MUST
      include a signature created with the private key belonging to the
      changed CP root certificate (which is part of the predecessor
      TRC).

   *  In order for a regular TRC update to be verifiable, all votes MUST
      be cast by _regular_ voting certificates.  That is, each index in
      the votes field of the regularly updated TRC MUST refer to a
      _regular_ voting certificate in the certificates field of the
      predecessor TRC.

3.1.5.5.   Sensitive TRC Update

   If a TRC update does not qualify as a regular update, it is
   considered a sensitive update.

   *  In order for a sensitive update to be verifiable, all votes MUST
      be cast by _sensitive_ voting certificates.  That is, each index
      in the votes field of the sensitively updated TRC MUST refer to a
      _sensitive_ voting certificate in the certificates field of the
      predecessor TRC.

3.1.5.6.   Signing a TRC Update

   As described above, a set of voters must cast votes on the updated
   TRC to make it verifiable.  The votingQuorum field of the predecessor
   TRC (see Section 3.1.2.2.7) defines the required number of voters,
   which will represent regular or sensitive voting certificates,
   respectively.  Furthermore, if one or more _new_ certificates are
   added to the updated TRC, the corresponding voting representatives
   must also sign the updated TRC, in order to show that they have
   access to the private keys listed in these fresh certificates.  This
   is called "showing proof-of-possession", and done by signing the TRC
   with the respective private key.  For the distinction between changed
   and new certificates in a TRC update, see Section 3.1.5.1.






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   It is up to the ISD members to decide how the "casting a vote"
   procedure for updated TRCs will take place.  Some ISDs will make a
   distinction between regular and sensitive updates.  These ISDs divide
   the regular and sensitive signing keys in different security classes
   and act accordingly.  For example, they keep the regular key in an
   online vault while the sensitive key would be stored offline in cold
   storage.  This way, the regular TRC update would lend itself to being
   automated (since the keys are accessible online) whereas the
   sensitive one would require manual actions to access the offline key.
   Other ISDs, however, keep both regular and sensitive keys online and
   perform both updates automatically.

3.1.5.7.   TRC Update Verification

   To verify a TRC update, the relying party must perform the following
   checks:

   *  Check that the specified update rules as described above are
      respected.

   *  Check whether the update is regular or sensitive.

      -  In case of a regular update,

         o  check that the signatures for the changing certificates are
            present and verifiable, and

         o  check that all votes are cast by a regular voting
            certificate.

      -  In case of a sensitive update, check that all votes are cast by
         a sensitive voting certificate.

   *  In both cases, check that all signatures are verifiable, and no
      superfluous signatures are attached.

   If one or more of the above checks gives a negative result, the
   updated TRC should be rejected.

3.2.  Initial TRC Signing Ceremony

   The very first base TRC of an ISD, called the initial TRC, is a
   special case of the base TRC where the number of the ISD is chosen.
   The initial TRC must be signed during a special signing ceremony--all
   voting representatives of the initial TRC need to take part in this
   signing ceremony to sign the agreed-upon TRC.  As part of the
   ceremony, the public keys of all voters are exchanged.  The TRC is
   then distributed throughout the ISD.  All entities within an ISD can



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   initially obtain an authentic TRC, by means of a secure off- or
   online mechanism.

   Appendix "Appendix A.  Signing Ceremony Initial TRC" describes a
   possible procedure for the signing ceremony of an ISD's initial TRC.
   It is in principle up to the initial members of an ISD how to shape
   the signing ceremony.  However, it is recommended having a process in
   line with the ceremony described in the Appendix.

4.  Deploying the CP PKI - Specifications

   This section provides several specifications regarding the deployment
   of the control-plane PKI.

4.1.  Deploying a TRC

4.1.1.   Base TRC

   Base TRCs are trust anchors and thus axiomatically trusted.  All ASes
   within an ISD must be pre-loaded with the currently valid base-
   version TRC of their own ISD.  For all specifications regarding the
   creation and distribution of initial/base TRCs, see Section 3.2.

