SPAKE2, a Password-Authenticated Key Exchange
RFC 9382



Internet Research Task Force (IRTF)                              W. Ladd
Request for Comments: 9382                                        Akamai
Category: Informational                                   September 2023
ISSN: 2070-1721

             SPAKE2, a Password-Authenticated Key Exchange

Abstract

   This document describes SPAKE2, which is a protocol for two parties
   that share a password to derive a strong shared key without
   disclosing the password.  This method is compatible with any group,
   is computationally efficient, and has a security proof.  This
   document predated the Crypto Forum Research Group (CFRG) password-
   authenticated key exchange (PAKE) competition, and it was not
   selected; however, given existing use of variants in Kerberos and
   other applications, it was felt that publication was beneficial.
   Applications that need a symmetric PAKE, but are unable to hash onto
   an elliptic curve at execution time, can use SPAKE2.  This document
   is a product of the Crypto Forum Research Group in the Internet
   Research Task Force (IRTF).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the individual opinion(s) of one or
   more members of the Crypto Forum Research Group of the Internet
   Research Task Force (IRTF).  Documents approved for publication by
   the IRSG are not candidates for any level of Internet Standard; see
   Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9382.

Copyright Notice

   Copyright (c) 2023 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
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Table of Contents

   1.  Introduction
   2.  Requirements Notation
   3.  Definition of SPAKE2
     3.1.  Protocol Flow
     3.2.  Setup
     3.3.  SPAKE2
   4.  Key Schedule and Key Confirmation
   5.  Per-User M and N and M=N
   6.  Ciphersuites
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Algorithm Used for Point Generation
   Appendix B.  SPAKE2 Test Vectors
   Acknowledgements
   Contributors
   Author's Address

1.  Introduction

   This document describes SPAKE2, which is a means for two parties that
   share a password to derive a strong shared key without disclosing the
   password.  This password-based key exchange protocol is compatible
   with any group (requiring only a scheme to map a random input of a
   fixed length per group to a random group element), is computationally
   efficient, and has a security proof.  Predetermined parameters for a
   selection of commonly used groups are also provided for use by other
   protocols.

   SPAKE2 was not selected as the result of the CFRG PAKE selection
   competition.  However, given existing use of variants in Kerberos and
   other applications, it was felt that publication was beneficial.
   This RFC represents the individual opinion(s) of one or more members
   of the Crypto Forum Research Group of the IRTF.

   Many of these applications predated methods to hash to elliptic
   curves being available or predated the publication of the PAKEs that
   were chosen as an outcome of the PAKE selection competition.  In
   cases where a symmetric PAKE is needed and hashing onto an elliptic
   curve at protocol execution time is not available, SPAKE2 is useful.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

3.  Definition of SPAKE2

3.1.  Protocol Flow

   SPAKE2 is a two-round protocol, wherein the first round establishes a
   shared secret between A and B, and the second round serves as key
   confirmation.  Prior to invocation, A and B are provisioned with
   information, such as the input password needed to run the protocol.
   We assume that the roles of A and B are agreed upon by both sides: A
   goes first and uses M, and B goes second and uses N.  If this
   assignment of roles is not possible, a symmetric variant MUST be
   used, as described later Section 5.  For instance, A may be the
   client when using TCP or TLS as an underlying protocol, and B may be
   the server.  Most protocols have such a distinction.  During the
   first round, A sends a public value pA to B, and B responds with its
   own public value pB.  Both A and B then derive a shared secret used
   to produce encryption and authentication keys.  The latter are used
   during the second round for key confirmation.  (Section 4 details the
   key derivation and confirmation steps.)  In particular, A sends a key
   confirmation message cA to B, and B responds with its own key
   confirmation message cB.  A MUST NOT consider the protocol complete
   until it receives and verifies cB.  Likewise, B MUST NOT consider the
   protocol complete until it receives and verifies cA.

   This sample flow is shown below.

