Internet DRAFT - draft-ietf-tls-semistatic-dh
draft-ietf-tls-semistatic-dh
TLS Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Standards Track N. Sullivan
Expires: 8 September 2020 Cloudflare
C.A. Wood
Apple Inc.
7 March 2020
Semi-Static Diffie-Hellman Key Establishment for TLS 1.3
draft-ietf-tls-semistatic-dh-01
Abstract
TLS 1.3 [RFC8446] specifies a signed Diffie-Hellman exchange modelled
after SIGMA [SIGMA]. This design is suitable for endpoints whose
certified credential is a signing key, which is the common situation
for current TLS servers. This document describes a mode of TLS 1.3
in which one or both endpoints have a certified DH key which is used
to authenticate the exchange.
Note to Readers
Source for this draft and an issue tracker can be found at
https://github.com/ekr/draft-rescorla-tls13-semistatic-dh
(https://github.com/ekr/draft-rescorla-tls13-semistatic-dh).
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 8 September 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Certificate Format . . . . . . . . . . . . . . . . . . . . . 5
5. Certificate Verify Computation . . . . . . . . . . . . . . . 5
6. Client Authentication . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
DISCLAIMER: This is a work-in-progress draft and has not yet seen
significant security analysis. Thus, this draft should not be used
as a basis for building production systems.
TLS 1.3 [RFC8446] specifies a signed Diffie-Hellman (DH) exchange
modeled after SIGMA [SIGMA]. This design is suitable for endpoints
whose certified credential is a signing key, which is the common
situation for current TLS servers, which is why it was selected for
TLS 1.3.
However, it is also possible - although currently rare - for
endpoints to have a credential which is an (EC)DH key. This can
happen in one of two ways:
* They may be issued a certificate with an (EC)DH key, as specified
for instance in [I-D.ietf-curdle-pkix]
* They may have a signing key which they use to generate a delegated
credential [I-D.ietf-tls-subcerts] containing an (EC)DH key.
In these situations, a signed DH exchange is not appropriate, and
instead a design in which the endpoint authenticates via its long-
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term (EC)DH key is suitable. This document describes such a design
modeled on that described in OPTLS [KW16].
This design has a number of potential advantages over the signed
exchange in TLS 1.3, specifically:
* If the end-entity certificate contains an (EC)DH key, TLS can
operate with a single asymmetric primitive (Diffie-Hellman). The
PKI component will still need signatures, but the TLS stack need
not have one. Note that this advantage is somewhat limited if the
(EC)DH key is in a delegated credential, but that allows for a
clean transition to (EC)DH certificates.
* If the endpoint has a comparatively slow signing cert (e.g.,
P-256) it can amortize that signature over a large number of
connections by creating a delegated credential with an (EC)DH key
from a faster group (e.g., X25519).
* Because there is no signature, the endpoint has deniability for
the existence of the communication. Note that it could always
have denied the contents of the communication.
This exchange is not generally faster than a signed exchange if
comparable groups are used. In fact, if delegated credentials are
used, it may be slower on the client as it has to validate the
delegated credential, though the result may be cached.
2. Protocol Overview
The overall protocol flow remains the same as that in ordinary TLS
1.3, as shown below:
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Client Server
Key ^ ClientHello
Exch | + key_share*
| + signature_algorithms*
| + psk_key_exchange_modes*
v + pre_shared_key* -------->
ServerHello ^ Key
+ key_share* | Exch
+ pre_shared_key* v
{EncryptedExtensions} ^ Server
{CertificateRequest*} v Params
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
[Application Data] <-------> [Application Data]
As usual, the client and server each supply an (EC)DH share in their
"key_share" extensions. However, in addition, the server supplies a
(signed) static (EC)DH share in its Certificate message, either
directly in its end-entity certificate or in a delegated credential.
The client and server then perform two (EC)DH exchanges:
* Between the client and server "key_share" values to form an
ephemeral secret (ES). This is the same value as is computed in
TLS 1.3 currently.
* Between the client's "key_share" and the server's static share, to
form a static secret (SS).
Note that this means that the server's static secret MUST be in the
same group as selected group for the ephemeral (EC)DH exchange.