4.1.2.   TRC Update

   All non-base TRCs of an ISD are updates of the ISD's base TRC(s).
   The TRC update chain consists of regular and sensitive TRC updates.
   The specifications and rules that apply to updating a TRC are
   described in Section 3.1.5.

4.1.2.1.   TRC Update Discovery

   Relying parties MUST have at least one valid TRC available.  Relying
   parties MUST discover TRC updates within the grace period defined in
   the updated TRC.  They SHOULD discover TRC updates in a matter of
   minutes to hours.  Additionally, the following requirement must be
   satisfied:

   *Requirement*
   Any entity sending information that is secured by the CP-PKI MUST be
   able to provide all the necessary trust material to verify said
   information.

   SCION provides the following mechanisms for discovering TRC updates
   and fulfilling the above requirement:






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   *  _Beaconing Process_
      The TRC version is announced in the beaconing process.  Each AS
      must announce what it considers to be the latest TRC.
      Furthermore, each AS must include the hash value of the TRC
      contents to facilitate the discovery of discrepancies.  Therefore,
      relying parties that are part of the beaconing process discover
      TRC updates passively.  That is, a core AS notices TRC updates for
      remote ISDs that are on the beaconing path.  A non-core AS only
      notices TRC updates for the local ISD through the beaconing
      process.  The creation of a new TRC should trigger the generation
      of new control-plane messages, as the propagation of control-plane
      messages will help other ASes rapidly discover the new TRC.

   *  _Path Lookup_
      In every path segment, all ASes must reference the latest TRC of
      their ISD.  Therefore, when resolving paths, every relying party
      will notice TRC updates, even remote ones.
      *Note:* The above mechanism only works when there is an active
      communication between the relying party and the ISD in question.

4.2.   Signing and Verifying Control-Plane Messages

   SCION requires that control-plane messages are signed.  The main
   purpose of the SCION control-plane PKI is providing a mechanism to
   distribute and authenticate public keys that are used to verify
   control-plane messages and information.  For example, each hop
   information in a path segment is signed by the respective AS.
   Consequently, all relying parties must be able to verify signatures
   with the help of the CP-PKI.

   The following sections specify the requirements that apply to the
   signing and verification of control-plane messages.

4.2.1.   Signing a Control-Plane Message

   An AS signs control-plane messages with the private key that
   corresponds to the (valid) AS' certificate.

   The AS MUST attach the following information as signature metadata.
   It is the minimum information a relying party requires to identify
   which certificate to use to verify the signed message.

   *  ISD-AS number: The ISD-AS number of the signing entity.  For
      specification details, see Section 2.2.1.2.1.

   *  Subject key identifier: The identifier of the public key that must
      be used to verify the message.  For specification details, see
      Section 2.2.2.2.



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   Additionally, the signer SHOULD include the following information:

   *  Serial and base number of the latest TRC: Including this
      information allows relying parties to discover TRC updates and
      trust resets.  For specification details, see Section 3.1.2.2.2.

   *  Timestamp: For many messages, the time at which it was signed is
      useful information to ensure freshness.

4.2.2.   Verifying a Control-Plane Message

   When the relying party receives a control-plane message they want to
   verify, the relying party first needs to identify the certificate
   needed to validate the corresponding signature on the message.

   AS certificates are bundled together with the corresponding signing
   CA certificate into certificate chains.  For efficiency, SCION
   distributes these certificate chains separately from the signed
   messages.  A certificate chain is verified against the CP root
   certificate.  However, the root certificate is *not* bundled in the
   chain, but with the TRC.  This makes it possible to extend the
   validity period of the root certificate, and to update the
   corresponding TRC, without having to modify the certificate chain.