                   A                       B
                   |                       |
                   |                       |
     (compute pA)  |          pA           |
                   |---------------------->|
                   |          pB           | (compute pB)
                   |<----------------------|
                   |                       |
                   |   (derive secrets)    |
                   |                       |
     (compute cA)  |          cA           |
                   |---------------------->|
                   |          cB           | (compute cB)
                   |                       | (check cA)
                   |<----------------------|
     (check cB)    |                       |

3.2.  Setup

   Let G be a group in which the gap Diffie-Hellman (GDH) problem is
   hard.  Suppose G has order p*h, where p is a large prime and h will
   be called the cofactor.  Let I be the unit element in G, e.g., the
   point at infinity if G is an elliptic curve group.  We denote the
   operations in the group additively.  We assume there is a
   representation of elements of G as byte strings: common choices would
   be SEC1 [SEC1] uncompressed or compressed for elliptic curve groups
   or big-endian integers of a fixed (per-group) length for prime field
   DH.  Applications MUST specify this encoding, typically by referring
   to the document defining the group.  We fix two elements, M and N, in
   the prime-order subgroup of G, as defined in Table 1 of this document
   for common groups, as well as generator P of the (large) prime-order
   subgroup of G.  In the case of a composite order group, we will work
   in the quotient group.  For common groups used in this document, P is
   specified in the document defining the group, so we do not repeat it
   here.

   For elliptic curves other than the ones in this document, the methods
   described in [RFC9380] SHOULD be used to generate M and N, e.g., via
   M = hash_to_curve("M SPAKE2 seed OID x") and N = hash_to_curve("N
   SPAKE2 seed OID x"), where x is an OID for the curve.  Applications
   MAY include a domain separation tag (DST) in this step, as specified
   in [RFC9380], though this is not required.

   || denotes concatenation of byte strings.  We also let len(S) denote
   the length of a string in bytes, represented as an eight-byte little-
   endian number.  Finally, let nil represent an empty string, i.e.,
   len(nil) = 0.  Text strings in double quotes are treated as their
   ASCII encodings throughout this document.

   KDF(ikm, salt, info, L) is a key-derivation function that takes as
   input a salt, input keying material (IKM), an info string, and
   derived key length L to derive a cryptographic key of length L.
   MAC(key, message) is a Message Authentication Code algorithm that
   takes a secret key and message as input to produce an output.  Let
   Hash be a hash function from arbitrary strings to bit strings of a
   fixed length that is at least 256 bits long.  Common choices for Hash
   are SHA-256 or SHA-512 [RFC6234].  Let MHF be a memory-hard hash
   function designed to slow down brute-force attackers.  Scrypt
   [RFC7914] is a common example of this function.  The output length of
   MHF matches that of Hash.  Parameter selection for MHF is out of
   scope for this document.  Section 6 specifies variants of KDF, MAC,
   and Hash that are suitable for use with the protocols contained
   herein.

   Let A and B be two parties.  A and B may also have digital
   representations of the parties' identities, such as Media Access
   Control addresses or other names (hostnames, usernames, etc.).  A and
   B may share additional authenticated data (AAD) of a length that is
   at most 2^16 - 128 bits and separate from their identities, which
   they may want to include in the protocol execution.  One example of
   AAD is a list of supported protocol versions if SPAKE2 were used in a
   higher-level protocol that negotiates use of a particular PAKE.
   Including this list would ensure that both parties agree upon the
   same set of supported protocols and therefore prevents downgrade
   attacks.  We also assume A and B share integer w; typically, w =
   MHF(pw) mod p for a user-supplied password, pw.  Standards, such as
   [NIST.SP.800-56Ar3], suggest taking mod p of a hash value that is 64
   bits longer than that needed to represent p to remove statistical
   bias introduced by the modulation.  Protocols using this
   specification MUST define the method used to compute w.  In some
   cases, it may be necessary to carry out various forms of
   normalization of the password before hashing [RFC8265].  The hashing
   algorithm SHOULD be an MHF so as to slow down brute-force attackers.

3.3.  SPAKE2

   To begin, A picks x randomly and uniformly from the integers in [0,p)
   and calculates X=x*P and pA=w*M+X.  Then, it transmits pA to B.

   B selects y randomly and uniformly from the integers in [0,p) and
   calculates Y=y*P and pB=w*N+Y.  Then, it transmits pB to A.

   Both A and B calculate group element K.  A calculates it as h*x*(pB-
   w*N), while B calculates it as h*y*(pA-w*M).  A knows pB because it
   has received it, and likewise B knows pA.  The multiplication by h
   prevents small subgroup confinement attacks by computing a unique
   value in the quotient group.

   K is a shared value, though it MUST NOT be used or output as a shared
   secret from the protocol.  Both A and B must derive two additional
   shared secrets from the protocol transcript, which includes K.  This
   use of the transcript ensures any manipulation of the messages sent
   is reflected in the keys.  The transcript TT is encoded as follows:

           TT = len(A)  || A
             || len(B)  || B
             || len(pA) || pA
             || len(pB) || pB
             || len(K)  || K
             || len(w)  || w

   Here, w is encoded as a big-endian number padded to the length of p.
   This representation prevents timing attacks that otherwise would
   reveal the length of w.  len(w) is thus a constant for a given group.
   We include it for consistency.

