Internet DRAFT - draft-fossati-tls-attestation
draft-fossati-tls-attestation
TLS H. Tschofenig
Internet-Draft
Intended status: Standards Track Y. Sheffer
Expires: 5 September 2024 Intuit
P. Howard
I. Mihalcea
Y. Deshpande
Arm Limited
A. Niemi
Huawei
4 March 2024
Using Attestation in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS)
draft-fossati-tls-attestation-05
Abstract
The TLS handshake protocol allows authentication of one or both peers
using static, long-term credentials. In some cases, it is also
desirable to ensure that the peer runtime environment is in a secure
state. Such an assurance can be achieved using attestation which is
a process by which an entity produces evidence about itself that
another party can use to appraise whether that entity is found in a
secure state. This document describes a series of protocol
extensions to the TLS 1.3 handshake that enables the binding of the
TLS authentication key to a remote attestation session. This enables
an entity capable of producing attestation evidence, such as a
confidential workload running in a Trusted Execution Environment
(TEE), or an IoT device that is trying to authenticate itself to a
network access point, to present a more comprehensive set of security
metrics to its peer. These extensions have been designed to allow
the peers to use any attestation technology, in any remote
attestation topology, and mutually.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-fossati-tls-attestation/.
Source for this draft and an issue tracker can be found at
https://github.com/yaronf/draft-tls-attestation.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Attestation Extensions . . . . . . . . . . . . . . . . . . . 5
5. Use of Remote Attestation Credentials in the TLS Handshake . 6
5.1. Handshake Overview . . . . . . . . . . . . . . . . . . . 6
5.2. TLS Client Authenticating Using Evidence . . . . . . . . 7
5.3. TLS Server Authenticating Using Evidence . . . . . . . . 8
5.4. TLS Client Authenticating Using Attestation Results . . . 9
5.5. TLS Server Authenticating Using Results . . . . . . . . . 10
6. Evidence Extensions (Background Check Model) . . . . . . . . 11
6.1. Attestation-only . . . . . . . . . . . . . . . . . . . . 12
6.2. Attestation Alongside X.509 Certificates . . . . . . . . 13
7. Attestation Results Extensions (Passport Model) . . . . . . . 15
8. TLS Client and Server Handshake Behavior . . . . . . . . . . 16
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8.1. Background Check Model . . . . . . . . . . . . . . . . . 17
8.1.1. Client Hello . . . . . . . . . . . . . . . . . . . . 17
8.1.2. Server Hello . . . . . . . . . . . . . . . . . . . . 18
8.2. Passport Model . . . . . . . . . . . . . . . . . . . . . 19
8.2.1. Client Hello . . . . . . . . . . . . . . . . . . . . 19
8.2.2. Server Hello . . . . . . . . . . . . . . . . . . . . 20
9. Background-Check Model Examples . . . . . . . . . . . . . . . 21
9.1. Cloud Confidential Computing . . . . . . . . . . . . . . 21
9.2. IoT Device Onboarding . . . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11.1. TLS Extensions . . . . . . . . . . . . . . . . . . . . . 26
11.2. TLS Alerts . . . . . . . . . . . . . . . . . . . . . . . 26
11.3. TLS Certificate Types . . . . . . . . . . . . . . . . . 27
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
12.1. Normative References . . . . . . . . . . . . . . . . . . 27
12.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Design Rationale: X.509 and Attestation Usage
Variants . . . . . . . . . . . . . . . . . . . . . . . . 30
Appendix B. Cross-protocol Binding Mechanism . . . . . . . . . . 31
B.1. Binding Mechanism . . . . . . . . . . . . . . . . . . . . 32
B.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix C. History . . . . . . . . . . . . . . . . . . . . . . 33
C.1. draft-fossati-tls-attestation-02 . . . . . . . . . . . . 33
C.2. draft-fossati-tls-attestation-01 . . . . . . . . . . . . 33
C.3. draft-fossati-tls-attestation-00 . . . . . . . . . . . . 33
Appendix D. Working Group Information . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
Attestation [RFC9334] is the process by which an entity produces
evidence about itself that another party can use to evaluate the
trustworthiness of that entity. This document describes a series of
protocol extensions to the TLS 1.3 handshake that enables the binding
of the TLS authentication key to a remote attestation session. As a
result, a peer can use "attestation credentials", consisting of
compound platform evidence and key attestation, to authenticate
itself to its peer during the setup of the TLS channel. This enables
an attester, such as a confidential workload running in a Trusted
Execution Environment (TEE) [I-D.ietf-teep-architecture], or an IoT
device that is trying to authenticate itself to a network access
point, to present a more comprehensive set of security metrics to its
peer. This, in turn, allows for the implementation of authorization
policies at the relying parties that are based on stronger security
signals.
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Given the variety of deployed and emerging attestation technologies
(e.g., [TPM1.2], [TPM2.0], [I-D.ietf-rats-eat]) these extensions have
been explicitly designed to be agnostic of the attestation formats.
This is achieved by reusing the generic encapsulation defined in
[I-D.ietf-rats-msg-wrap] for transporting evidence and attestation
result payloads in the TLS Certificate message.
The proposed design supports both background-check and passport
topologies, as described in Sections 5.2 and 5.1 of [RFC9334]. This
is detailed in Section 6 and Section 7. This specification provides
both one-way (server-only) and mutual (client and server)
authentication using attestation credentials, and allows the
attestation topologies at each peer to be independent of each other.
This document does not specify any attestation technology. Companion
documents are expected to define specific attestation mechanisms.