   To verify a control-plane message, the relying party must perform the
   following steps:

   1.  Build a collection of root certificates from the latest TRC of
       the relevant ISD (that is, the ISD referenced in the signature
       metadata of the message).  If the grace period (see
       Section 3.1.2.2.4) introduced by the latest TRC is still on-
       going, the root certificates in the second-to-latest TRC must
       also be included.  For a description on how to build the correct
       collection of certificates, see Section 3.1.4.1.

   2.  If the signature metadata of the message contains the serial and
       base number of the latest TRC, the relying party must check that
       they have this latest TRC.  If not, the relying party must
       request the latest TRC.

   3.  After constructing the pool of root certificates, the relying
       party must select the certificate chain used to verify the
       message.  The AS certificate included in this certificate chain
       MUST have the following properties:

       *  The ISD-AS number in the subject of the AS certificate MUST
          match the ISD-AS number in the signature metadata.  See also
          Section 2.2.1.2.1.



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       *  The subject key identifier of the AS certificate MUST match
          the subject key identifier in the signature metadata.  See
          also Section 2.2.2.2.

       *  The AS certificate MUST be valid at verification time.
          Normally, this will be the current time.  In special cases,
          e.g., auditing, the time can be set to the past to check if
          the message was verifiable at the given time.

   4.  After selecting a certificate chain to verify the control-plane
       messages, the relying party must verify the certificate chain,
       by:

       *  Executing the regular X.509 verification procedure.  For
          details, see X.509
          (https://handle.itu.int/11.1002/1000/13031).

       *  Checking that

          -  all subjects of the certificates in the chain carry the
             same ISD number (see also Section 2.2.1.2.1,

          -  each certificate is of the correct type (see also
             Section 2.1), and

          -  the CA certificate validity period covers the AS
             certificate validity period.

   5.  If the verification of the certificate chain was successful, the
       relying party can now verify the control-plane messages, with the
       root certificates from the certificate chain.

   If any cryptographic material is missing in the process, the relying
   party must query the originator of the message for the missing
   material.  If it cannot be resolved, the verification process fails.

   *Important:* An implication of the above procedure is that path
   segments should be verifiable at time of use.  It is not enough to
   rely on path segments being verified on insert, since TRC updates
   that change the root key can invalidate a certificate chain.

4.3.  Creating a New Control-Plane AS Certificate

   The steps required to create a new AS certificate are the following:

   1.  The AS creates a new key pair and a certificate signing request
       (CSR) using that key pair.




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   2.  The AS sends the certificate signing request to the relevant CA
       within the ISD.

   3.  The CA uses its CA key and the CSR to create the new AS
       certificate.

   4.  The CA sends the AS certificate back to the AS.

   When an AS joins an ISD, the first CSR is sent out of band to one of
   the CAs as part of the formalities to join the ISD.  Subsequent
   certificate renewals may be automated and can leverage the control-
   plane communication infrastructure.

5.  Security Considerations

   The goal of SCION is to provide a secure inter-domain network
   architecture, therefore this paragraph focuses on _inter_-AS security
   considerations.  Specifically, it covers potential attacks and
   mitigations for the control-plane PKI.  All _intra_-AS trust- and
   security aspects are out of scope.

5.1.  Dependency on Certificates

   In public key infrastructures, certificate authorities have both the
   responsibility and power to issue and revoke certificates.  If a CA
   is compromised or malicious, this power can be abused.  A misbehaving
   CA could refuse to issue certificates to legitimate entities, or even
   issue illegitimate certificates to allow impersonation of another
   entity.  In the context of the SCION control-plane PKI, refusing to
   issue or renew a certificate to an AS will ultimately cut that AS off
   from the network, turning the CP-PKI into a potential network "kill
   switch".  In order to prevent this, within each ISD there are usually
   multiple independent CAs.

