TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track March 13, 2017
Expires: September 14, 2017
Exported Authenticators in TLS
draft-sullivan-tls-exported-authenticator-01
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
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This Internet-Draft will expire on September 14, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 2
3. API considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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 is useful in the following situations:
o servers that are authoritative for multiple domains the same
connection but do not have a certificate that is simultaneously
authoritative for all of them
o servers that have resources that require client authentication to
access and need to request client authentication after the
connection has started
o clients that want to assert ownership over an identity to a server
after a connection has been established
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.
2. Authenticator
The authenticator is a structured message that can be exported from
either party of a TLS connection. It can be sent out-of-band to the
other party of a TLS connection to be validated.
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
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structures from section 4.4. of [I-D.ietf-tls-tls13], but different
parameters. Also, unlike the Certificate and CertificateRequest
messages in TLS 1.3, the messages described in this draft are not
encryped with a handshake key.
Each Authenticator is computed using a Handshake Context and Finished
MAC Key derived from the TLS session. The Handshake Context is
identical for both parties of the TLS connection, the Finished MAC
Key is dependent on whether the Authenticator is created by the
client or the server.
o The Handshake Context is an [RFC5705] (for TLS 1.2) or
[I-D.ietf-tls-tls13] exporter value derived using the label
"authenticator handshake context" and length 64 bytes.
o The Finished MAC Key is an exporter value derived using the label
"server authenticator finished key" or "client authenticator
finished key", depending on the sender. The length of this key is
equal to the length of the output of the hash function negotiated
in TLS. For TLS 1.3, it's the hash algorithm of the cipher suite.
For TLS 1.2, it's the hash algorithm selected for the PRF for AEAD
ciphers, or the hash algorithm used as the HMAC in non-AEAD
ciphers.
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.
The certificate message contains an opaque string called
certificate_request_context which MUST be unique for a given
connection. Its format should be defined by the application layer
protocol and MUST be non-zero length. For example, it may be a
randomly chosen identifier used by the higher-level protocol during
the transport of the Authenticator to the other party.
CertificateVerify A signature over the value Hash(Handshake
Context || Certificate)
Finished A HMAC over the value Hash(Handshake Context ||
Certificate || CertificateVerify) 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
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negotiated. This is described in section 4.2.3. of
[I-D.ietf-tls-tls13] and sections 7.4.2. and 7.4.6. of [RFC5246].
The exported authenticator message is the concatenation of messages:
Certificate || CertificateVerify || Finished
3. API considerations
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
obtain an authenticator by providing the following:
o certificate_request_context (from 1 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)
Given an established connection and an exported authenticator
message, the application should be able to provide the authenticator
to the connection. If the Finished and CertificateVerify messages
verify, the TLS library should return the following:
o certificate chain and extensions
o certificate_request_context
In order for the application layer to communicate which certificates
it will accept, an API should be exposed that returns an array of TLS
1.3 SignatureScheme objects that corresponds to the signature
algorithms that the library is willing to validate in an exported
authenticator message.
4. 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.
From a formal security perspective, one drawback of this mechanism is
that there is no explicit signaling mechanism for one party to
acknowledge an Authenticator to the party who computed it. Nothing
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about the state of the connection is changed when a new Authenticator
is exported, and the Handshake Context of the TLS connection is
unchanged after creating or validating an authenticator. This
property makes it difficult to formally prove that a server is
jointly authoritative over multiple certificates, rather than
individually authoritative on each certificate.
Another result of the unidirectional nature of Authenticator messages
is that the view of which certificates the other party is
authoritative over does not reside in the TLS state machine. Not
knowing when the exported authenticator was created or validated at
the TLS layer also means that assumptions about when the other party
is considered authoritative can not be determined purely from where
in the in the TLS record layer it was sent. A valid authenticator
can be created at any time during the connection. If it matters to
the application whether or not an authenticator was acknowledged
before or after a particular piece of data, it should be tracked as
part of the application layer semantics.
5. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for the security considerations section were provided by
Karthikeyan Bhargavan.
6. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-19 (work in progress),
March 2017.
[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>.
[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>.
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[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 September 14, 2017 [Page 6]
