library/dhm.c

/*
 *  Diffie-Hellman-Merkle key exchange
 *
 *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *  This file is part of mbed TLS (https://tls.mbed.org)
 */
/*
 *  The following sources were referenced in the design of this implementation
 *  of the Diffie-Hellman-Merkle algorithm:
 *
 *  [1] Handbook of Applied Cryptography - 1997, Chapter 12
 *      Menezes, van Oorschot and Vanstone
 *
 */

#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
#else
#include MBEDTLS_CONFIG_FILE
#endif

#if defined(MBEDTLS_DHM_C)

#include "mbedtls/dhm.h"

#include <string.h>

#if defined(MBEDTLS_PEM_PARSE_C)
#include "mbedtls/pem.h"
#endif

#if defined(MBEDTLS_ASN1_PARSE_C)
#include "mbedtls/asn1.h"
#endif

#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdlib.h>
#include <stdio.h>
#define mbedtls_printf     printf
#define mbedtls_calloc    calloc
#define mbedtls_free       free
#endif

/*
 * Diffie-Hellman groups from RFC 5114
 *
 * \warning The origin of the primes in RFC 5114 is not documented and
 *          their use therefore constitutes a security risk!
 *
 * \deprecated The primes from RFC 5114 are superseded by the primes
 *             from RFC 3526 and RFC 7919 and should no longer be used.
 *             They will be removed in the next major version.
 */

const char * mbedtls_dhm_rfc5114_modp_2048_p =
    "AD107E1E9123A9D0D660FAA79559C51FA20D64E5683B9FD1"
    "B54B1597B61D0A75E6FA141DF95A56DBAF9A3C407BA1DF15"
    "EB3D688A309C180E1DE6B85A1274A0A66D3F8152AD6AC212"
    "9037C9EDEFDA4DF8D91E8FEF55B7394B7AD5B7D0B6C12207"
    "C9F98D11ED34DBF6C6BA0B2C8BBC27BE6A00E0A0B9C49708"
    "B3BF8A317091883681286130BC8985DB1602E714415D9330"
    "278273C7DE31EFDC7310F7121FD5A07415987D9ADC0A486D"
    "CDF93ACC44328387315D75E198C641A480CD86A1B9E587E8"
    "BE60E69CC928B2B9C52172E413042E9B23F10B0E16E79763"
    "C9B53DCF4BA80A29E3FB73C16B8E75B97EF363E2FFA31F71"
    "CF9DE5384E71B81C0AC4DFFE0C10E64F";
const char * mbedtls_dhm_rfc5114_modp_2048_g =
    "AC4032EF4F2D9AE39DF30B5C8FFDAC506CDEBE7B89998CAF"
    "74866A08CFE4FFE3A6824A4E10B9A6F0DD921F01A70C4AFA"
    "AB739D7700C29F52C57DB17C620A8652BE5E9001A8D66AD7"
    "C17669101999024AF4D027275AC1348BB8A762D0521BC98A"
    "E247150422EA1ED409939D54DA7460CDB5F6C6B250717CBE"
    "F180EB34118E98D119529A45D6F834566E3025E316A330EF"
    "BB77A86F0C1AB15B051AE3D428C8F8ACB70A8137150B8EEB"
    "10E183EDD19963DDD9E263E4770589EF6AA21E7F5F2FF381"
    "B539CCE3409D13CD566AFBB48D6C019181E1BCFE94B30269"
    "EDFE72FE9B6AA4BD7B5A0F1C71CFFF4C19C418E1F6EC0179"
    "81BC087F2A7065B384B890D3191F2BFA";

/*
 * Diffie-Hellman groups from RFC 3526
 */

const char * mbedtls_dhm_rfc3526_modp_2048_p =
    "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1"
    "29024E088A67CC74020BBEA63B139B22514A08798E3404DD"
    "EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245"
    "E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED"
    "EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D"
    "C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F"
    "83655D23DCA3AD961C62F356208552BB9ED529077096966D"
    "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B"
    "E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9"
    "DE2BCBF6955817183995497CEA956AE515D2261898FA0510"
    "15728E5A8AACAA68FFFFFFFFFFFFFFFF";
const char * mbedtls_dhm_rfc3526_modp_2048_g = "02";