   SCION’s trust architecture fundamentally differs from a global
   monopolistic trust model.  In SCION, each ISD manages its own trust
   roots instead of a single global entity providing those roots.  This
   structure gives each ISD autonomy in terms of key management and in
   terms of trust, and prevents the occurrence of a global kill switch
   affecting all ISDs at once.  However, each ISD is still susceptible
   of compromises that could affect or halt other components (control
   plane and forwarding).  The following sections explain these cases
   and possible countermeasures.








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5.1.1.  Compromise of an ISD

   Compared to other trust architectures, in SCION there is no central
   authority that could "switch off" an ISD, as each ISD relies on its
   own independent cryptographic trust roots.  Each AS within an ISD is
   therefore dependant on its ISD's PKI for its functioning.  This
   section discusses potential compromises of the PKI at various levels
   of the hierarchy.

   *  On TRC level: The private root keys of the root certificates
      contained in an TRC are used to sign CA certificates.  If one of
      these private root keys is compromised, the adversary could issue
      illegitimate CA certificates which may be used in further attacks.
      To maliciously perform a TRC update, an attacker would need to
      compromise multiple voting keys.  This number depends on the
      voting quorum set in the TRC: the higher the quorum, the more
      unlikely a malicious update will be.

   *  On CA level: The private keys of an ISD's CA certificates are used
      to sign the AS certificates.  All ASes within an ISD obtain
      certificates directly from the CAs.  If one of the CA’s keys is
      compromised, an adversary could issue illegitimate AS
      certificates, which may be used in further attacks.

   *  On AS level: Each AS within an ISD signs control-plane messages,
      such as the Path Construction Beacons (PCBs) used for path
      discovery, with their AS private key.  If the keys of an AS are
      compromised by an adversary, this adversary can illegitimately
      sign control-plane messages, including PCBs.  This means that the
      adversary can also manipulate the PCBs, and propagate the
      manipulated PCBs to neighboring ASes or register/store them as
      path segments.

5.1.2.  Recovery from Compromise

   This section deals with possible recovery from the compromises
   discussed in the previous paragraph.  As described in Section 1.4.2,
   there is no revocation in the CP-PKI.

   *  On TRC level: If any of the root keys or voting keys contained in
      the TRC are compromised, the TRC must be updated as described in
      Section 3.1.5.  Note that this is a sensitive TRC update, as the
      certificate related to the compromised private key must be
      replaced with an entirely new certificate (and not just changed).
      A trust reset is only required in the case the number of
      compromised keys at the same time is greater or equal than the
      TRC's quorum (see Section 3.1.2.2.7).




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   *  On CA level: If the private key related to a CA certificate is
      compromised, the impacted CA AS must obtain a new CA certificate
      from the corresponding root AS.  CA certificates are generally
      short lived to limit the impact of compromise.  Alternatively,
      with a TRC update, a new root keys can also be forced,
      invalidating the compromised CA.

   *  On AS level: In the event of a key compromise of a (non-core) AS,
      the impacted AS needs to obtain a new certificate from its CA.
      This process will vary depending on internal issuance protocols.

5.2.  Denial of Service Attacks

   As described previously, the SCION's control-plane PKI lays the
   foundation for the authentication procedures in SCION.  It provides
   each AS within a specific ISD with a certified key pair.  These keys
   enable the authentication of all control-plane messages - every AS
   and endpoint can verify all control-plane messages by following the
   certificate chain.

   The relying party must be able to discover and obtain new or updated
   cryptographic material.  For the control plane messages, this is
   simplified by the observation that the sender of a message (e.g. of a
   path construction beacon during path exploration or a path segment
   during a path lookup) always has all the cryptographic material to
   verify it.  Thus, the receiver can always immediately obtain all the
   cryptographic material from the message originator.  As the
   corresponding PKI messaging thus only occurs when the control plane
   is already communicating, these requests to obtain cryptographic
   material are not prone to additional denial of service attacks.  We
   therefore refer to [I-D.scion-cp] for a more detailed description of
   DoS vulnerabilities of control-plane messages.

