| Internet Engineering Task Force | A. Popov, Ed. |
| Internet-Draft | M. Nyström |
| Intended status: Standards Track | Microsoft Corp. |
| Expires: August 20, 2017 | D. Balfanz |
| A. Langley | |
| Google Inc. | |
| J. Hodges | |
| Paypal | |
| February 16, 2017 |
The Token Binding Protocol Version 1.0
draft-ietf-tokbind-protocol-13
This document specifies Version 1.0 of the Token Binding protocol. The Token Binding protocol allows client/server applications to create long-lived, uniquely identifiable TLS [RFC5246] bindings spanning multiple TLS sessions and connections. Applications are then enabled to cryptographically bind security tokens to the TLS layer, preventing token export and replay attacks. To protect privacy, the Token Binding identifiers are only conveyed over TLS and can be reset by the user at any time.
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This Internet-Draft will expire on August 20, 2017.
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Often, servers generate various security tokens (e.g. HTTP cookies, OAuth tokens) for applications to present when accessing protected resources. In general, any party in possession of bearer security tokens gain access to certain protected resource(s). Attackers take advantage of this by exporting bearer tokens from user’s application connections or machines, presenting them to application servers, and impersonating authenticated users. The idea of Token Binding is to prevent such attacks by cryptographically binding application security tokens to the underlying TLS layer.
A Token Binding is established by a user agent generating a private-public key pair (possibly, within a secure hardware module, such as TPM) per target server, providing the public key to the server, and proving possession of the corresponding private key, on every TLS connection to the server. The proof of possession involves signing the exported keying material (EKM) [RFC5705] from the TLS connection with the private key. The corresponding public key is included in the Token Binding identifier structure (described in the Section 3.2 "TokenBinding.tokenbindingid"). Token Bindings are long-lived, i.e., they encompass multiple TLS connections and TLS sessions between a given client and server. To protect privacy, Token Binding IDs are never conveyed over insecure connections and can be reset by the user at any time, e.g., when clearing browser cookies.
When issuing a security token to a client that supports Token Binding, a server includes the client’s Token Binding ID in the token. Later on, when a client presents a security token containing a Token Binding ID, the server ensures the ID in the token matches the ID of the Token Binding established with the client. In the case of a mismatch, the server rejects the token (details are application-specific).
In order to successfully export and replay a bound security token, an attacker needs to also be able to export the client’s private key, which is hard to do if the key is specially protected, e.g., generated in a secure hardware module.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
In the course of a TLS handshake, a client and server use the Token Binding Negotiation TLS Extension [I-D.ietf-tokbind-negotiation] to negotiate the Token Binding protocol version and the parameters (signature algorithm, length) of the Token Binding key. This negotiation does not require additional round-trips.
The Token Binding protocol consists of one message sent by the client to the server, proving possession of one or more client-generated asymmetric private keys. This message is not sent if the Token Binding Negotiation has been unsuccessful. The Token Binding message is sent with the application protocol data over TLS.
A server receiving the Token Binding message verifies that the key parameters in the message match the Token Binding parameters negotiated via [I-D.ietf-tokbind-negotiation], and then validates the signatures contained in the Token Binding message. If either of these checks fails, the server rejects the binding, along with all associeted bound tokens. Otherwise the Token Binding is successfully established with the ID contained in the Token Binding message.
When a server supporting the Token Binding protocol receives a bound token, the server compares the Token Binding ID in the token with the Token Binding ID established with the client. If the bound token came from a TLS connection without a Token Binding, or if the Token Binding IDs do not match, the token is rejected.
This document defines the format of the Token Binding protocol message, the process of establishing a Token Binding, the format of the Token Binding ID, and the process of validating a bound token. Token Binding Negotiation TLS Extension [I-D.ietf-tokbind-negotiation] describes the negotiation of the Token Binding protocol and key parameters. Token Binding over HTTP [I-D.ietf-tokbind-https] explains how the Token Binding message is encapsulated within HTTP/1.1 [RFC7230] or HTTP/2 [RFC7540] messages. [I-D.ietf-tokbind-https] also describes Token Binding between multiple communicating parties: User Agent, Identity Provider and Relying Party.
The Token Binding message is sent by the client to prove possession of one or more private keys held by the client. This message MUST be sent if the client and server successfully negotiated the use of the Token Binding protocol via [I-D.ietf-tokbind-negotiation], and MUST NOT be sent otherwise. This message MUST be sent in the client's first application protocol message. This message MAY also be sent in subsequent application protocol messages, proving possession of additional private keys held by the same client, which can be used to facilitate token binding between more than two communicating parties. For example, Token Binding over HTTP [I-D.ietf-tokbind-https] specifies an encapsulation of the Token Binding message in HTTP application protocol messages, as well as scenarios involving more than two communicating parties.
