Network Working Group                                          R. Barnes
Internet-Draft                                                  O. Friel
Intended status: Informational                                     Cisco
Expires: December 31, 2018                                 June 29, 2018

                      Usage of SPAKE with TLS 1.3


   The pre-shared key mechanism available in TLS 1.3 is not suitable for
   usage with low-entropy keys, such as passwords entered by users.
   This document describes an extension that enables the use of
   password-authenticated key exchange protocols with TLS 1.3.

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 December 31, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  TLS Extensions  . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Compatible PAKE Protocols . . . . . . . . . . . . . . . . . .   5
   6.  SPAKE2+ Implementation  . . . . . . . . . . . . . . . . . . .   6
   7.  Pre-provisioned Parameters  . . . . . . . . . . . . . . . . .   6
   8.  Content of the TLS Extensions . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     9.1.  Security when using SPAKE2+ . . . . . . . . . . . . . . .   8
   10. Open Items  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  PAKE Algorithm Negotiation . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   DISCLAIMER: This is a work-in-progress draft and has not yet seen
   significant security analysis.  It should not be used as a basis for
   building production systems.

   In some applications, it is desireable to enable a client and server
   to authenticate to one another using a low-entropy pre-shared value,
   such as a user-entered password.

   In prior versions of TLS, this functionality has been provided by the
   integration of the Secure Remote Password PAKE protocol (SRP)
   [RFC5054].  The specific SRP integration described in RFC 5054 does
   not immediately extend to TLS 1.3 because it relies on the Client Key
   Exchange and Server Key Exchange messages, which no longer exist in

   TLS 1.3 itself provides a mechanism for authentication with pre-
   shared keys (PSKs).  However, PSKs used with this protocol need to be
   "full-entropy", because the binder values used for authentication can
   be used to mount a dictionary attack on the PSK.  So while the TLS
   1.3 PSK mechanism is suitable for the session resumption cases for
   which it is specified, it cannot be used when the client and server
   share only a low-entropy secret.

   Enabling TLS to address this use case effectively requires the TLS
   handshake to execute a password-authenticated key establishment

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   (PAKE) protocol.  This document describes a TLS extension "pake" that
   can carry data necessary to execute a PAKE.

   This extension is generic, in that it can be used to carry key
   exchange information for multiple different PAKEs.  We assume that
   the client and server have pre-negotiated a choice of PAKE (and any
   required parameters) in addition to the password itself.  As a first
   case, this document defines a concrete protocol for executing the
   SPAKE2+ PAKE protocol [I-D.irtf-cfrg-spake2].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The mechanisms described in this document also apply to DTLS 1.3
   [I-D.ietf-tls-dtls13], but for brevity, we will refer only to TLS

3.  Setup

   In order to use this protocol, a TLS client and server need to have
   pre-provisioned the values required to execute the protocol:

   o  A choice of PAKE protocol

   o  Any parameters required by the PAKE protocol

   o  A password (or a derived value as described by the PAKE protocol)

   Servers will of course have multiple instances of this configuration
   information for different clients.  Clients may also have multiple
   identities, even within a given server.  We assume that in either
   case, a single opaque "identity" value is sufficient to identify the
   required parameters.

4.  TLS Extensions

   A client offers to authenticate with PAKE by including an "pake"
   extension in its ClientHello.  The content of this exension is a
   "PAKEClientHello" value, providing a list of identities under which
   the client can authenticate, and for each identity, the client's
   first message from the underlying PAKE protocol.

   If a client sends the "pake" extension, then it MAY also send the
   "key_share" and "pre_shared_key" extensions, to allow the server to
   choose an authentication mode.  Unlike PSK-based authentication,

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   however, authentication with PAKE cannot be combined with the normal
   TLS ECDH mechanism.  Forward secrecy is provided by the PAKE itself.

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

   struct {
       PAKEShare client_shares<0..2^16-1>;
   } PAKEClientHello;

   A server that receives an "pake" extension examines the list of
   client shares to see if there is one with an identity the server
   recognizes.  If so, the server may indicate its choice of PAKE
   authentication by including an "pake" extension in its ServerHello.
   The content of this exension is a "PAKEServerHello" value, specifying
   the identity value for the password the server has selected, and the
   server's first message in the PAKE protocol.

   Use of PAKE authenication is compatible with standard certificate-
   based authentication of both clients and servers.  If a server
   includes an "pake" extension in its ServerHello, it may still send
   the Certificate and CertificateVerify messages, and/or send a
   CertificateRequest message to the client.

   If a server uses PAKE authentication, then it MUST NOT send an
   extension of type "key_share", "pre_shared_key", or "early_data".

   struct {
       PAKEShare server_share;
   } PAKEServerHello;

   Based on the messages exchanged in the ClientHello and ServerHello,
   the client and server execute the specified PAKE protocol to derive a
   shared key.  This key is used as the "ECHD(E)" input to the TLS 1.3
   key schedule.

