srp.c

/*
 * Secure Remote Password 6a implementation based on mbedtls.
 *
 * Copyright (c) 2015 Dieter Wimberger
 * https://github.com/dwimberger/mbedtls-csrp
 * 
 * Derived from:
 * Copyright (c) 2010 Tom Cocagne. All rights reserved.
 * https://github.com/cocagne/csrp
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2015 Tom Cocagne, Dieter Wimberger
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is furnished to do
 * so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 */

#include <time.h>

#include <stdlib.h>
#include <string.h>

#include "mbedtls/bignum.h"
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/sha1.h"
#include "mbedtls/sha256.h"
#include "mbedtls/sha512.h"


#include "srp.h"

static int g_initialized = 0;
static mbedtls_entropy_context entropy_ctx;
static mbedtls_ctr_drbg_context ctr_drbg_ctx;
static mbedtls_mpi * RR;

typedef struct
{
    BIGNUM     *N;
    BIGNUM     *g;
} NGConstant;

struct NGHex
{
    const char * n_hex;
    const char * g_hex;
};

/* All constants here were pulled from Appendix A of RFC 5054 */
static struct NGHex global_Ng_constants[] = {
 { /* 512 */
  "D66AAFE8E245F9AC245A199F62CE61AB8FA90A4D80C71CD2ADFD0B9DA163B29F2A34AFBDB3B"
  "1B5D0102559CE63D8B6E86B0AA59C14E79D4AA62D1748E4249DF3",
  "2"
 },
 { /* 768 */
   "B344C7C4F8C495031BB4E04FF8F84EE95008163940B9558276744D91F7CC9F402653BE7147F"
   "00F576B93754BCDDF71B636F2099E6FFF90E79575F3D0DE694AFF737D9BE9713CEF8D837ADA"
   "6380B1093E94B6A529A8C6C2BE33E0867C60C3262B",
   "2"
 },
 { /* 1024 */
   "EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256576D674DF7496"
   "EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D6089DAD15DC7D7B46154D6B6CE8E"
   "F4AD69B15D4982559B297BCF1885C529F566660E57EC68EDBC3C05726CC02FD4CBF4976EAA"
   "9AFD5138FE8376435B9FC61D2FC0EB06E3",
   "2"
 },
 { /* 2048 */
   "AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050A37329CBB4"
   "A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50E8083969EDB767B0CF60"
   "95179A163AB3661A05FBD5FAAAE82918A9962F0B93B855F97993EC975EEAA80D740ADBF4FF"
   "747359D041D5C33EA71D281E446B14773BCA97B43A23FB801676BD207A436C6481F1D2B907"
   "8717461A5B9D32E688F87748544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB37861"
   "60279004E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DB"
   "FBB694B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73",
   "2"
 },
 { /* 4096 */
   "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
   "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
   "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
   "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
   "49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8"
   "FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D"
   "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C"
   "180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718"
   "3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D"
   "04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D"
   "B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226"
   "1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
   "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC"
   "E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26"
   "99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB"
   "04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2"
   "233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127"
   "D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199"
   "FFFFFFFFFFFFFFFF",
   "5"
 },
 { /* 8192 */
   "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
   "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
   "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
   "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
   "49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8"
   "FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D"
   "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C"
   "180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718"
   "3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D"
   "04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D"
   "B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226"
   "1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
   "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC"
   "E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26"
   "99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB"
   "04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2"
   "233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127"
   "D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934028492"
   "36C3FAB4D27C7026C1D4DCB2602646DEC9751E763DBA37BDF8FF9406"
   "AD9E530EE5DB382F413001AEB06A53ED9027D831179727B0865A8918"
   "DA3EDBEBCF9B14ED44CE6CBACED4BB1BDB7F1447E6CC254B33205151"
   "2BD7AF426FB8F401378CD2BF5983CA01C64B92ECF032EA15D1721D03"
   "F482D7CE6E74FEF6D55E702F46980C82B5A84031900B1C9E59E7C97F"
   "BEC7E8F323A97A7E36CC88BE0F1D45B7FF585AC54BD407B22B4154AA"
   "CC8F6D7EBF48E1D814CC5ED20F8037E0A79715EEF29BE32806A1D58B"
   "B7C5DA76F550AA3D8A1FBFF0EB19CCB1A313D55CDA56C9EC2EF29632"
   "387FE8D76E3C0468043E8F663F4860EE12BF2D5B0B7474D6E694F91E"
   "6DBE115974A3926F12FEE5E438777CB6A932DF8CD8BEC4D073B931BA"
   "3BC832B68D9DD300741FA7BF8AFC47ED2576F6936BA424663AAB639C"
   "5AE4F5683423B4742BF1C978238F16CBE39D652DE3FDB8BEFC848AD9"
   "22222E04A4037C0713EB57A81A23F0C73473FC646CEA306B4BCBC886"
   "2F8385DDFA9D4B7FA2C087E879683303ED5BDD3A062B3CF5B3A278A6"
   "6D2A13F83F44F82DDF310EE074AB6A364597E899A0255DC164F31CC5"
   "0846851DF9AB48195DED7EA1B1D510BD7EE74D73FAF36BC31ECFA268"
   "359046F4EB879F924009438B481C6CD7889A002ED5EE382BC9190DA6"
   "FC026E479558E4475677E9AA9E3050E2765694DFC81F56E880B96E71"
   "60C980DD98EDD3DFFFFFFFFFFFFFFFFF",
   "13"
 },
 {0,0} /* null sentinel */
};


