Internet Engineering Task Force J. Hoyland, Ed.
Internet-Draft Royal Holloway, University of London
Intended status: Standards Track June 25, 2018
Expires: December 27, 2018
Layered Exported Authenticators in TLS
draft-hoyland-tls-layered-exported-authenticator-00
Abstract
This document describes an extension that allows for Exported
Authenticators (EAs) to authenticate each other. The extension
includes a reference to a previous EA. An EA containing this
extension constitues an attestation of the authenticity of the
referenced EA.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Extension Format . . . . . . . . . . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Exported Authenticators (EAs)[EA] provide a method for authenticating
one party of a Transport Layer Security (TLS) communication to the
other after the session has been established. EAs are defined for
TLS 1.3[TLS13] and TLS 1.2 with extended master secret, RFC 7627
[RFC7627]. Multiple EAs sent on the same channel do not prove joint
authentication. They prove that the sender is individually
authoritative over each certificate, but not jointly authoritative
over all certificates. By including this extension a sender can
prove joint authentication. This extension can be included in
CertificateRequest messages and Certificate messages.
Joint authentication could be used, for example, to securely update
pinned certificates. When a client connects to a server for which it
has a pinned certificate, the server could send the new certificate
to be pinned, and then bind the previously pinned certificate to it.
This proves to the client that the server is jointly authoritative
over both certificates. To defeat this mechanism an attacker is
required to both compromise the key of the old certificate and
improperly obtain a certificate from the PKI.
Another potential use is to provide proof that a certificate has been
accepted. Because EAs do not have a response mechanism, the sender
of an EA does not know the receiver's view of its authentication
status. By using this extension to reference EAs sent by its peer, a
party can prove to its peer that it has accepted a particular
certificate.
By constructing a chain of referenced EAs complex joint
authentication properties can be achieved.
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1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Extension Format
The "extension_data" field of this extension SHALL contain:
struct {
opaque prev_certificate_request_context<0..2^8-1>;
opaque binding[Hash.length];
} LayeredEA;
where "prev_certificate_request_context" is the certificate request
context of the EA you are referencing, and "binding" is the Finished
message of that same EA. The hash used is that used in the exported
authenticator, which is the hash function used by the TLS connection.
A party who wishes its peer to prove it is jointly authoritative over
multiple certificates can request a sequence of certificates, each
bound to its predecessor. Receipt of a series of EAs binding these
certificates into a chain proves the sender is jointly authoritative
over all those certificates.
A party who receives a CertificateRequest with this extension MUST
verify that it previously received or sent an EA with the appropriate
certificate request context and Finished message. If so then the
party MAY respond with a Certificate fulfilling the request, or it
MAY choose to not fulfil the request.
A party who receives a request from its peer for which it does not
recognise the referenced certificate or does not want to link to the
referenced certificate for some other reason, but still wishes to
respond with an EA MAY send an EA omitting the extension, or it MAY
choose to not fulfil the request. If the peer receives an EA with
the extension omitted it proves the sender is authoritative over the
certificate in the EA, but makes no claims about the previous EA
referenced in the request.
For spontaneous certificates The server MUST include a unique (within
the context of the connection) certificate_request_context for any EA
it may wish to bind to. To be able to verify bindings both parties
must keep a list of accepted EAs they are willing to bind to,
including certificate_request_contexts and Finished messages. A
client that receives a spontaneous EA with a
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certificate_request_context that it has already seen and for which it
is willing to receive a binding MUST ignore it.
3. Acknowledgements
4. IANA Considerations
This document requests IANA to update the TLS ExtensionsType
registry, defined in [TLS13], to include the
layered_exported_authenticator extension.
5. Security Considerations
For the authentication guarantees to apply, requests, and thus
responses, must unambiguously identify previous EAs. Because EAs do
not place a restriction on both parties to a connection using the
same certificate_request_context, the certificate_request_context is
not sufficient to unambiguously identify previous EAs. Because EAs
are unidirectional, and the Finished message is dependent on the
labels used to enforce this, the Finished message is sufficient to
identify previous EAS unambiguously. In the case of spontaneous EAs
a malicious server or an attacker who had compromised the TLS channel
could send two identical spontaneous EAs. To militate against this a
client receiving such an EA MUST check that it has not already
accepted an EA with the same certificate_request_context that it is
willing to bind to. If it previously accepted such a certificate but
did not add it to the list of certificates which it was willing to
bind to, adding it to the list is still secure. The
certificate_request_context is included in the request to ease
identification of the previous EA, but is not sufficient alone.
Both parties can be sure the Finished messages that are used to
reference previous EAs are unique. For requested EAs the inclusion
of the certificate_request_context, which is generated by the
requestor, guarantees this is the case. For spontaneous certificates
the client may only accept EAs after checking it does not have any
EAs it is willing to bind to with the same
certificate_request_context.
The Finished messages amount to channel bindings as defined in
RFC5056 [RFC5056], and thus publication of them should not weaken the
security of either the referenced EA or the TLS channel.
This extension only authenticates prior EAs. Thus, an attacker who
is able to compromise a TLS connection could append authentications
to the connection. Any attempt to bind to these certificates by an
honest agent would not be accepted by the peer.
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6. References
6.1. Normative References
[EA] Sullivan, N., "Exported Authenticators in TLS", draft-
ietf-tls-exported-authenticator-07 (work in progress),
June 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc7627>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
6.2. Informative References
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
Author's Address
Jonathan Hoyland (editor)
Royal Holloway, University of London
Egham
UK
Email: jonathan.hoyland@gmail.com
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