INTERNET-DRAFT                                 Editor: Stephen Farrell
expires in six months                           Baltimore Technologies
                                                         February 2002

                  Securely Available Credentials Protocol
                  <draft-ietf-sacred-protocol-bss-02.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document describes an SRP-based protocol for securely available
   credentials. Discussion of this draft is taking place on the SACRED
   mailing list of the IETF SACRED working group (see
   http://www.imc.org/ietf-sacred for subscription information).

Table Of Contents

   Status of this Memo.............................................1
   Abstract........................................................1
   Table Of Contents...............................................1
   1. Introduction.................................................2
   2. The protocol.................................................3
   3. BEEP Profile for SACRED......................................7
   4. IANA Considerations.........................................10
   5. Security Considerations.....................................10
   References.....................................................12
   Acknowledgements...............................................13
   Editor's Address...............................................13
   Full Copyright Statement.......................................13
   Appendix A: XML Schema.........................................14
   Appendix B: DTD................................................17
   Appendix C: An Example of Tuning with BEEP.....................18
   Appendix D: Provision SACRED using other Protocols.............19
   Appendix E: Changes & Open Issues..............................19


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1. Introduction

   <<Open issues/editorial notes are in brackets, like this.>>

   We describe a protocol whereby a user can acquire cryptographic
   credentials (e.g., private keys, PKCS#15 structures) from a
   workstation which has locally trusted software installed, but with
   no user-specific configuration. This is somewhat less secure than a
   smart card, but can be used until smart cards and smart card readers
   on workstations become ubiquitous, and can be useful even after
   smart cards are ubiquitous, as a backup strategy when a user's smart
   card is lost or malfunctioning.

   The protocol uses [BEEP] for its exchange model with payloads
   consisting of the XML messages defined in this specification. The
   protocol sets out to meet the requirements in [REQS]. In particular,
   security requirements in [REQS] are met by mandating support for TLS
   [TLS] and/or SRP [SASL-SRP].

   We assume the only authentication information available to the user
   is a username and password.

   Many user-chosen passwords are vulnerable to dictionary attacks. So
   this protocol is designed to give no information with which an
   attacker can acquire information for launching a dictionary attack,
   whether by eavesdropping or by impersonating either the client or
   server.

   The protocol also allows a user to create or delete an account,
   change her account password and/or credentials and upload the new
   values to the server. The protocol ensures that only someone that
   knew the old account password is able to modify the credentials as
   stored on the credential server. The protocol does not preclude
   configuring a server to disallow some operations (e.g. credential
   upload) for some users. The account management operations as a whole
   are OPTIONAL to implement for both credential servers and clients.

   Note that there are potentially two "passwords" involved when using
   this protocol - the first used to authenticate the user to the
   credential server, and the second to decrypt (parts of) the
   credential following a download operation. Where the context
   requires it, we refer to the former as the account password and the
   latter as the credential password.









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2. The protocol

   This section defines the account management and "run-time"
   operations for the SACRED protocol.

   It also describes the message formats used, which are based on XML.
   Appendices A & B provide schema and DTD for these elements.

   The approach taken here is to define SACRED elements that are
   compatible with the elements used in [XKMS] and [XMLDSIG], so that
   an implementation of this protocol can easily also support XKMS, and
   vice versa.

   It is also intended that other SACRED protocol instances (e.g. using
   a different authentication scheme, credential format or transport
   protocol) could re-use many of the definitions here.

2.1 Account management operations

   These operations MAY be implemented, that is, they are OPTIONAL.

2.1.1   Information Request

   This operation REQUIRES no authentication.

   The purpose of this operation is to provide to the client the values
   required for account creation.

   The client sends an InfoRequest message (which has no content).

   The server responds with an InfoResponse message which contains the
   authentication mechanism parameters for the server and the list of
   supported ProcessInfo types.

2.1.2   Create Account

   This operation REQUIRES TLS server authentication.

   The purpose of this operation is to setup a new account on the
   server.  The information required for a "new" account will depend on
   the SASL mechanism used.  For [SRP], it consists simply of the
   username and SRP password verifier.

   The client sends a CreateAccountRequest which contains the account
   name (e.g. username). It also contains the elements required to
   create an account for a particular authentication mechanism.  The
   actual information is defined according to the authentication
   mechanism. For SASL-SRP this consists of the salt and password
   verifier.

   The server responds with an error or acknowledgement message.



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2.1.3   Remove Account

   This operation REQUIRES one of SRP or TLS mutual authentication.