static NGConstant * new_ng( SRP_NGType ng_type, const char * n_hex, const char * g_hex )
{
    NGConstant * ng   = (NGConstant *) malloc( sizeof(NGConstant) );
    ng->N = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    ng->g = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(ng->N);
    mbedtls_mpi_init(ng->g);

    if( !ng || !ng->N || !ng->g )
       return 0;

    if ( ng_type != SRP_NG_CUSTOM )
    {
        n_hex = global_Ng_constants[ ng_type ].n_hex;
        g_hex = global_Ng_constants[ ng_type ].g_hex;
    }

    mbedtls_mpi_read_string( ng->N, 16, n_hex);
    mbedtls_mpi_read_string( ng->g, 16, g_hex);

    return ng;
}

static void delete_ng( NGConstant * ng )
{
   if (ng)
   {
   	  mbedtls_mpi_free( ng->N );
   	  mbedtls_mpi_free( ng->g );
   	  free(ng->N);
   	  free(ng->g);
      free(ng);
   }
}


typedef union
{
    mbedtls_sha1_context   sha;
    mbedtls_sha256_context sha256;
    mbedtls_sha512_context sha512;
} HashCTX;


struct SRPVerifier
{
    SRP_HashAlgorithm  hash_alg;
    NGConstant        *ng;

    const char          * username;
    const unsigned char * bytes_B;
    int                   authenticated;

    unsigned char M           [SHA512_DIGEST_LENGTH];
    unsigned char H_AMK       [SHA512_DIGEST_LENGTH];
    unsigned char session_key [SHA512_DIGEST_LENGTH];
};


struct SRPUser
{
    SRP_HashAlgorithm  hash_alg;
    NGConstant        *ng;

    BIGNUM *a;
    BIGNUM *A;
    BIGNUM *S;

    const unsigned char * bytes_A;
    int                   authenticated;

    const char *          username;
    const unsigned char * password;
    int                   password_len;

    unsigned char M           [SHA512_DIGEST_LENGTH];
    unsigned char H_AMK       [SHA512_DIGEST_LENGTH];
    unsigned char session_key [SHA512_DIGEST_LENGTH];
};


static void hash_init( SRP_HashAlgorithm alg, HashCTX *c )
{
    switch (alg)
    {
      case SRP_SHA1  : mbedtls_sha1_init( &c->sha );
      case SRP_SHA256: mbedtls_sha256_init( &c->sha256 );
      case SRP_SHA512: mbedtls_sha512_init( &c->sha512 );
      default:
        return;
    };
}

static void hash_start( SRP_HashAlgorithm alg, HashCTX *c )
{
    switch (alg)
    {
      case SRP_SHA1  : mbedtls_sha1_starts( &c->sha );
      case SRP_SHA224: mbedtls_sha256_starts( &c->sha256, 1 );
      case SRP_SHA256: mbedtls_sha256_starts( &c->sha256, 0 );
      case SRP_SHA384: mbedtls_sha512_starts( &c->sha512, 1 );
      case SRP_SHA512: mbedtls_sha512_starts( &c->sha512, 0 );
      default:
        return;
    };
}


