Internet DRAFT - draft-rescorla-tls13-semistatic-dh
draft-rescorla-tls13-semistatic-dh
TLS Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Standards Track N. Sullivan
Expires: September 6, 2018 Cloudflare
March 05, 2018
Semi-Static Diffie-Hellman Key Establishment for TLS 1.3
draft-rescorla-tls13-semistatic-dh-00
Abstract
TLS 1.3 [I-D.ietf-tls-tls13] 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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Certificate Format . . . . . . . . . . . . . . . . . . . . . 5
5. Cryptographic Details . . . . . . . . . . . . . . . . . . . . 5
5.1. Certificate Verify Computation . . . . . . . . . . . . . 5
5.2. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 6
6. Client Authentication . . . . . . . . . . . . . . . . . . . . 6
7. 0-RTT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
DISCLAIMER: This is a work-in-progress draft and is currenty totally
handwavy, so it has not yet seen significant security analysis. It
should not be used as a basis for building production systems.
TLS 1.3 [I-D.ietf-tls-tls13] 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, 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:
o They may be issued a certificate with an (EC)DH key, as specified
for instance in [I-D.ietf-curdle-pkix]
o 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 server authenticates via its long-term
(EC)DH key is suitable. This document describes such a design
modelled on that described in OPTLS [KW16].
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This design has a number of potential advantages over the signed
exchange in TLS 1.3, specifically:
o 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.
o It is more resistant to random number generation failures on the
server because the attacker needs to have both the server's long-
term (EC)DH key and the ephemeral (EC)DH key in order to compute
the traffic secrets. [Note:
[I-D.cremers-cfrg-randomness-improvements] describes a technique
for accomplishing this with a signed exchange.]
o If the server 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).
o Because there is no signature, the server 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 this operation is cacheable.
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
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:
o 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.
o 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:
o Instead of containing a signature, the CertificateVerify contains
a MAC of the handshake transcript, computed based on SS.
o SS is mixed into the key schedule at the last HKDF-Extract stage
(where currently a 0 is used as the IKM input).
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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:
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.
4. Certificate Format
Like 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] and [[TBD: P-256, etc.]] for specific details
about these formats.
5. Cryptographic Details
5.1. 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 - SS-Base-Key - is derived from SS as follows:
SS-Base-Key = HKDF-Extract(0, SS)
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The MAC is then computed using the Finished computation described in
[I-D.ietf-tls-tls13] Section 4.4, with SS-Base-Key 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
Finished using the Master Secret. These MACs serve different
purposes: the first authenticates the handshake and the second proves
possession of the ephemeral secret. [[OPEN ISSUE: Verify that this
is OK because neither MAC is computed with the mixed key. At least
one version of OPTLS was somewhat like that, however.]]
5.2. Key Schedule
The final HKDF-Extract stage of the TLS 1.3 key schedule has an HKDF-
Extract with the IKM of 0. When static key exchange is negotiated,
that 0 is replaced with SS, as shown below.
...
Derive-Secret(., "derived", "")
|
v
SS -> HKDF-Extract = Master Secret
|
+-----> Derive-Secret(., "c ap traffic",
| ClientHello...server Finished)
| = client_application_traffic_secret_0
|
...
6. Client Authentication
[[OPEN ISSUE]] In principle, we can do client authentication the same
way, with the client's DH key in Certificate and a MAC in
CertificateVerity. However, it's less good because the client's
static key doesn't get mixed in at all. Also, client DH keys seem
even further off.
7. 0-RTT
[[OPEN ISSUE]] It seems like one ought to be able to publish the
server's static key and use it for 0-RTT, but actually we don't know
how to do the publication piece, so I think we should leave this out
for now.
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8. Security Considerations
[[OPEN ISSUE: This is a -00, so the security considerations are kind
of sketchy.]]
o This is intended to have roughly equivalent security properties to
current TLS 1.3, except for the points raised in the introduction.
o There are open questions about how much key mixing we want to do,
especially with respect to client authentication.
o I'm not sure I like the double extract of SS. I've looked it over
and the SS-Base-Key and the HKDF-Extract to make the MS should be
independent, but I'd like to give it another look-over to see if
there is a cleaner way to do it.
9. 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.
10. References
10.1. 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", draft-ietf-curdle-
pkix-07 (work in progress), November 2017.
[I-D.ietf-tls-subcerts]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS", draft-ietf-tls-
subcerts-00 (work in progress), October 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-26 (work in progress),
March 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>.
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10.2. Informative References
[I-D.cremers-cfrg-randomness-improvements]
Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", draft-cremers-cfrg-randomness-improvements-00
(work in progress), March 2018.
[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
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