   If an identity is absent, it is encoded as a zero-length string.
   This MUST only be done for applications in which identities are
   implicit.  Otherwise, the protocol risks unknown key-share attacks,
   where both sides of a connection disagree over who is authenticated.

   Upon completion of this protocol, A and B compute shared secrets Ke,
   KcA, and KcB, as specified in Section 4.  A MUST send B a key
   confirmation message so that both parties agree upon these shared
   secrets.  The confirmation message cA is computed as a MAC over the
   protocol transcript TT, using KcA as follows: cA = MAC(KcA, TT).
   Similarly, B MUST send A a confirmation message using a MAC that is
   computed equivalently, except with the use of KcB.  Key confirmation
   verification requires computing cA (or cB, respectively) and checking
   for equality against that which was received.

4.  Key Schedule and Key Confirmation

   The protocol transcript TT, as defined in Section 3.3, is unique and
   secret to A and B (though it contains some substrings that are not
   secret).  Both parties use TT to derive shared symmetric secrets Ke
   and Ka as Ke || Ka = Hash(TT), with |Ke| = |Ka|.  The length of each
   key is equal to half of the digest output, e.g., 128 bits for SHA-
   256.  Keys MUST be at least 128 bits in length.

   Both endpoints use Ka to derive subsequent MAC keys for key
   confirmation messages.  Specifically, KcA and KcB are the MAC keys
   used by A and B, respectively.  A and B compute them as KcA || KcB =
   KDF(Ka, nil, "ConfirmationKeys" || AAD, L), where AAD is the
   associated data given to each endpoint or AAD is nil if none was
   provided.  The length of each of KcA and KcB is equal to half of the
   underlying hash output length, e.g., |KcA| = |KcB| = 128 bits for
   HKDF(SHA256), with L=256 bits.

   The resulting key schedule for this protocol, given transcript TT and
   AAD, is as follows.

       TT  -> Hash(TT) = Ke || Ka
       AAD -> KDF(Ka, nil, "ConfirmationKeys" || AAD) = KcA || KcB

   A and B output Ke as the shared secret from the protocol.  Ka and its
   derived keys are not used for anything except key confirmation.

5.  Per-User M and N and M=N

   To avoid concerns that an attacker needs to solve a single Elliptic
   Curve Diffie-Hellman (ECDH) instance to break the authentication of
   SPAKE2, it is possible to vary M and N using [RFC9380] as follows:

       M = hash_to_curve(Hash("M SPAKE2" || len(A) || A || len(B) || B))
       N = hash_to_curve(Hash("N SPAKE2" || len(A) || A || len(B) || B))

   There is also a symmetric variant where M=N.  For this variant, we
   set:

       M = hash_to_curve(Hash("M AND N SPAKE2"))

   This variant MUST be used when it is not possible to determine
   whether A or B should use M (or N), due to asymmetries in the
   protocol flows or the desire to use only a single shared secret with
   nil identities for authentication.  The security of these variants is
   examined in [MNVAR].  The variant with per-user M and N may not be
   suitable for protocols that require the initial messages to be
   generated by each party at the same time and that do not know the
   exact identity of the parties before the flow begins.

6.  Ciphersuites

   This section documents SPAKE2 ciphersuite configurations.  A
   ciphersuite indicates a group, cryptographic hash function, and pair
   of KDF and MAC functions, e.g., SPAKE2-P256-SHA256-HKDF-HMAC.  This
   ciphersuite indicates a SPAKE2 protocol instance over P-256 that uses
   SHA-256, along with HMAC-based Key Derivation Function (HKDF)
   [RFC5869] and Hashed Message Authentication Code (HMAC) [RFC2104] for
   G, Hash, KDF, and MAC functions, respectively.  For Ed25519, the
   compressed encoding is used [RFC8032]; all others use the
   uncompressed SEC1 encoding.