static void hash_update( SRP_HashAlgorithm alg, HashCTX *c, const void *data, size_t len )
{
    switch (alg)
    {
      case SRP_SHA1  : mbedtls_sha1_update( &c->sha, data, len ); break;
      case SRP_SHA224: mbedtls_sha256_update( &c->sha256, data, len ); break;
      case SRP_SHA256: mbedtls_sha256_update( &c->sha256, data, len ); break;
      case SRP_SHA384: mbedtls_sha512_update( &c->sha512, data, len ); break;
      case SRP_SHA512: mbedtls_sha512_update( &c->sha512, data, len ); break;
      default:
        return;
    };
}
static void hash_final( SRP_HashAlgorithm alg, HashCTX *c, unsigned char *md )
{
    switch (alg)
    {
      case SRP_SHA1  : mbedtls_sha1_finish( &c->sha, md ); break;
      case SRP_SHA224: mbedtls_sha256_finish( &c->sha256, md ); break;
      case SRP_SHA256: mbedtls_sha256_finish( &c->sha256, md ); break;
      case SRP_SHA384: mbedtls_sha512_finish( &c->sha512, md ); break;
      case SRP_SHA512: mbedtls_sha512_finish( &c->sha512, md ); break;
      default:
        return;
    };
}
static void hash( SRP_HashAlgorithm alg, const unsigned char *d, size_t n, unsigned char *md )
{
    switch (alg)
    {
      case SRP_SHA1  : mbedtls_sha1( d, n, md ); break;
      case SRP_SHA224: mbedtls_sha256( d, n, md, 1); break;
      case SRP_SHA256: mbedtls_sha256( d, n, md, 0); break;
      case SRP_SHA384: mbedtls_sha512( d, n, md, 1 ); break;
      case SRP_SHA512: mbedtls_sha512( d, n, md, 0 ); break;
      default:
        return;
    };
}
static int hash_length( SRP_HashAlgorithm alg )
{
    switch (alg)
    {
      case SRP_SHA1  : return SHA1_DIGEST_LENGTH;
      case SRP_SHA224: return SHA224_DIGEST_LENGTH;
      case SRP_SHA256: return SHA256_DIGEST_LENGTH;
      case SRP_SHA384: return SHA384_DIGEST_LENGTH;
      case SRP_SHA512: return SHA512_DIGEST_LENGTH;
      default:
        return -1;
    };
}


static BIGNUM * H_nn( SRP_HashAlgorithm alg, const BIGNUM * n1, const BIGNUM * n2 )
{
    unsigned char   buff[ SHA512_DIGEST_LENGTH ];
    int             len_n1 = mbedtls_mpi_size(n1);
    int             len_n2 = mbedtls_mpi_size(n2);
    int             nbytes = len_n1 + len_n2;
    unsigned char * bin    = (unsigned char *) malloc( nbytes );
    if (!bin)
       return 0;

    mbedtls_mpi_write_binary( n1, bin, len_n1 );
    mbedtls_mpi_write_binary( n2, bin+len_n1, len_n2 );
    hash( alg, bin, nbytes, buff );
    free(bin);
    BIGNUM * bn;
    bn = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(bn);
    mbedtls_mpi_read_binary( bn, buff, hash_length(alg) );
    return bn;
}

static BIGNUM * H_ns( SRP_HashAlgorithm alg, const BIGNUM * n, const unsigned char * bytes, int len_bytes )
{
    unsigned char   buff[ SHA512_DIGEST_LENGTH ];
    int             len_n  = mbedtls_mpi_size(n);
    int             nbytes = len_n + len_bytes;
    unsigned char * bin    = (unsigned char *) malloc( nbytes );
    if (!bin)
       return 0;
    mbedtls_mpi_write_binary( n, bin, len_n );
    memcpy( bin + len_n, bytes, len_bytes );
    hash( alg, bin, nbytes, buff );
    free(bin);

