Network Working Group                                        N. Sullivan
Internet-Draft                                                Cloudflare
Intended status: Standards Track                             H. Krawczyk
Expires: 26 August 2021                                     IBM Research
                                                                O. Friel
                                                               R. Barnes
                                                        22 February 2021

                          OPAQUE with TLS 1.3


   This document describes two mechanisms for enabling the use of the
   OPAQUE password-authenticated key exchange in TLS 1.3.

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
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   This Internet-Draft will expire on 26 August 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  OPAQUE  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Password Registration . . . . . . . . . . . . . . . . . . . .   4
   5.  Password Authentication . . . . . . . . . . . . . . . . . . .   4
     5.1.  TLS Extensions  . . . . . . . . . . . . . . . . . . . . .   5
   6.  Use of extensions in TLS handshake flows  . . . . . . . . . .   7
     6.1.  OPAQUE-KEX  . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  OPAQUE-Sign . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Integration into Exported Authenticators  . . . . . . . . . .   9
   8.  Summary of properties . . . . . . . . . . . . . . . . . . . .  10
   9.  Privacy considerations  . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Note that this draft has not received significant security review and
   should not be the basis for production systems.

   OPAQUE [opaque-paper] is a mutual authentication method that enables
   the establishment of an authenticated cryptographic key between a
   client and server based on a user's password, without ever exposing
   the password to servers or other entities other than the client
   machine and without relying on a Public Key Infrastructure (PKI).
   OPAQUE leverages a primitive called a Strong symmetric Password
   Authenticated Key Exchange (Strong aPAKE) to provide desirable
   properties including resistance to pre-computation attacks in the
   event of a server compromise.

   In some cases, it is desirable to combine password-based
   authentication with traditional PKI-based authentication as a
   defense-in-depth measure.  For example, in the case of IoT devices,
   it may be useful to validate that both parties were issued a
   certificate from a certain manufacturer.  Another desirable property
   for password-based authentication systems is the ability to hide the
   client's identity from the network.  This document describes the use
   of OPAQUE in TLS 1.3 [TLS13] both as part of the TLS handshake and
   post-handshake facilitated by Exported Authenticators
   [I-D.ietf-tls-exported-authenticator], how the different approaches

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   satisfy the above properties and the trade-offs associated with each

   The in-handshake instantiations of OPAQUE can be used to authenticate
   a TLS handshake with a password alone, or in conjunction with
   certificate-based (mutual) authentication but does not provide
   identity hiding for the client.  The Exported Authenticators
   instantiation of OPAQUE provides client identity hiding by default
   and allows the application to do password authentication at any time
   during the connection, but requires PKI authentication for the
   initial handshake and application-layer semantics to be defined for
   transporting authentication messages.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.


   OPAQUE [opaque-paper] is a Strong Asymmetric Password-Authenticated
   Key Exchange (Strong aPAKE) built on an oblivious pseudo-random
   function (OPRF) and authenticated key exchange protocol that is
   secure against key-compromise impersonation (KCI) attacks.  Unlike
   previous PAKE methods such as SRP [RFC2945] and SPAKE-2
   [I-D.irtf-cfrg-spake2], which require a public salt value, a Strong
   aPAKE leverages the OPRF private key as salt, making it resistant to
   pre-computation attacks on the password database stored on the

   TLS 1.3 provides a KCI-secure key agreement algorithm suitable for
   use with OPAQUE.  This document describes two instantiations of
   OPAQUE in TLS 1.3: one based on digital signatures, called OPAQUE-
   Sign, and one on Diffie-Hellman key agreement, called OPAQUE-KEX.