   +==============+==================+================+================+
   |      G       |       Hash       |      KDF       |      MAC       |
   +==============+==================+================+================+
   |    P-256     | SHA256 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |              |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |    P-256     | SHA512 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |              |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |    P-384     | SHA256 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |              |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |    P-384     | SHA512 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |              |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |    P-521     | SHA512 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |              |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   | edwards25519 | SHA256 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |  [RFC8032]   |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |  edwards448  | SHA512 [RFC6234] | HKDF [RFC5869] |      HMAC      |
   |  [RFC8032]   |                  |                |   [RFC2104]    |
   +--------------+------------------+----------------+----------------+
   |    P-256     | SHA256 [RFC6234] | HKDF [RFC5869] |  CMAC-AES-128  |
   |              |                  |                |   [RFC4493]    |
   +--------------+------------------+----------------+----------------+
   |    P-256     | SHA512 [RFC6234] | HKDF [RFC5869] |  CMAC-AES-128  |
   |              |                  |                |   [RFC4493]    |
   +--------------+------------------+----------------+----------------+

                        Table 1: SPAKE2 Ciphersuites

   The following points represent permissible point generation seeds for
   the groups listed in Table 1, using the algorithm presented in
   Appendix A.  These byte strings are compressed points, as in [SEC1],
   for curves from [SEC1].

   For P-256:

   M =
   02886e2f97ace46e55ba9dd7242579f2993b64e16ef3dcab95afd497333d8fa12f
   seed: 1.2.840.10045.3.1.7 point generation seed (M)

   N =
   03d8bbd6c639c62937b04d997f38c3770719c629d7014d49a24b4f98baa1292b49
   seed: 1.2.840.10045.3.1.7 point generation seed (N)

   For P-384:

   M =
   030ff0895ae5ebf6187080a82d82b42e2765e3b2f8749c7e05eba366434b363d3dc
   36f15314739074d2eb8613fceec2853
   seed: 1.3.132.0.34 point generation seed (M)

   N =
   02c72cf2e390853a1c1c4ad816a62fd15824f56078918f43f922ca21518f9c543bb
   252c5490214cf9aa3f0baab4b665c10
   seed: 1.3.132.0.34 point generation seed (N)

   For P-521:

   M =
   02003f06f38131b2ba2600791e82488e8d20ab889af753a41806c5db18d37d85608
   cfae06b82e4a72cd744c719193562a653ea1f119eef9356907edc9b56979962d7aa
   seed: 1.3.132.0.35 point generation seed (M)

   N =
   0200c7924b9ec017f3094562894336a53c50167ba8c5963876880542bc669e494b25
   32d76c5b53dfb349fdf69154b9e0048c58a42e8ed04cef052a3bc349d95575cd25
   seed: 1.3.132.0.35 point generation seed (N)

   For edwards25519:

   M =
   d048032c6ea0b6d697ddc2e86bda85a33adac920f1bf18e1b0c6d166a5cecdaf
   seed: edwards25519 point generation seed (M)

   N =
   d3bfb518f44f3430f29d0c92af503865a1ed3281dc69b35dd868ba85f886c4ab
   seed: edwards25519 point generation seed (N)

   For edwards448:

   M =
   b6221038a775ecd007a4e4dde39fd76ae91d3cf0cc92be8f0c2fa6d6b66f9a12
   942f5a92646109152292464f3e63d354701c7848d9fc3b8880
   seed: edwards448 point generation seed (M)

   N =
   6034c65b66e4cd7a49b0edec3e3c9ccc4588afd8cf324e29f0a84a072531c4db
   f97ff9af195ed714a689251f08f8e06e2d1f24a0ffc0146600
   seed: edwards448 point generation seed (N)

7.  Security Considerations

   A security proof of SPAKE2 for prime order groups is found in [REF],
   reducing the security of SPAKE2 to the GDH assumption.  Note that the
   choice of M and N is critical for the security proof.  The generation
   methods specified in this document are designed to eliminate concerns
   related to knowing discrete logs of M and N.

   Elements received from a peer MUST be checked for group membership.
   Failure to properly deserialize and validate group elements can lead
   to attacks.  An endpoint MUST abort the protocol if any received
   public value is not a member of G.

   The choices of random numbers MUST be uniform.  Randomly generated
   values, e.g., x and y, MUST NOT be reused; such reuse violates the
   security assumptions of the protocol and results in significant
   insecurity.  It is RECOMMENDED to generate these uniform numbers
   using rejection sampling.

   Some implementations of elliptic curve multiplication may leak
   information about the length of the scalar.  These MUST NOT be used.
   All operations on elliptic curve points must take time independent of
   the inputs.  Hashing of the transcript may take time depending only
   on the length of the transcript but not the contents.

