TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track October 30, 2017
Expires: May 3, 2018
Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-04
Abstract
This document describes a mechanism in Transport Layer Security (TLS)
to provide an exportable proof of ownership of a certificate that can
be transmitted out of band and verified by the other party.
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 May 3, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 3
4. API considerations . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
This document provides a way to authenticate one party of a Transport
Layer Security (TLS) communication to another using a certificate
after the session has been established. This allows both the client
and server to prove ownership of additional identities at any time
after the handshake has completed. This proof of authentication can
be exported and transmitted out of band from one party to be
validated by the other party.
This mechanism provides two advantages over the authentication that
TLS natively provides:
multiple identities - Endpoints that are authoritative for multiple
identities - but do not have a single certificate that includes
all of the identities - can authenticate with those identities
over a single connection.
spontaneous authentication - Endpoints can authenticate after a
connection is established, in response to events in a higher-layer
protocol, as well as integrating more context.
This document intends to replace much of the functionality of
renegotiation in previous versions of TLS. It has the advantages
over renegotiation of not requiring additional on-the-wire changes
during a connection. For simplicity, only TLS 1.2 and later are
supported.
Post-handshake authentication is defined in TLS 1.3, but it has the
disadvantage of requiring additional state to be stored in the TLS
state machine and it composes poorly with multiplexed connection
protocols like HTTP/2. It is also only available for client
authentication. This mechanism is intended to be used as part of a
replacement for post-handshake authentication in applications.
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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. Authenticator
The authenticator is a structured message that can be exported from
either party of a TLS connection. It can be transmitted to the other
party of the TLS connection at the application layer. The
application layer protocol used to send the authenticator SHOULD use
TLS as its underlying transport.
An authenticator message can be constructed by either the client or
the server given an established TLS connection, a certificate, and a
corresponding private key. This authenticator uses the message
structures from Section 4.4 of [TLS13], but different parameters.
Also, unlike the Certificate and CertificateRequest messages in TLS
1.3, the messages described in this draft are not encrypted with a
handshake key.
Each authenticator is computed using a Handshake Context and Finished
MAC Key derived from the TLS session. These values are derived using
an exporter as described in [RFC5705] (for TLS 1.2) or [TLS13] (for
TLS 1.3). These values use different labels depending on the role of
the sender:
o The Handshake Context is an exporter value that is derived using
the label "EXPORTER-client authenticator handshake context" or
"EXPORTER-server authenticator handshake context" for
authenticators sent by the client and server respectively.
o The Finished MAC Key is an exporter value derived using the label
"EXPORTER-client authenticator finished key" or "EXPORTER-server
authenticator finished key" for authenticators sent by the client
and server respectively.
The context_value used for the exporter is absent (length zero) for
all four values. The length of the exported value is equal to the
length of the output of the hash function selected in TLS for the
pseudorandom function (PRF). Cipher suites that do not use the TLS
PRF MUST define a hash function that can be used for this purpose or
they cannot be used.
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If the connection is TLS 1.2, the master secret MUST have been
computed with the extended master secret [RFC7627] to avoid key
synchronization attacks.
Certificate The certificate to be used for authentication and any
supporting certificates in the chain. This structure is defined
in [TLS13], Section 4.4.2.
The certificate message contains an opaque string called
certificate_request_context. The format of
certificate_request_context is defined by the application layer
protocol and its value can be used to differentiate exported
authenticators. For example, the application may use a sequence
number used by the higher-level protocol during the transport of the
authenticator to the other party. Using a unique and unpredictable
value ties the authenticator to a given context, allowing the
application to prevent authenticators from being replayed or
precomputed by an attacker with temporary access to a private key.
CertificateVerify This message is used to provide explicit proof
that an endpoint possesses the private key corresponding to its
certificate.
struct {
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} CertificateVerify;
The algorithm field specifies the signature algorithm used (see
Section 4.2.3 of [TLS13] for the definition of this field). The
signature is a digital signature using that algorithm. The signature
scheme MUST be a valid signature scheme for TLS 1.3. This excludes
all RSASSA-PKCS1-v1_5 algorithms and ECDSA algorithms that are not
supported in TLS 1.3. For servers, this signature scheme must match
one of the signature and hash algorithms advertised in the
signature_algorithms extension of the ClientHello. The signature is
computed using the over the concatenation of:
o A string that consists of octet 32 (0x20) repeated 64 times
o The context string "Exported Authenticator" (which is not NULL-
terminated)
o A single 0 byte which serves as the separator
o The value Hash(Handshake Context || Certificate) where Hash is the
hash function for the handshake.
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Finished A HMAC over the value Hash(Handshake Context ||
Certificate || CertificateVerify) where Hash is the hash function
for the handshake, and the HMAC is computed using the hash
function from the handshake and the Finished MAC Key as a key.
The certificates used in the Certificate message MUST conform to the
requirements of a Certificate message in the version of TLS
negotiated. This is described in Section 4.2.3 of [TLS13] and
Sections 7.4.2 and 7.4.6 of [RFC5246]. Alternative certificate
formats such as [RFC7250] Raw Public Keys are not supported.
The exported authenticator message is the concatenation of messages:
Certificate || CertificateVerify || Finished
A given exported authenticator can be validated by checking the
validity of the CertificateVerify message and recomputing the
Finished message to see if it matches.
4. API considerations
The creation and validation of exported authenticators SHOULD be
implemented inside TLS library even if it is possible to implement it
at the application layer. TLS implementations supporting the use of
exported authenticators MUST provide application programming
interfaces by which clients and servers may request and verify
exported authenticator messages.
Given an established connection, the application SHOULD be able to
call an "authenticate" API which takes as input:
o certificate_request_context (from 0 to 255 bytes)
o valid certificate chain for the connection and associated
extensions (OCSP, SCT, etc.)
o signer (either the private key associated with the certificate, or
interface to perform private key operation)
o signature scheme
The API returns the exported authenticator as output.
Given an established connection and an exported authenticator
message, the application SHOULD be able to call a "validate" API that
takes an exported authenticator as an input. If the Finished and
CertificateVerify messages verify correctly, the API returns the
following as output:
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o certificate chain and extensions
o certificate_request_context
In order for the application layer to be able to choose the
certificates and signature schemes to use when constructing an
authenticator, a TLS server SHOULD expose an API that returns the
content of the signature_algorithms extension of client's ClientHello
message.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
The Certificate/Verify/Finished pattern intentionally looks like the
TLS 1.3 pattern which now has been analyzed several times. In the
case where the client presents an authenticator to a server, [SIGMAC]
presents a relevant framework for analysis.
Authenticators are independent and unidirectional. There is no
explicit state change inside TLS when an authenticator is either
created or validated.
o This property makes it difficult to formally prove that a server
is jointly authoritative over multiple certificates, rather than
individually authoritative over each.
o There is no indication in the TLS layer about which point in time
an authenticator was computed. Any feedback about the time of
creation or validation of the authenticator should be tracked as
part of the application layer semantics if required.
The signatures generated with this API cover the context string
"Exported Authenticator" and therefore cannot be transplanted into
other protocols.
7. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 6 were provided by Karthikeyan Bhargavan.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <http://www.rfc-editor.org/info/rfc5705>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<http://www.rfc-editor.org/info/rfc7627>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
8.2. Informative References
[SIGMAC] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client
Authentication in TLS 1.3)", 2016,
<https://eprint.iacr.org/2016/711.pdf>.
Author's Address
Nick Sullivan
Cloudflare Inc.
Email: nick@cloudflare.com
Sullivan Expires May 3, 2018 [Page 7]