	BIGNUM * bn;
	bn = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(bn);
    mbedtls_mpi_read_binary( bn, buff, hash_length(alg) );
    return bn;
}

static BIGNUM * calculate_x( SRP_HashAlgorithm alg, const BIGNUM * salt, const char * username, const unsigned char * password, int password_len )
{
    unsigned char ucp_hash[SHA512_DIGEST_LENGTH];
    HashCTX       ctx;

    hash_init( alg, &ctx );

    hash_update( alg, &ctx, username, strlen(username) );
    hash_update( alg, &ctx, ":", 1 );
    hash_update( alg, &ctx, password, password_len );

    hash_final( alg, &ctx, ucp_hash );

    return H_ns( alg, salt, ucp_hash, hash_length(alg) );
}

static void update_hash_n( SRP_HashAlgorithm alg, HashCTX *ctx, const BIGNUM * n )
{
    unsigned long len = mbedtls_mpi_size(n);
    unsigned char * n_bytes = (unsigned char *) malloc( len );
    if (!n_bytes)
       return;
    mbedtls_mpi_write_binary( n, n_bytes, len );
    hash_update(alg, ctx, n_bytes, len);
    free(n_bytes);
}

static void hash_num( SRP_HashAlgorithm alg, const BIGNUM * n, unsigned char * dest )
{
    int             nbytes = mbedtls_mpi_size(n);
    unsigned char * bin    = (unsigned char *) malloc( nbytes );
    if(!bin)
       return;
    mbedtls_mpi_write_binary( n, bin, nbytes );
    hash( alg, bin, nbytes, dest );
    free(bin);
}

static void calculate_M( SRP_HashAlgorithm alg, NGConstant *ng, unsigned char * dest, const char * I, const BIGNUM * s,
                         const BIGNUM * A, const BIGNUM * B, const unsigned char * K )
{
    unsigned char H_N[ SHA512_DIGEST_LENGTH ];
    unsigned char H_g[ SHA512_DIGEST_LENGTH ];
    unsigned char H_I[ SHA512_DIGEST_LENGTH ];
    unsigned char H_xor[ SHA512_DIGEST_LENGTH ];
    HashCTX       ctx;
    int           i = 0;
    int           hash_len = hash_length(alg);

    hash_num( alg, ng->N, H_N );
    hash_num( alg, ng->g, H_g );

    hash(alg, (const unsigned char *)I, strlen(I), H_I);


    for (i=0; i < hash_len; i++ )
        H_xor[i] = H_N[i] ^ H_g[i];

    hash_init( alg, &ctx );

    hash_update( alg, &ctx, H_xor, hash_len );
    hash_update( alg, &ctx, H_I,   hash_len );
    update_hash_n( alg, &ctx, s );
    update_hash_n( alg, &ctx, A );
    update_hash_n( alg, &ctx, B );
    hash_update( alg, &ctx, K, hash_len );

    hash_final( alg, &ctx, dest );
}

static void calculate_H_AMK( SRP_HashAlgorithm alg, unsigned char *dest, const BIGNUM * A, const unsigned char * M, const unsigned char * K )
{
    HashCTX ctx;

    hash_init( alg, &ctx );

    update_hash_n( alg, &ctx, A );
    hash_update( alg, &ctx, M, hash_length(alg) );
    hash_update( alg, &ctx, K, hash_length(alg) );

    hash_final( alg, &ctx, dest );
}


static void init_random()
{
    if (g_initialized)
        return;

     mbedtls_entropy_init( &entropy_ctx );
     mbedtls_ctr_drbg_init( &ctr_drbg_ctx );

     unsigned char hotBits[128] = {
    82, 42, 71, 87, 124, 241, 30, 1, 54, 239, 240, 121, 89, 9, 151, 11, 60,
    226, 142, 47, 115, 157, 100, 126, 242, 132, 46, 12, 56, 197, 194, 76,
    198, 122, 90, 241, 255, 43, 120, 209, 69, 21, 195, 212, 100, 251, 18,
    111, 30, 238, 24, 199, 238, 236, 138, 225, 45, 15, 42, 83, 114, 132,
    165, 141, 32, 185, 167, 100, 131, 23, 236, 9, 11, 51, 130, 136, 97, 161,
    36, 174, 129, 234, 2, 54, 119, 184, 70, 103, 118, 109, 122, 15, 24, 23,
    166, 203, 102, 160, 77, 100, 17, 4, 132, 138, 215, 204, 109, 245, 122,
    9, 184, 89, 70, 247, 125, 97, 213, 240, 85, 243, 91, 226, 127, 64, 136,
    37, 154, 232
};

     mbedtls_ctr_drbg_seed(
     	&ctr_drbg_ctx,
     	mbedtls_entropy_func,
     	&entropy_ctx,
        hotBits,
        128
    );