2. Conventions and Terminology
The reader is assumed to be familiar with the vocabulary and concepts
defined in Section 4 of [RFC9334], and those in Section 2 of
[I-D.bft-rats-kat].
The following terms are used in this document:
TLS Identity Key (TIK):
A cryptographic key used by one of the peers to authenticate
itself during the TLS handshake.
TIK-C, TIK-S:
The TIK that identifies the client or the server, respectively.
TIK-C-ID, TIK-S-ID:
An identifier for TIK-C or respectively, TIK-S. This may be a
fingerprint (cryptographic hash) of the public key, but other
implementations are possible.
"Remote attestation credentials", or "attestation credentials", is
used to refer to both attestation evidence and attestation results,
when no distinction needs to be made between them.
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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3. Overview
The basic functional goal is to link the authenticated key exchange
of TLS with an interleaved remote attestation session in such a way
that the key used to sign the handshake can be proven to be residing
within the boundaries of an attested TEE. The requirement is that
the attester can provide evidence containing the security status of
both the signing key and the platform that is hosting it. The
associated security goal is to obtain such binding so that no replay,
relay or splicing from an adversary is possible.
Throughout the document, we will assume the conceptual attester model
described in Section 3 of [I-D.bft-rats-kat], where TEE attestation
is provided by a Platform Attestation Token (PAT) signed by the
attester's "attesting environment". Among other security metrics,
the PAT contains evidence about the integrity of a "Key Attestation
Service" executing within the TEE which issues a Key Attestation
Token (KAT) for the TLS handshake signing key (TIK) as described in
Section 5.1.
The protocol's security relies on the verifiable binding between
these two logically separate units of evidence.
4. Attestation Extensions
As typical with new features in TLS, the client indicates support for
the new extension in the ClientHello message. The newly introduced
extensions allow remote attestation credentials and nonces to be
exchanged. The nonces are used for guaranteeing freshness of the
exchanged evidence when the background check model is in use.
When either the evidence or the attestation results extension is
successfully negotiated, the content of the corresponding Certificate
message contains a payload that is encoded based on the wrapper
defined in [I-D.ietf-rats-msg-wrap]. Both JSON and CBOR
serializations are allowed in CMW, with the emitter choosing which
serialization to use.
In TLS a client has to demonstrate possession of the private key via
the CertificateVerify message, when client-based authentication is
requested. The attestation payload must contain assertions relating
to the client's TLS Identity Key (TIK-C), which associate the private
key with the attestation information. These assertions may come in
the form of a Key Attestation Token (KAT), or of specific claims in
an attestation result document. An example of a KAT format utilizing
the EAT format can be found in [I-D.bft-rats-kat].
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The relying party can obtain and appraise the remote attestation
results either directly from the Certificate message (in the passport
model), or by relaying the evidence from the Certificate message to
the verifier and receiving the attestation results. Subsequently,
the attested key is used to verify the CertificateVerify message.
When using the passport model, the remote attestation results
obtained by the attester from its trusted verifiers can be cached and
used for any number of subsequent TLS handshakes, as long as the
freshness policy requirements are satisfied.
This protocol supports both monolithic and split implementations. In
a monolithic implementation, the TLS stack is completely embedded
within the TEE. In a split implementation, the TLS stack is located
outside the TEE, but any private keys (and in particular, the TIK)
only exist within the TEE. In order to support both options, only
the TIK's identity and its public component are ever passed between
the Client or Server TLS stack and its Attestation Service.
5. Use of Remote Attestation Credentials in the TLS Handshake
For both the passport model (described in section 5.1 of [RFC9334])
and background check model (described in Section 5.2 of [RFC9334])
the following modes of operation are allowed when used with TLS,
namely:
* TLS client is the attester,
* TLS server is the attester, and
* TLS client and server mutually attest towards each other.
We will show the message exchanges of the first two cases in sub-
sections below. Mutual authentication via attestation combines these
two (non-interfering) flows, including cases where one of the peers
uses the passport model for its attestation, and the other uses the
background check model.
5.1. Handshake Overview
The handshake defined here is analogous to certificate-based
authentication in a regular TLS handshake. Instead of the
certificate's private key, we use the TIK identity key. This key is
attested, with attestation being carried by the Certificate message.
Following that, the peer being attested proves possession of the
private key using the CertificateVerify message.
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Depending on the use case, the protocol supports peer authentication
using attestation only, or using both attestation and a regular
public key certificate.
The current version of the document assumes the KAT/PAT construct of
[I-D.bft-rats-kat]. Not all platforms support this model, and a
document that defines private key attestation for use in TLS
Attestation as defined here, must specify:
* The format and the lifetime of TIK (e.g. an ephemeral, per session
TIK vs. a long lived one).
* How the key is attested using a structure carried by the
Certificate message.
* How proof of possession is performed.
5.2. TLS Client Authenticating Using Evidence
In this use case, the TLS server (acting as a relying party)
challenges the TLS client (as the attester) to provide evidence. The
TLS server needs to provide a nonce in the EncryptedExtensions
message to the TLS client so that the attestation service can feed
the nonce into the generation of the evidence. The TLS server, when
receiving the evidence, will have to contact the verifier (which is
not shown in the diagram).
An example of this flow can be found in device onboarding where the
client initiates the communication with cloud infrastructure to get
credentials, firmware and other configuration data provisioned to the
device. For the server to consider the device genuine it needs to
present evidence.