   The purpose of this operation is to delete the entire account.

   The client sends a RemoveAccountRequest message (which has no
   content) to the server.

   The server MUST delete all information relating to the account and
   respond with an error or acknowledgement message.

2.1.4   Modify Account

   This operation REQUIRES one of SRP or TLS mutual authentication.

   The purpose of this operation is to allow the client to change the
   information required for authentication.  The information required
   will depend on the authentication method used. For [SRP] it will
   consist of a salt and a password verifier value.  Once the account
   information has been changed, the server will respond with an error
   or acknowledgement message.

   The client sends a ModifyAccountRequest message which cntains the
   elements required to change the authentication information for the
   account, for a particular authentication mechanism.  The actual
   information is defined according to the authentication mechanism.
   For SASL-SRP this consists of the salt and password verifier.

   The server responds with an error or acknowledgement message.

2.2 "Run-time" operations

   These operations MUST be supported by all conformant
   implementations.

2.2.1   Credential Upload

   This operation REQUIRES one of SRP or TLS mutual authentication.

   The purpose of this operation is to allow the client to deposit a
   credential with the server, or to delete one or more credentials
   associated with the account.

   The client sends an UploadRequest message to the server which can
   contain zero or one Credential. If the UploadRequest contains no
   Credential then the all the credentials associated with that account
   are deleted.

   If a credential with the same credential selector field as in the
   UploadRequest, (a "matching" credential), already exists for the
   account, then that credential is replaced with the new credential


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   from the UploadRequest. Otherwise a "new" credential is associated
   with that account.

   If the new credential from the UploadRequest contains no PayLoad
   field then the (one and only) "matching" credential is deleted.

   If no "matching" credential exists, the server returns an error.

   If any change is made to the stored credentials associated with the
   account then the server MUST update the LastModified value (returned
   in DownloadResponse messages) to the current time (at the server).

   The LastModified value can also be of use in detecting conflicts.
   For example, download to platform A, download to platform B, update
   from B, update from A.  The server could detect a conflict on the
   second upload.

   The server responds with an error or acknowledgement message.

2.2.2   Credential Download

   This operation REQUIRES one of SRP or TLS mutual authentication.

   The purpose of this operation is to allow a client to get one or
   more credentials from a server (the purpose of the entire protocol
   really!).

   The client sends a DownloadRequest message to the server which MAY
   contain a credential selector string for the credential. No, or an
   enmpty, credential selector means the request is for all credentials
   associated with the account.

   The server responds with a DownloadResponse or an error message. A
   DownloadResponse contains one or more credential payloads plus the
   LastModified time which represents the time (at the server) when the
   last change was made to the set of credentials associated with the
   account (e.g. via an UploadRequest).

2.3 Miscellaneous

2.3.1   Session security

   Six SACRED operations are defined above. In this section we specify
   the requirements for security for each of the operations (where
   supported).

        Operation                 Security REQUIRED
        ---------                 -----------------
        Information request       NONE
        Create account            Server authentication,
                                  Privacy, Integrity
        Remove account            Mutual authentication,
                                  Privacy, Integrity

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        Modify account            Mutual authentication,
                                  Privacy, Integrity
        Credential upload         Mutual authentication,
                                  Privacy, Integrity
        Credential download       Mutual authentication,
                                  Privacy, Integrity

   The security requirements can be met by several mechanisms. This
   document REQUIRES credential servers to support TLS and SASL-SRP.
   Clients MUST support SASL-SRP or TLS.

   The mandatory-to-implement TLS cipher suite for SACRED is
   TLS_RSA_WITH_3DES-EDE_CBC_SHA.

   For SASL-SRP, all three security services (privacy, replay
   protection, and integrity) MUST be used.

2.3.2   Handling multiple credentials for an account

   When more than one credential is stored under a single account, the
   client can select a single credential using the optional credential
   selector string.

   There is no concept of a "default credential" - all credentials MUST
   have an associated selector unique for that account.  The selector
   is REQUIRED for upload requests and OPTIONAL for download requests.
   If the selector is omitted in a download request it MUST be
   interpreted as a request for all the stored credentials.

   An empty selector string value (i.e. "") in a credential download
   request, is to be interpreted as if the selector string were
   omitted, i.e. a download request containing this is a request for
   all credentials.

   It is an error to have more than one credential stored under the
   same account where both have the same credential selector string.