    RR = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(RR);
    g_initialized = 1;

}


/***********************************************************************************************************
 *
 *  Exported Functions
 *
 ***********************************************************************************************************/

void srp_random_seed( const unsigned char * random_data, int data_length )
{
    g_initialized = 1;


   if( mbedtls_ctr_drbg_seed( &ctr_drbg_ctx, mbedtls_entropy_func, &entropy_ctx,
                           (const unsigned char *) random_data,
                           data_length )  != 0 )
	{
    	return;
    }

}


void srp_create_salted_verification_key( SRP_HashAlgorithm alg,
                                         SRP_NGType ng_type, const char * username,
                                         const unsigned char * password, int len_password,
                                         const unsigned char ** bytes_s, int * len_s,
                                         const unsigned char ** bytes_v, int * len_v,
                                         const char * n_hex, const char * g_hex )
{


    BIGNUM     * s;
    BIGNUM     * v;
    BIGNUM     * x   = 0;

    s = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
	mbedtls_mpi_init(s);
    v = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
	mbedtls_mpi_init(v);

    NGConstant * ng  = new_ng( ng_type, n_hex, g_hex );

    if( !s || !v || !ng )
       goto cleanup_and_exit;

    init_random(); /* Only happens once */

    mbedtls_mpi_fill_random( s, 32,
                     &mbedtls_ctr_drbg_random,
                     &ctr_drbg_ctx );


    x = calculate_x( alg, s, username, password, len_password );

    if( !x )
       goto cleanup_and_exit;

	mbedtls_mpi_exp_mod(v, ng->g, x, ng->N, RR);

    *len_s   = mbedtls_mpi_size(s);
    *len_v   = mbedtls_mpi_size(v);

    *bytes_s = (const unsigned char *) malloc( *len_s );
    *bytes_v = (const unsigned char *) malloc( *len_v );

    if (!bytes_s || !bytes_v)
       goto cleanup_and_exit;

    mbedtls_mpi_write_binary( s, *bytes_s, *len_s );
    mbedtls_mpi_write_binary( v, *bytes_v, *len_v );

 cleanup_and_exit:
    delete_ng( ng );
    mbedtls_mpi_free(s);
    free(s);
    mbedtls_mpi_free(v);
    free(v);
    mbedtls_mpi_free(x);
    free(x);
    //TODO: BN_CTX_free(ctx);
}



/* Out: bytes_B, len_B.
 *
 * On failure, bytes_B will be set to NULL and len_B will be set to 0
 */
struct SRPVerifier *  srp_verifier_new( SRP_HashAlgorithm alg, SRP_NGType ng_type, const char * username,
                                        const unsigned char * bytes_s, int len_s,
                                        const unsigned char * bytes_v, int len_v,
                                        const unsigned char * bytes_A, int len_A,
                                        const unsigned char ** bytes_B, int * len_B,
                                        const char * n_hex, const char * g_hex )
{

    BIGNUM *s;
    BIGNUM *v;
    BIGNUM *A;

	  s = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(s);
    mbedtls_mpi_read_binary(s, bytes_s, len_s);
    v = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(v);
    mbedtls_mpi_read_binary(v, bytes_v, len_v);
    A = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(A);
	  mbedtls_mpi_read_binary(A, bytes_A, len_A);

    BIGNUM             *u    = 0;
    BIGNUM             *B;
    B = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(B);
    BIGNUM             *S;
    S = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(S);
    BIGNUM             *b;
    b = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(b);
    BIGNUM             *k    = 0;
    BIGNUM             *tmp1;
    tmp1 = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(tmp1);
    BIGNUM             *tmp2;
    tmp2 = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(tmp2);
    int                 ulen = strlen(username) + 1;
    NGConstant         *ng   = new_ng( ng_type, n_hex, g_hex );
    struct SRPVerifier *ver  = 0;

