Token Binding Working Group G. Mandyam
Internet-Draft L. Lundblade
Intended status: Standards Track J. Azen
Expires: January 18, 2019 Qualcomm Technologies Inc.
July 17, 2018
Attested TLS Token Binding
draft-mandyam-tokbind-attest-05
Abstract
Token binding allows HTTP servers to bind bearer tokens to TLS
connections. In order to do this, clients or user agents must prove
possession of a private key. However, proof-of-possession of a
private key becomes truly meaningful to a server when accompanied by
an attestation statement. This specification describes extensions to
the existing token binding protocol to allow for attestation
statements to be sent along with the related token binding messages.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Attestation Enhancement to TLS Token Binding Message . . . . 3
2.1. Token Binding Attestation Registry . . . . . . . . . . . 4
2.2. KeyStore Attestation . . . . . . . . . . . . . . . . . . 4
2.2.1. Verification Procedures . . . . . . . . . . . . . . . 5
2.3. TPMv2 Attestation . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Verification Procedures . . . . . . . . . . . . . . . 5
3. Extension Support Negotiation . . . . . . . . . . . . . . . . 6
3.1. Negotiating Token Binding Protocol Extensions . . . . . . 7
4. Example - Platform Attestation for Anomaly Detection . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. TLS Extensions Registry . . . . . . . . . . . . . . . . . 8
5.2. Token Binding Extension for Attestation . . . . . . . . . 8
5.3. Token Binding Attestation Type Registry . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[I-D.ietf-tokbind-protocol] and [I-D.ietf-tokbind-negotiation]
describe a framework whereby servers can leverage cryptographically-
bound authentication tokens to verify TLS connections. This is
useful for prevention of man-in-the-middle attacks on TLS sessions,
and provides a mechanism by which identity federation systems can be
leveraged by a relying party to verify a client based on proof-of-
possession of a private key.
Once the use of token binding is negotiated as part of the TLS
handshake, an application layer message (the Token Binding message)
may be sent from the client to the relying party whose primary
purpose is to encapsulate a signature over a value associated with
the current TLS session (Exported Key Material, i.e. EKM - see
[I-D.ietf-tokbind-protocol]).
Proof-of-possession of a private key is useful to a relying party,
but the associated signature in the Token Binding message does not
provide an indication as to how the private key is stored and in what
kind of environment the associated cryptographic operation takes
place. This information may be required by a relying party in order
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to satisfy requirements regarding client platform integrity.
Therefore, attestations are sometimes required by relying parties in
order for them to accept signatures from clients. As per the
definition in [I-D.birkholz-tuda], "remote attestation describes the
attempt to determine the integrity and trustworthiness of an endpoint
-- the attestee -- over a network to another endpoint -- the verifier
-- without direct access." Attestation statements are therefore
widely used in any server verification operation that leverages
client cryptography.
TLS token binding can therefore be enhanced with remote attestation
statements. The attestation statement can be used to augment Token
Binding message. This could be used by a relying party for several
different purpose, including (1) to determine whether to accept token
binding messages from the associated client, or (2) require an
additional mechanism for binding the TLS connection to an
authentication operation by the client.
2. Attestation Enhancement to TLS Token Binding Message
The attestation statement can be processed 'in-band' as part of the
Token Binding Message itself. This document leverages the
TokenBinding.extensions field of the Token Binding Message as
described in Section 3.4 of [I-D.ietf-tokbind-protocol], where the
extension data conforms to the guidelines of Section 6.3 of the same
document. The value of the extension, as required by this same
section, is TBD. The extension data takes the form of a CBOR
(compact binary object representation) Data Definition Language
construct, i.e. CDDL.
extension_data = {attestation}
attestation = (
attestation_type: tstr,
attestation_data: bstr,
)
The attestation data is determined according to the attestation type.
In this document, the following types are defined: "KeyStore" (where
the corresponding attestation data defined in [Keystore]) and "TPMv2"
(where the corresponding attestation data defined in [TPMv2]).
Additional attestation types may be accepted by the token binding
implementation (for instance, see Section 8 of [webauthn]).
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2.1. Token Binding Attestation Registry
It is recommended that a registry be created for additional
attestation types, with corresponding specifications required to
define the attestation data.
Entries in this registry must include the following fields:
o Value: The text string that uniquely identifies the attestation
type, e.g. "TPMv2"
o Description: A human-readable description of the attestation, e.g.
"TCG TPM Version 2 compliant attestation"
o Specification: A reference to a specification that defines the
attestation data.