const char * mbedtls_dhm_rfc3526_modp_3072_p =
    "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1"
    "29024E088A67CC74020BBEA63B139B22514A08798E3404DD"
    "EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245"
    "E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED"
    "EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D"
    "C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F"
    "83655D23DCA3AD961C62F356208552BB9ED529077096966D"
    "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B"
    "E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9"
    "DE2BCBF6955817183995497CEA956AE515D2261898FA0510"
    "15728E5A8AAAC42DAD33170D04507A33A85521ABDF1CBA64"
    "ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7"
    "ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6B"
    "F12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
    "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB31"
    "43DB5BFCE0FD108E4B82D120A93AD2CAFFFFFFFFFFFFFFFF";
const char * mbedtls_dhm_rfc3526_modp_3072_g = "02";

const char * mbedtls_dhm_rfc3526_modp_4096_p =
    "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1"
    "29024E088A67CC74020BBEA63B139B22514A08798E3404DD"
    "EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245"
    "E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED"
    "EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D"
    "C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F"
    "83655D23DCA3AD961C62F356208552BB9ED529077096966D"
    "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B"
    "E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9"
    "DE2BCBF6955817183995497CEA956AE515D2261898FA0510"
    "15728E5A8AAAC42DAD33170D04507A33A85521ABDF1CBA64"
    "ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7"
    "ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6B"
    "F12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
    "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB31"
    "43DB5BFCE0FD108E4B82D120A92108011A723C12A787E6D7"
    "88719A10BDBA5B2699C327186AF4E23C1A946834B6150BDA"
    "2583E9CA2AD44CE8DBBBC2DB04DE8EF92E8EFC141FBECAA6"
    "287C59474E6BC05D99B2964FA090C3A2233BA186515BE7ED"
    "1F612970CEE2D7AFB81BDD762170481CD0069127D5B05AA9"
    "93B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199"
    "FFFFFFFFFFFFFFFF";
const char * mbedtls_dhm_rfc3526_modp_4096_g = "02";
/* Implementation that should never be optimized out by the compiler */
static void mbedtls_zeroize( void *v, size_t n ) {
    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
}

/*
 * helper to validate the mbedtls_mpi size and import it
 */
static int dhm_read_bignum( mbedtls_mpi *X,
                            unsigned char **p,
                            const unsigned char *end )
{
    int ret, n;

    if( end - *p < 2 )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    n = ( (*p)[0] << 8 ) | (*p)[1];
    (*p) += 2;

    if( (int)( end - *p ) < n )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    if( ( ret = mbedtls_mpi_read_binary( X, *p, n ) ) != 0 )
        return( MBEDTLS_ERR_DHM_READ_PARAMS_FAILED + ret );

    (*p) += n;

    return( 0 );
}

/*
 * Verify sanity of parameter with regards to P
 *
 * Parameter should be: 2 <= public_param <= P - 2
 *
 * For more information on the attack, see:
 *  http://www.cl.cam.ac.uk/~rja14/Papers/psandqs.pdf
 *  http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2005-2643
 */
static int dhm_check_range( const mbedtls_mpi *param, const mbedtls_mpi *P )
{
    mbedtls_mpi L, U;
    int ret = MBEDTLS_ERR_DHM_BAD_INPUT_DATA;

    mbedtls_mpi_init( &L ); mbedtls_mpi_init( &U );

    MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &L, 2 ) );
    MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( &U, P, 2 ) );

    if( mbedtls_mpi_cmp_mpi( param, &L ) >= 0 &&
        mbedtls_mpi_cmp_mpi( param, &U ) <= 0 )
    {
        ret = 0;
    }

cleanup:
    mbedtls_mpi_free( &L ); mbedtls_mpi_free( &U );
    return( ret );
}

void mbedtls_dhm_init( mbedtls_dhm_context *ctx )
{
    memset( ctx, 0, sizeof( mbedtls_dhm_context ) );
}

/*
 * Parse the ServerKeyExchange parameters
 */
int mbedtls_dhm_read_params( mbedtls_dhm_context *ctx,
                     unsigned char **p,
                     const unsigned char *end )
{
    int ret;

    if( ( ret = dhm_read_bignum( &ctx->P,  p, end ) ) != 0 ||
        ( ret = dhm_read_bignum( &ctx->G,  p, end ) ) != 0 ||
        ( ret = dhm_read_bignum( &ctx->GY, p, end ) ) != 0 )
        return( ret );

    if( ( ret = dhm_check_range( &ctx->GY, &ctx->P ) ) != 0 )
        return( ret );

    ctx->len = mbedtls_mpi_size( &ctx->P );

    return( 0 );
}

/*
 * Setup and write the ServerKeyExchange parameters
 */
int mbedtls_dhm_make_params( mbedtls_dhm_context *ctx, int x_size,
                     unsigned char *output, size_t *olen,
                     int (*f_rng)(void *, unsigned char *, size_t),
                     void *p_rng )
{
    int ret, count = 0;
    size_t n1, n2, n3;
    unsigned char *p;

    if( mbedtls_mpi_cmp_int( &ctx->P, 0 ) == 0 )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    /*
     * Generate X as large as possible ( < P )
     */
    do
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->X, x_size, f_rng, p_rng ) );

        while( mbedtls_mpi_cmp_mpi( &ctx->X, &ctx->P ) >= 0 )
            MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &ctx->X, 1 ) );

        if( count++ > 10 )
            return( MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED );
    }
    while( dhm_check_range( &ctx->X, &ctx->P ) != 0 );