    *len_B   = 0;
    *bytes_B = 0;

    if( !s || !v || !A || !B || !S || !b || !tmp1 || !tmp2 || !ng ) {
       goto cleanup_and_exit;
    }

    ver = (struct SRPVerifier *) malloc( sizeof(struct SRPVerifier) );

    if (!ver) {
       goto cleanup_and_exit;
	}
    init_random(); /* Only happens once */

    ver->username = (char *) malloc( ulen );
    ver->hash_alg = alg;
    ver->ng       = ng;

    if (!ver->username)
    {
       free(ver);
       ver = 0;
       goto cleanup_and_exit;
    }

    memcpy( (char*)ver->username, username, ulen );

    ver->authenticated = 0;

    /* SRP-6a safety check */
    mbedtls_mpi_mod_mpi( tmp1, A, ng->N );
    if ( mbedtls_mpi_cmp_int( tmp1, 0 )  != 0)
    {
        mbedtls_mpi_fill_random( b, 256,
                     &mbedtls_ctr_drbg_random,
                     &ctr_drbg_ctx );

       k = H_nn(alg, ng->N, ng->g);

       /* B = kv + g^b */
       mbedtls_mpi_mul_mpi( tmp1, k, v);
       mbedtls_mpi_exp_mod( tmp2, ng->g, b, ng->N, RR );
       mbedtls_mpi_add_mpi( B, tmp1, tmp2 );

       u = H_nn(alg, A, B);

       /* S = (A *(v^u)) ^ b */
       mbedtls_mpi_exp_mod(tmp1, v, u, ng->N, RR);
       mbedtls_mpi_mul_mpi(tmp2, A, tmp1);
       mbedtls_mpi_exp_mod(S, tmp2, b, ng->N, RR);

       hash_num(alg, S, ver->session_key);

       calculate_M( alg, ng, ver->M, username, s, A, B, ver->session_key );
       calculate_H_AMK( alg, ver->H_AMK, A, ver->M, ver->session_key );

       *len_B   = mbedtls_mpi_size(B);
       *bytes_B = malloc( *len_B );

       if( !*bytes_B )
       {
          free( (void*) ver->username );
          free( ver );
          ver = 0;
          *len_B = 0;
          goto cleanup_and_exit;
       }

       mbedtls_mpi_write_binary( B, *bytes_B, *len_B );
       ver->bytes_B = *bytes_B;
    }

 cleanup_and_exit:
    mbedtls_mpi_free(s);
    free(s);
    mbedtls_mpi_free(v);
    free(v);
    mbedtls_mpi_free(A);
    free(A);
    if (u) {mbedtls_mpi_free(u); free(u);}
    if (k) {mbedtls_mpi_free(k); free(k);}
    mbedtls_mpi_free(B);
    free(B);
    mbedtls_mpi_free(S);
    free(S);
    mbedtls_mpi_free(b);
    free(b);
    mbedtls_mpi_free(tmp1);
    free(tmp1);
    mbedtls_mpi_free(tmp2);
    free(tmp2);

    return ver;
}




void srp_verifier_delete( struct SRPVerifier * ver )
{
   if (ver)
   {
      delete_ng( ver->ng );
      free( (char *) ver->username );
      if(ver->bytes_B !=0) {
      	free( (unsigned char *) ver->bytes_B );
      }
      memset(ver, 0, sizeof(*ver));
      free( ver );
   }
}



int srp_verifier_is_authenticated( struct SRPVerifier * ver )
{
    return ver->authenticated;
}


const char * srp_verifier_get_username( struct SRPVerifier * ver )
{
    return ver->username;
}


const unsigned char * srp_verifier_get_session_key( struct SRPVerifier * ver, int * key_length )
{
    if (key_length)
        *key_length = hash_length( ver->hash_alg );
    return ver->session_key;
}


int                   srp_verifier_get_session_key_length( struct SRPVerifier * ver )
{
    return hash_length( ver->hash_alg );
}


