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Versions: (draft-sullivan-tls-exported-authenticator)
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TLS N. Sullivan
Internet-Draft Cloudflare Inc.
Intended status: Standards Track December 18, 2019
Expires: June 20, 2020
Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-11
Abstract
This document describes a mechanism in Transport Layer Security (TLS)
for peers to provide a proof of ownership of a certificate. This
proof can be exported by one peer, transmitted out-of-band to the
other peer, and verified by the receiving peer.
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 https://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 20, 2020.
Copyright Notice
Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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. Message Sequences . . . . . . . . . . . . . . . . . . . . . . 3
4. Authenticator Request . . . . . . . . . . . . . . . . . . . . 4
5. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Authenticator Keys . . . . . . . . . . . . . . . . . . . 6
5.2. Authenticator Construction . . . . . . . . . . . . . . . 6
5.2.1. Certificate . . . . . . . . . . . . . . . . . . . . . 7
5.2.2. CertificateVerify . . . . . . . . . . . . . . . . . . 8
5.2.3. Finished . . . . . . . . . . . . . . . . . . . . . . 9
5.2.4. Authenticator Creation . . . . . . . . . . . . . . . 9
6. Empty Authenticator . . . . . . . . . . . . . . . . . . . . . 9
7. API considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. The "request" API . . . . . . . . . . . . . . . . . . . . 10
7.2. The "get context" API . . . . . . . . . . . . . . . . . . 10
7.3. The "authenticate" API . . . . . . . . . . . . . . . . . 10
7.4. The "validate" API . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8.1. Update of the TLS ExtensionType Registry . . . . . . . . 11
8.2. Update of the TLS Exporter Labels Registry . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
This document provides a way to authenticate one party of a Transport
Layer Security (TLS) connection to its peer 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 its peer.
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 additional identities
over a single connection.
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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.
Versions of TLS prior to TLS 1.3 used renegotiation as a way to
enable post-handshake client authentication given an existing TLS
connection. The mechanism described in this document may be used to
replace the post-handshake authentication functionality provided by
renegotiation. Unlike renegotiation, exported Authenticator-based
post-handshake authentication does not require any changes at the TLS
layer.
Post-handshake authentication is defined in TLS 1.3, but it has the
disadvantage of requiring additional state to be stored as part of
the TLS state machine. Furthermore, the authentication boundaries of
TLS 1.3 post-handshake authentication align with TLS record
boundaries, which are often not aligned with the authentication
boundaries of the higher-layer protocol. For example, multiplexed
connection protocols like HTTP/2 [RFC7540] do not have a notion of
which TLS record a given message is a part of.
Exported Authenticators are meant to be used as a building block for
application protocols. Mechanisms such as those required to
advertise support and handle authentication errors are not handled at
the TLS layer.
TLS (or DTLS) version 1.2 or later are REQUIRED to implement the
mechanisms described in this document.
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. Message Sequences
There are two types of messages defined in this document:
Authenticator Requests and Authenticators. These can be combined in
the following three sequences:
Client Authentication
o Server generates Authenticator Request
o Client generates Authenticator from Server's Authenticator Request
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o Server validates Client's Authenticator
Server Authentication
o Client generates Authenticator Request
o Server generates Authenticator from Client's Authenticator Request
o Client validates Server's Authenticator
Spontaneous Server Authentication
o Server generates Authenticator
o Client validates Server's Authenticator
4. Authenticator Request
The authenticator request is a structured message that can be created
by either party of a TLS connection using data exported from that
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 request SHOULD use TLS as its
underlying transport to keep the request confidential. The
application MAY use the existing TLS connection to transport the
authenticator.
An authenticator request message can be constructed by either the
client or the server. Server-generated authenticator requests use
the CertificateRequest message from Section 4.3.2 of [TLS13].
Client-generated authenticator requests use a new message, called the
ClientCertificateRequest, which uses the same structure as
CertificateRequest. These messages structures are used even if the
TLS connection protocol is TLS 1.2.
The CertificateRequest and ClientCertificateRequest messages are used
to define the parameters in a request for an authenticator. These
messages do not include any TLS framing and are not encrypted with a
handshake key.
The structures are defined to be:
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struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} ClientCertificateRequest;
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 be unique for
each authenticator request within the scope of a connection
(preventing replay and context confusion). 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 set of extensions allowed in the CertificateRequest
structure are those defined in the TLS ExtensionType Values IANA
registry containing CR in the TLS 1.3 column. The extensions
allowed in the ClientCertificateRequest are those containing CR in
the TLS 1.3 column, along with the server_name [RFC6066]
extension.
The uniqueness requirements of the certificate_request_context apply
only to CertificateRequest and ClientCertificateRequest messages that
are used as part of authenticator requests. There is no impact if
the value of a certificate_request_context used in an authenticator
request matches the value of a certificate_request_context in the
handshake or in a post-handshake message.
5. 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 or a protocol with comparable security properties as its
underlying transport to keep the certificate confidential. The
application MAY use the existing TLS connection to transport the
authenticator.
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
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without a preceding authenticator request; for servers an
authenticator request is optional. For authenticators that do not
correspond to authenticator requests, the certificate_request_context
is chosen by the server.
