Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9261.
|
|
|---|---|---|---|
| Author | Nick Sullivan | ||
| Last updated | 2017-12-13 | ||
| Replaces | draft-sullivan-tls-exported-authenticator | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
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by Christer Holmberg
Ready w/issues
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9261 (Proposed Standard) | |
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draft-ietf-tls-exported-authenticator-05
TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track December 13, 2017
Expires: June 16, 2018
Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-05
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 16, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Authenticator Request . . . . . . . . . . . . . . . . . . . . 3
4. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 4
5. API considerations . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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
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authentication. This mechanism is intended to be used as part of a
replacement for post-handshake authentication in applications.
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 Request
The authenticator request 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 to keep the
request confidential.
An authenticator request message can be constructed by either the
client or the server. This authenticator request uses the
CertificateRequest message structure from Section 4.3.2 of [TLS13].
This message does not include the TLS record layer and is therefore
not encrypted with a handshake key.
CertificateRequest This message is used to define the parameters in
a request for an authenticator.
struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} CertificateRequest;
certificate_request_context An opaque string which identifies the
certificate request and which will be echoed in the authenticator
message. The certificate_request_context MUST be unique within
the scope of this connection (thus preventing replay of
authenticators). The certificate_request_context SHOULD be chosen
to be unpredictable to the peer (e.g., by randomly generating it)
in order to prevent an attacker who has temporary access to the
peer's private key from pre-computing valid authenticators.
extensions The extensions that are allowed in this structure include
the extensions defined for CertificateRequest messages defined in
Section 4.2. of [TLS13] and the server_name [RFC6066] extension,
which is allowed for client-generated authenticator requests.
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4. 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 to keep the certificate confidential.
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. For clients, an authenticator request is
required; for servers an authenticator request is optional. The
authenticator uses the message structures from Section 4.4 of
[TLS13], but different parameters. These messages do not include the
TLS record layer and are therefore 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.
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.
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The certificate message contains an opaque string called
certificate_request_context, which is extracted from the
authenticator request if present. If no authenticator request is
provided, it is zero-length.
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. If an authenticator request is present, the
signature algorithm MUST be chosen from one of the signature schemes
in the authenticator request. Otherwise, the signature algorithm
used should be chosen from the "signature_algorithms" extension of
the ClientHello used in the connection handshake.
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 If the authenticator request is present, the value "Hash(Handshake
Context || authenticator request || Certificate)"
o If an authenticator request is not present, the value
"Hash(Handshake Context || Certificate)"
where Hash is the hash function for the handshake.
Finished A HMAC over the value Hash(Handshake Context ||
Certificate || CertificateVerify) if an authenticator is present,
or Hash(Handshake Context || authenticator request ||
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.
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The certificates chosen in the Certificate message MUST conform to
the requirements of a Certificate message in the version of TLS
negotiated. If an authenticator request is present, the signature
algorithms used to choose the algorithm are taken from the
"signature_algorithms" in the from the authenticator. If there is no
authenticator request, the signature algorithms are chosen from the
"signature_algorithms" extension from the ClientHello used in the
connection. 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
"server_name" [RFC6066], "certificate_authorities" (Section 4.2.4. of
[TLS13]), or "oid_filters" (Section 4.2.5. of [TLS13]) extensions are
used to guide certificate selection, with the extensions provided in
the authenticator request taking precedence over the extensions
provided in the connection handshake.
If an authenticator request was provided, the Certificate message
MUST contain only extensions present in the authenticator request.
Otherwise, the Certificate message MUST contain only extensions
present in the ClientHello.
The authenticator message is the concatenation of messages:
Certificate || CertificateVerify || Finished
A given authenticator can be validated by checking the validity of
the CertificateVerify message given the authenticator request (if
used) and recomputing the Finished message to see if it matches.
5. API considerations
The creation and validation of both authenticator requests and
authenticators SHOULD be implemented inside the 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 the following APIs:
"request", which takes as input:
o certificate_request_context (from 0 to 255 bytes)
o set of extensions to include (this MUST include
signature_algorithms)
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It returns an authenticator request, which is a sequence of octets
that includes a CertificateRequest message.
"get context", which takes as input
o authenticator
It returns the certificate_request_context.
"authenticate", which takes as input:
o a set of certificate chains and associated extensions (OCSP, SCT,
etc.)
o a signer (either the private key associated with the certificate,
or interface to perform private key operation) for each chain
o an optional authenticator request
It returns the exported authenticator as output. It is RECOMMENDED
that the logic for selecting the certificates and extensions to
include in the exporter is implemented in the TLS library.
Implementing this in the TLS library lets the implementer take
advantage of existing extension and certificate selection logic.
It is also possible to implement this API outside of the TLS library
using TLS exporters. This may be preferable in cases where the
application does not have access to a TLS library with these APIs or
when TLS is handled independently of the application layer protocol.
"validate", which takes as input: * an optional authenticator request
* an authenticator
It returns the certificate chain and extensions.
6. IANA Considerations
This document has no IANA actions.
7. 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.
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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.
8. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 7 were provided by Karthikeyan Bhargavan.
9. References
9.1. 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, <https://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, <https://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, <https://www.rfc-editor.org/info/rfc5705>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, <https://www.rfc-
editor.org/info/rfc6066>.
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[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, <https://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,
<https://www.rfc-editor.org/info/rfc7627>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-22 (work in progress),
November 2017.
9.2. 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
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