2.3.3   Common fields

   The type "ds:CryptoBinary" (inherited from [XMLDSIG]) is used for
   almost all binary values. The value in such elements MUST be the
   base64 encoding of the binary value in network byte order. See
   [XMLDSIG] for further details and example. The exception to this is
   the "salt" field, which is of type base64Binary instead. The reason
   for this is that leading zeros are stripped from ds:CryptoBinary,
   which is correct in most cases, but since the salt is a direct input
   to a hash function, leading zeros are significant and so have to be
   preserved.

   All messages sent to the server MAY contain ProcessInfo values. This
   field MAY be used by other specifications or for vendor extensions.
   For example, a server might require clients to include a phone
   number in this field. The information response message contains a

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   list of the types of ProcessInfo that the server supports. This
   extensibility scheme is the same as that used in [XKMS] and [XBULK].

   Where no specific response message is defined for an operation (e.g.
   for UploadRequest) then the transport will indicate success or
   failure.

   All of the response messages defined here MAY contain a Status
   string, containing a value intended for human consumption.

2.3.4   Credential Format

   A number of messages involve the Credential element. It has the
   following fields (all optional fields may occur exactly zero or one
   times unless otherwise stated):

   - CredentialSelector contains a string by which this particular
     credential (for this account) can be identified.
   - PayLoad contains either a ds:KeyInfo or some other form of
     credential. Implementations MUST support the pkcs#15 form of
     ds:KeyInfo defined below (the SacredPKCS15 element).
   - TimeToLive (optional) is a hint which clients SHOULD honor, which
     specifies the number of seconds for which the downloaded
     credential is to be usable.
   - ProcessInfo (optional) MAY contain any (typed) information that
     the server is intended to process. If the server doesn't support
     any of the ProcessInfo data, it MAY ignore that data.
   - ClientInfo (optional) MAY contain any (typed) information that the
     client is intended to process, but which the server MUST ignore.
     If the client doesn't support any of the ClientInfo data, it MAY
     ignore that data (e.g. if the ClientInfo is device specific).

3. BEEP Profile for SACRED

   The protocol described in this memo is realized as a [BEEP] profile.

   Future memos may define alternative versions of the BEEP profile for
   SACRED. When a BEEP peer sends its greeting, it indicates which
   profiles it is willing to support. Accordingly, when the BEEP client
   asks to start a channel, it indicates the versions it supports, and
   if any of these are acceptable to the BEEP server, the latter
   specifies which profile it is starting.

   Profile Identification: http://iana.org/beep/transient/sacred/bss

   This profile URI is consistent with [TRANS].

   Messages Exchanged during Channel Creation:
        InfoRequest,
        CreateAccountRequest,
        RemoveAccountRequest,
        ModifyAccountRequest,
        DownloadRequest,

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        UploadRequest,
        InfoResponse,
        DownloadResponse,
        error,
        ok

   Messages starting one-to-one exchanges:
        InfoRequest,
        CreateAccountRequest,
        RemoveAccountRequest,
        ModifyAccountRequest,
        DownloadRequest,
        UploadRequest

   Messages in positive replies:
        ok,
        InfoResponse,
        DownloadResponse

   Messages in negative replies: error

   Messages in one-to-many changes: none

   Message Syntax: c.f.,Section 3

   Message Semantics: c.f., Section 2

   Contact Information: c.f., the editor's address section of this memo

3.1 Profile Initialization

   Because all but one of the operations of the SACRED profile have
   security requirements (cf., Section 2.3.1), before starting the
   SACRED profile, the BEEP session will likely be tuned using either

       http://iana.org/beep/TLS
       or
       http://iana.org/SASL/SRP

   (Appendix C gives an example of tuning a BEEP session using SRP).

   Regardless, upon completion of the negotiation process, a tuning
   reset occurs in which both BEEP peers issue a new greeting. Consult
   Section 3 of [BEEP] for an example of how a BEEP peer may choose to
   issue different greetings based on whether privacy is in use.

   Any of the messages listed in section 4.2 below may be exchanged
   during channel initialization (c.f., Section 2.3.1.2 of [BEEP]),
   e.g.,

        C: <start number='1'>
        C:   <profile uri='http://iana.org/beep/transient/sacred/bss'>
        C:             <![CDATA[<DownloadRequest ...>]]>

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        C:     </profile>
        C: </start>

        S: <profile uri='http://iana.org/beep/transient/sacred/bss'>
        S:   <![CDATA[<DownloadResponse ...>]]>
        S: </profile>

   Note that BEEP imposes both encoding and length limitations on the
   messages that are piggybacked during channel initialization.

