[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00

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

   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 September 6, 2018.

Copyright Notice

   Copyright (c) 2018 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
   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



Rescorla & Sullivan     Expires September 6, 2018               [Page 1]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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].




Rescorla & Sullivan     Expires September 6, 2018               [Page 2]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


   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:
















Rescorla & Sullivan     Expires September 6, 2018               [Page 3]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


       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).







Rescorla & Sullivan     Expires September 6, 2018               [Page 4]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


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)




Rescorla & Sullivan     Expires September 6, 2018               [Page 5]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


   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.






Rescorla & Sullivan     Expires September 6, 2018               [Page 6]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


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>.





Rescorla & Sullivan     Expires September 6, 2018               [Page 7]


Internet-Draft           TLS 1.3 Semi-Static KX               March 2018


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























Rescorla & Sullivan     Expires September 6, 2018               [Page 8]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/