Using Attestation in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)
draft-fossati-tls-attestation-03
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Hannes Tschofenig , Yaron Sheffer , Paul Howard , Ionuț Mihalcea , Yogesh Deshpande | ||
| Last updated | 2023-03-13 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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draft-fossati-tls-attestation-03
TLS H. Tschofenig
Internet-Draft
Intended status: Standards Track Y. Sheffer
Expires: 14 September 2023 Intuit
P. Howard
I. Mihalcea
Y. Deshpande
Arm Limited
13 March 2023
Using Attestation in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS)
draft-fossati-tls-attestation-03
Abstract
Attestation is the process by which an entity produces evidence about
itself that another party can use to evaluate the trustworthiness of
that entity.
In use cases that require the use of remote attestation, such as
confidential computing or device onboarding, an attester has to
convey evidence or attestation results to a relying party. This
information exchange may happen at different layers in the protocol
stack.
This specification provides a generic way of passing evidence and
attestation results in the TLS handshake. Functionality-wise this is
accomplished with the help of key attestation.
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 14 September 2023.
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Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Use of Evidence with the Background Check Model . . . . . . . 5
4.1. TLS Client as Attester . . . . . . . . . . . . . . . . . 5
4.2. TLS Server as Attester . . . . . . . . . . . . . . . . . 6
5. Evidence Extensions (Background Check Model) . . . . . . . . 7
5.1. Attestation-only . . . . . . . . . . . . . . . . . . . . 8
5.2. Attestation alongside X.509 certificates . . . . . . . . 9
6. TLS Client and Server Handshake Behavior . . . . . . . . . . 11
6.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . 13
7. Background-Check Model Examples . . . . . . . . . . . . . . . 14
7.1. Cloud Confidential Computing . . . . . . . . . . . . . . 14
7.2. IoT Device Onboarding . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. TLS Extensions . . . . . . . . . . . . . . . . . . . . . 19
9.2. TLS Alerts . . . . . . . . . . . . . . . . . . . . . . . 19
9.3. TLS Certificate Types . . . . . . . . . . . . . . . . . . 20
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10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Design Rationale: X.509 and Attestation Usage
Variants . . . . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Cross-protocol binding mechanism . . . . . . . . . . 24
B.1. Binding mechanism . . . . . . . . . . . . . . . . . . . . 24
B.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix C. History . . . . . . . . . . . . . . . . . . . . . . 25
C.1. draft-fossati-tls-attestation-02 . . . . . . . . . . . . 25
C.2. draft-fossati-tls-attestation-01 . . . . . . . . . . . . 26
C.3. draft-fossati-tls-attestation-00 . . . . . . . . . . . . 26
Appendix D. Working Group Information . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Remote ATtestation ProcedureS (RATS) architecture defines two
basic types of topological patterns to communicate between an
attester, a relying party, and a verifier, namely the background-
check model and the passport model. These two models are
fundamentally different and require a different treatment when
incorporated into the TLS handshake. For better readability we
suggest to use different extensions for these two models.
The two models can be summarized as follows:
* In the background check model, the attester conveys evidence to
the relying party, which then forwards the evidence to the
verifier for appraisal; the verifier computes the attestation
result and sends it back to the relying party.
* In the passport model, the attester transmits evidence to the
verifier directly and receives attestation results, which are then
relayed to the relying party.
This specification supports both patterns.
Several formats for encoding evidence are available, such as: - the
Entity Attestation Token (EAT) [I-D.ietf-rats-eat], - the Trusted
Platform Modules (TPMs) [TPM1.2] [TPM2.0], - the Android Key
Attestation, and - Apple Key Attestation.
Likewise, there are different encodings available for attestation
results. One such encoding, AR4SI [I-D.ietf-rats-ar4si] is being
standardized by the RATS working group.
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This version of the specification defines how to support the
background check model in the TLS handshake, such that the details
about the attestation technology are agnostic to the TLS handshake
itself. Later versions of the specification will support the
passport model as well.
To give the peer information that the handshake signing key is
properly secured, the associated evidence has to be verified by that
peer. Hence, attestation evidence about the security state of the
signing key is needed, which is typically associated with evidence
about the overall platform state. The platform attestation service
ensures that the key attestation service has not been tampered with.
The platform attestation service issues the Platform Attestation
Token (PAT) and the key attestation service issues the Key
Attestation Token (KAT). The security of the protocol critically
depends on the verifiable binding between these two logically
separate units of evidence.