   As with client authentication via certificates, the server has not
   authenticated the client until after it has received the client's
   Finished message.  When a server negotiates the use of this mechanism
   for authentication, it MUST NOT send application data before it has
   received the client's Finished message.

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5.  Compatible PAKE Protocols

   In order to be usable with the "pake" extension, a PAKE protocol must
   specify some syntax for its messages, and the protocol itself must be
   compatible with the message flow described above.  A specification
   describing the use of a particular PAKE protocol with TLS must
   provide the following details:

   o  Parameters that must be pre-provisioned

   o  Content of the "pake_message" field in a ClientHello

   o  Content of the "pake_message" field in a ServerHello

   o  How the PAKE protocol is executed based on those messages

   o  How the outputs are of the PAKE protocol are used to populate the
      "PSK" and "ECDH(E)" inputs to the TLS key schedule.

   The underlying cryptographic protocol must be compatible with the
   message flow described above:

   o  It must be possible to execute in one round-trip, with the client
      speaking first

   o  The Finished MAC must provide sufficient key confirmation for the
      protocol, taking into account the contents of the handshake

   In addition, to be compatible with the security requirements of TLS
   1.3, PAKE protocols defined for use with TLS 1.3 MUST provide forward

   Several current PAKE protocols satisfy these requirements, for

   o  SPAKE2+ (described below) [I-D.irtf-cfrg-spake2]

   o  SPEKE and derivatives such as Dragonfly [speke]

   o  OPAQUE [opaque]

   o  SRP [RFC2945]

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6.  SPAKE2+ Implementation

7.  Pre-provisioned Parameters

   In order to use SPAKE2+, a TLS client and server need to have pre-
   provisioned the values required to execute the SPAKE2+ protocol (see
   Section 3.1 of [I-D.irtf-cfrg-spake2]):

   o  A DH group of order "p*h", with "p" a large prime, and generator

   o  Fixed elements "M" and "N" for the group

   o  A hash function "H"

   o  A password "pw"

   Note that the hash function "H" might be different from the hash
   function associated with the ciphersuite negotiated by the two
   parties.  The hash function "H" MUST be a hash function suitable for
   hashing passwords, e.g., Argon2 or scrypt [I-D.irtf-cfrg-argon2]

   The TLS client and server roles map to the "A" and "B" roles in the
   SPAKE2+ specification, respectively.  The identity of the server is
   the domain name sent in the "server_name" extension of the
   ClientHello message.  The identity of the client is an opaque octet
   string, specified in the "spake2" ClientHello extension, defined

   From the shared password, each party computes two shared integers
   "w0" and "w1" by running the following algorithm twice (changing the
   "context" value each time):

   struct {
     uint16 context;
     opaque client\_identity<0..255>;
     opaque server\_name<0..255>;
     opaque password<0..255>;
   } PasswordInput;

   o  Encode the following values into a "PasswordInput" structure:

      *  "client_identity": The client's identity, as described above.

      *  "server_name": The server's identity, as described above.

      *  "password": The password "pw"

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      *  "context": One of the following values:

         +  0x7730, when generating "w0"

         +  0x7731, when generating "w1"

   o  Use the hash function "H" with the encoded "PasswordInput"
      structure as input to derive an "n"-byte string, where "n" is the
      byte-length of "p".

   o  Interpret the "n"-bit string as an integer "w" in network byte
      order.  Return the result "(w % p) * h" of reducing "w" mod p and
      multiplying it by "h".

   Servers MUST store only the value "w0" and the product "L = w1*G",
   where "G" is the fixed generator of the group.  Clients will need to
   have access to the values "w0" and "w1" directly, but SHOULD generate
   these values dynamically, rather than caching them.

8.  Content of the TLS Extensions

   The content of a "pake_message" in a ClientHello is the client's key
   share "T".  The value "T" is computed as specified in
   [I-D.irtf-cfrg-spake2], as "T = w*M + X", where "M" is a fixed value
   for the DH group and "X" is the public key of a fresh DH key pair.
   The format of the key share "T" is the same as for a
   "KeyShareEntry.key_exchange" value from the same group.

   The content of a "pake_message" in a ServerHello is the server's key
   share "S".  The value "S" is computed as specified in
   [I-D.irtf-cfrg-spake2], as "S = w*N + Y", where "N" is a fixed value
   for the DH group and "Y" is the public key of a fresh DH key pair.
   The format of the key share "S" is the same as for a
   "KeyShareEntry.key_exchange" value from the same group.

   Based on these messages, both the client and server can compute the
   two shared values as specified in [I-D.irtf-cfrg-spake2].

             | Name | Value  | Client        | Server       |
             | Z    | x*y*G  | x*(S - w0*N)  | x*(T - w0*M) |
             |      |        |               |              |
             | V    | w1*y*G | w1*(S - w0*N) | y*L          |

   The following value is used as the "(EC)DHE" input to the TLS 1.3 key

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   K = H(Z || V)

   Here "H" is the hash function corresponding to the TLS cipher suite
   in use and "||" represents concatenation of octet strings.