   For certificate renewal, on the other hand, this does not apply.
   Denial of Service on the CA infrastructure or on the communication
   links from the individual ASes to the CA, could be used by an
   attacker to prevent victim ASes from renewing their certificates,
   halting the path discovery process.  This risk can be mitigated in
   multiple ways:

   *  CAs only need to be accessible from ASes within the ISD, reducing
      the potential DoS attack surface

   *  ISDs usually rely on multiple CAs

   *  ISDs could create policies and processes to renew certificates
      out-of-band




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6.  IANA Considerations

   This document has no IANA actions.

   The SCION AS and ISD number are SCION-specific numbers.  They are
   currently allocated by Anapaya Systems, a provider of SCION-based
   networking software and solutions (see Anapaya ISD AS assignments
   (https://docs.anapaya.net/en/latest/resources/isd-as-assignments/)).
   This task is currently being transitioned from Anapaya to the SCION
   Association.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC5398]  Huston, G., "Autonomous System (AS) Number Reservation for
              Documentation Use", RFC 5398, DOI 10.17487/RFC5398,
              December 2008, <https://www.rfc-editor.org/rfc/rfc5398>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/rfc/rfc5480>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

   [RFC5758]  Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
              Polk, "Internet X.509 Public Key Infrastructure:
              Additional Algorithms and Identifiers for DSA and ECDSA",
              RFC 5758, DOI 10.17487/RFC5758, January 2010,
              <https://www.rfc-editor.org/rfc/rfc5758>.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
              2013, <https://www.rfc-editor.org/rfc/rfc6996>.



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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

7.2.  Informative References

   [BARRERA17]
              Barrera, D., Chuat, L., Perrig, A., Reischuk, R., and P.
              Szalachowski, "The SCION internet architecture",
              Association for Computing Machinery (ACM), Communications
              of the ACM vol. 60, no. 6, pp. 56-65, DOI 10.1145/3085591,
              May 2017, <https://doi.org/10.1145/3085591>.

   [CHUAT22]  Chuat, L., Legner, M., Basin, D., Hausheer, D., Hitz, S.,
              Mueller, P., and A. Perrig, "The Complete Guide to SCION",
              ISBN 978-3-031-05287-3, 2022,
              <https://doi.org/10.1007/978-3-031-05288-0>.

   [I-D.scion-components]
              Rustignoli, N. and C. de Kater, "SCION Components
              Analysis", 2022, <https://datatracker.ietf.org/doc/draft-
              rustignoli-panrg-scion-components/>.

   [I-D.scion-cp]
              de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control
              Plane", 2023, <https://datatracker.ietf.org/doc/draft-
              dekater-scion-controlplane/>.

   [I-D.scion-overview]
              de Kater, C., Rustignoli, N., and A. Perrig, "SCION
              Overview", 2022, <https://datatracker.ietf.org/doc/draft-
              dekater-panrg-scion-overview/>.

Acknowledgments

   Many thanks go to Fritz Steinmann (SIX Group AG), Juan A.  Garcia
   Prado (ETH Zurich) and Russ Housley (IETF) for reviewing this
   document.  We are also very grateful to Adrian Perrig (ETH Zurich),
   for providing guidance and feedback about each aspect of SCION.
   Finally, we are indebted to the SCION development teams of Anapaya
   and ETH Zurich, for their practical knowledge and for the
   documentation about the CP PKI, as well as to the authors of
   [CHUAT22] - the book is an important source of input and inspiration
   for this draft.







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Appendix A.  Signing Ceremony Initial TRC

   The following sections describe a possible signing ceremony for the
   first (initial) base TRC of an ISD.  Although each ISD is free to
   decide how to shape this signing ceremony, it is recommended
   establishing a procedure similar to the one below.