2.2. KeyStore Attestation
KeyStore attestation is relevant to the Android operating system.
The Android Keystore mechanism allows for an application (such as a
browser implementing the Token Binding stack) to create a key pair,
export the public key, and protect the private key in a hardware-
backed keystore. The Android Keystore can then be used to verify a
keypair using the Keystore Attestation mechanism, which involves
signing a payload according to a public key that chains to a root
certificate signed by an attestation root key that is specific to the
device manufacturer.
KeyStore attestation provides a signature over a payload generated by
the application. Since in this case the application is the Token
Binding stack resident on the device, the payload is the Exported Key
Material (EKM) corresponding to the current TLS connection (see
Section 3.3 of [I-D.ietf-tokbind-protocol]). Then the attestation
takes the form of a signature accompanies by a chain of DER-encoded
x.509 certificates:
attestation_data = (
sig: bytes,
x5c: [credCert: bytes, *(caCert: bytes)]
)
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2.2.1. Verification Procedures
The steps at the server for verifying a Token Binding KeyStore
Attestation are:
o Extract EKM for current TLS connection.
o Verify that attestation_data is in the expected CBOR format.
o Parse the first certificate listed in x5c and extract the public
key, algorithm and challenge. If the challenge does not match the
EKM then the attestation is invalid.
o If the challenge matches the EKM, verify the sig with respect to
the extracted public key and algorithm from the previous step.
o Verify the rest of the certificate chain up to the root. The root
certificate must match the expected root for the device.
2.3. TPMv2 Attestation
Version 2 of the Trusted Computing Group's Trusted Platform Module
(TPM) specification provides for an attestation generated within the
context of a TPM. The attestation then is defined as
attestation_data = (
tpmt_sig: bytes,
tpms_attest: bytes,
x5c: [credCert: bytes, *(caCert: bytes)]
)
The tpmt_sig is generated over a tpms_attest structure signed with
respect to the certificate chain provided in the x5c array. It is
derived from the TPMT_SIGNATURE data structure defined in
Section 11.3.4 of [TPMv2]. tpms_attest is derived from the
TPMS_ATTEST data structure in Section 10.2.8 of [TPMv2], specifically
with the extraData field being set to a SHA-256 hash of the EKM.
2.3.1. Verification Procedures
The steps for verifying a Token Binding TPMv2 Attestation are:
o Extract EKM for current TLS connection.
o Verify that attestation_data is in the expected CBOR format.
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o Parse the first certificate listed in x5c and extract the public
key.
o Verify the tpms_attest structure,which includes
* Verify that the type field is set to TPM_ST_ATTEST_CERTIFY.
* Verify that extraData is equivalent to the EKM.
* Verify that magic is set to the expected TPM_GENERATED_VALUE
for the expected command sequence used to generate the
attestation.
* Verification of additonal TPMS_ATTEST data fields is optional.
o Verify tpmt_sig with respect to the public key provided in the
first certifcate in x5c, using the algorithm as specified in the
sigAlg field (see Sections 11.3.4, 11.2.1.5 and 9.29 of [TPMv2]).
3. Extension Support Negotiation
Even if the client supports a Token Binding extension, it may not be
desirable to send the extension if the server does not support it.
The benefits of client-suppression of an extension could include
saving of bits "over the wire" or simplified processing of the Token
Binding message at the server. Currently, extension support is not
communicated as part of the Token Binding extensions to TLS (see
[I-D.ietf-tokbind-negotiation]).
It is proposed that the Client and Server Hello extensions defined in
Sections 3 and 4 of [I-D.ietf-tokbind-negotiation] be extended so
that endpoints can communicate their support for specific
TokenBinding.extensions. With reference to Section 3, it is
recommended that the "token_binding" TLS extension be augmented by
the client to include supported TokenBinding.extensions as follows:
enum {
attestation(0), (255)
} TokenBindingExtensions;
struct {
TB_ProtocolVersion token_binding_version;
TokenBindingKeyParameters key_parameters_list<1..2^8-1>;
TokenBindingExtensions supported_extensions_list<1..2^8-1>
} TokenBindingParameters;
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The "supported_extensions_list" contains the list of identifiers of
all token binding message extensions supported by the client. A
server supporting token binding extensions will respond in the server
hello with an appropriate "token_binding" extension that includes a
"supported_extensions_list". This list must be a subset of the the
extensions provided in the client hello.