This document does not define how different attestation technologies
are encoded. This is accomplished by companion specifications.
2. Conventions and Terminology
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 RFC
2119 [RFC2119].
3. Overview
The Remote Attestation Procedures (RATS) architecture
[I-D.ietf-rats-architecture] defines two types of interaction models
for attestation, namely the passport model and the background check
model. The subsections below explain the difference in their
interactions.
As typical with new features in TLS, the client indicates support for
the new extension in the ClientHello message. The newly introduced
extensions allow evidence and nonces to be exchanged. The nonces are
used for guaranteeing freshness of the exchanged evidence.
When the evidence extension is successfully negotiated, the content
of the Certificate message contains a payload that is encoded based
on the wrapper defined in [I-D.ftbs-rats-msg-wrap].
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 a key attestation
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token, which associates a private key with the attestation
information. An example of a key attestation token format utilizing
the EAT format can be found in [I-D.bft-rats-kat].
The recipient extracts evidence from the Certificate message and
relays it to the verifier to obtain attestation results.
Subsequently, the attested key is used to verify the
CertificateVerify message.
4. Use of Evidence with the Background Check Model
The background check model is described in Section 5.2 of
[I-D.ietf-rats-architecture] and allows the following modes of
operation 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 three cases in sub-sections
below.
4.1. TLS Client as Attester
In this use case the TLS client, as the attester, is challenged by
the TLS server to provide evidence. The TLS client is the attester
and the the TLS server acts as a relying party. 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.
4.2. TLS Server as Attester
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 Client Providing Evidence to TLS Server.
5. Evidence Extensions (Background Check Model)
This document defines two new extensions, the evidence_request and
the evidence_proposal, for use with the background check model.
The EvidenceType structure encodes either a media type or as a
content format. The media type is a string-based identifier while
the content format uses a number. The former is more flexible and
does not necessarily require a registration through IANA while the
latter is more efficient over-the-wire.
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 { NUMERIC(0), STRING(1) } encodingType;
enum { ATTESTATION(0), CERT_ATTESTATION(1) } credentialType;
struct {
encodingType type;
credentialType cred_type;
select (encodingType) {
case NUMERIC:
uint16 content_format;
case STRING:
opaque media_type<0..2^16-1>;
};
} EvidenceType;
struct {
select(ClientOrServerExtension) {
case client:
EvidenceType supported_evidence_types<1..2^8-1>;
opaque nonce<0..2^8-1>;
case server:
EvidenceType selected_evidence_type;
}
} evidenceRequestTypeExtension;
struct {
select(ClientOrServerExtension) {
case client:
EvidenceType supported_evidence_types<1..2^8-1>;
case server:
EvidenceType selected_evidence_type;
opaque nonce<0..2^8-1>;
}
} evidenceProposalTypeExtension;
Figure 3: TLS Structure for Evidence.
5.1. 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 4, 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>;
/* payload used to convey evidence */
case attestation:
opaque evidence<1..2^24-1>;
case X509:
opaque cert_data<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 4: Certificate Message when using only attestation.
The encoding of the evidence structure is defined in
[I-D.ftbs-rats-msg-wrap].
5.2. 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 5.
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struct {
select (certificate_type) {
case RawPublicKey:
/* From RFC 7250 ASN.1_subjectPublicKeyInfo */
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
/* X.509 certificate conveyed as usual */
case X509:
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 5: Certificate Message when using PKIX and attestation.
The encoding of the evidence structure is defined in
[I-D.ftbs-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 6.
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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 6: Format of TLS channel binder.
* Nonce is the value provided as a challenge by the relying party.
* The identity key public 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 partial transcript of the
TLS handshake, up to (but not including) the Certificate message.
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.
6. TLS Client and Server Handshake Behavior
The high-level message exchange in Figure 7 shows the
evidence_proposal and evidence_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
v + evidence_request
-------->
ServerHello ^ Key
+ key_share* | Exch
+ pre_shared_key* v
{EncryptedExtensions} ^ Server
+ evidence_proposal |
+ evidence_request |
{CertificateRequest*} v Params
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
[Application Data] <-------> [Application Data]
Figure 7: Attestation Message Overview.
6.1. 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.
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.
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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.
6.2. 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
client_attestation_type 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
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.
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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.
7. Background-Check Model Examples
7.1. 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 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 8 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 kinds of evidence
it understands and a nonce that will be used to challenge the
attester.