 Ceremony Participants

   A signing ceremony includes participants from member organizations of
   the respective Isolation Domain.  The participants of the signing
   ceremony fulfill different roles:

   *  The *ceremony administrator* is in charge of moderating the
      signing process.  He/she guides all participants through the steps
      they need to take.  The ceremony administrator may also act as an
      intermediary between participants when they share information with
      each other.

   *  A *voting AS representative* is capable of creating voting
      signatures on the TRC.  This means the voting representative is in
      possession of a device with the private keys of the respective
      certificates in the TRC.

   *  A *witness* is any person that participates in the ceremony as a
      passive entity.  The witness has no active role in any of the
      steps of the ceremony, but can stop the process and inquire for
      more information if they feel the integrity of the process might
      have been compromised.

   *Note:* It is assumed that the member organizations of the ISD have
   decided in advance, before the signing ceremony, on the roles of the
   ceremony participants.  That is, they have reached agreement about
   the Certificate Authority (CA) ASes (that will also issue the root
   certificates), the voting ASes, the representatives of the voting
   ASes, the ceremony administrator and the witnesses.

   *Note:* For the signing ceremony, it is assumed that all parties are
   trustworthy.  Issues encountered during the ceremony are assumed to
   be caused by honest mistakes, and not by malicious intent.  Hash
   comparison checks are included to counter mistakes, such that every
   participant is sure that they operate on the same data.  Furthermore,
   the private keys of each participant never leave their machine.  The
   ceremony administrator does not have to be entrusted with private
   keys.






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Ceremony Preparations

   Prior to the ceremony, participants decide on the physical location
   of the ceremony, the devices that will be used during the ceremony
   and the policy of the ISD.  Specifically, the voting entities agree
   on the following parameters:

   *  validity of the TRC,

   *  voting quorum,

   *  core ASes/authoritative ASes,

   *  description, and

   *  list of CP root certificates.

   When these values are agreed upon, a number of voters, equal to or
   larger than the specified voting quorum, needs to execute the signing
   ceremony.  For the base TRC, all voting entities need to be present
   with both their sensitive and regular voting keys.  The ceremony
   process is structured in multiple rounds of data sharing.  The
   ceremony administrator leads the interaction and gives instructions
   to each participant.

 Location

   The location must provide electricity and enough power sockets for
   each participant.  Furthermore, it should provide a monitor (or
   projector) that allows the ceremony administrator to screencast.

 Devices

   Each party brings their own device that is provisioned with the
   required material, as described below.

   *  Device to exchange data.  This device can either be provided by
      the ceremony administrator, or, if preferable, by any of the
      voting representatives.

   *  Ceremony administrator's device: The ceremony administrator should
      bring a machine that is capable of creating and verifying a TRC.
      Furthermore, it needs to be able to compute the SHA-512 digest
      (hash value) of files.

   *  Voting representative's device: The voting representative should
      bring a machine that is capable of signing and verifying TRCs.
      Thus, the machine needs to have access to all the voting private



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      keys.  Furthermore, it needs to be able to compute the SHA-512
      digest (hash value) of the files.  The exact binaries that are
      required are described in a separate document.

   *Important:* It is very important that all devices, especially the
   data exchange device, are not compromised.  Therefore, the ceremony
   should ideally include a procedure to verify that the devices are
   secure.

Preparation Steps

   Each party involved in a TRC signing ceremony must go through a few
   steps in preparation for the ceremony.  This section outlines these
   steps.

 Preparatory Tasks of the Ceremony Administrator

   In the preparation phase of the TRC Signing Ceremony, the ceremony
   administrator has the following tasks:

   1.  Send out the high-level TRC Signing Ceremony description and the
       document describing the TRC Signing Ceremony Phases to the
       participants, all in digital form.

   2.  Remind all representatives of the voting ASes that they need to
       agree on a common TRC policy before scheduling the TRC ceremony.