    /*
     * Calculate GX = G^X mod P
     */
    MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->GX, &ctx->G, &ctx->X,
                          &ctx->P , &ctx->RP ) );

    if( ( ret = dhm_check_range( &ctx->GX, &ctx->P ) ) != 0 )
        return( ret );

    /*
     * export P, G, GX
     */
#define DHM_MPI_EXPORT(X,n)                     \
    MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( X, p + 2, n ) ); \
    *p++ = (unsigned char)( n >> 8 );           \
    *p++ = (unsigned char)( n      ); p += n;

    n1 = mbedtls_mpi_size( &ctx->P  );
    n2 = mbedtls_mpi_size( &ctx->G  );
    n3 = mbedtls_mpi_size( &ctx->GX );

    p = output;
    DHM_MPI_EXPORT( &ctx->P , n1 );
    DHM_MPI_EXPORT( &ctx->G , n2 );
    DHM_MPI_EXPORT( &ctx->GX, n3 );

    *olen  = p - output;

    ctx->len = n1;

cleanup:

    if( ret != 0 )
        return( MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED + ret );

    return( 0 );
}

/*
 * Import the peer's public value G^Y
 */
int mbedtls_dhm_read_public( mbedtls_dhm_context *ctx,
                     const unsigned char *input, size_t ilen )
{
    int ret;

    if( ctx == NULL || ilen < 1 || ilen > ctx->len )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    if( ( ret = mbedtls_mpi_read_binary( &ctx->GY, input, ilen ) ) != 0 )
        return( MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED + ret );

    return( 0 );
}

/*
 * Create own private value X and export G^X
 */
int mbedtls_dhm_make_public( mbedtls_dhm_context *ctx, int x_size,
                     unsigned char *output, size_t olen,
                     int (*f_rng)(void *, unsigned char *, size_t),
                     void *p_rng )
{
    int ret, count = 0;

    if( ctx == NULL || olen < 1 || olen > ctx->len )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    if( mbedtls_mpi_cmp_int( &ctx->P, 0 ) == 0 )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    /*
     * generate X and calculate GX = G^X mod P
     */
    do
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->X, x_size, f_rng, p_rng ) );

        while( mbedtls_mpi_cmp_mpi( &ctx->X, &ctx->P ) >= 0 )
            MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &ctx->X, 1 ) );

        if( count++ > 10 )
            return( MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED );
    }
    while( dhm_check_range( &ctx->X, &ctx->P ) != 0 );

    MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->GX, &ctx->G, &ctx->X,
                          &ctx->P , &ctx->RP ) );

    if( ( ret = dhm_check_range( &ctx->GX, &ctx->P ) ) != 0 )
        return( ret );

    MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &ctx->GX, output, olen ) );

cleanup:

    if( ret != 0 )
        return( MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED + ret );

    return( 0 );
}

/*
 * Use the blinding method and optimisation suggested in section 10 of:
 *  KOCHER, Paul C. Timing attacks on implementations of Diffie-Hellman, RSA,
 *  DSS, and other systems. In : Advances in Cryptology-CRYPTO'96. Springer
 *  Berlin Heidelberg, 1996. p. 104-113.
 */
static int dhm_update_blinding( mbedtls_dhm_context *ctx,
                    int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
    int ret, count;

    /*
     * Don't use any blinding the first time a particular X is used,
     * but remember it to use blinding next time.
     */
    if( mbedtls_mpi_cmp_mpi( &ctx->X, &ctx->pX ) != 0 )
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &ctx->pX, &ctx->X ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &ctx->Vi, 1 ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &ctx->Vf, 1 ) );

        return( 0 );
    }

    /*
     * Ok, we need blinding. Can we re-use existing values?
     * If yes, just update them by squaring them.
     */
    if( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) != 0 )
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vi, &ctx->Vi ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->P ) );

        MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vf, &ctx->Vf, &ctx->Vf ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vf, &ctx->Vf, &ctx->P ) );

        return( 0 );
    }

    /*
     * We need to generate blinding values from scratch
     */

    /* Vi = random( 2, P-1 ) */
    count = 0;
    do
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vi, mbedtls_mpi_size( &ctx->P ), f_rng, p_rng ) );

        while( mbedtls_mpi_cmp_mpi( &ctx->Vi, &ctx->P ) >= 0 )
            MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &ctx->Vi, 1 ) );

        if( count++ > 10 )
            return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
    }
    while( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) <= 0 );

    /* Vf = Vi^-X mod P */
    MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) );
    MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vf, &ctx->Vf, &ctx->X, &ctx->P, &ctx->RP ) );

cleanup:
    return( ret );
}

/*
 * Derive and export the shared secret (G^Y)^X mod P
 */
int mbedtls_dhm_calc_secret( mbedtls_dhm_context *ctx,
                     unsigned char *output, size_t output_size, size_t *olen,
                     int (*f_rng)(void *, unsigned char *, size_t),
                     void *p_rng )
{
    int ret;
    mbedtls_mpi GYb;

    if( ctx == NULL || output_size < ctx->len )
        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );

    if( ( ret = dhm_check_range( &ctx->GY, &ctx->P ) ) != 0 )
        return( ret );

    mbedtls_mpi_init( &GYb );

    /* Blind peer's value */
    if( f_rng != NULL )
    {
        MBEDTLS_MPI_CHK( dhm_update_blinding( ctx, f_rng, p_rng ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &GYb, &ctx->GY, &ctx->Vi ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &GYb, &GYb, &ctx->P ) );
    }
    else
        MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &GYb, &ctx->GY ) );

    /* Do modular exponentiation */
    MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->K, &GYb, &ctx->X,
                          &ctx->P, &ctx->RP ) );

    /* Unblind secret value */
    if( f_rng != NULL )
    {
        MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->K, &ctx->K, &ctx->Vf ) );
        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->K, &ctx->K, &ctx->P ) );
    }

    *olen = mbedtls_mpi_size( &ctx->K );

    MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &ctx->K, output, *olen ) );

cleanup:
    mbedtls_mpi_free( &GYb );

    if( ret != 0 )
        return( MBEDTLS_ERR_DHM_CALC_SECRET_FAILED + ret );

    return( 0 );
}

/*
 * Free the components of a DHM key
 */
void mbedtls_dhm_free( mbedtls_dhm_context *ctx )
{
    mbedtls_mpi_free( &ctx->pX); mbedtls_mpi_free( &ctx->Vf ); mbedtls_mpi_free( &ctx->Vi );
    mbedtls_mpi_free( &ctx->RP ); mbedtls_mpi_free( &ctx->K ); mbedtls_mpi_free( &ctx->GY );
    mbedtls_mpi_free( &ctx->GX ); mbedtls_mpi_free( &ctx->X ); mbedtls_mpi_free( &ctx->G );
    mbedtls_mpi_free( &ctx->P );

    mbedtls_zeroize( ctx, sizeof( mbedtls_dhm_context ) );
}

#if defined(MBEDTLS_ASN1_PARSE_C)
/*
 * Parse DHM parameters
 */
int mbedtls_dhm_parse_dhm( mbedtls_dhm_context *dhm, const unsigned char *dhmin,
                   size_t dhminlen )
{
    int ret;
    size_t len;
    unsigned char *p, *end;
#if defined(MBEDTLS_PEM_PARSE_C)
    mbedtls_pem_context pem;

    mbedtls_pem_init( &pem );

    /* Avoid calling mbedtls_pem_read_buffer() on non-null-terminated string */
    if( dhminlen == 0 || dhmin[dhminlen - 1] != '\0' )
        ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
    else
        ret = mbedtls_pem_read_buffer( &pem,
                               "-----BEGIN DH PARAMETERS-----",
                               "-----END DH PARAMETERS-----",
                               dhmin, NULL, 0, &dhminlen );

    if( ret == 0 )
    {
        /*
         * Was PEM encoded
         */
        dhminlen = pem.buflen;
    }
    else if( ret != MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT )
        goto exit;

    p = ( ret == 0 ) ? pem.buf : (unsigned char *) dhmin;
#else
    p = (unsigned char *) dhmin;
#endif /* MBEDTLS_PEM_PARSE_C */
    end = p + dhminlen;