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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 the local API to
request the attestation. The returned evidence binds the identity
key with the platform identity and security state. The server then
signs the handshake transcript 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
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) | | |
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| | | ) | | |
| | +----------------------->| | |
| | | ServerHello | | |
| | | {...} | | |
| | | EncryptedExtensions | | |
| | | {...} | | |
| | | evidence_request( | | |
| | | type(a) | | |
| | | ) | | |
| | |<-----------------------+ | |
| | | | attest_key( | |
| | | | nonce, | |
| | | | TIK | |
| | | | ) | |
| | | +-------------->| |
| | | | CAB(KAT, PAT) | |
| | | |<--------------+ |
| | | | sign(TIK,hs) | |
| | | +-------------->| |
| | | | sig | |
| | | |<--------------+ |
| | | Certificate(KAT,PAT) | | |
| | | CertificateVerify(sig) | | |
| | | Finished | | |
| | |<-----------------------+ | |
| | POST /76839A9E | | | |
| | Body: { | | | |
| | type(a), | | | |
| | CAB | | | |
| | } | | | |
| |<-------------------+ | | |
| | Body: { | | | |
| | att-result: AR{} | | | |
| | } | | | |
| +------------------->| | | |
| | +---. | | |
| | | | verify AR{} | | |
| | |<--' | | |
| | +---. | | |
| | | | verify sig | | |
| | |<--' | | |
| | | Finished | | |
| | +----------------------->| | |
| | | | | |
'-+--------------------+------------------------+---------------+---'
| application data |
|<---------------------->|
| |
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Figure 8: Example Exchange with Server as Attester.
7.2. 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 9 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 with
the workload and platform identity and security state. The client
then signs the handshake transcript 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 | | |
| | attest_key( | CertificateVerify | | |
| | nonce, | Finished | | |
| | TIK |<-----------------------+ | |
| | ) | | | |
| |<---------------+ | | |
| | CAB(KAT, PAT) | | | |
| +--------------->| | | |
| | sign(TIK,hs) | | | |
| |<---------------+ | | |
| | sig | | | |
| +--------------->| Certificate(KAT,PAT) | | |
| | | CertificateVerify(sig) | | |
| | | Finished | | |
| | +----------------------->| | |
| | | | | |
| | | | POST /76839A9E | |
| | | | Body: { | |
| | | | type(a), | |
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| | | | CAB | |
| | | | } | |
| | | +------------------>| |
| | | | Body: { | |
| | | | att-result: AR{} | |
| | | | } | |
| | | |<------------------+ |
| | | +---. | |
| | | | | verify AR{} | |
| | | |<--' | |
| | | +---. | |
| | | | | verify sig | |
| | | |<--' | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
'--+----------------+------------------------+-------------------+---'
| application data |
|<---------------------->|
| |
Figure 9: Example Exchange with Client as Attester.
8. Security Considerations
TBD.
9. IANA Considerations
9.1. TLS Extensions
IANA is asked to allocate two new TLS extensions, evidence_request
and evidence_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.
9.2. 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 entry:
* Value: TBD1
* Description: unsupported_evidence
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* DTLS-OK: Y
* Reference: [This document]
* Comment:
9.3. 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]
10. References
10.1. Normative References
[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-00, 21 October 2022,
<https://datatracker.ietf.org/doc/html/draft-bft-rats-kat-
00>.
[I-D.ftbs-rats-msg-wrap]
Birkholz, H., Smith, N., Fossati, T., and H. Tschofenig,
"RATS Conceptual Messages Wrapper", Work in Progress,
Internet-Draft, draft-ftbs-rats-msg-wrap-02, 7 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ftbs-rats-
msg-wrap-02>.
[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>.
[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>.
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[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>.
10.2. 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-00, 12 December
2022, <https://datatracker.ietf.org/doc/html/draft-acme-
device-attest-00>.
[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-
04, 2 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-ar4si-04>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-rats-architecture-22, 28 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
architecture-22>.
[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-19, 19
December 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-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>.
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[TLS-Ext-Registry]
IANA, "Transport Layer Security (TLS) Extensions",
<https://www.iana.org/assignments/tls-extensiontype-
values>.
[TLS-Param-Registry]
IANA, "Transport Layer Security (TLS) Parameters",
<https://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/>.
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:
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- 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.
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 5.2.
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 5.1. A possible
instantiation of the KAT is described in [I-D.bft-rats-kat].
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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.
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
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.
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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
}
Figure 10: 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
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* 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
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
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Yogesh Deshpande
Arm Limited
Email: Yogesh.Deshpande@arm.com
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