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Client Server
Key ^ ClientHello
Exch | + evidence_proposal
| + key_share*
| + signature_algorithms*
v -------->
ServerHello ^ Key
+ key_share* | Exch
v
{EncryptedExtensions} ^ Server
+ evidence_proposal | Params
(nonce) |
{CertificateRequest} v
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 1: TLS Client Providing Evidence to TLS Server.
5.3. TLS Server Authenticating Using Evidence
In this use case the TLS client challenges the TLS server to present
evidence. The TLS server acts as an attester while the TLS client is
the relying party. The TLS client, when receiving the evidence, will
have to contact the verifier (which is not shown in the diagram).
An example of this flow can be found in confidential computing where
a compute workload is only submitted to the server infrastructure
once the client/user is assured that the confidential computing
platform is genuine.
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Client Server
Key ^ ClientHello
Exch | + evidence_request
| (nonce)
| + key_share*
| + signature_algorithms*
v -------->
ServerHello ^ Key
+ key_share* | Exch
v
{EncryptedExtensions} ^ Server
+ evidence_request | Params
|
{CertificateRequest} v
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 2: TLS Server Providing Evidence to TLS Client.
5.4. TLS Client Authenticating Using Attestation Results
In this use case the TLS client, as the attester, provides
attestation results to the TLS server. The TLS client is the
attester and the the TLS server acts as a relying party. Prior to
delivering its Certificate message, the client must contact the
verifier (not shown in the diagram) to receive the attestation
results that it will use as credentials.
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Client Server
Key ^ ClientHello
Exch | + results_proposal
| + key_share*
| + signature_algorithms*
v -------->
ServerHello ^ Key
+ key_share* | Exch
v
{EncryptedExtensions} ^ Server
+ results_proposal | Params
{CertificateRequest} v
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 3: TLS Client Providing Results to TLS Server.
5.5. TLS Server Authenticating Using Results
In this use case the TLS client, as the relying party, requests
attestation results from the TLS server. Prior to delivering its
Certificate message, the server must contact the verifier (not shown
in the diagram) to receive the attestation results that it will use
as credentials.
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Client Server
Key ^ ClientHello
Exch | + results_request
| + key_share*
| + signature_algorithms*
v -------->
ServerHello ^ Key
+ key_share* | Exch
v
{EncryptedExtensions} ^ Server
+ results_request | Params
|
{CertificateRequest} v
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 4: TLS Server Providing Attestation Results to TLS Client.
6. Evidence Extensions (Background Check Model)
The EvidenceType structure also contains an indicator for the type of
credential expected in the Certificate message. The credential can
either contain attestation evidence alone, or an X.509 certificate
alongside attestation evidence.
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enum { CONTENT_FORMAT(0), MEDIA_TYPE(1) } typeEncoding;
enum { ATTESTATION(0), CERT_ATTESTATION(1) } credentialKind;
struct {
credentialKind credential_kind;
typeEncoding type_encoding;
select (EvidenceType.type_encoding) {
case CONTENT_FORMAT:
uint16 content_format;
case MEDIA_TYPE:
opaque media_type<0..2^16-1>;
};
} EvidenceType;
struct {
select(Handshake.msg_type) {
case client_hello:
EvidenceType supported_evidence_types<1..2^8-1>;
opaque nonce<8..2^8-1>;
case server_hello:
EvidenceType selected_evidence_type;
}
} evidenceRequestTypeExtension;
struct {
select(Handshake.msg_type) {
case client_hello:
EvidenceType supported_evidence_types<1..2^8-1>;
case server_hello:
EvidenceType selected_evidence_type;
opaque nonce<8..2^8-1>;
}
} evidenceProposalTypeExtension;
Figure 5: TLS Extension Structure for Evidence.
6.1. Attestation-only
When the chosen evidence type indicates the sole use of attestation
for authentication, the Certificate payload is used as a container
for attestation evidence, as shown in Figure 6, and follows the model
of [RFC8446].
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struct {
select (certificate_type) {
case RawPublicKey:
/* From RFC 7250 ASN.1_subjectPublicKeyInfo */
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X509:
opaque cert_data<1..2^24-1>;
case attestation:
/* payload used to convey evidence */
opaque evidence<1..2^24-1>;
};
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
Figure 6: Certificate Message when using only attestation.
The encoding of the evidence structure is defined in
[I-D.ietf-rats-msg-wrap].
6.2. Attestation Alongside X.509 Certificates
When the chosen evidence type indicates usage of both attestation and
PKIX, the X.509 certificate will serve as the main payload in the
Certificate message, while the attestation evidence will be carried
in the CertificateEntry extension, as shown in Figure 7.
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struct {
select (certificate_type) {
case RawPublicKey:
/* From RFC 7250 ASN.1_subjectPublicKeyInfo */
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X509:
/* X.509 certificate conveyed as usual */
opaque cert_data<1..2^24-1>;
};
/* attestation evidence conveyed as an extension, see below */
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
struct {
ExtensionType extension_type;
/* payload used to convey evidence */
opaque extension_data<0..2^16-1>;
} Extension;
enum {
/* other extension types defined in the IANA TLS
ExtensionType Value registry */
/* variant used to identify attestation evidence */
attestation_evidence(60),
(65535)
} ExtensionType;
Figure 7: Certificate Message when using PKIX and attestation.
The encoding of the evidence structure is defined in
[I-D.ietf-rats-msg-wrap].
As described in Appendix A, this authentication mechanism is meant
primarily for carrying platform attestation evidence to provide more
context to the relying party. This evidence must be
cryptographically bound to the TLS handshake to prevent relay
attacks. An Attestation Channel Binder as described in Appendix B is
therefore used when the attestation scheme does not allow the binding
data to be part of the token. The structure of the binder is given
in Figure 8.