   OPAQUE consists of two distinct phases: password registration and
   authentication.  We will describe the mechanisms for password
   registration in this document but it is assumed to have been done
   outside of a TLS connection.  During password registration, the
   client and server establish a shared set of parameters for future
   authentication and two private-public key pairs are generated, one
   for the client and one for the server.  The server keeps its private
   key and stores an encapsulated copy of the client's key pair along
   with its own public key in an "envelope" that is encrypted with the
   result of the OPRF operation.  Note that it is possible for the

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   server to use the same key for multiple clients.  It may be necessary
   to permit multiple simultaneous server keys in the even of a key
   rollover.  The client does not store any state nor any PKI

   In OPAQUE-Sign, the key pairs generated at password registration time
   are digital signature keys.  These signature keys are used in place
   of certificate keys for both server and client authentication in a
   TLS handshake.  Client authentication is technically optional, though
   in practice is almost universally required.  OPAQUE-Sign cannot be
   used alongside certificate-based handshake authentication.  This
   instantiation can also be leveraged to do part of a post-handshake
   authentication using Exported Authenticators
   [I-D.ietf-tls-exported-authenticator] given an established TLS
   connection protected with certificate-based authentication.

   In OPAQUE-KEX, the key pairs are Diffie-Hellman keys and are used to
   establish a shared secret that is fed into the key schedule for the
   handshake.  The handshake continues to use Certificate-based
   authentication and establishes the shared key using Diffie-Hellman.
   This instantiations is best suited to use cases in which both
   password and certificate-based authentication are needed during the
   initial handshake, which is useful in some scenarios.  There is no
   unilateral authentication in this context, mutual authentication is
   demonstrated explicitly through the finished messages.

4.  Password Registration

   Password registration is run between a client U and a server S.  It
   is assumed that U can authenticate S during this registration phase
   (this is the only part in OPAQUE that requires some form of
   authenticated channel, either physical, out-of-band, PKI-based, etc.)
   During this phase, clients run the registration flow in
   [I-D.irtf-cfrg-opaque] using a specific OPAQUE configuration
   consisting of a tuple (OPRF, Hash, MHF, AKE).  The specific AKE is
   not used during registration.  It is only used during login.

   During this phase, a specific OPAQUE configuration is chosen, which
   consists of a tuple (OPRF, Hash, MHF, AKE).  See
   [I-D.irtf-cfrg-opaque] for details about configuration parameters.
   In this context, AKE is either OPAQUE-Sign or OPAQUE-KEX.

5.  Password Authentication

   Password authentication integrates TLS into OPAQUE in such a way that
   clients prove knowledge of a password to servers.  In this section,
   we describe TLS extensions that support this integration for both

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5.1.  TLS Extensions

   We define several TLS extensions to signal support for OPAQUE and
   transport the parameters.  The extensions used here have a similar
   structure to those described in Usage of PAKE with TLS 1.3
   [I-D.barnes-tls-pake].  The permitted messages that these extensions
   are allowed and the expected protocol flows are described below.

   First, this document specifies extensions used to convey OPAQUE
   client and server messages, called "opaque_client_auth" and
   "opaque_server_auth" respectively.

     enum {
     } ExtensionType;

   The "opaque_client_auth" extension contains a
   "PAKEClientAuthExtension" struct and can only be included in the
   "CertificateRequest" and "Certificate" messages.

     struct {
       opaque identity<0..2^16-1>;
     } PAKEClientAuthExtension;

   The "opaque_server_auth" extension contains a
   "PAKEServerAuthExtension" struct and can only be included in the
   "ClientHello", "EncryptedExtensions", "CertificateRequest" and
   "Certificate" messages, depending on the type.

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     struct {
       opaque idU<0..2^16-1>;
       CredentialRequest request;
     } PAKEShareClient;

     struct {
       opaque idS<0..2^16-1>;
       CredentialResponse response;
     } PAKEShareServer;

     struct {
       select (Handshake.msg_type) {
           PAKEShareClient client_shares<0..2^16-1>;
           OPAQUEType types<0..2^16-1>;
         EncryptedExtensions, Certificate:
           PAKEShareServer server_share;
           OPAQUEType type;
     } PAKEServerAuthExtension;

   This document also defines the following set of types;

     enum {
     } OPAQUEType;

   Servers use PAKEShareClient.idU to index the user's record on the
   server and create the PAKEShareServer.response.  The types field
   indicates the set of supported auth types by the client.
   PAKEShareClient.request and PAKEShareServer.response, of type
   CredentialRequest and CredentialResponse, respectively, are defined
   in [I-D.irtf-cfrg-opaque].