The Token Binding message format is defined using TLS Presentation Language (see Section 4 of [RFC5246]):
enum {
rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255)
} TokenBindingKeyParameters;
struct {
opaque modulus<1..2^16-1>;
opaque publicexponent<1..2^8-1>;
} RSAPublicKey;
struct {
opaque point <1..2^8-1>;
} ECPoint;
struct {
TokenBindingKeyParameters key_parameters;
uint16 key_length; /* Length (in bytes) of the following
TokenBindingID.TokenBindingPublicKey */
select (key_parameters) {
case rsa2048_pkcs1.5:
case rsa2048_pss:
RSAPublicKey rsapubkey;
case ecdsap256:
ECPoint point;
} TokenBindingPublicKey;
} TokenBindingID;
enum {
(255) /* No initial ExtensionType registrations */
} ExtensionType;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
provided_token_binding(0), referred_token_binding(1), (255)
} TokenBindingType;
struct {
TokenBindingType tokenbinding_type;
TokenBindingID tokenbindingid;
opaque signature<0..2^16-1>; /* Signature over the concatenation
of tokenbinding_type,
key_parameters and exported
keying material (EKM) */
Extension extensions<0..2^16-1>;
} TokenBinding;
struct {
TokenBinding tokenbindings<0..2^16-1>;
} TokenBindingMessage;
The Token Binding message consists of a series of TokenBinding structures, each containing the type of the token binding, the TokenBindingID, a signature using the Token Binding key, optionally followed by Extension structures.
This document defines two Token Binding types:
Token Binding over HTTP [I-D.ietf-tokbind-https] describes a use case for referred_token_binding where Token Bindings are established between multiple communicating parties: User Agent, Identity Provider and Relying Party. User Agent sends referred_token_binding to the Identity Provider in order to prove possession of the Token Binding key it uses with the Relying Party. The Identity Provider can then bind the token it is supplying (for presentation to the Relying Party) to the Token Binding ID contained in the referred_token_binding. Such a bound token enjoys the protections discussed below in Section 7 "Security Considerations".
The ID of the Token Binding established as a result of Token Binding message processing contains the identifier of the key parameters negotiated via [I-D.ietf-tokbind-negotiation], the length (in bytes) of the Token Binding public key, and the Token Binding public key itself. Token Binding ID can be obtained from the TokenBinding structure by discarding the Token Binding type, signature and extensions.
When rsa2048_pkcs1.5 or rsa2048_pss is used, RSAPublicKey.modulus and RSAPublicKey.publicexponent contain the modulus and exponent of a 2048-bit RSA public key represented in big-endian format, with leading zero bytes omitted.
When ecdsap256 is used, ECPoint.point contains the X coordinate followed by the Y coordinate of a Curve P-256 key. The X and Y coordinates are unsigned 32-byte integers encoded in big-endian format, preserving any leading zero bytes. Future specifications may define Token Binding keys using other elliptic curves with their corresponding signature and point formats.
Token Binding protocol implementations SHOULD make Token Binding IDs available to the application as opaque byte sequences. E.g., server applications will use Token Binding IDs when generating and verifying bound tokens.
When rsa2048_pkcs1.5 is used, TokenBinding.signature contains the signature generated using the RSASSA-PKCS1-v1_5 signature scheme defined in [RFC3447] with SHA256 as the hash function.
When rsa2048_pss is used, TokenBinding.signature contains the signature generated using the RSASSA-PSS signature scheme defined in [RFC3447] with SHA256 as the hash function. MGF1 with SHA256 MUST be used as the mask generation function, and the salt length MUST equal 32 bytes.
When ecdsap256 is used, TokenBinding.signature contains a pair of 32-byte integers, R followed by S, generated with ECDSA using Curve P-256 and SHA256 as defined in [ANSI.X9-62.2005] and [FIPS.186-4.2013]. R and S are encoded in big-endian format, preserving any leading zero bytes.
The signature is computed over the byte string representing the concatenation of:
Please note that TLS 1.2 and earlier versions support renegotiation, which produces a new TLS master secret for the same connection, with associated session keys and EKM value. TokenBinding.signature MUST be a signature of the EKM value derived from the TLS master secret that produced the session keys encrypting the TLS application_data record(s) containing this TokenBinding. Such use of the current EKM for the TLS connection makes replay of bound tokens within renegotiated TLS sessions detectable, but requires the application to synchronize Token Binding message generation and verification with the TLS handshake state.