/* user_M must be exactly SHA512_DIGEST_LENGTH bytes in size */
void srp_verifier_verify_session( struct SRPVerifier * ver, const unsigned char * user_M, const unsigned char ** bytes_HAMK )
{
    if ( memcmp( ver->M, user_M, hash_length(ver->hash_alg) ) == 0 )
    {
        ver->authenticated = 1;
        *bytes_HAMK = ver->H_AMK;
    }
    else
        *bytes_HAMK = NULL;
}

/*******************************************************************************/

struct SRPUser * srp_user_new( SRP_HashAlgorithm alg, SRP_NGType ng_type, const char * username,
                               const unsigned char * bytes_password, int len_password,
                               const char * n_hex, const char * g_hex )
{
    struct SRPUser  *usr  = (struct SRPUser *) malloc( sizeof(struct SRPUser) );
    int              ulen = strlen(username) + 1;

    if (!usr)
       goto err_exit;

    init_random(); /* Only happens once */

    usr->hash_alg = alg;
    usr->ng       = new_ng( ng_type, n_hex, g_hex );

    usr->a = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    usr->A = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    usr->S = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));

	mbedtls_mpi_init(usr->a);
	mbedtls_mpi_init(usr->A);
	mbedtls_mpi_init(usr->S);

    if (!usr->ng || !usr->a || !usr->A || !usr->S)
       goto err_exit;

    usr->username     = (const char *) malloc(ulen);
    usr->password     = (const unsigned char *) malloc(len_password);
    usr->password_len = len_password;

    if (!usr->username || !usr->password)
       goto err_exit;

    memcpy((char *)usr->username, username,       ulen);
    memcpy((char *)usr->password, bytes_password, len_password);

    usr->authenticated = 0;

    usr->bytes_A = 0;

    return usr;

 err_exit:
    if (usr)
    {
       mbedtls_mpi_free(usr->a);
       mbedtls_mpi_free(usr->A);
       mbedtls_mpi_free(usr->S);
       if (usr->username)
          free((void*)usr->username);
       if (usr->password)
       {
          memset((void*)usr->password, 0, usr->password_len);
          free((void*)usr->password);
       }
       free(usr);
    }

    return 0;
}



void srp_user_delete( struct SRPUser * usr )
{
   if( usr )
   {
      mbedtls_mpi_free( usr->a );
      mbedtls_mpi_free( usr->A );
      mbedtls_mpi_free( usr->S );

      delete_ng( usr->ng );

      memset((void*)usr->password, 0, usr->password_len);

      free((char *)usr->username);
      free((char *)usr->password);

      if (usr->bytes_A)
         free( (char *)usr->bytes_A );

      memset(usr, 0, sizeof(*usr));
      free( usr );
   }
}



int srp_user_is_authenticated( struct SRPUser * usr)
{
    return usr->authenticated;
}


const char * srp_user_get_username( struct SRPUser * usr )
{
    return usr->username;
}



const unsigned char * srp_user_get_session_key( struct SRPUser * usr, int * key_length )
{
    if (key_length)
        *key_length = hash_length( usr->hash_alg );
    return usr->session_key;
}


int                   srp_user_get_session_key_length( struct SRPUser * usr )
{
    return hash_length( usr->hash_alg );
}



/* Output: username, bytes_A, len_A */
void  srp_user_start_authentication( struct SRPUser * usr, const char ** username,
                                     const unsigned char ** bytes_A, int * len_A )
{

    mbedtls_mpi_fill_random( usr->a, 256, &mbedtls_ctr_drbg_random, &ctr_drbg_ctx);
    mbedtls_mpi_exp_mod(usr->A, usr->ng->g, usr->a, usr->ng->N, RR);