   This document also describes a new CertificateEntry structure that
   corresponds to an authentication via a signature derived using
   OPAQUE.  This structure serves as a placeholder for the
   PAKEServerAuthExtension extension.

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     struct {
       select (certificate_type) {
         case OPAQUESign:
           /* Defined in this document */
           opaque null<0>

         case RawPublicKey:
           /* From RFC 7250 ASN.1_subjectPublicKeyInfo */
           opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;

         case X509:
           opaque cert_data<1..2^24-1>;
       Extension extensions<0..2^16-1>;
     } CertificateEntry;

   We request that IANA add an additional type to the "TLS Certificate
   Types" registry for this OPAQUESign.

   Support for the OPAQUESign Certificate type for server authentication
   can be negotiated using the server_certificate_type [RFC7250] and the
   Certificate type for client authentication can be negotiated using
   the client_certificate_type extension [RFC7250].

   Note that there needs to be a change to the client_certificate_type
   row in the IANA "TLS ExtensionType Values" table to allow
   client_certificate_type extension to be used as an extension to the
   CertificateRequest message.

6.  Use of extensions in TLS handshake flows


   In this mode, OPAQUE private keys are used for key agreement
   algorithm and the result is fed into the TLS key schedule.  Password
   validation is confirmed by the validation of the finished message.
   These modes can be used in conjunction with optional Certificate-
   based authentication.

   It should be noted that since the identity of the client it is not
   encrypted as it is sent as an extension to the ClientHello.  This may
   present a privacy problem unless a mechanism like Encrypted Client
   Hello [ECH] is created to protect it.

   Upon receiving a PAKEServerAuth extension, the server checks to see
   if it has a matching record for this identity.  If the record does
   not exist, the handshake is aborted with a "illegal_parameter" alert.
   If the record does exist, but the key type of the record does not

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   match any of the supported_groups sent in the key_share extension of
   the ClientHello, an HRR is sent containing the set of valid key types
   that it found records for.

   Given a matching key_share and an identity with a matching
   supported_group, the server returns its PAKEServerAuth as an
   extension to its EncryptedExtensions.  Both parties then derive a
   shared OPAQUE key as follows:

    U computes
      K = H(g^y ^ PrivU || PubU ^ x || PubS ^ PrivU || IdU || IdS )
    S computes
      K = H(g^x ^ PrivS || PubS ^ y || PubU ^ PrivS || IdU || IdS )

   IdU, IdS represent the identities of user (sent as identity in
   PAKEShareClient) and server (Certificate message).  H is the HKDF
   function agreed upon in the TLS handshake.

   The result, K, is then added as an input to the Master Secret in
   place of the 0 value defined in TLS 1.3.  Specifically,

     0 -> HKDF-Extract = Master Secret


     K -> HKDF-Extract = Master Secret

   In this construction, the finished messages cannot be validated
   unless the OPAQUE computation was done correctly on both sides,
   authenticating both client and server.

6.2.  OPAQUE-Sign

   In this modes of operation, the OPAQUE private keys are used for
   digital signatures and are used to define a new Certificate type and
   CertificateVerify algorithm.  Like the OPAQUE-KEX instantiations
   above, the identity of the client is sent in the clear in the
   client's first flight unless a mechanism like Encrypted Client Hello
   [ECH] is created to protect it.

   Upon receiving a PAKEServerAuth extension, the server checks to see
   if it has a matching record for this identity.  If the record does
   not exist, the handshake is aborted with a TBD error message.  If the
   record does exist, but the key type of the record does not match any
   of the supported_signatures sent in the the ClientHello, the
   handshake must be aborted with a "illegal_parameter" error.

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   We define a new Certificate message type for an OPAQUE-Sign
   authenticated handshake.

   enum {
   } CertificateType;

   Certificates of this type have CertificateEntry structs of the form:

   struct {
     Extension extensions<0..2^16-1>;
   } CertificateEntry;

   Given a matching signature_scheme and an identity with a matching key
   type, the server returns a certificate message with type OPAQUE-Sign
   with PAKEServerAuth as an extension.  The private key used in the
   CertificateVerify message is set to the private key used during
   account registration, and the client verifies it using the server
   public key contained in the client's envelope.