Specifications defining the use of Token Binding with application protocols, such as Token Binding over HTTP [I-D.ietf-tokbind-https], MAY prohibit the use of TLS renegotiation in combination with Token Binding, obviating the need for such synchronization. Alternatively, such specifications need to define a way to determine which EKM value corresponds to a given TokenBindingMessage, and a mechanism preventing a TokenBindingMessage from being split across TLS renegotiation boundaries (i.e., due to TLS message fragmentation - see Section 6.2.1 of [RFC5246]). Note that application layer messages conveying a TokenBindingMessage may cross renegotiation boundaries in ways that make processing difficult.
The EKM is obtained using the Keying Material Exporters for TLS defined in [RFC5705], by supplying the following input values:
A Token Binding message may optionally contain a series of Extension structures, each consisting of an extension_type and extension_data. The structure and meaning of extension_data depends on the specific extension_type.
Initially, no extension types are defined (see Section 6.3 "Token Binding Extensions Registry"). One of the possible uses of extensions envisioned at the time of this writing is attestation: cryptographic proof that allows the server to verify that the Token Binding key is hardware-bound. The definitions of such Token Binding protocol extensions are outside the scope of this specification.
An implementation MUST ignore any unknown Token Binding types.
The client MUST include at least one TokenBinding structure in the Token Binding message. The key parameters used in the provided_token_binding MUST match those negotiated with the server via [I-D.ietf-tokbind-negotiation].
The client SHOULD generate and store Token Binding keys in a secure manner that prevents key export. In order to prevent cooperating servers from linking user identities, different keys SHOULD be used by the client for connections to different servers, according to the token scoping rules of the application protocol.
When the client needs to send a referred_token_binding to the Identity Provider, the client SHALL construct the referred TokenBinding structure in the following manner:
Conveying referred Token Bindings in this fashion allows the Identity Provider to verify that the client controls the Token Binding key used with the Relying Party.
The triple handshake vulnerability in TLS 1.2 and older TLS versions affects the security of the Token Binding protocol, as described in Section 7 "Security Considerations". Therefore, the server MUST NOT negotiate the use of the Token Binding protocol with these TLS versions, unless the server also negotiates the Extended Master Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS extensions.
If the use of the Token Binding protocol was not negotiated, but the client sends the Token Binding message, the server MUST reject any contained bindings. If the Token Binding type is "provided_token_binding", the server MUST verify that the signature algorithm (including elliptic curve in the case of ECDSA) and key length in the Token Binding message match those negotiated via [I-D.ietf-tokbind-negotiation]. In the case of a mismatch, the server MUST reject the binding. Token Bindings of type "referred_token_binding" may use different key parameters than those negotiated with this client.
If the Token Binding message does not contain at least one TokenBinding structure, or if a signature contained in any TokenBinding structure is invalid, the server MUST reject the binding.
Servers MUST ignore any unknown extensions. Initially, no extension types are defined (see Section 6.3 "Token Binding Extensions Registry").
If all checks defined above have passed successfully, the Token Binding between this client and server is established. The Token Binding ID(s) conveyed in the Token Binding Message can be provided to the server-side application. The application may then use the Token Binding IDs for bound security token creation and validation, see Section 5.
If a Token Binding is rejected, any associated bound tokens MUST also be rejected by the server. The effect of this is application-specific, e.g. failing requests, a requirement for the client to re-authenticate and present a different token, or connection termination.
Security tokens can be bound to the TLS layer either by embedding the Token Binding ID in the token, or by maintaining a database mapping tokens to Token Binding IDs. The specific method of generating bound security tokens is application-defined and beyond the scope of this document. Note that applicable security considerations are outlined in Section 7.
Either or both clients and servers MAY create bound security tokens. For example, HTTPS servers employing Token Binding for securing their HTTP cookies will bind the cookies. In the case of a server-initiated challenge-response protocol employing Token Binding and TLS, the client can, for example, incorporate the Token Binding ID within the signed object it returns, thus binding the object.
Upon receipt of a security token, the server attempts to retrieve Token Binding ID information from the token and from the TLS connection with the client. Application-provided policy determines whether to honor non-bound (bearer) tokens. If the token is bound and a Token Binding has not been established for the client connection, the server MUST discard the token. If the Token Binding ID for the token does not match the Token Binding ID established for the client connection, the server MUST discard the token.