3.2 Profile Exchange

   All messages are exchanged as "application/beep+xml" (c.f., Section
   6.4 of [BEEP]):

   Role         MSG                   RPY                     ERR
   ----         ---                   ---                     ---
   I            InfoRequest           InfoResponse            error
   I            CreateAccountRequest  ok                      error
   I            RemoveAccountRequest  ok                      error
   I            ModifyAccountRequest  ok                      error
   I            DownloadRequest       DownloadResponse        error
   I            UploadRequest         ok                      error

3.3 Error handling

   The "error" message from Section 2.3.1.5 of [BEEP] is used to convey
   error information. Typically, after flagging an error, a peer will
   initiate a graceful release of the BEEP session.

   The following BEEP error reply codes from [BEEP] are to be used:

   code  meaning
   ====  =======
   421   service not available
   450   requested action not taken (e.g., lock already in
          use)
   451   requested action aborted (e.g., local error in
          processing)
   454   temporary authentication failure
   500   general syntax error (e.g., poorly-formed XML)
   501   syntax error in parameters (e.g., non-valid XML)
   504   parameter not implemented
   530   authentication required
   534   authentication mechanism insufficient (e.g., too
          weak, sequence exhausted, etc.)
   535   authentication failure
   537   action not authorized for user
   538   authentication mechanism requires encryption
   550   requested action not taken (e.g., no requested
          profiles are acceptable)
   553   parameter invalid
   554   transaction failed (e.g., policy violation)

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   The following SACRED-specific error reply codes can also be used:

   code  meaning
   ====  =======
   555   Extension (ProcessInfo) used not supported
   556   Required extension (ProcessInfo) not present

4. IANA Considerations

   If the IANA approves this memo for standards-track publication, then
   the IANA registers the BEEP profile specified in Section 4, and
   selects an appropriate standards-track URI, e.g.,

        http://iana.org/beep/sacred/bss

   <<Q: should we apply for a port number? [yes]>>

5. Security Considerations

   [REQS] calls for specifications to state how they address the
   vulnerabilities listed below.

    V1.   A passive attacker can watch all packets on the network and
           later carry out a dictionary attack.
           - The use of SRP or TLS counters this vulnerability.
    V2.   An attacker can attempt to masquerade as a credential server
           in an attempt to get a client to reveal information on line
           that allows for a later dictionary attack.
           - The use of server or mutual authentication counters this
           vulnerability.
    V3.   An attacker can attempt to get a client to decrypt a chosen
           "ciphertext" and get the client to make use of the resulting
           plaintext - the attacker may then be able to carry out a
           dictionary attack (e.g. if the plaintext resulting from
           "decryption" of a random string is used as a DSA private
           key).
           - The use of server or mutual authentication counters this
           vulnerability.
    V4.   An attacker could overwrite a repository entry so that when
           a user subsequently uses what they think is a good
           credential, they expose information about their password
           (and hence the "real" credential).
           - Server implementations SHOULD take measures to protect the
           database. Clients MAY use the ClientInfo field to store e.g.
           a signature over the Credential, which they then verify
           before using the private component.
    V5.   An attacker can copy a credential server's repository and
           carry out a dictionary attack.
           - Server implementations SHOULD take measures to protect the
           database.
    V6.   An attacker can attempt to masquerade as a client in an
           attempt to get a server to reveal information that allows

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           for a later dictionary attack.
           - The use of the SRP strong password algorithm counters this
           to a great extent. Additionally, credential servers MAY
           choose to provide mechanisms that protect against online
           dictionary attacks against user account passwords, either by
           repeated access attempts to a single user account (varying
           the password) or by attempting to access many user accounts
           using the same password.
    V7.   An attacker can persuade a server that a successful login
           has occurred, even if it hasn't.
           - Client authentication prevents this.
    V8.   (Upload) An attacker can overwrite someone else's
           credentials on the server.
           - Only if they know the account password already (thanks to
           mutual authentication).
    V9.   (When using password-based authentication) An attacker can
           force a password change to a known (or "weak") password.
           - Client authentication counters this.
    V10.  An attacker can attempt a man-in-the-middle attack for lots
           of reasons...
           - Mutual authentication plus the encryption of subsequent
           messages prevents this.
    V11.  User enters password instead of name.
           - The SASL-SRP scheme allows clients and server to use a
           hash of the user id, which would make it very hard for an
           attacker to detect this happening. However, using this
           technique damages interoperability since all clients dealing
           with the credential in question MUST know in advance that
           the extra hashing step is to be used. If a client uses this
           technique, then it SHOULD use the SHA-1 algorithm and base64
           encode the output. Servers MUST be able to handle usernames
           which are formed in this manner (essentially servers MUST be
           able to support random-looking 28 byte usernames).
    V12.  An attacker could attempt various denial-of-service attacks.
           - No specific countermeasures against DoS are proposed.