   3.  Bring all digitally distributed documents as a printout for all
       parties that take part.

 Preparatory Tasks of the Voting AS Representatives

   The preparatory task of the representatives of the voting ASes
   (short: the voters) is to generate the necessary certificates.

   *Important:* Before generating the certificates, all voters need to
   agree on a preliminary TRC policy, in particular on the *validity
   period of the TRC*. This is necessary because all the certificates
   that are generated in advance must *cover the full TRC validity
   period*. The other policy values could be amended during the ceremony
   itself.

   Each representative of a voting AS must create the following keys and
   certificates:

   *  A sensitive voting private key, and a certificate holding the
      corresponding public key.




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   *  A regular voting private key, and a certificate holding the
      corresponding public key.

Preparatory Tasks of the Certificate Authority ASes

   Each AS that will be a Certificate Authority (a so-called CA AS) must
   ensure that the following key and certificate is available:

   *  A control-plane root private key, and a certificate holding the
      corresponding public key.

   This implies that there will be one control-plane root certificate
   per CA AS.

   *Note*: Representatives of CA ASes must not be present at the signing
   ceremony themselves, as they do not have to put a signature on the
   TRC.  However, if a CA AS does not attend the signing ceremony in
   person, it must ensure that the corresponding root certificate is
   available at the ceremony to be shared.

 Ceremony Process

   The ceremony process for the initial base TRC is structured in
   multiple rounds of data sharing.  The ceremony administrator leads
   the interaction and instructs each participant with what to do.

   The ceremony process contains the following phases:

   *  Phase 1: Certificate Exchange (Appendix "Phase 1: Certificate
      Exchange").  In the first phase of the ceremony, all voting
      parties share the certificates that must be part of the TRC with
      the ceremony administrator.

   *  Phase 2: Generation of the TRC Payload (Appendix "Phase 2:
      Generation of the TRC Payload").  In the second phase, the
      ceremony administrator generates the TRC payload based on the
      bundled certificates and the agreed-upon ISD policy.

   *  Phase 3: TRC Signing (Appendix "Phase 3: TRC Signing").  In the
      third phase, each voting representative attaches the required
      signatures to the TRC.

   *  Phase 4: TRC Validation (Appendix "Phase 4: TRC Validation").  In
      the final phase of the ceremony, all voting representatives share
      the signed TRC with the ceremony administrator, who aggregates it
      in a single signed TRC document.

   A detailed description of each phase follows below.



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 Phase 1: Certificate Exchange

   In Phase 1 of the signing ceremony, all parties share the
   certificates that must be part of the TRC with the ceremony
   administrator.  For the representatives of the voting ASes, these are
   the sensitive and the regular voting certificates.  For the
   representatives of the CA ASes, these are the CP root certificates.
   If a CA AS does not attend the signing ceremony in person, it must
   ensure that the corresponding root certificate is available at the
   ceremony to be shared.

   The actual sharing happens over the data exchange device, which goes
   from one voting representative to the next.  Each representative
   copies the requested certificates from their own machine onto the
   data exchange device, before forwarding the device to the next voter.
   The last representative returns the device to the ceremony
   administrator.

   *Important:* Note that only the *certificates* must be shared during
   this step, *not* the private keys.  Copying a private key by mistake
   invalidates the security of the ceremony.

   For each provided certificate, the ceremony administrator checks that
   its validity period covers the previously agreed-upon TRC validity,
   that the signature algorithms are correct, and that the certificate
   is of the valid type (root, sensitive voting or regular voting
   certificate).  If the results of these checks are as expected, the
   ceremony administrator computes the SHA256 sum for each certificate.

   The ceremony administrator then aggregates and bundles the provided
   certificates, and calculates the hash value (SHA-512 digest) over the
   entire bundle.  Additionally, the ceremony administrator displays all
   hash values on the monitor.

