| Internet-Draft | Attestation in TLS/DTLS | October 2022 |
| Tschofenig, et al. | Expires 27 April 2023 | [Page] |
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.¶
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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 to use different extensions for these two models.¶
The two models can be summarized as follows:¶
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.¶
Like-wise, 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.¶
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 has either already been done is done accomplished by companion specifications.¶
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].¶
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. 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 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.¶
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¶
We will show the message exchanges of the three cases in sub-sections below.¶
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 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.¶
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]
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 ensured that the confidential computing platform is genuine.¶
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]
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.¶
enum { NUMERIC(0), STRING(1) } encodingType;
struct {
encodingType 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;
The Certificate payload is used as a container, as shown in Figure 4, and follows the model of [RFC8446].¶
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;
The encoding of the evidence structure is defined in [I-D.ftbs-rats-msg-wrap].¶
The high-level message exchange in Figure 5 shows the evidence_proposal and evidence_request extensions added to the ClientHello and the EncryptedExtensions messages.¶
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]
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 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 indicates the types of 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.¶
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 cient is not configured to use the proposed evidence type with the given server. If the client has no evidenence 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.¶
If the server receives a ClientHello that contains the evidence_proposal extension and/or the evidence_request extension, then three outcomes are possible:¶
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.¶
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.¶
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:¶
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 6 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.¶
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 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.¶
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 remote 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 emulated, or cloned.¶
The protocol flow is shown in Figure 7 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 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.¶
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.¶
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:¶
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:¶
RFC EDITOR: PLEASE REMOVE THE THIS SECTION¶
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