TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track June 05, 2018
Expires: December 7, 2018
Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-07
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 December 7, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Authenticator Request . . . . . . . . . . . . . . . . . . . . 3
4. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Authenticator Keys . . . . . . . . . . . . . . . . . . . 4
4.2. Authenticator Construction . . . . . . . . . . . . . . . 5
4.2.1. Certificate . . . . . . . . . . . . . . . . . . . . . 5
4.2.2. CertificateVerify . . . . . . . . . . . . . . . . . . 6
4.2.3. Finished . . . . . . . . . . . . . . . . . . . . . . 7
4.2.4. Authenticator Creation . . . . . . . . . . . . . . . 7
5. Empty Authenticator . . . . . . . . . . . . . . . . . . . . . 8
6. API considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. The "request" API . . . . . . . . . . . . . . . . . . . . 8
6.2. The "get context" API . . . . . . . . . . . . . . . . . . 9
6.3. The "authenticate" API . . . . . . . . . . . . . . . . . 9
6.4. The "validate" API . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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.
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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 [RFC7540]. 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.
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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.
The CertificateRequest is used to define the parameters in a request
for an authenticator. The definition for TLS 1.3 is:
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. A certificate_request_context value MUST NOT be repeated
for multiple authenticator requests generated by the same party
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within the scope of a 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. This
value is unrelated to the certificate_request_context used in
post-handshake authentication and collisions do not need to be
avoided.
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.
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. Clients MUST NOT send an authenticator
without a preceding authenticator request; 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.
4.1. Authenticator Keys
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
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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. For TLS 1.3 symmetric cipher suites, the hash
algorithm used with HKDF is 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.
4.2. Authenticator Construction
An authenticator is formed from the concatenation of TLS 1.3 [TLS13]
Certificate, CertificateVerify, and Finished messages.
If an authenticator request is present, the extensions used to guide
the construction of these messages are taken from the authenticator
request. If there is no authenticator request, the extensions are
chosen from the TLS handshake. Only servers can provide an
authenticator without a corresponding request. In such cases,
ClientHello extensions are used to determine permissible extensions
in the Certificate message.
4.2.1. 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, which is extracted from the
authenticator request if present. If no authenticator request is
provided, the certificate_request_context can be chosen arbitrarily.
The certificates chosen in the Certificate message MUST conform to
the requirements of a Certificate message in the negotiated version
of TLS. In particular, the certificate chain MUST be valid for the a
signature algorithms indicated by the peer in the
"signature_algorithms" and "signature_algorithms_cert" extension, as
described in Section 4.2.3 of [TLS13] for TLS 1.3 or the
"signature_algorithms" extension from Sections 7.4.2 and 7.4.6 of
[RFC5246] for TLS 1.2.
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In addition to "signature_algorithms" and
"signature_algorithms_cert", the "server_name" [RFC6066],
"certificate_authorities" (Section 4.2.4. of [TLS13]), and
"oid_filters" (Section 4.2.5. of [TLS13]) extensions are used to
guide certificate selection. The extensions, or others that might
affect certificate selection, are taken from the authenticator
request if present, or the TLS handshake if not.
Alternative certificate formats such as [RFC7250] Raw Public Keys are
not supported in this version of the specification.
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 TLS handshake.
4.2.2. CertificateVerify
This message is used to provide explicit proof that an endpoint
possesses the private key corresponding to its certificate. The
definition for TLS 1.3 is:
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 combinations of
ECDSA and hash 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" sent by the peer in the TLS
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)
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o A single 0 byte which serves as the separator
o The hashed authenticator transcript
The authenticator transcript is the hash of the concatenated
Handshake Context, authenticator request (if present), and
Certificate message:
Hash(Handshake Context || authenticator request || Certificate)
Where Hash is the hash function negotiated by TLS. If the
authenticator request is not present, it is omitted from this
construction (that is, it is zero length).
If the party that generates the exported authenticator does so with a
different connection than the party that is validating it, then the
Handshake Context will not match, resulting in a CertificateVerify
message that does not validate. This includes situations in which
the application data is sent via TLS-terminating proxy. Given a
failed CertificateVerify validation, it may be helpful for the
application to confirm that both peers share the same connection
using a value derived from the connection secrets before taking a
user-visible action.
4.2.3. Finished
A HMAC [HMAC] over the hashed authenticator transcript, which is the
concatenated Handshake Context, authenticator request (if present),
Certificate, and CertificateVerify:
Hash(Handshake Context || authenticator request ||
Certificate || CertificateVerify)
The HMAC is computed using the same hash function using the Finished
MAC Key as a key.
4.2.4. Authenticator Creation
An endpoint constructs an authenticator by serializing the
Certificate, CertificateVerify, and Finished as TLS handshake
messages and concatenating the octets:
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.
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5. Empty Authenticator
If, given an authenticator request, the endpoint does have an
appropriate certificate or does not want to return one, it constructs
an authenticated refusal called an empty authenticator. This is an
HMAC over the hashed authenticator transcript with a Certificate
message containing no CertificateEntries and the CertificateVerify
message omitted:
"Hash(Handshake Context || authenticator request || Certificate) "
The HMAC is computed using the same hash function using the Finished
MAC Key as a key.
6. 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.
Notwithstanding the success conditions described below, all APIs MUST
fail if:
o the connection uses a TLS version of 1.1 or earlier, or
o the connection is TLS 1.2 and the extended master secret [RFC7627]
was not used
The following sections describes APIs that are considered necessary
to implement exported authenticators. These are informative only.
6.1. The "request" API
The "request" API takes as input:
o certificate_request_context (from 0 to 255 bytes)
o set of extensions to include (this MUST include
signature_algorithms)
It returns an authenticator request, which is a sequence of octets
that includes a CertificateRequest message.
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6.2. The "get context" API
The "get context" API takes as input:
o authenticator
It returns the certificate_request_context.
6.3. The "authenticate" API
The "authenticate" 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 or certificate_request_context
(from 0 to 255 bytes)
It returns either the exported authenticator or an empty
authenticator as a sequence of octets. 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.
6.4. The "validate" API
The "validate" API takes as input:
o an optional authenticator request
o an authenticator
It returns the certificate chain and extensions and a status to
indicate whether the authenticator is valid or not. If the
authenticator was empty - that is, it did not contain a certificate -
the certificate chain will contain no certificates.
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7. IANA Considerations
This document has no IANA actions.
8. 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.
9. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 8 were provided by Karthikeyan Bhargavan.
10. References
10.1. Normative References
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[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>.
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[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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
10.2. Informative References
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[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>.
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Author's Address
Nick Sullivan
Cloudflare Inc.
Email: nick@cloudflare.com
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