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attestation_channel_binder = {
&(nonce: 1) => bstr .size (8..64)
&(ik_pub_fingerprint: 2) => bstr .size (16..64)
&(channel_binder: 3) => bstr .size (16..64)
}
Figure 8: Format of TLS channel binder.
* Nonce is the value provided as a challenge by the relying party.
* The identity key fingerprint (ik_pub_fingerprint) is a hash of the
Subject Public Key Info from the leaf X.509 certificate
transmitted in the handshake.
* The channel binder (channel_binder) is a value obtained from the
early exporter mechanism offered by the TLS implementation
(Section 7.5 of [RFC8446]). This Early Exporter Value (EEV) must
be obtained immediately following the ServerHello message, using
'attestation-binder' as the label, an empty context, and with the
key length set to 32 bytes. Figure 9 shows this computation using
the notation from [RFC8446].
TLS-Early-Exporter(label, context_value, key_length) =
HKDF-Expand-Label(
Derive-Secret(early_exporter_master_secret, label, ""),
"exporter", Hash(context_value), key_length)
channel_binder = TLS-Early-Exporter(label = "attestation-binder",
context_value = "", key_length = 32)
Figure 9: Usage of TLS v1.3 early exporter for channel binding.
A hash of the binder must be included in the attestation evidence.
Previous to hashing, the binder must be encoded as described in
Appendix B.
The hash algorithm negotiatied within the handshake must be used
wherever hashing is required for the binder.
7. Attestation Results Extensions (Passport Model)
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struct {
opaque verifier_identity<0..2^16-1>;
} VerifierIdentityType;
struct {
select(Handshake.msg_type) {
case client_hello:
VerifierIdentityType trusted_verifiers<1..2^8-1>;
case server_hello:
VerifierIdentityType selected_verifier;
}
} resultsRequestTypeExtension;
struct {
select(Handshake.msg_type) {
case client_hello:
VerifierIdentityType trusted_verifiers<1..2^8-1>;
case server_hello:
VerifierIdentityType selected_verifier;
}
} resultsProposalTypeExtension;
Figure 10: TLS Extension Structure for Attestation Results.
8. TLS Client and Server Handshake Behavior
The high-level message exchange in Figure 11 shows the
evidence_proposal, evidence_request, results_proposal, and
results_request extensions added to the ClientHello and the
EncryptedExtensions messages.
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Client Server
Key ^ ClientHello
Exch | + key_share*
| + signature_algorithms*
| + psk_key_exchange_modes*
| + pre_shared_key*
| + evidence_proposal*
| + evidence_request*
| + results_proposal*
v + results_request*
-------->
ServerHello ^ Key
+ key_share* | Exch
+ pre_shared_key* v
{EncryptedExtensions} ^ Server
+ evidence_proposal* |
+ evidence_request* |
+ results_proposal* |
+ results_request* |
{CertificateRequest*} v Params
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 11: Attestation Message Overview.
8.1. Background Check Model
8.1.1. Client Hello
To indicate the support for passing evidence in TLS following the
background check model, clients include the evidence_proposal and/or
the evidence_request extensions in the ClientHello.
The evidence_proposal extension in the ClientHello message indicates
the evidence types the client is able to provide to the server, when
requested using a CertificateRequest message.
The evidence_request extension in the ClientHello message indicates
the evidence types the client challenges the server to provide in a
subsequent Certificate payload.
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The evidence_proposal and evidence_request extensions sent in the
ClientHello each carry a list of supported evidence types, sorted by
preference. When the client supports only one evidence type, it is a
list containing a single element.
The client MUST omit evidence types from the evidence_proposal
extension in the ClientHello if it cannot respond to a request from
the server to present a proposed evidence type, or if the client is
not configured to use the proposed evidence type with the given
server. If the client has no evidence types to send in the
ClientHello it MUST omit the evidence_proposal extension in the
ClientHello.
The client MUST omit evidence types from the evidence_request
extension in the ClientHello if it is not able to pass the indicated
verification type to a verifier. If the client does not act as a
relying party with regards to evidence processing (as defined in the
RATS architecture) then the client MUST omit the evidence_request
extension from the ClientHello.
8.1.2. Server Hello
If the server receives a ClientHello that contains the
evidence_proposal extension and/or the evidence_request extension,
then three outcomes are possible:
* The server does not support the extensions defined in this
document. In this case, the server returns the
EncryptedExtensions without the extensions defined in this
document.
* The server supports the extensions defined in this document, but
it does not have any evidence type in common with the client.
Then, the server terminates the session with a fatal alert of type
"unsupported_evidence".
* The server supports the extensions defined in this document and
has at least one evidence type in common with the client. In this
case, the processing rules described below are followed.
The evidence_proposal extension in the ClientHello indicates the
evidence types the client is able to provide to the server, when
challenged using a certificate_request message. If the server wants
to request evidence from the client, it MUST include the
evidence_proposal extension in the EncryptedExtensions. This
evidence_proposal extension in the EncryptedExtensions then indicates
what evidence format the client is requested to provide in a
subsequent Certificate message. The value conveyed in the
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evidence_proposal extension by the server MUST be selected from one
of the values provided in the evidence_proposal extension sent in the
ClientHello. The server MUST also send a certificate_request
message.
If the server does not send a certificate_request message or none of
the evidence types supported by the client (as indicated in the
evidence_proposal extension in the ClientHello) match the server-
supported evidence types, then the evidence_proposal extension in the
ServerHello MUST be omitted.