   The ceremony administrator now shares the bundle with the
   representatives of the voting and CA ASes.  This could happen again
   via the data exchange device, which goes from one representative to
   the next.  Each representative verifies that the certificates they
   contributed have the same hash value as the displayed value on the
   monitor.  Furthermore, all representatives must confirm that the hash
   value of the bundled certificates on their machine is equal to the
   value on the monitor.

   Phase 1 is concluded when every representative has confirmed that the
   SHA256 sums are correct.

   *Note:* If there is a mismatch in any of the SHA256 sums, Phase 1
   needs to be repeated.



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Phase 2: Generation of the TRC Payload

   In Phase 2 of the ceremony, the ceremony administrator generates the
   TRC payload based on the bundled certificates and the agreed-upon ISD
   policy.  The result is displayed on the monitor along with a hash
   value (SHA-512 digest).

   To be able to generate the payload, the ceremony administrator must
   ask the voting representatives for

   *  The ISD number of the ISD.  The number (identifier, ID) of an ISD
      must be chosen and agreed upon by the participants during the
      signing ceremony of the ISD's initial TRC.  The ceremony
      administrator needs the ISD number to specify the identifier (ID)
      of the initial TRC.  This iD is part of the TRC payload.  For more
      information, see Section 3.1.2.2.2.

   *  The description of the TRC.  For more information, see
      Section 3.1.2.2.10.

   *  The AS numbers of the core ASes of the ISD.  For more information,
      see Section 3.1.2.2.8.

   *  The AS numbers of the authoritative ASes of the ISD.  For more
      information, see Section 3.1.2.2.9.

   *  The voting quorum for the next TRC update.  For more information,
      see Section 3.1.2.2.7.

   *  The validity period of the new TRC.  For more information, see
      Section 3.1.2.2.3.

   *Note:* It is assumed that the voting ASes have agreed on the answers
   to the above questions in advance, before the signing ceremony.

   The ceremony administrator can now specify the TRC payload variables
   in the payload template file, and show the filled-in template on the
   monitor.  When the voters have verified the data, the ceremony
   administrator can compute the DER encoding of the TRC data as well as
   the SHA256 sum of the TRC payload file.  The ceremony administrator
   then distributes the TRC payload (via the data exchange device) to
   all voting representatives, who verify the payload's hash value.  The
   voters do this by computing the hash value of the TRC payload on
   their machine and checking whether their value matches the one on the
   monitor.

   Phase 2 successfully concludes once every voting representative
   confirms that the contents of the TRC payload are correct.



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Phase 3: TRC Signing

   In Phase 3, each voting representative attaches a signature created
   with each one of their private voting keys to the TRC (payload file).
   They do this on their own machine.  The purpose of signing a TRC that
   contains newly introduced public keys with the corresponding private
   keys is to prove the possession of the private keys.

   Phase 3 concludes after all voting representatives have cast their
   votes.

 Phase 4: TRC Validation

   In Phase 4, all voting representatives share the signed TRC with the
   ceremony administrator.  This happens again over the data exchange
   device, which goes from one voter to the next.  Each voting
   representative copies the TRC payload signed with the voter's private
   keys from their own machine onto the data exchange device.  The last
   voter returns the device to the ceremony administrator, who assembles
   the final TRC by aggregating the payload data with the votes
   (signatures) cast by the voting representatives.

   The signed TRC is validated by inspecting its contents on the monitor
   and verifying the signatures based on the exchanged certificates in
   Phase 1.  The ceremony administrator then shares the signed TRC with
   all participants.  Each of them must then inspect it once more, and
   verify it based on the certificates exchanged in Phase 1.  At this
   point, the ceremony is completed.  All participants have the signed
   TRC, and can use it to distribute the trust anchors for their ISD.

Authors' Addresses

   Corine de Kater
   SCION Association
   Email: c_de_kater@gmx.ch


   Nicola Rustignoli
   SCION Association
   Email: nic@scion.org


   Samuel Hitz
   Anapaya Systems
   Email: hitz@anapaya.net






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