/* $Id: crypt_prot.x,v 1.4 2001/08/20 04:43:23 dm Exp $ */

/*
 * This file was written by David Mazieres.  Its contents is
 * uncopyrighted and in the public domain.  Of course, standards of
 * academic honesty nonetheless prevent anyone in research from
 * falsely claiming credit for this work.
 */

%#include "bigint.h"


/*
 * These structures define the raw byte formats of messages exchanged
 * by the SRP protocol, as published in:
 *
 * T. Wu, The Secure Remote Password Protocol, in Proceedings of the
 * 1998 Internet Society Network and Distributed System Security
 * Symposium, San Diego, CA, Mar 1998, pp. 97-111.
 *
 *   sessid is a session identifier known by the user and server to be fresh
 *
 *   N is a prime number such that (N-1)/2 is also prime
 *   g is a generator of Z_N^*
 *
 *   x is a function of the user's password and salt
 *   v is g^x mod N
 *
 *   a is a random element of Z_N^* selected by the user (client)
 *   A = g^a mod N
 *
 *   b and u are random elements of Z_N^* picked by the server
 *   B = v + g^b mod N
 *
 *   S = g^{ab} * g^{xub}
 *   M = SHA-1 (sessid, N, g, user, salt, A, B, S)
 *   H = SHA-1 (sessid, A, M, S)
 *
 * The protocol proceeds as follows:
 *
 *   User -> Server:  username
 *   Server -> User:  salt, N, g
 *   User -> Server:  A
 *   Server -> User:  B, u
 *   User -> Server:  M
 *   Server -> User:  H
 *
 * After this, K can be used to generate secret session keys for use
 * between the user and server.
 */

typedef opaque _srp_hash[20];

/* server to client */
struct srp_msg1 {
  string salt<>;
  bigint N;
  bigint g;
};

/* client to server */
struct srp_msg2 {
  bigint A;
};

/* server to client */
struct srp_msg3 {
  bigint B;
  bigint u;
};

/* hashed, then client to server */
struct srp_msg4_src {
  _srp_hash sessid;
  bigint N;
  bigint g;
  string user<>;
  string salt<>;
  bigint A;
  bigint B;
  bigint S;
};

/* hashed, then server to client */
struct srp_msg5_src {
  _srp_hash sessid;
  bigint A;
  _srp_hash M;
  bigint S;
};

#if 0
/* Info stored by server */
struct srp_info {
  bigint n;
  bigint g;
  string salt<>;
  bigint v;
};
#endif