The evidence_request extension in the ClientHello indicates what
types of evidence the client can challenge the server to return in a
subsequent Certificate message. With the evidence_request extension
in the EncryptedExtensions, the server indicates the evidence type
carried in the Certificate message sent by the server. The evidence
type in the evidence_request extension MUST contain a single value
selected from the evidence_request extension in the ClientHello.
8.2. Passport Model
8.2.1. Client Hello
To indicate the support for passing attestation results in TLS
following the passport model, clients include the results_proposal
and/or the results_request extensions in the ClientHello message.
The results_proposal extension in the ClientHello message indicates
the verifier identities from which it can relay attestation results,
when requested using a CertificateRequest message.
The results_request extension in the ClientHello message indicates
the verifier identities from which the client expects the server to
provide attestation results in a subsequent Certificate payload.
The results_proposal and results_request extensions sent in the
ClientHello each carry a list of supported verifier identities,
sorted by preference. When the client supports only one verifier, it
is a list containing a single element.
The client MUST omit verifier identities from the results_proposal
extension in the ClientHello if it cannot respond to a request from
the server to present attestation results from a proposed verifier,
or if the client is not configured to relay the results from the
proposed verifier with the given server. If the client has no
verifier identities to send in the ClientHello it MUST omit the
results_proposal extension in the ClientHello.
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The client MUST omit verifier identities from the results_request
extension in the ClientHello if it is not configured to trust
attestation results issued by said verifiers. If the client does not
act as a relying party with regards to the processing of attestation
results (as defined in the RATS architecture) then the client MUST
omit the results_request extension from the ClientHello.
8.2.2. Server Hello
If the server receives a ClientHello that contains the
results_proposal extension and/or the results_request extension, then
three outcomes are possible:
* The server does not support the extensions defined in this
document. In this case, the server returns the
EncryptedExtensions without the extensions defined in this
document.
* The server supports the extensions defined in this document, but
it does not have any trusted verifiers in common with the client.
Then, the server terminates the session with a fatal alert of type
"unsupported_verifiers".
* The server supports the extensions defined in this document and
has at least one trusted verifier in common with the client. In
this case, the processing rules described below are followed.
The results_proposal extension in the ClientHello indicates the
verifier identities from which the client is able to provide
attestation results to the server, when challenged using a
certificate_request message. If the server wants to request evidence
from the client, it MUST include the results_proposal extension in
the EncryptedExtensions. This results_proposal extension in the
EncryptedExtensions then indicates what verifier the client is
requested to provide attestation results from in a subsequent
Certificate message. The value conveyed in the results_proposal
extension by the server MUST be selected from one of the values
provided in the results_proposal extension sent in the ClientHello.
The server MUST also send a certificate_request message.
If the server does not send a certificate_request message or none of
the verifier identities proposed by the client (as indicated in the
results_proposal extension in the ClientHello) match the server-
trusted verifiers, then the results_proposal extension in the
ServerHello MUST be omitted.
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The results_request extension in the ClientHello indicates what
verifiers the client trusts as issuers of attestation results for the
server. With the results_request extension in the
EncryptedExtensions, the server indicates the identity of the
verifier who issued the attestation results carried in the
Certificate message sent by the server. The verifier identity in the
results_request extension MUST contain a single value selected from
the results_request extension in the ClientHello.
9. Background-Check Model Examples
9.1. Cloud Confidential Computing
In this example, a confidential workload is executed on computational
resources hosted at a cloud service provider. This is a typical
scenario for secure, privacy-preserving multiparty computation,
including anti-money laundering, drug development in healthcare,
contact tracing in pandemic times, etc.
In such scenarios, the users (e.g., the party providing the data
input for the computation, the consumer of the computed attestation
results, the party providing a proprietary ML model used in the
computation) have two goals:
* Identifying the workload they are interacting with,
* Making sure that the platform on which the workload executes is a
Trusted Execution Environment (TEE) with the expected features.
A convenient arrangement is to verify that the two requirements are
met at the same time that the secure channel is established.
The protocol flow, alongside all the involved actors, is captured in
Figure 12 where the TLS client is the user (the relying party) while
the TLS server is co-located with the TEE-hosted confidential
workload (the attester).
The flow starts with the client initiating a verification session
with a trusted verifier. The verifier returns the evidence types it
understands and a nonce that will be used to challenge the attester.
The client starts the TLS handshake with the server by supplying the
attestation-related parameters it has obtained from the verifier. If
the server supports one of the offered evidence types, it will echo
it in the specular extension and proceed by invoking a local API
(represented by attest_key(...) in the figure below) to request
attestation using the nonce supplied by the verifier. The returned
evidence binds the identity key (TIK-S) with the platform identity
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and security state, packaged into a CAB. The server then signs a
transcript hash (represented by hs in the figure below) of the
handshake context and the server's Certificate message with the
(attested) identity key, and sends the attestation evidence together
with the signature over to the client.
The client forwards the attestation evidence to the verifier using
the previously established session, obtains the attestation result
(AR) and checks whether it is acceptable according to its local
policy. If so, it proceeds and verifies the handshake signature
using the corresponding public key (for example, using the PoP key in
the KAT evidence [I-D.bft-rats-kat]).
The attestation evidence verification combined with the verification
of the CertificateVerify signature provide confirmation that the
presented cryptographic identity is bound to the workload and
platform identity, and that the workload and platform are
trustworthy. Therefore, after the handshake is finalized, the client
can trust the workload on the other side of the established secure
channel to provide the required confidential computing properties.