   SPAKE2 does not support augmentation.  As a result, the server has to
   store a password equivalent.  This is considered a significant
   drawback in some use cases.  Applications that need augmented PAKEs
   should use [CFRG-OPAQUE].

   The HMAC keys in this document are shorter than recommended in
   [RFC8032].  This is appropriate, as the difficulty of the discrete
   logarithm problem is comparable with the difficulty of brute forcing
   the keys.

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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>.

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
              2006, <https://www.rfc-editor.org/info/rfc4493>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/info/rfc5480>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7914]  Percival, C. and S. Josefsson, "The scrypt Password-Based
              Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
              August 2016, <https://www.rfc-editor.org/info/rfc7914>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9380]  Faz-Hernandez, A., Scott, S., Sullivan, N., Wahby, R. S.,
              and C. A. Wood, "Hashing to Elliptic Curves", RFC 9380,
              DOI 10.17487/RFC9380, August 2023,
              <https://www.rfc-editor.org/info/rfc9380>.

9.2.  Informative References

   [CFRG-OPAQUE]
              Bourdrez, D., Krawczyk, H., Lewi, K., and C. A. Wood, "The
              OPAQUE Asymmetric PAKE Protocol", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-opaque-11, 8 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              opaque-11>.

   [MNVAR]    Abdalla, M., Barbosa, M., Bradley, T., Jarecki, S., Katz,
              J., and J. Xu, "Universally Composable Relaxed Password
              Authenticated Key Exchange", in Advances in Cryptology -
              CRYPTO 2020, Lecture Notes in Computer Science, Volume
              12170, Springer, DOI 10.1007/978-3-030-56784-2_10, August
              2020, <https://doi.org/10.1007/978-3-030-56784-2_10>.

   [NIST.SP.800-56Ar3]
              National Institute of Standards and Technology,
              "Recommendation for Pair-Wise Key-Establishment Schemes
              Using Discrete Logarithm Cryptography", Revision 3, NIST
              Special Publication 800-56A,
              DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Ar3>.

   [REF]      Abdalla, M. and D. Pointcheval, "Simple Password-Based
              Encrypted Key Exchange Protocols", Cryptography-CT-RSA
              2005, Lecture Notes in Computer Science, Volume 3376,
              pages 191-208, Springer, DOI 10.1007/978-3-540-30574-3_14,
              February 2005,
              <https://doi.org/10.1007/978-3-540-30574-3_14>.

   [RFC8265]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 8265,
              DOI 10.17487/RFC8265, October 2017,
              <https://www.rfc-editor.org/info/rfc8265>.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", May 2009.

Appendix A.  Algorithm Used for Point Generation

   This section describes the algorithm that was used to generate points
   M and N in Table 1.

   For each curve in Table 1, we construct a string using the curve OID
   from [RFC5480] (as an ASCII string) or its name, combined with the
   needed constant, e.g., "1.3.132.0.35 point generation seed (M)" for
   P-521.  This string is turned into a series of blocks by hashing with
   SHA-256 and hashing that output again to generate the next 32 bytes
   and so on.  This pattern is repeated for each group and value, with
   the string modified appropriately.

   A byte string of a length equal to that of an encoded group element
   is constructed by concatenating as many blocks as are required,
   starting from the first block and truncating to the desired length.
   The byte string is then formatted as required for the group.  In the
   case of Weierstrass curves, we take the desired length as the length
   for representing a compressed point (Section 2.3.4 of [SEC1]) and use
   the low-order bit of the first byte as the sign bit.  In order to
   obtain the correct format, the value of the first byte is set to 0x02
   or 0x03 (clearing the first six bits and setting the seventh bit),
   leaving the sign bit as it was in the byte string constructed by
   concatenating hash blocks.  For the curves in [RFC8032], a different
   procedure is used.  For edwards448, the 57-byte input has the least-
   significant 7 bits of the last byte set to zero, and for
   edwards25519, the 32-byte input is not modified.  For both the curves
   in [RFC8032], the (modified) input is then interpreted as the
   representation of the group element.  If this interpretation yields a
   valid group element with the correct order (p), the (modified) byte
   string is the output.  Otherwise, the initial hash block is discarded
   and a new byte string is constructed from the remaining hash blocks.
   The procedure of constructing a byte string of the appropriate
   length, formatting it as required for the curve, and checking if it
   is a valid point of the correct order is repeated until a valid
   element is found.