   If the CreateAccountRequest message were sent over a cleartext
   channel (or otherwise exposed) then an attacker could mount a
   dictionary attack and recover the account password. This is why the
   server authenticated TLS transport is REQUIRED for this operation.

   If someone steals the server database they can launch a dictionary
   attack.  If the dictionary attack is successful, the attacker can
   decrypt the user's credentials. An attacker that has learned the
   user's account password can also upload new credentials, assuming
   the user is authorized to modify the credentials, because someone
   who knows the user's account password is assumed to be the user.
   However, if someone steals the server database and is unsuccessful
   at obtaining the user's account password through a dictionary
   attack, they will be unable to upload new credentials.

   Credential servers SHOULD incorporate measures that act to counter
   denial of service attacks. In particular, they SHOULD drop inactive

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   connections and minimize the use of resources by un-authenticated
   connections. A number of recommendations are listed at [DDOS].
   <<Anyone got a nicer/better reference?>>

References

   Normative:

       [BEEP]     Rose, M., "The Blocks Extensible Exchange Protocol
                   Core", RFC 3080.
       [PKCS15]   "PKCS #15 v1.1: Cryptographic Token Information
                   Syntax Standard," RSA Laboratories, June 2000.
       [REQS]     Arsenault, A., Farrell, S., "Securely Available
                   Credentials - Requirements", RFC 3157.
       [RFC2026]  Bradner, S., "The Internet Standards Process --
                   Revision 3", RFC 2026.
       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", RFC 2119.
       [SASL]     Myers, J., "Simple Authentication and Security Layer
                   (SASL)", RFC 2222.
       [SASL-SRP] Burdis, K.R. & Naffah, R., "Secure Remote Password
                   SASL Mechanism", draft-burdis-cat-srp-sasl-06,
                   January 2002, work-in-progress
       [SRP]      Wu, T., "The SRP Authentication and Key Exchange
                   System", RFC 2945.
       [TLS]      Dierks, T., "The TLS Protocol - Version 1.0", RFC
                   2246.
       [XMLDSIG]  Eastlake, D., et al. "XML-Signature Syntax and
                   Processing", RFC 3075

   Informative:

       [DDOS]     "Recommendations for the Protection against
                   Distributed Denial-of-Service Attacks in the
                   Internet",
                   http://www.iwar.org.uk/comsec/resources/dos/ddos_en.
                   htm
       [TRANS]    Rose, M., "A Transient Prefix for Identifying
                   Profiles under Development by the Working Groups of
                   the IETF", draft-mrose-beep-transientid-01,
                   September 2001, work-in-progress
       [XKMS]     Hallam-Baker, P. (ed), "XML Key Management
                   Specification",
                   http://www.w3.org/2001/XKMS/Drafts/XKMS/, work-in-
                   progress
       [XBULK]    Hughes, M (ed), "XML Key Management Specification -
                   Bulk Operation",
                   http://www.w3.org/2001/XKMS/Drafts/xkms-req.html,
                   work-in-progress





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Acknowledgements

   Radia Perlman (radia.perlman@sun.com) and Charlie Kaufman
   (ckaufman@iris.com) co-authored earlier versions of this document.
   Michael Zolotarev (mzolotar@tpg.com.au) did much of the initial work
   adapting an earlier draft to the use of SRP. Marshall Rose
   (mrose@dbc.mtview.ca.us) helped out, in particular, with the BEEP
   profile. The following people were actively involved in the mailing
   list discussions leading to this draft:

        David Chizmadia (vze2729k@verizon.net),
        Dave Crocker (dcrocker@brandenburg.com),
        Lawrence Greenfield (leg+@andrew.cmu.edu),
        Dale Gustafson (dale.gustafson@bpsi.net),
        Mike Just (Mike.Just@entrust.com),
        John Linn (jlinn@rsasecurity.com),
        Neal McBurnett (neal@bcn.boulder.co.us),
        Keith Moore (moore@cs.utk.edu),
        Bob Morgan (rlmorgan@washington.edu),
        Eamon O'Tuathail (eamon.otuathail@clipcode.com),
        Magnus Nystrom (magnus@rsasecurity.com),
        Gareth Richards (grichards@rsasecurity.com)

   Of course, any and all errors remain the editor's responsibility.