5.1. Authenticator Keys
Each authenticator is computed using a Handshake Context and Finished
MAC Key derived from the TLS connection. These values are derived
using an exporter as described in [RFC5705] (for TLS 1.2) or Sec. 7.5
of [TLS13] (for TLS 1.3). For TLS 1.3, the exporter_master_secret
MUST be used, not the early_exporter_master_secret. 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 empty (zero length) for
all four values. There is no need to include additional context
information at this stage since the application-supplied context is
included in the authenticator itself. 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). Exported
authenticators cannot be used with cipher suites that do not use the
TLS PRF and have not defined a hash function for this purpose. This
hash is referred to as the authenticator hash.
To avoid key synchronization attacks, Exported Authenticators MUST
NOT be generated or accepted on TLS 1.2 connectons that did not
negotiate the extended master secret [RFC7627].
5.2. Authenticator Construction
An authenticator is formed from the concatenation of TLS 1.3 [TLS13]
Certificate, CertificateVerify, and Finished messages.
If the peer creating the certificate_request_context has already
created or correctly validated an authenticator with the same value,
then no authenticator should be constructed. If there is no
authenticator request, the extensions are chosen from those presented
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in the TLS handshake's ClientHello. Only servers can provide an
authenticator without a corresponding request.
ClientHello extensions are used to determine permissible extensions
in the Certificate message. This follows the general model for
extensions in TLS in which extensions can only be included as part of
a Certificate message if they were previously sent as part of a
CertificateRequest message or ClientHello message, to ensure that the
recipient will be able to process such extensions.
5.2.1. Certificate
The Certificate message contains 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
but MUST be unique within the scope of the connection and be
unpredictable to the peer.
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.
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.
Only the X509 certificate type defined in [TLS13] is supported.
Alternative certificate formats such as [RFC7250] Raw Public Keys are
not supported in this version of the specification and their use in
this context has not yet been analysed.
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. Unrecognized extensions in the
authenticator request MUST be ignored.
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5.2.2. 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 present in the
authenticator request. Otherwise, the signature algorithm used
should be chosen from the "signature_algorithms" sent by the peer in
the ClientHello of the TLS handshake. If there are no available
signature algorithms, then no authenticator should be constructed.
The signature is computed using the chosen signature scheme 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 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 authenticator hash defined in section 4.1. If the
authenticator request is not present, it is omitted from this
construction (that is, it is zero length).
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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.
5.2.3. Finished
A HMAC [HMAC] over the hashed authenticator transcript, which is the
concatenated Handshake Context, authenticator request (if present),
Certificate, and CertificateVerify. The HMAC is computed using the
authenticator hash, using the Finished MAC Key as a key.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate || CertificateVerify))
5.2.4. 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
An authenticator is valid if the CertificateVerify message is
correctly constructed given the authenticator request (if used) and
the Finished message matches the expected value. When validating an
authenticator, a constant-time comparison SHOULD be used.
6. Empty Authenticator
If, given an authenticator request, the endpoint does not have an
appropriate certificate or does not want to return one, it constructs
an authenticated refusal called an empty authenticator. This is a
Finished message sent without a Certificate or CertificateVerify.
This message is an HMAC over the hashed authenticator transcript with
a Certificate message containing no CertificateEntries and the
CertificateVerify message omitted. The HMAC is computed using the
authenticator hash, using the Finished MAC Key as a key. This
message does not include any TLS framing.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate))
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7. 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 SHOULD
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 extension
[RFC7627] was not negotiated
The following sections describes APIs that are considered necessary
to implement exported authenticators. These are informative only.
7.1. 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 comprises a CertificateRequest or ClientCertificateRequest
message.
7.2. The "get context" API
The "get context" API takes as input:
o authenticator or authenticator request
It returns the certificate_request_context.
7.3. The "authenticate" API
The "authenticate" API takes as input:
o a reference to an active connection
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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 operations) for each chain
o an 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 and more easily remember which
extensions were sent in the ClientHello.
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.
7.4. The "validate" API
The "validate" API takes as input:
o a reference to an active connection
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. The API SHOULD
return a failure if the certificate_request_context of the
authenticator was used in a previously validated authenticator.
Well-formed empty authenticators are returned as valid.
8. IANA Considerations
8.1. Update of the TLS ExtensionType Registry
IANA is requested to update the entry for server_name(0) in the
registry for ExtensionType (defined in [TLS13]) by replacing the
value in the "TLS 1.3" column with the value "CH, EE, CR".
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8.2. Update of the TLS Exporter Labels Registry
IANA is requested to add the following entries to the registry for
Exporter Labels (defined in [RFC5705]): "EXPORTER-server
authenticator handshake context", "EXPORTER-client authenticator
finished key" and "EXPORTER-server authenticator finished key".
9. Security Considerations
The Certificate/Verify/Finished pattern intentionally looks like the
TLS 1.3 pattern which now has been analyzed several times. For
example, [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. The application in possession of a validated
authenticator can rely on any semantics associated with data in the
certificate_request_context.
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.
10. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 9 were provided by Karthikeyan Bhargavan.
11. References
11.1. 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>.
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[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>.
[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>.
[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>.
[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", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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11.2. 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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