.------------------------.
.----------. .--------. | Server | Attestation |
| Verifier | | Client | | | Service |
'--+-------' '---+----' '---+---------------+----'
| | | |
| POST /newSession | | |
|<-------------------+ | |
| 201 Created | | |
| Location: /76839A9 | | |
| Body: { | | |
| nonce, | | |
| supp-media-types | | |
| } | | |
+------------------->| | |
| | | |
.--+-----------. | | |
| TLS handshake | | | |
+--+------------+-------+------------------------+---------------+---.
| | | ClientHello | | |
| | | {...} | | |
| | | evidence_request( | | |
| | | nonce, | | |
| | | types(a,b,c) | | |
| | | ) | | |
| | +----------------------->| | |
| | | | attest_key( | |
| | | | nonce, | |
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| | | | TIK-S-ID | |
| | | | ) | |
| | | +-------------->| |
| | | | CAB(KAT, PAT) | |
| | | |<--------------+ |
| | | ServerHello | | |
| | | {...} | | |
| | | EncryptedExtensions | | |
| | | {...} | | |
| | | evidence_request( | | |
| | | type(a) | | |
| | | ) | | |
| | | Certificate(KAT,PAT) | | |
| | |<-----------------------+ | |
| | | |sign(TIK-S-ID,hs) |
| | | +-------------->| |
| | | | sig | |
| | | |<--------------+ |
| | | CertificateVerify(sig) | | |
| | | Finished | | |
| | |<-----------------------+ | |
| | POST /76839A9E | | | |
| | Body: { | | | |
| | type(a), | | | |
| | CAB | | | |
| | } | | | |
| |<-------------------+ | | |
| | Body: { | | | |
| | att-result: AR{} | | | |
| | } | | | |
| +------------------->| | | |
| | +---. | | |
| | | | verify AR{} | | |
| | |<--' | | |
| | +---. | | |
| | | | verify sig | | |
| | |<--' | | |
| | | Finished | | |
| | +----------------------->| | |
| | | | | |
'-+--------------------+------------------------+---------------+---'
| application data |
|<---------------------->|
| |
Figure 12: Example Exchange with Server as Attester.
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9.2. IoT Device Onboarding
In this example, an IoT is onboarded to a cloud service provider (or
to a network operator). In this scenario there is typically no a
priori relationship between the device and the cloud service provider
that will remotely manage the device.
In such scenario, the cloud service provider wants to make sure that
the device runs the correct version of firmware, has not been rooted,
is not emulated or cloned.
The protocol flow is shown in Figure 13 where the client is the
attester while the server is the relying party.
The flow starts with the client initiating a TLS exchange with the
TLS server operated by the cloud service provider. The client
indicates what evidence types it supports.
The server obtains a nonce from the verifier, in real-time or from a
reserved nonce range, and returns it to the client alongside the
selected evidence type. Since the evidence will be returned in the
Certificate message the server has to request mutual authentication
via the CertificateRequest message.
The client, when receiving the EncryptedExtension with the
evidence_proposal, will proceed by invoking a local API to request
the attestation. The returned evidence binds the identity key (TIK-
C) with the workload and platform identity and security state,
packaged into a CAB. The client then signs a transcript hash of the
handshake context and the client's Certificate message with the
(attested) identity key, and sends the evidence together with the
signature over to the server.
The server forwards the attestation evidence to the verifier, obtains
the attestation result and checks that it is acceptable according to
its local policy. The evidence verification combined with the
verification of the CertificateVerify signature provide confirmation
that the presented cryptographic identity is bound to the platform
identity, and that the platform is trustworthy.
If successful, the server proceeds with the application layer
protocol exchange. If, for some reason, the attestation result is
not satisfactory the TLS server will terminate the exchange.
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.--------------------------.
| Attestation | Client | .--------. .----------.
| Service | | | Server | | Verifier |
'--+----------------+------' '----+---' '-----+----'
| | | |
.--+-----------. | | |
| TLS handshake | | | |
+--+------------+---+------------------------+-------------------+---.
| | | | | |
| | | ClientHello | | |
| | | {...} | | |
| | | evidence_proposal( | | |
| | | types(a,b,c) | | |
| | | ) | | |
| | +----------------------->| | |
| | | | | |
| + | ServerHello | POST /newSession | |
| | | {...} +------------------>| |
| | | | 201 Created | |
| | | | Location: /76839 | |
| | | | Body: { | |
| | | | nonce, | |
| | | EncryptedExtensions | types(a,b,c) | |
| | | {...} | } | |
| | | evidence_proposal( |<------------------+ |
| | | nonce, | | |
| | | type(a) | | |
| | | ) | | |
| | | CertificateRequest | | |
| | | Certificate | | |
| | | CertificateVerify | | |
| | attest_key( | Finished | | |
| | nonce, |<-----------------------+ | |
| | TIK-C-ID | | | |
| | ) | | | |
| |<---------------+ | | |
| | | | | |
| | CAB(KAT, PAT) | | | |
| +--------------->| Certificate(KAT,PAT) | | |
| | +----------------------->| | |
| |sign(TIK-C-ID,hs) | | |
| |<---------------+ | | |
| | sig | CertificateVerify(sig) | | |
| +--------------->| Finished | | |
| | +----------------------->| | |
| | | | | |
| | | | POST /76839A9E | |
| | | | Body: { | |
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| | | | type(a), | |
| | | | CAB | |
| | | | } | |
| | | +------------------>| |
| | | | Body: { | |
| | | | att-result: AR{} | |
| | | | } | |
| | | |<------------------+ |
| | | +---. | |
| | | | | verify AR{} | |
| | | |<--' | |
| | | +---. | |
| | | | | verify sig | |
| | | |<--' | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
'--+----------------+------------------------+-------------------+---'
| application data |
|<---------------------->|
| |
Figure 13: Example Exchange with Client as Attester.