Since a TLS extension cannot itself be extended, the "token_binding"
TLS extension cannot be reused. Therefore it is proposed that a new
TLS extension be defined - "token_binding_with_extensions". This TLS
extension codepoint is identical to the existing "token_binding"
extension except for the additional data structures defined above.
3.1. Negotiating Token Binding Protocol Extensions
The negotation described in Section 4 of
[I-D.ietf-tokbind-negotiation] still applies, except now the
"token_binding_with_extensions" codepoint would be used if the client
supports any token binding extension. In addition, a client can
receive a "supported_extensions_list" from the server as part of the
server hello. The client must terminate the handshake if the
"supported_extensions_list" received from the server is not a subset
of the "supported_extensions_list" sent by the client in the client
hello. If the server hello list of supported extensions is a subset
of the client supported extensions, then the client must only send
those extensions specified in the server hello in the Token Binding
protocol. If the server hello does not include a
"supported_extensions_list", then the client must not send any
extensions along with the Token Binding Message.
4. Example - Platform Attestation for Anomaly Detection
An example of where a platform-based attestation is useful can be for
remote attestation based on client traffic anomaly detection. Many
network infrastructure deployments employ network traffic monitors
for anomalous pattern detection. Examples of anomalous patterns
detectable in the TLS handshake could be unexpected cipher suite
negotiation for a given source/destination pairing. In this case, it
may be desirable for a client-enhanced attestation reflecting for
instance that an expected offered cipher suite in the client hello
message is present or the originating browser integrity is intact
(e.g. through a hash over the browser application package). If the
network traffic monitor can interpret the atttestation included in
the token binding message, then it can verify the attestation and
potentially emit alerts based on an unexpected attestation.
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5. IANA Considerations
This memo includes the following requests to IANA.
5.1. TLS Extensions Registry
This document proposes an update of the TLS "ExtensionType Values"
registry. The following addition to the registry is requested:
Value: TBD
Extension name: token_binding_with_extensions
Reference: this document
Recommended: Yes
5.2. Token Binding Extension for Attestation
This document proposes an extension conformant with Section 6.3 of
[I-D.ietf-tokbind-protocol], with the following specifics:
o Value: TBD, an octet value between 0 and 255
o Description: Token binding attestation extension
o Specification: This document
5.3. Token Binding Attestation Type Registry
This document proposes the establishment of an attestation registry
for a Token Binding extension focused on attestation. Entries in
this registry must include the following fields:
o Value: The text string that uniquely identifies the attestation
type
o Description: A human-readable description of the attestation
o Specification: A reference to a specification that defines the
attestation data.
6. Acknowledgments
Thanks to Andrei Popov for his detailed review and recommendations.
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7. References
7.1. Normative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-greevenbosch-appsawg-
cbor-cddl-11 (work in progress), July 2017.
[I-D.ietf-tokbind-https]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
tokbind-https-12 (work in progress), January 2018.
[I-D.ietf-tokbind-negotiation]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", draft-ietf-tokbind-
negotiation-10 (work in progress), October 2017.
[I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "The Token Binding Protocol Version 1.0", draft-
ietf-tokbind-protocol-16 (work in progress), October 2017.
[Keystore]
Google Inc., "Verifying hardware-backed key pairs with Key
Attestation",
<https://developer.android.com/training/articles/
security-key-attestation>.
[TPMv2] The Trusted Computing Group, "Trusted Platform Module
Library, Part 2: Structures", September 2016,
<http://www.trustedcomputinggroup.org/wp-content/uploads/
TPM-Rev-2.0-Part-2-Structures-01.38.pdf>.
[webauthn]
The Worldwide Web Consortium, "Web Authentication: An API
for accessing Scoped Credentials",
<https://www.w3.org/TR/webauthn/>.
7.2. Informative References
[I-D.birkholz-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
tuda-02 (work in progress), July 2016.
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Authors' Addresses
Giridhar Mandyam
Qualcomm Technologies Inc.
5775 Morehouse Drive
San Diego, California 92121
USA
Phone: +1 858 651 7200
Email: mandyam@qti.qualcomm.com
Laurence Lundblade
Qualcomm Technologies Inc.
5775 Morehouse Drive
San Diego, California 92121
USA
Phone: +1 858 658 3584
Email: llundbla@qti.qualcomm.com
Jon Azen
Qualcomm Technologies Inc.
5775 Morehouse Drive
San Diego, California 92121
USA
Phone: +1 858 651 9476
Email: jazen@qti.qualcomm.com
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