Token Binding Working Group G. Mandyam
Internet-Draft Qualcomm Technologies Inc.
Intended status: Standards Track L. Lundblade
Expires: July 28, 2019 Security Theory LLC
J. Azen
Qualcomm Technologies Inc.
January 24, 2019
Attested TLS Token Binding
draft-mandyam-tokbind-attest-07
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. KeyStore Attestation . . . . . . . . . . . . . . . . . . 4
2.1.1. Verification Procedures . . . . . . . . . . . . . . . 4
2.2. TPMv2 Attestation . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Verification Procedures . . . . . . . . . . . . . . . 6
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 Extensions for Attestation . . . . . . . . 8
6. Security and Privacy Considerations . . . . . . . . . . . . . 9
6.1. Attestation Privacy Considerations . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[RFC8471] and [RFC8472] describe a framework whereby servers can
leverage cryptographically-bound authentication tokens in part to
create uniquely-identifiable TLS bindings that can span multiple
connections between a client and a server. 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. The
payload used for the signature is the token binding public key (see
[RFC8471]). Use of the token binding public key allows for
generation of the attestation signature once over the lifetime of the
public key.
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 [RFC8471], 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 assigned
per attestation type. 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]).
The attestation data will likely include a signature over a challenge
(depenting on the attestation type). The challenge can be used to
prevent replay of the attestation. However since the attestation is
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itself part of the token binding message (which has its own anti-
replay protection mechanism), the attestation signature need only be
generated over a known payload associated with the TLS token binding
session - the token binding public key. As a result, the token
binding client only needs to send the attestation once during the
lifetime of the token binding public key. In other words, if an
attestation is included in the token binding message, it should only
be sent in the initial token binding message following the creation
of the token binding key pair.
2.1. 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.
The octet value of the token binding extension that serves as
identifiaction for the Keystore attestation type is requested to be
0.
KeyStore attestation provides a signature over a payload generated by
the application. The payload is a SHA-256 hash of the token binding
public key corresponding to the current TLS connection (see
Section 3.3 of [RFC8471]). Then the attestation takes the form of a
signature, a signature-generation algorithmic identifier
corresponding to the COSE algorithm registry ([cose_iana]), and a
chain of DER-encoded x.509 certificates:
attestation_data = (
alg: int,
sig: bytes,
x5c: [credCert: bytes, *(caCert: bytes)]
)
2.1.1. Verification Procedures
The steps at the server for verifying a Token Binding KeyStore
Attestation are:
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o Retrieve token binding public key for the current TLS connection,
and compute is SHA-256 hash.
o Verify that attestation_data is in the expected CBOR format.
o Parse the first certificate listed in x5c and extract the public
key and challenge. If the challenge does not match the SHA-256
hash of the token binding public key then the attestation is
invalid.
o If the challenge matches the expected hash of the token binding
public key, 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.
2.2. 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 = (
alg: int,
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, and the
algorithmic identifier corresponding to the COSE algorithm registry
([cose_iana]). 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 token
binding public key.
The octet value of the token binding extension that serves as
identifiaction for the TPMv2 attestation type is requested to be 1.
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2.2.1. Verification Procedures
The steps for verifying a Token Binding TPMv2 Attestation are:
o Extract the token binding public key for the 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.
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 SHA-256 hash of the
token binding public key for the current TLS connection.
* 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
[RFC8472]).
It is proposed that the Client and Server Hello extensions defined in
Sections 3 and 4 of [RFC8472] 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:
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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;
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 [RFC8472] 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
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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.
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 Extensions for Attestation
This document proposes two extensions conformant with Section 6.3 of
[RFC8471], with the following specifics:
Androoid Keystore Attestation:
o Value: 0
o Description: Android Keystore Attestation
o Specification: This document
TPM v2 Attestation:
o Value: 1
o Description: TPMv2 Attestation
o Specification: This document
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6. Security and Privacy Considerations
The security and privacy considerations provided in Section 7 of
[RFC8471] are applicable to the attestation extensions proposed in
this document. Additional considerations are provided in this
section.
6.1. Attestation Privacy Considerations
The root signing key for the certificate chain used in verifying an
attestation can be unique to the device. As a result, this can be
used to track a device and/or end user. This potential privacy issue
can be mitigated by the use of batch keys as an alternative to unique
keys, or by generation of origin-specific attestation keys.
The attestation data may also contain device-specific identifiers, or
information that can be used to fingerprint a device. Sensitive
information can be excluded from the attestation data when this is a
concern.
7. Acknowledgments
Thanks to Andrei Popov for his detailed review and recommendations.
8. References
8.1. Normative References
[cose_iana]
Internet Assigned Numbers Authority, "COSE Algorithms",
<https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>.
[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.
[Keystore]
Google Inc., "Verifying hardware-backed key pairs with Key
Attestation",
<https://developer.android.com/training/articles/
security-key-attestation>.
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[RFC8471] Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
"The Token Binding Protocol Version 1.0", RFC 8471,
DOI 10.17487/RFC8471, October 2018,
<https://www.rfc-editor.org/info/rfc8471>.
[RFC8472] Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
Layer Security (TLS) Extension for Token Binding Protocol
Negotiation", RFC 8472, DOI 10.17487/RFC8472, October
2018, <https://www.rfc-editor.org/info/rfc8472>.
[RFC8473] Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
J. Hodges, "Token Binding over HTTP", RFC 8473,
DOI 10.17487/RFC8473, October 2018,
<https://www.rfc-editor.org/info/rfc8473>.
[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/>.
8.2. 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.
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
Security Theory LLC
Email: lgl@island-resort.com
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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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