   The following Python snippet generates the above points, assuming an
   elliptic curve implementation follows the interface of
   Edwards25519Point.stdbase() and Edwards448Point.stdbase() in
   Appendix A of [RFC8032]:

   def iterated_hash(seed, n):
       h = seed
       for i in range(n):
           h = hashlib.sha256(h).digest()
       return h

   def bighash(seed, start, sz):
       n = -(-sz // 32)
       hashes = [iterated_hash(seed, i)
                 for i in range(start, start + n)]
       return b''.join(hashes)[:sz]

   def canon_pointstr(ecname, s):
       if ecname == 'edwards25519':
           return s
       elif ecname == 'edwards448':
           return s[:-1] + bytes([s[-1] & 0x80])
       else:
           return bytes([(s[0] & 1) | 2]) + s[1:]

   def gen_point(seed, ecname, ec):
       for i in range(1, 1000):
           hval = bighash(seed, i, len(ec.encode()))
           pointstr = canon_pointstr(ecname, hval)
           try:
               p = ec.decode(pointstr)
               if p != ec.zero_elem() and p * p.l() == ec.zero_elem():
                   return pointstr, i
           except Exception:
               pass

Appendix B.  SPAKE2 Test Vectors

   This section contains test vectors for SPAKE2, using the P256-SHA256-
   HKDF-HMAC ciphersuite.  (The choice of MHF is omitted, and the values
   for w, x, and y are provided directly.)  All points are encoded using
   the uncompressed format, i.e., with a 0x04 octet prefix, specified in
   [SEC1].  A and B identity strings are provided in the protocol
   invocation.

   Line breaks have been added due to line-length limitations.

 spake2: A='server', B='client'
 w = 0x2ee57912099d31560b3a44b1184b9b4866e904c49d12ac5042c97dca461b1a5f
 x = 0x43dd0fd7215bdcb482879fca3220c6a968e66d70b1356cac18bb26c84a78d729
 pA = 0x04a56fa807caaa53a4d28dbb9853b9815c61a411118a6fe516a8798434751470
 f9010153ac33d0d5f2047ffdb1a3e42c9b4e6be662766e1eeb4116988ede5f912c
 y = 0xdcb60106f276b02606d8ef0a328c02e4b629f84f89786af5befb0bc75b6e66be
 pB = 0x0406557e482bd03097ad0cbaa5df82115460d951e3451962f1eaf4367a420676
 d09857ccbc522686c83d1852abfa8ed6e4a1155cf8f1543ceca528afb591a1e0b7
 K = 0x0412af7e89717850671913e6b469ace67bd90a4df8ce45c2af19010175e37eed
 69f75897996d539356e2fa6a406d528501f907e04d97515fbe83db277b715d3325
 TT = 0x06000000000000007365727665720600000000000000636c69656e744100000
 00000000004a56fa807caaa53a4d28dbb9853b9815c61a411118a6fe516a8798434751
 470f9010153ac33d0d5f2047ffdb1a3e42c9b4e6be662766e1eeb4116988ede5f912c4
 1000000000000000406557e482bd03097ad0cbaa5df82115460d951e3451962f1eaf43
 67a420676d09857ccbc522686c83d1852abfa8ed6e4a1155cf8f1543ceca528afb591a
 1e0b741000000000000000412af7e89717850671913e6b469ace67bd90a4df8ce45c2a
 f19010175e37eed69f75897996d539356e2fa6a406d528501f907e04d97515fbe83db2
 77b715d332520000000000000002ee57912099d31560b3a44b1184b9b4866e904c49d1
 2ac5042c97dca461b1a5f
 HASH(TT) = 0x0e0672dc86f8e45565d338b0540abe6915bdf72e2b35b5c9e5663168e9
 60a91b
 Ke = 0x0e0672dc86f8e45565d338b0540abe69
 Ka = 0x15bdf72e2b35b5c9e5663168e960a91b
 KcA = 0x00c12546835755c86d8c0db7851ae86f
 KcB = 0xa9fa3406c3b781b93d804485430ca27a
 A conf = 0x58ad4aa88e0b60d5061eb6b5dd93e80d9c4f00d127c65b3b35b1b5281f
 ee38f0
 B conf = 0xd3e2e547f1ae04f2dbdbf0fc4b79f8ecff2dff314b5d32fe9fcef2fb26
 dc459b