10. Security Considerations
TBD.
11. IANA Considerations
11.1. TLS Extensions
IANA is asked to allocate four new TLS extensions, evidence_request,
evidence_proposal, results_request, results_proposal, from the "TLS
ExtensionType Values" subregistry of the "Transport Layer Security
(TLS) Extensions" registry [TLS-Ext-Registry]. These extensions are
used in the ClientHello and the EncryptedExtensions messages. The
values carried in these extensions are taken from TBD.
11.2. TLS Alerts
IANA is requested to allocate a value in the "TLS Alerts" subregistry
of the "Transport Layer Security (TLS) Parameters" registry
[TLS-Param-Registry] and populate it with the following entries:
* Value: TBD1
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* Description: unsupported_evidence
* DTLS-OK: Y
* Reference: [This document]
* Comment:
* Value: TBD2
* Description: unsupported_verifiers
* DTLS-OK: Y
* Reference: [This document]
* Comment:
11.3. TLS Certificate Types
IANA is requested to allocate a new value in the "TLS Certificate
Types" subregistry of the "Transport Layer Security (TLS) Extensions"
registry [TLS-Ext-Registry], as follows:
* Value: TBD2
* Description: Attestation
* Reference: [This document]
12. References
12.1. Normative References
[I-D.ietf-rats-msg-wrap]
Birkholz, H., Smith, N., Fossati, T., and H. Tschofenig,
"RATS Conceptual Messages Wrapper (CMW)", Work in
Progress, Internet-Draft, draft-ietf-rats-msg-wrap-04, 27
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-msg-wrap-04>.
[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>.
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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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
12.2. Informative References
[DICE-Layering]
Trusted Computing Group, "DICE Layering Architecture
Version 1.00 Revision 0.19", July 2020,
<https://trustedcomputinggroup.org/resource/dice-layering-
architecture/>.
[I-D.acme-device-attest]
Weeks, B., "Automated Certificate Management Environment
(ACME) Device Attestation Extension", Work in Progress,
Internet-Draft, draft-acme-device-attest-02, 22 February
2024, <https://datatracker.ietf.org/doc/html/draft-acme-
device-attest-02>.
[I-D.bft-rats-kat]
Brossard, M., Fossati, T., and H. Tschofenig, "An EAT-
based Key Attestation Token", Work in Progress, Internet-
Draft, draft-bft-rats-kat-03, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-bft-rats-kat-
03>.
[I-D.ietf-rats-ar4si]
Voit, E., Birkholz, H., Hardjono, T., Fossati, T., and V.
Scarlata, "Attestation Results for Secure Interactions",
Work in Progress, Internet-Draft, draft-ietf-rats-ar4si-
05, 30 August 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
ar4si-05>.
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[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Wallace, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-25, 15
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-25>.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. M. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-19, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
architecture-19>.
[RA-TLS] Knauth, T., Steiner, M., Chakrabarti, S., Lei, L., Xing,
C., and M. Vij, "Integrating Remote Attestation with
Transport Layer Security", January 2018,
<https://arxiv.org/abs/1801.05863>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
[TLS-Ext-Registry]
IANA, "Transport Layer Security (TLS) Extensions",
<http://www.iana.org/assignments/tls-extensiontype-
values>.
[TLS-Param-Registry]
IANA, "Transport Layer Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters>.
[TPM1.2] Trusted Computing Group, "TPM Main Specification Level 2
Version 1.2, Revision 116", March 2011,
<https://trustedcomputinggroup.org/resource/tpm-main-
specification/>.
[TPM2.0] Trusted Computing Group, "Trusted Platform Module Library
Specification, Family "2.0", Level 00, Revision 01.59",
November 2019,
<https://trustedcomputinggroup.org/resource/tpm-library-
specification/>.
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Appendix A. Design Rationale: X.509 and Attestation Usage Variants
The inclusion of attestation results and evidence as part of the TLS
handshake offers the relying party information about the state of the
system and its cryptographic keys, but lacks the means to specify a
stable endpoint identifier. While it is possible to solve this
problem by including an identifier as part of the attestation result,
some use cases require the use of a public key infrastructure (PKI).
It is therefore important to consider the possible approaches for
conveying X.509 certificates and attestation within a single
handshake.
In general, the following combinations of X.509 and attestation usage
are possible:
1. X.509 certificates only: In this case no attestation is exchanged
in the TLS handshake. Authentication relies on PKI alone, i.e.
TLS with X.509 certificates.
2. X.509 certificates containing attestation extension: The X.509
certificates in the Certificate message carry attestation as part
of the X.509 certificate extensions. Several proposals exist
that enable this functionality:
* Custom X.509 extension:
- Attester-issued certificates (e.g., RA-TLS [RA-TLS]): The
attester acts as a certification authority (CA) and
includes the attestation evidence within an X.509
extension.
- DICE defines extensions that include attestation
information in the "Embedded CA" certificates (See
Section 8.1.1.1 of [DICE-Layering]).
- Third party CA-issued certificates (e.g., ACME Device
Attestation [I-D.acme-device-attest]): Remote attestation
is performed between the third party CA and the attester
prior to certificate issuance, after which the CA adds an
extension indicating that the certificate key has fulfilled
some verification policy.