Editor's Address

   Stephen Farrell,
   Baltimore Technologies,
   39 Parkgate Street,
   Dublin 8,
   IRELAND
   Phone: +353-1-881-6000
   Email: stephen.farrell@baltimore.ie

Full Copyright Statement

   Copyright (C) The Internet Society (date).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  In addition,
   the ASN.1 module presented in Appendix B may be used in whole or in
   part without inclusion of the copyright notice.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process shall be


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   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.  This
   document and the information contained herein is provided on an "AS
   IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
   FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
   NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
   WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Appendix A: XML Schema

   <?xml version="1.0"?>
   <!--DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
      "XMLSchema.dtd" [
       <!ATTLIST schema
       xmlns:sacred CDATA #FIXED "sacred-2002-02-22"
      >
       <!ENTITY sacred 'sacred-2002-02-22'>
       <!ENTITY % p ''>
       <!ENTITY % s ''>
      ] -->
   <schema
     targetNamespace="sacred-2002-02-22"
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
     xmlns:sacred="sacred-2002-02-22"
     xmlns="http://www.w3.org/2000/10/XMLSchema">
     <!-- extensibility holes -->
     <complexType name="ProcessInfoType">
       <sequence maxOccurs="unbounded">
         <any namespace="##other"/>
       </sequence>
     </complexType>
     <element name="ProcessInfo" type="sacred:ProcessInfoType"/>
     <complexType name="ClientInfoType">
       <sequence maxOccurs="unbounded">
         <any namespace="##other"/>
       </sequence>
     </complexType>
     <element name="ClientInfo" type="sacred:ClientInfoType"/>
     <!-- Where to put authenentication information -->
     <complexType name="AuthInfoType">
       <choice>
         <element name="SRPAuthInfo">
           <complexType>
             <sequence>
               <element name="salt" type="sacred:base64Binary"/>
               <element name="PasswordVerifier"type="ds:CryptoBinary"/>
             </sequence>
           </complexType>
         </element>

Editor: Farrell                                              [Page 14]

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         <sequence maxOccurs="unbounded">
           <any namespace="##other"/>
         </sequence>
       </choice>
     </complexType>
     <element name="AuthInfo" type="sacred:AuthInfoType"/>
     <!-- authentication mechanism parameters -->
     <complexType name="AuthParamsType">
       <choice>
         <element name="SRPAuthParams">
           <complexType>
             <sequence>
               <element name="N" type="ds:CryptoBinary"/>
               <element name="g" type="ds:CryptoBinary"/>
               <element name="HashAlg" type="uriReference"/>
             </sequence>
           </complexType>
         </element>
         <sequence maxOccurs="unbounded">
           <any namespace="##other"/>
         </sequence>
       </choice>
     </complexType>
     <element name="AuthParams" type="sacred:AuthParamsType"/>
     <!-- Protocol messsages -->
     <!-- "account handling" operations -->
     <!-- Information request -->
     <element name="InfoRequest"/>
     <element name="InfoResponse">
       <complexType>
         <sequence>
           <element name="Status" type="string" minOccurs="0"/>
           <element name="ServerId" type="string"/>
           <element ref="sacred:AuthParams"/>
           <element ref="sacred:ProcessInfo" minOccurs="0"/>
           <!-- SRP stuff -->
         </sequence>
       </complexType>
     </element>
     <!-- Create Account Request -->
     <element name="CreateAccountRequest">
       <complexType>
         <sequence>
           <element name="UserId" type="string"/>
           <element ref="sacred:AuthInfo"/>
           <element ref="sacred:ProcessInfo" minOccurs="0"/>
         </sequence>
       </complexType>
     </element>
     <!-- reomve account request -->
     <element name="RemoveAccountRequest">
       <complexType>
         <sequence>

Editor: Farrell                                              [Page 15]