 spake2: A='', B='client'
 w = 0x0548d8729f730589e579b0475a582c1608138ddf7054b73b5381c7e883e2efae
 x = 0x403abbe3b1b4b9ba17e3032849759d723939a27a27b9d921c500edde18ed654b
 pA = 0x04a897b769e681c62ac1c2357319a3d363f610839c4477720d24cbe32f5fd8
 5f44fb92ba966578c1b712be6962498834078262caa5b441ecfa9d4a9485720e918a
 y = 0x903023b6598908936ea7c929bd761af6039577a9c3f9581064187c3049d87065
 pB = 0x04e0f816fd1c35e22065d5556215c097e799390d16661c386e0ecc84593974
 a61b881a8c82327687d0501862970c64565560cb5671f696048050ca66ca5f8cc7fc
 K = 0x048f83ec9f6e4f87cc6f9dc740bdc2769725f923364f01c84148c049a39a735e
 bda82eac03e00112fd6a5710682767cff5361f7e819e53d8d3c3a2922e0d837aa6
 TT = 0x00000000000000000600000000000000636c69656e74410000000000000004a
 897b769e681c62ac1c2357319a3d363f610839c4477720d24cbe32f5fd85f44fb92ba9
 66578c1b712be6962498834078262caa5b441ecfa9d4a9485720e918a4100000000000
 00004e0f816fd1c35e22065d5556215c097e799390d16661c386e0ecc84593974a61b8
 81a8c82327687d0501862970c64565560cb5671f696048050ca66ca5f8cc7fc4100000
 000000000048f83ec9f6e4f87cc6f9dc740bdc2769725f923364f01c84148c049a39a7
 35ebda82eac03e00112fd6a5710682767cff5361f7e819e53d8d3c3a2922e0d837aa62
 0000000000000000548d8729f730589e579b0475a582c1608138ddf7054b73b5381c7e
 883e2efae
 Hash(TT) = 0x642f05c473c2cd79909f9a841e2f30a70bf89b18180af97353ba198789
 c2b963
 Ke = 0x642f05c473c2cd79909f9a841e2f30a7
 Ka = 0x0bf89b18180af97353ba198789c2b963
 KcA = 0xc6be376fc7cd1301fd0a13adf3e7bffd
 KcB = 0xb7243f4ae60440a49b3f8cab3c1fba07
 A conf = 0x47d29e6666af1b7dd450d571233085d7a9866e4d49d2645e2df9754895
 21232b
 B conf = 0x3313c5cefc361d27fb16847a91c2a73b766ffa90a4839122a9b70a2f6b
 d1d6df

 spake2: A='server', B=''
 w = 0x626e0cdc7b14c9db3e52a0b1b3a768c98e37852d5db30febe0497b14eae8c254
 x = 0x07adb3db6bc623d3399726bfdbfd3d15a58ea776ab8a308b00392621291f9633
 pA = 0x04f88fb71c99bfffaea370966b7eb99cd4be0ff1a7d335caac4211c4afd855e2
 e15a873b298503ad8ba1d9cbb9a392d2ba309b48bfd7879aefd0f2cea6009763b0
 y = 0xb6a4fc8dbb629d4ba51d6f91ed1532cf87adec98f25dd153a75accafafedec16
 pB = 0x040c269d6be017dccb15182ac6bfcd9e2a14de019dd587eaf4bdfd353f031101
 e7cca177f8eb362a6e83e7d5e729c0732e1b528879c086f39ba0f31a9661bd34db
 K = 0x0445ee233b8ecb51ebd6e7da3f307e88a1616bae2166121221fdc0dadb986afa
 f3ec8a988dc9c626fa3b99f58a7ca7c9b844bb3e8dd9554aafc5b53813504c1cbe
 TT = 0x06000000000000007365727665720000000000000000410000000000000004f
 88fb71c99bfffaea370966b7eb99cd4be0ff1a7d335caac4211c4afd855e2e15a873b2
 98503ad8ba1d9cbb9a392d2ba309b48bfd7879aefd0f2cea6009763b04100000000000
 000040c269d6be017dccb15182ac6bfcd9e2a14de019dd587eaf4bdfd353f031101e7c
 ca177f8eb362a6e83e7d5e729c0732e1b528879c086f39ba0f31a9661bd34db4100000
 0000000000445ee233b8ecb51ebd6e7da3f307e88a1616bae2166121221fdc0dadb986
 afaf3ec8a988dc9c626fa3b99f58a7ca7c9b844bb3e8dd9554aafc5b53813504c1cbe2
 000000000000000626e0cdc7b14c9db3e52a0b1b3a768c98e37852d5db30febe0497b1
 4eae8c254
 Hash(TT) = 0x005184ff460da2ce59062c87733c299c3521297d736598fc0a1127600e
 fa1afb
 Ke = 0x005184ff460da2ce59062c87733c299c
 Ka = 0x3521297d736598fc0a1127600efa1afb
 KcA = 0xf3da53604f0aeecea5a33be7bddf6edf
 KcB = 0x9e3f86848736f159bd92b6e107ec6799
 A conf = 0xbc9f9bbe99f26d0b2260e6456e05a86196a3307ec6663a18bf6ac8257365
 33b2
 B conf = 0xc2370e1bf813b086dff0d834e74425a06e6390f48f5411900276dcccc5a2
 97ec