9.  Security Considerations

   Many of the security properties of this protocol will derive from the
   PAKE protocol being used.  Security considerations for PAKE protocols
   are noted in Section 5.

   The mechanism defined in this document does not provide protection
   for the client's identity, in contrast to TLS client authentication
   with certificates.

   TLS servers that offer this mechanism can be used by third party
   attackers as an oracle for two questions:

   1.  Whether the server knows about a given identity

   2.  Whether the server recognizes a given (identity, password) pair

   The former is signaled by whether the server returns a "pake"

   [[TODO: Similar to, the server could run through a complete handshake
   calculation and fail at the end so that the attacker only knows that
   the identity/password pair is incorrect, but does not know if the
   identity is recognized or not.  This requires that the server can
   interpret the pake_message and ascertain the associated PAKE
   algorithm, group parameters, etc., which requires a reworking of some
   text in this draft as the identity is currently defined as providing
   a map to said group parameters.  This is related to the discussion in
   the Open Items section.]]

   The latter is signaled by whether the connection succeeds.  These
   oracles are all-or-nothing: If the attacker does not have the correct
   identity or password, he does not learn anything about the correct

9.1.  Security when using SPAKE2+

   For the most part, the security properties of the password-based
   authentication described in this document are the same as those
   described in the Security Considerations of [I-D.irtf-cfrg-spake2].
   The TLS Finished MAC provides the key confirmation required for the
   security of the protocol.  Note that all of the elements covered by

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   the example confirmation hash listed in that document are also
   covered by the Finished MAC:

   o  "A", "B", and "T" are included via the ClientHello

   o  "S" via the ServerHello

   o  "K", and "w" via the TLS key schedule

   The "x" and "y" values used in the SPAKE2 protocol MUST have the same
   ephemerality properties as the key shares sent in the "key_shares"
   extension.  In particular, "x" and "y" MUST NOT be equal to zero.
   This ensures that TLS sessions using SPAKE2 have the same forward
   secrecy properties as sessions using the normal TLS (EC)DH mechanism.

10.  Open Items

10.1.  PAKE Algorithm Negotiation

   It is possible that a client may know the password to use, but may
   not know in advance which PAKE protocols(s) a particular server
   supports.  A potential solution to this is similar to TLS1.3
   ClientHello "key_share" operation: the client may send an empty
   "client_shares" vector in its PAKEClientHello extension.  The server
   can then send an HelloRetryRequest indicating which PAKE protocol,
   and associated group parameters, the client should use.  The client
   then sends another ClientHello that includes "pake_message" in the
   PAKEClientHello extension calculated using the correct algorithm.
   This requires definition of a suitable field for transporting PAKE
   algorithm and group parameters.

   As an optimaisation, similar to TLS1.3 key_share operation, the
   client could guess the PAKE protocol and include a "pake_message"
   derived from its guess in the initial ClientHello.  If the server
   does not support the selected PAKE protcol (or protocol group
   parameter, etc.), the server can send an HelloRetryRequest indicating
   the supported PAKE protocol and group parameters.  Note: it is TBD if
   sending two different "pake_messages" derived from two different
   protocol and/or group parameters in two different ClientHello
   messages constitutes a significant attack vector.  This needs
   cryptographic review.

11.  IANA Considerations

   This document requests that IANA add a value to the TLS ExtensionType
   Registry with the following contents:

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             | Value | Extension Name | TLS 1.3 | Reference |
             | TBD   | pake           |  CH, SH |  RFC XXXX |

   [[ RFC EDITOR: Please replace "TBD" in the above table with the value
   assigned by IANA, and replace "XXXX" with the RFC number assigned to
   this document. ]]

12.  References

12.1.  Normative References

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-26 (work in progress), March

              Ladd, W. and B. Kaduk, "SPAKE2, a PAKE", draft-irtf-cfrg-
              spake2-05 (work in progress), February 2018.

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

12.2.  Informative References

              Harkins, D., "Secure Password Ciphersuites for Transport
              Layer Security (TLS)", draft-harkins-tls-dragonfly-02
              (work in progress), August 2017.

              Biryukov, A., Dinu, D., Khovratovich, D., and S.
              Josefsson, "The memory-hard Argon2 password hash and
              proof-of-work function", draft-irtf-cfrg-argon2-03 (work
              in progress), August 2017.

   [opaque]   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,

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   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, DOI 10.17487/RFC5054, November
              2007, <>.

   [RFC7914]  Percival, C. and S. Josefsson, "The scrypt Password-Based
              Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
              August 2016, <>.

   [speke]    Jablon, D., "Extended Password Key Exchange Protocols
              Immune to Dictionary Attacks", 1997.

Authors' Addresses

   Richard Barnes


   Owen Friel


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