The handshake then proceeds as usual, except that instead of
containing a signature, the CertificateVerify contains a MAC of the
handshake transcript, computed based on SS.
3. Negotiation
In order to negotiate this mode, we treat the (EC)DH MAC as if it
were a signature and negotiate it with a set of new signature scheme
values:
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enum {
sig_p256(0x0901),
sig_p384(0x0902),
sig_p521(0x0903),
sig_x52219(0x0904),
sig_x448(0x0905),
} SignatureScheme;
When present in the "signature_algorithms" extension or
CertificateVerify.signature_scheme, these values indicate DH MAC with
the specified key exchange mode. These values MUST NOT appear in
"signature_algorithms_cert".
Before sending and upon receipt, endpoints MUST ensure that the
signature scheme is consistent with the ephemeral (EC)DH group in
use. Clients MUST NOT advertise signature scheme values that are
inconsistent with the "named_groups" extension they offer.
4. Certificate Format
Similar to signing keys, static DH keys are carried in the
Certificate message, either directly in the EE certificate, or in a
delegated credential. In either case, the OID for the
SubjectPublicKeyInfo MUST be appropriate for use with (EC)DH key
establishment. If in a certificate, the key usage and EKU MUST also
be set appropriately. See [I-D.ietf-curdle-pkix] for specific
details about these formats.
5. Certificate Verify Computation
Instead of a signature, the server proves knowledge of the private
key associated with its static share by computing a MAC over the
handshake transcript using SS. The transcript thus far includes all
messages up to and including Certificate, i.e.:
Transcript-Hash(Handshake Context, Certificate)
The MAC key, xSS, is derived from SS as follows:
xSS = HKDF-Extract(0, SS)
The MAC is then computed using the Finished computation described in
[RFC8446] Section 4.4, with xSS as the Base Key value. Receivers
MUST validate the MAC and terminate the handshake with a
"decrypt_error" alert upon failure.
Note that this means that the server sends two MAC computations in
the handshake, one in CertificateVerify using SS and the other in
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Finished using the Master Secret. These MACs serve different
purposes: the first authenticates the handshake and the second proves
possession of the ephemeral secret.
6. Client Authentication
Client authentication works similar to that of server authentication
described in Section 2. In particular, servers indicate support of
semi-static keys by sending one of the relevant SignatureScheme
values defined in Section 3 inside the CertificateRequest
"signature_algorithms" extension. If applicable, clients reply with
a non-empty Certificate message carrying a corresponding certificate
with static DH key matching the chosen signature algorithm. Clients
then also compute the CertificateVerify message using the procedure
of Section 5, over the transcript hash Handshake Context described in
[RFC8446], Section 4.4.
If no matching certificate is available, clients send an empty
Certificate message as per [RFC8446]; Section 4.4.2.
7. Security Considerations
[[OPEN ISSUE: This design requires formal analysis.]]
This is intended to have roughly equivalent security properties to
current TLS 1.3, except for the points raised in the introduction.
Open questions:
* Should semi-static key shares be mixed into the key schedule?
8. IANA Considerations
IANA [SHOULD add/has added] the new code points specified in
Section 3 to the TLS 1.3 signature scheme registry, with a
"recommended" value of TBD.
9. References
9.1. Normative References
[I-D.ietf-curdle-pkix]
Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519 and X448 for use in the Internet
X.509 Public Key Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-curdle-pkix-10, 8 May 2018,
<http://www.ietf.org/internet-drafts/draft-ietf-curdle-
pkix-10.txt>.
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[I-D.ietf-tls-subcerts]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS", Work in Progress,
Internet-Draft, draft-ietf-tls-subcerts-06, 5 February
2020, <http://www.ietf.org/internet-drafts/draft-ietf-tls-
subcerts-06.txt>.
[RFC8446] 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>.
9.2. Informative References
[KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3",
Proceedings of Euro S"P 2016 , 2016,
<https://eprint.iacr.org/2015/978>.
[SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' approach to
authenticated Diffie-Hellman and its use in the IKE
protocols", Proceedings of CRYPTO 2003 , 2003.
Authors' Addresses
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Christopher A. Wood
Apple Inc.
Email: cawood@apple.com
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