   It is RECOMMENDED that the server includes a CertificateRequest
   message with a PAKEClientAuth and the identity originally sent in the
   PAKEServerAuth extension from the client hello.  On receiving a
   CertificateRequest message with a PAKEClientAuth extension, the
   client returns a CertificateVerify message signed by PrivC which is
   validated by the server using PubC.

7.  Integration into Exported Authenticators

   Neither of the above mechanisms provides privacy for the user during
   the authentication phase, as the user id is sent in the clear.
   Additionally, OPAQUE-Sign has the drawback that it cannot be used in
   conjunction with certificate-based authentication.

   It is possible to address both the privacy concerns and the
   requirement for certificate-based authentication by using OPAQUE-Sign
   in an Exported Authenticator [I-D.ietf-tls-exported-authenticator]
   flow, since exported authenticators are sent over a secure channel
   that is typically established with certificate-based authentication.
   Using Exported Authenticators for OPAQUE has the additional benefit
   that it can be triggered at any time after a TLS session has been
   established, which better fits modern web-based authentication

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   The ClientHello contains PAKEServerAuth, PAKEClientAuth with empty
   identity values to indicate support for these mechanisms.

   1.  Client creates Authenticator Request with CR extension

   2.  Server creates Exported Authenticator with OPAQUE-Sign
       (PAKEServerAuth) and CertificateVerify (signed with the OPAQUE
       private key).

   If the server would like to then establish mutual authentication, it
   can do the following:

   1.  Server creates Authenticator Request with CH extension
       PAKEClientAuth (identity)

   2.  Client creates Exported Authenticator with OPAQUE-Sign
       Certificate and CertificateVerify (signed with user private key
       derived from the envelope).

   Support for Exported Authenticators is negotiated at the application

8.  Summary of properties

   | Variant \ |Identity|Certificate|Server-only|Post-handshake|Minimum|
   | Property  | hiding | auth      | auth      | auth         | round |
   |           |        |           |           |              | trips |
   |OPAQUE-Sign| yes    | yes       | yes       | yes          | 2-RTT |
   | with EA   |        |           |           |              |       |
   |OPAQUE-Sign| no     | no        | yes       | no           | 1-RTT |
   |OPAQUE-KEX | no     | no        | no        | no           | 1-RTT |

                                  Table 1

9.  Privacy considerations

   TBD: cleartext identity, etc

10.  Security Considerations

   TODO: protecting against user enumeration

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11.  IANA Considerations

   *  Existing IANA references have not been updated yet to point to
      this document.

      IANA is asked to register a new value in the "TLS Certificate
      Types" registry of Transport Layer Security (TLS) Extensions (TLS-
      Certificate-Types-Registry), as follows:

   *  Value: 4 Description: OPAQUE Authentication Reference: This RFC

   Correction request: The client_certificate_type row in the IANA TLS
   ExtensionType Values table to allow client_certificate_type extension
   to be used as an extension to the CertificateRequest message.

12.  References

12.1.  Normative References

              Sullivan, N., "Exported Authenticators in TLS", Work in
              Progress, Internet-Draft, draft-ietf-tls-exported-
              authenticator-14, 25 January 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

12.2.  Informative References

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   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-09, 16 December 2020,

              Barnes, R. and O. Friel, "Usage of PAKE with TLS 1.3",
              Work in Progress, Internet-Draft, draft-barnes-tls-pake-
              04, 16 July 2018, <

              Krawczyk, H., Lewi, K., and C. A. Wood, "The OPAQUE
              Asymmetric PAKE Protocol", Work in Progress, Internet-
              Draft, draft-irtf-cfrg-opaque-03, 21 February 2021,

              Ladd, W. and B. Kaduk, "SPAKE2, a PAKE", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-spake2-18, 17 January
              2021, <

              Xu, J., "OPAQUE: An Asymmetric PAKE Protocol Secure
              Against Pre-Computation Attacks", 2018.

   [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",
              RFC 2945, DOI 10.17487/RFC2945, September 2000,

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,

Appendix A.  Acknowledgments

Authors' Addresses

   Nick Sullivan


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   Hugo Krawczyk
   IBM Research


   Owen Friel


   Richard Barnes


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