    *len_A   = mbedtls_mpi_size(usr->A);
    *bytes_A = malloc( *len_A );

    if (!*bytes_A)
    {
       *len_A = 0;
       *bytes_A = 0;
       *username = 0;
       return;
    }

    mbedtls_mpi_write_binary( usr->A, (unsigned char *) *bytes_A, *len_A );

    usr->bytes_A = *bytes_A;
    *username = usr->username;
}


/* Output: bytes_M. Buffer length is SHA512_DIGEST_LENGTH */
void  srp_user_process_challenge( struct SRPUser * usr,
                                  const unsigned char * bytes_s, int len_s,
                                  const unsigned char * bytes_B, int len_B,
                                  const unsigned char ** bytes_M, int * len_M )
{
    BIGNUM *s;
    BIGNUM *B;
    BIGNUM *u    = 0;
    BIGNUM *x    = 0;
    BIGNUM *k    = 0;
    BIGNUM *v;
    BIGNUM *tmp1;
    BIGNUM *tmp2;
    BIGNUM *tmp3;

    s = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(s);
    mbedtls_mpi_read_binary(s, bytes_s, len_s);
    B = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
    mbedtls_mpi_init(B);
    mbedtls_mpi_read_binary(B, bytes_B, len_B);
    v = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
 	mbedtls_mpi_init(v);
 	tmp1 = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
 	mbedtls_mpi_init(tmp1);
 	tmp2 = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
 	mbedtls_mpi_init(tmp2);
 	tmp3 = (mbedtls_mpi *) malloc(sizeof(mbedtls_mpi));
 	mbedtls_mpi_init(tmp3);

    *len_M = 0;
    *bytes_M = 0;

    if( !s || !B || !v || !tmp1 || !tmp2 || !tmp3 )
       goto cleanup_and_exit;

    u = H_nn(usr->hash_alg, usr->A, B);

    if (!u)
       goto cleanup_and_exit;

    x = calculate_x( usr->hash_alg, s, usr->username, usr->password, usr->password_len );

    if (!x)
       goto cleanup_and_exit;

    k = H_nn(usr->hash_alg, usr->ng->N, usr->ng->g);

    if (!k)
       goto cleanup_and_exit;

    /* SRP-6a safety check */
    if( mbedtls_mpi_cmp_int( B, 0 ) != 0 && mbedtls_mpi_cmp_int( u, 0 ) !=0 )
    {
	    mbedtls_mpi_exp_mod(v, usr->ng->g, x, usr->ng->N, RR);
        /* S = (B - k*(g^x)) ^ (a + ux) */
        mbedtls_mpi_mul_mpi( tmp1, u, x );
        mbedtls_mpi_add_mpi( tmp2, usr->a, tmp1);
        /* tmp2 = (a + ux)      */
        mbedtls_mpi_exp_mod( tmp1, usr->ng->g, x, usr->ng->N, RR);        
        mbedtls_mpi_mul_mpi( tmp3, k, tmp1 );
        /* tmp3 = k*(g^x)       */
        mbedtls_mpi_sub_mpi(tmp1, B, tmp3);
        /* tmp1 = (B - K*(g^x)) */
        mbedtls_mpi_exp_mod( usr->S, tmp1, tmp2, usr->ng->N, RR);        

        hash_num(usr->hash_alg, usr->S, usr->session_key);

        calculate_M( usr->hash_alg, usr->ng, usr->M, usr->username, s, usr->A, B, usr->session_key );
        calculate_H_AMK( usr->hash_alg, usr->H_AMK, usr->A, usr->M, usr->session_key );

        *bytes_M = usr->M;
        if (len_M) {
            *len_M = hash_length( usr->hash_alg );
        }
    }
    else
    {
        *bytes_M = NULL;
        if (len_M)
            *len_M   = 0;
    }

 cleanup_and_exit:

    mbedtls_mpi_free(s);
    mbedtls_mpi_free(B);
    mbedtls_mpi_free(u);
    mbedtls_mpi_free(x);
    mbedtls_mpi_free(k);
    mbedtls_mpi_free(v);
    mbedtls_mpi_free(tmp1);
    mbedtls_mpi_free(tmp2);
    mbedtls_mpi_free(tmp3);
    free(s);
    free(B);
    free(u);
    free(x);
    free(k);
    free(v);
    free(tmp1);
    free(tmp2);
    free(tmp3);
}


void srp_user_verify_session( struct SRPUser * usr, const unsigned char * bytes_HAMK )
{
    if ( memcmp( usr->H_AMK, bytes_HAMK, hash_length(usr->hash_alg) ) == 0 )
        usr->authenticated = 1;
}