* Explicit signalling via existing methods, e.g. using a policy
OID in the end-entity certificate.
* Implicit signalling, e.g. via the issuer name.
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3. X.509 certificates alongside a PAT: This use case assumes that a
keypair with a corresponding certificate already exists and that
the owner wishes to continue using it. As a consequence, there
is no cryptographic linkage between the certificate and the PAT.
This approach is described in Section 6.2.
4. X.509 certificates alongside the PAT and KAT: The addition of key
attestation implies that the TLS identity key must have been
generated and stored securely by the attested platform. Unlike
in variant (3), the certificate, the KAT, and the PAT must be
cryptographically linked. This variant is currently not
addressed in this document.
5. Combined PAT/KAT: With this variant the attestation token carries
information pertaining to both platform and key. No X.509
certificate is transmitted during the handshake. This approach
is currently not addressed in this document.
6. PAT alongside KAT: This variant is similar to (5) with the
exception that the key and the platform attestations are stored
in separate tokens, cryptographically linked together. This
approach is covered by this document in Section 6.1. A possible
instantiation of the KAT is described in [I-D.bft-rats-kat].
Appendix B. Cross-protocol Binding Mechanism
Note: This section describes a protocol-agnostic mechanism which is
used in the context of TLS within the body of the draft. The
mechanism might, in the future, be spun out into its own document.
One of the issues that must be addressed when using remote
attestation as an authentication mechanism is the binding to the
outer protocol (i.e., the protocol requiring authentication). For
every instance of the combined protocol, the remote attestation
credentials must be verifiably linked to the outer protocol. The
main reason for this requirement is security: a lack of binding can
result in the attestation credentials being relayed.
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If the attestation credentials can be enhanced freely and in a
verifiable way, the binding can be performed by inserting the
relevant data as new claims. If the ways of enhancing the
attestation credentials are more restricted, ad-hoc solutions can be
devised which address the issue. For example, many roots of trust
only allow a small amount (32-64 bytes) of user-provided data which
will be included in the attestation token. If more data must be
included, it must therefore be compressed. In this case, the problem
is compounded by the need to also include a challenge value coming
from the relying party. The verification steps also become more
complex, as the binding data must be returned from the verifier and
checked by the relying party.
However, regardless of how the binding and verification are
performed, similar but distinct approaches need to be taken for every
protocol into which remote attestation is embedded, as the type or
semantics of the binding data could differ. A more standardised way
of tackling this issue would therefore be beneficial. This appendix
presents a solution to this problem, in the context of attestation
evidence.
B.1. Binding Mechanism
The core of the binding mechanism consists of a new token format -
the Attestation Channel Binder - that represents a set of binders as
a CBOR map. Binders are individual pieces of data with an
unambiguous definition. Each binder is a name/value pair, where the
name must be an integer and the value must be a byte string.
Each protocol using the Attestation Channel Binder to bind
attestation credentials must define its Attestation Channel Binder
using CDDL. The only mandated binder is the challenger nonce which
must use the value 1 as a name. Every other name/value pair must
come with a text description of its semantics. The byte strings
forming the values of binders can be size-restricted where this value
is known.
Attestation Channel Binders are encoded in CBOR, following the CBOR
core deterministic encoding requirements (Section 4.2.1 of
[RFC8949]).
An example Attestation Channel Binder is shown below.
attestation_channel_binder = {
&(nonce: 1) => bstr .size (8..64)
&(ik_pub_fingerprint: 2) => bstr .size 32
&(session_key_binder: 3) => bstr .size 32
}
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Figure 14: Format of a possible TLS Attestation Channel Binder.
B.2. Usage
When a Attestation Channel Binder is used to compress data to fit the
space afforded by an attestation scheme, the encoded binder must be
hashed. Since the relying party has access to all the data expected
in the binder, the binder itself need not be conveyed. How the
hashing algorithm is chosen, used, and conveyed must be defined per
outer protocol. If the digest size does not match the user data size
mandated by the attestation scheme, the digest is truncated or
expanded appropriately.
The verifier must first hash the encoded token received from the
relying party and then compare the hashes. The challenge value
included in the binder can then be extracted and verified. If
verification is successful, binder correctness can also be assumed by
the relying party, as verification was done with the values it
expected.
Appendix C. History
RFC EDITOR: PLEASE REMOVE THIS SECTION
C.1. draft-fossati-tls-attestation-02
* Focus on the background check model
* Added examples
* Updated introduction
* Moved attestation format-specific content to related drafts.
C.2. draft-fossati-tls-attestation-01
* Added details about TPM attestation
C.3. draft-fossati-tls-attestation-00
* Initial version
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Internet-Draft Attestation in TLS/DTLS March 2024
Appendix D. Working Group Information
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org (mailto:tls@ietf.org). Information on
the group and information on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/tls
(https://www1.ietf.org/mailman/listinfo/tls)
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html (https://www.ietf.org/mail-
archive/web/tls/current/index.html)
Authors' Addresses
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
Yaron Sheffer
Intuit
Email: yaronf.ietf@gmail.com
Paul Howard
Arm Limited
Email: Paul.Howard@arm.com
Ionut Mihalcea
Arm Limited
Email: Ionut.Mihalcea@arm.com
Yogesh Deshpande
Arm Limited
Email: Yogesh.Deshpande@arm.com
Arto Niemi
Huawei
Email: arto.niemi@huawei.com
Tschofenig, et al. Expires 5 September 2024 [Page 34]