This section establishes three IANA registries: "Token Binding Key Parameters", "Token Binding Types" and "Token Binding Extensions". It also registers a new TLS exporter label in the TLS Exporter Label Registry.
This document establishes a registry for identifiers of Token Binding key parameters entitled "Token Binding Key Parameters" under the "Token Binding Protocol" heading.
Entries in this registry require the following fields:
This registry operates under the "Expert Review" policy as defined in [RFC5226]. The designated expert is advised to encourage the inclusion of a reference to a permanent and readily available specification that enables the creation of interoperable implementations using the identified set of Token Binding key parameters.
An initial set of registrations for this registry follows:
This document establishes a registry for Token Binding type identifiers entitled "Token Binding Types" under the "Token Binding Protocol" heading.
Entries in this registry require the following fields:
This registry operates under the "Expert Review" policy as defined in [RFC5226]. The designated expert is advised to encourage the inclusion of a reference to a permanent and readily available specification that enables the creation of interoperable implementations using the identified Token Binding type.
An initial set of registrations for this registry follows:
This document establishes a registry for Token Binding extensions entitled "Token Binding Extensions" under the "Token Binding Protocol" heading.
Entries in this registry require the following fields:
This registry operates under the "Expert Review" policy as defined in [RFC5226]. The designated expert is advised to encourage the inclusion of a reference to a permanent and readily available specification that enables the creation of interoperable implementations using the identified Token Binding extension. This document creates no initial registrations in the "Token Binding Extensions" registry.
This document adds a registration for the "EXPORTER-Token-Binding" value in the TLS Exporter Label Registry to correspond to this specification.
The goal of the Token Binding protocol is to prevent attackers from exporting and replaying security tokens, thereby impersonating legitimate users and gaining access to protected resources. Bound tokens can be replayed by the malware present in User Agents, which may be undetectable by a server. However, in order to export bound tokens to other machines and successfully replay them, attackers also need to export corresponding Token Binding private keys. Token Binding private keys are therefore high-value assets and SHOULD be strongly protected, ideally by generating them in a hardware security module that prevents key export.
The manner in which a token is bound to the TLS layer is application-defined and beyond the scope of this document. However, the resulting bound token needs to be integrity-protected, so that an attacker cannot remove the binding or substitute a Token Binding ID of their choice without detection.
The Token Binding protocol does not prevent cooperating clients from sharing a bound token. A client could intentionally export a bound token with the corresponding Token Binding private key, or perform signatures using this key on behalf of another client.
The Token Binding protocol is only used when negotiated via [I-D.ietf-tokbind-negotiation] within the TLS handshake. TLS prevents active attackers from modifying the messages of the TLS handshake, therefore it is not possible for the attacker to remove or modify the Token Binding Negotiation TLS Extension used to negotiate the Token Binding protocol and key parameters. The signature algorithm and key length used in the TokenBinding of type "provided_token_binding" MUST match the parameters negotiated via [I-D.ietf-tokbind-negotiation].
The Token Binding protocol uses persistent, long-lived Token Binding IDs. To protect privacy, Token Binding IDs are never transmitted in clear text and can be reset by the user at any time, e.g. when clearing browser cookies. Some applications offer a special privacy mode where they don't store or use tokens supplied by the server, e.g. "in private" browsing. When operating in this special privacy mode, applications SHOULD use newly generated Token Binding keys and delete them when exiting this mode, or else SHOULD NOT negotiate Token Binding at all.
In order to prevent cooperating servers from linking user identities, different keys MUST be used by the client for connections to different servers, according to the token scoping rules of the application protocol.
A server can use tokens and Token Binding IDs to track clients. Client applications that automatically limit the lifetime or scope of tokens to maintain user privacy SHOULD apply the same validity time and scope limits to Token Binding keys.
Existing systems provide a variety of platform-specific mechanisms for certain applications to share tokens, e.g. to enable single sign-on scenarios. For these scenarios to keep working with bound tokens, the applications that are allowed to share tokens will need to also share Token Binding keys. Care must be taken to restrict the sharing of Token Binding keys to the same group(s) of applications that share the same tokens.
The Token Binding protocol relies on the TLS Exporters [RFC5705] to associate a TLS connection with a Token Binding. The triple handshake attack [TRIPLE-HS] is a known vulnerability in TLS 1.2 and older TLS versions, allowing the attacker to synchronize keying material between TLS connections. The attacker can then successfully replay bound tokens. For this reason, the Token Binding protocol MUST NOT be negotiated with these TLS versions, unless the Extended Master Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS extensions have also been negotiated.
This document incorporates comments and suggestions offered by Eric Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell, and others.