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           <element ref="sacred:ProcessInfo" minOccurs="0"/>
         </sequence>
       </complexType>
     </element>
     <!-- password change request -->
     <element name="ModifyAccountRequest">
       <complexType>
         <sequence>
           <element ref="sacred:AuthInfo"/>
           <element ref="sacred:ProcessInfo" minOccurs="0"/>
         </sequence>
       </complexType>
     </element>
     <!-- "run-time" operations -->
     <!-- DownLoad Request -->
     <element name="DownloadRequest">
       <complexType>
         <sequence>
           <element name="CredentialSelector" type="string"
   minOccurs="0"/>
           <element ref="sacred:ProcessInfo" minOccurs="0"/>
         </sequence>
       </complexType>
     </element>
     <!-- Download Response -->
     <element name="DownloadResponse">
       <complexType>
         <sequence>
           <element name="Status" type="string" minOccurs="0"/>
           <element name="LastModified" type="dateTime"/>
           <element name="Credential" type="sacred:CredentialType"
             maxOccurs="unbounded"/>
         </sequence>
       </complexType>
     </element>
     <!-- Upload request -->
     <element name="UploadRequest">
       <complexType>
         <sequence>
           <element name="Credential" type="sacred:CredentialType"
             minOccurs="0" maxOccurs="1" />
         </sequence>
       </complexType>
     </element>
     <!-- Credential related structures -->
     <!-- A new ds:KeyInfo thing -->
     <element name="SacredPKCS15" type="ds:CryptoBinary"/>
     <!-- credential -->
     <complexType name="CredentialType">
       <sequence>
         <element name="CredentialSelector" type="string"/>
         <element name="Payload" type="ds:KeyInfoType"/>
         <element name="TimeToLive" type="string" minOccurs="0"/>

Editor: Farrell                                              [Page 16]

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         <element ref="sacred:ProcessInfo" minOccurs="0"/>
         <element ref="sacred:ClientInfo" minOccurs="0"/>
       </sequence>
     </complexType>
     <!-- if base64Binary isn't in your schema editor then add this -->
     <simpleType name="base64Binary">
       <restriction base="string"/>
     </simpleType>
   </schema>

Appendix B: DTD

   <<This is semi-automatically generated from the schema above and is
   probably therefore inaccurate.

   << Stuff like <!ELEMENT ClientInfo ()+> is wrong, I know. Will copy
   some trick from xmldsig to fix in next rev.>>

   <?xml version="1.0" encoding="UTF-8"?>

   <!ELEMENT AuthInfo (SRPAuthInfo | ()+)>
   <!ELEMENT SRPAuthInfo (salt, PasswordVerifier)>
   <!ELEMENT salt (#PCDATA)>
   <!ELEMENT PasswordVerifier (#PCDATA)>

   <!ELEMENT AuthParams (SRPAuthParams | ()+)>
   <!ELEMENT SRPAuthParams (N, g, HashAlg)>
   <!ELEMENT N (#PCDATA)>
   <!ELEMENT g (#PCDATA)>
   <!ELEMENT HashAlg (#PCDATA)>

   <!ELEMENT ClientInfo ()+>
   <!ELEMENT ProcessInfo ()+>

   <!ELEMENT CreateAccountRequest (UserId, AuthInfo, ProcessInfo?)>
   <!ELEMENT DownloadRequest (CredentialSelector?, ProcessInfo?)>
   <!ELEMENT DownloadResponse (Status?, LastModified, Credential+)>
   <!ELEMENT InfoRequest (#PCDATA)>
   <!ELEMENT InfoResponse (Status?, ServerId, AuthParams,
             ProcessInfo?)>
   <!ELEMENT ModifyAccountRequest (AuthInfo, ProcessInfo?)>
   <!ELEMENT RemoveAccountRequest (ProcessInfo?)>
   <!ELEMENT UploadRequest (Credential?)>

   <!ELEMENT SacredPKCS15 (#PCDATA)>
   <!ELEMENT UserId (#PCDATA)>
   <!ELEMENT Status (#PCDATA)>
   <!ELEMENT ServerId (#PCDATA)>
   <!ELEMENT LastModified (#PCDATA)>

   <!ELEMENT Credential (CredentialSelector, Payload, TimeToLive?,
             ProcessInfo?, ClientInfo?)>
   <!ELEMENT CredentialSelector (#PCDATA)>

Editor: Farrell                                              [Page 17]

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   <!ELEMENT TimeToLive (#PCDATA)>
   <!ELEMENT Payload ()+>
   <!ATTLIST Payload
     Id ID #IMPLIED
   >

Appendix C: An Example of Tuning with BEEP

   Here is what tuning BEEP for authentication and privacy looks like
   using SASL and SRP:

       L: <wait for incoming connection>
       I: <open connection>
       L: RPY 0 0 . 0 170
       L: Content-Type: application/beep+xml
       L:
       L: <greeting>
       L:    <profile uri='http://iana.org/beep/TLS' />
       L:    <profile uri='http://iana.org/beep/SASL/SRP-SHA-160' />
       L: </greeting>
       L: END
       I: RPY 0 0 . 0 52
       I: Content-Type: application/beep+xml
       I:
       I: <greeting />
       I: END
       I: MSG 0 1 . 52 184
       I: Content-Type: application/beep+xml
       I:
       I: <start number='1'>
       I:    <profile uri='http://iana.org/beep/SASL/SRP-SHA-160'>
       I:        <![CDATA[<blob>...</blob>]]>
       I:    </profile>
       I: </start>
       I: END
       L: RPY 0 1 . 170 783
       L: Content-Type: application/beep+xml
       L:
       L: <profile uri='http://iana.org/beep/SASL/SRP-SHA-160'>
       L:     <![CDATA[<blob>...</blob>]]>
       L: </profile>
       L: END
       I: MSG 1 0 . 0 559
       I: Content-Type: application/beep+xml
       I:
       I: <blob>...</blob>
       I: END
       L: RPY 1 0 . 0 429
       L: Content-Type: application/beep+xml
       L:
       L: <blob>...</blob>
       L: END


Editor: Farrell                                              [Page 18]

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       I: MSG 1 1 . 559 82
       I: Content-Type: application/beep+xml
       I:
       I: <blob>...</blob>
       I: END
       L: RPY 1 2 . 429 66
       L: Content-Type: application/beep+xml
       L:
       L: <blob status='complete' />
       L: END

           ... successful transport security negotiation ...

       L: RPY 0 0 . 0 127
       L: Content-Type: application/beep+xml
       L:
       L: <greeting>
       L:   <profile uri='http://iana.org/beep/transient/sacred/bss' />
       L: </greeting>
       L: END
       I: RPY 0 0 . 0 52
       I: Content-Type: application/beep+xml
       I:
       I: <greeting />
       I: END

Appendix D: Provision SACRED using other Protocols

   <<This appendix represents the "no other substrates" position.>>

   SACRED may be implemented in a non-BEEP environment, providing that
   before any SACRED PDUs are sent, the application protocol must be
   protected according to the security mandates provided in Section
   2.3.

   For example, if SACRED is provisioned as the payload of an
   application protocol that supports SASL and TLS, then the
   appropriate SASL and/or TLS negotiation must successfully occur
   before exchanging Sacred PDUs.

   Alternatively, if the application protocol doesn't support SASL,
   then one or more PDUs are defined to facilitate a SASL negotiation,
   and the appropriate negotiation must occur before exchanging Sacred
   PDUs.

Appendix E: Changes & Open Issues

   <<Note to RFC editor. Please remove this appendix and its TOC entry
   prior to RFC publication.>>

   Open Issues:



Editor: Farrell                                              [Page 19]

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   - Multiple substrates issue
   - SASL authorization identity issue
   - Should we apply for a port number? (probably)
   - Should the DTD or schema be normative? I'd usually go for the
     schema, but in this case the DTD seems much simpler.
     - Fix DTD extensibility scheme
   - Should we specify a max value where "unbounded" is in the schema
     or "+"/"*" in the DTD?

   -02:
        - Changes as per mailing list discussion:
          - deleted (previous) section 2.4: session mgt & added sec.
             cons about DoS
          - Applied changes from the following threads:
             - "BEEP adjustments"
             - "MultipleCredentials"
             - "SRP adjustments"
             - "LastModified"
        - Other changes:
          - clarified how UploadRequest can delete one or more
             credential payloads
          - merged sections 2 and 3 since its much clearer that way
          - changed Credential structure about a bit due to moving
             LastModified

   -01: Changes as per mailing list discussion:
        - Change from authors to editor + acks
        - Included resolved comments from list:
          - password -> account pwd or cred pwd as appropriate
          - account mgt separated and optional
          - added example beep tuning
          - selector: no default, omit in d/l means all
          - changed LastModified scheme as per list comments
        - Excluded administrative operations (was an open issue)
        - Demoted hashed(username) concept to a note under security
          considerations (see V11).
        - Dropped idea of specifying a mapping between SRP id and cTLS
          certificate.
        - Dropped xkms & xbulk as normative references, but copied some
          stuff from them.

   -00: This version is adapted from draft-ietf-SACRED-protocol-beep-
   pdm-00.txt, the main changes are:
        - PDM -> SRP &/or TLS
        - Payload security -> SASL or TLS
        - Dropped username hashing
        - Dropped away-from-home







Editor: Farrell                                              [Page 20]