 spake2: A='', B=''
 w = 0x7bf46c454b4c1b25799527d896508afd5fc62ef4ec59db1efb49113063d70cca
 x = 0x8cef65df64bb2d0f83540c53632de911b5b24b3eab6cc74a97609fd659e95473
 pA = 0x04a65b367a3f613cf9f0654b1b28a1e3a8a40387956c8ba6063e8658563890f4
 6ca1ef6a676598889fc28de2950ab8120b79a5ef1ea4c9f44bc98f585634b46d66
 y = 0xd7a66f64074a84652d8d623a92e20c9675c61cb5b4f6a0063e4648a2fdc02d53
 pB = 0x04589f13218822710d98d8b2123a079041052d9941b9cf88c6617ddb2fcc0494
 662eea8ba6b64692dc318250030c6af045cb738bc81ba35b043c3dcb46adf6f58d
 K = 0x041a3c03d51b452537ca2a1fea6110353c6d5ed483c4f0f86f4492ca3f378d40
 a994b4477f93c64d928edbbcd3e85a7c709b7ea73ee97986ce3d1438e135543772
 TT = 0x00000000000000000000000000000000410000000000000004a65b367a3f613
 cf9f0654b1b28a1e3a8a40387956c8ba6063e8658563890f46ca1ef6a676598889fc28
 de2950ab8120b79a5ef1ea4c9f44bc98f585634b46d66410000000000000004589f132
 18822710d98d8b2123a079041052d9941b9cf88c6617ddb2fcc0494662eea8ba6b6469
 2dc318250030c6af045cb738bc81ba35b043c3dcb46adf6f58d4100000000000000041
 a3c03d51b452537ca2a1fea6110353c6d5ed483c4f0f86f4492ca3f378d40a994b4477
 f93c64d928edbbcd3e85a7c709b7ea73ee97986ce3d1438e1355437722000000000000
 0007bf46c454b4c1b25799527d896508afd5fc62ef4ec59db1efb49113063d70cca
 Hash(TT) = 0xfc6374762ba5cf11f4b2caa08b2cd1b9907ae0e26e8d6234318d91583c
 d74c86
 Ke = 0xfc6374762ba5cf11f4b2caa08b2cd1b9
 Ka = 0x907ae0e26e8d6234318d91583cd74c86
 KcA = 0x5dbd2f477166b7fb6d61febbd77a5563
 KcB = 0x7689b4654407a5faeffdc8f18359d8a3
 A conf = 0xdfb4db8d48ae5a675963ea5e6c19d98d4ea028d8e898dad96ea19a80ade9
 5dca
 B conf = 0xd0f0609d1613138d354f7e95f19fb556bf52d751947241e8c7118df5ef0a
 e175

Acknowledgements

   Special thanks to Nathaniel McCallum and Greg Hudson for generating M
   and N and Chris Wood for generating test vectors.  Thanks to Mike
   Hamburg for advice on how to deal with cofactors.  Greg Hudson also
   suggested the addition of warnings on the reuse of x and y.  Thanks
   to Fedor Brunner, Adam Langley, Liliya Akhmetzyanova, and the members
   of the CFRG for comments and advice.  Thanks to Scott Fluhrer and
   those Crypto Panel experts involved in the PAKE selection process
   (https://github.com/cfrg/pake-selection) who have provided valuable
   comments.  Chris Wood contributed substantial text and reformatting
   to address the excellent review comments from Kenny Paterson.

Contributors

   Benjamin Kaduk
   Akamai Technologies
   Email: kaduk@mit.edu

Author's Address

   Watson Ladd
   Akamai Technologies
   Email: watsonbladd@gmail.com