    /*
     *  DHParams ::= SEQUENCE {
     *      prime              INTEGER,  -- P
     *      generator          INTEGER,  -- g
     *      privateValueLength INTEGER OPTIONAL
     *  }
     */
    if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
            MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
    {
        ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
        goto exit;
    }

    end = p + len;

    if( ( ret = mbedtls_asn1_get_mpi( &p, end, &dhm->P  ) ) != 0 ||
        ( ret = mbedtls_asn1_get_mpi( &p, end, &dhm->G ) ) != 0 )
    {
        ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
        goto exit;
    }

    if( p != end )
    {
        /* This might be the optional privateValueLength.
         * If so, we can cleanly discard it */
        mbedtls_mpi rec;
        mbedtls_mpi_init( &rec );
        ret = mbedtls_asn1_get_mpi( &p, end, &rec );
        mbedtls_mpi_free( &rec );
        if ( ret != 0 )
        {
            ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
            goto exit;
        }
        if ( p != end )
        {
            ret = MBEDTLS_ERR_DHM_INVALID_FORMAT +
                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
            goto exit;
        }
    }

    ret = 0;

    dhm->len = mbedtls_mpi_size( &dhm->P );

exit:
#if defined(MBEDTLS_PEM_PARSE_C)
    mbedtls_pem_free( &pem );
#endif
    if( ret != 0 )
        mbedtls_dhm_free( dhm );

    return( ret );
}

#if defined(MBEDTLS_FS_IO)
/*
 * Load all data from a file into a given buffer.
 *
 * The file is expected to contain either PEM or DER encoded data.
 * A terminating null byte is always appended. It is included in the announced
 * length only if the data looks like it is PEM encoded.
 */
static int load_file( const char *path, unsigned char **buf, size_t *n )
{
    FILE *f;
    long size;

    if( ( f = fopen( path, "rb" ) ) == NULL )
        return( MBEDTLS_ERR_DHM_FILE_IO_ERROR );

    fseek( f, 0, SEEK_END );
    if( ( size = ftell( f ) ) == -1 )
    {
        fclose( f );
        return( MBEDTLS_ERR_DHM_FILE_IO_ERROR );
    }
    fseek( f, 0, SEEK_SET );

    *n = (size_t) size;

    if( *n + 1 == 0 ||
        ( *buf = mbedtls_calloc( 1, *n + 1 ) ) == NULL )
    {
        fclose( f );
        return( MBEDTLS_ERR_DHM_ALLOC_FAILED );
    }

    if( fread( *buf, 1, *n, f ) != *n )
    {
        fclose( f );
        mbedtls_free( *buf );
        return( MBEDTLS_ERR_DHM_FILE_IO_ERROR );
    }

    fclose( f );

    (*buf)[*n] = '\0';

    if( strstr( (const char *) *buf, "-----BEGIN " ) != NULL )
        ++*n;

    return( 0 );
}

/*
 * Load and parse DHM parameters
 */
int mbedtls_dhm_parse_dhmfile( mbedtls_dhm_context *dhm, const char *path )
{
    int ret;
    size_t n;
    unsigned char *buf;

    if( ( ret = load_file( path, &buf, &n ) ) != 0 )
        return( ret );

    ret = mbedtls_dhm_parse_dhm( dhm, buf, n );

    mbedtls_zeroize( buf, n );
    mbedtls_free( buf );

    return( ret );
}
#endif /* MBEDTLS_FS_IO */
#endif /* MBEDTLS_ASN1_PARSE_C */

#if defined(MBEDTLS_SELF_TEST)

static const char mbedtls_test_dhm_params[] =
"-----BEGIN DH PARAMETERS-----\r\n"
"MIGHAoGBAJ419DBEOgmQTzo5qXl5fQcN9TN455wkOL7052HzxxRVMyhYmwQcgJvh\r\n"
"1sa18fyfR9OiVEMYglOpkqVoGLN7qd5aQNNi5W7/C+VBdHTBJcGZJyyP5B3qcz32\r\n"
"9mLJKudlVudV0Qxk5qUJaPZ/xupz0NyoVpviuiBOI1gNi8ovSXWzAgEC\r\n"
"-----END DH PARAMETERS-----\r\n";

static const size_t mbedtls_test_dhm_params_len = sizeof( mbedtls_test_dhm_params );

/*
 * Checkup routine
 */
int mbedtls_dhm_self_test( int verbose )
{
    int ret;
    mbedtls_dhm_context dhm;

    mbedtls_dhm_init( &dhm );

    if( verbose != 0 )
        mbedtls_printf( "  DHM parameter load: " );

    if( ( ret = mbedtls_dhm_parse_dhm( &dhm,
                    (const unsigned char *) mbedtls_test_dhm_params,
                    mbedtls_test_dhm_params_len ) ) != 0 )
    {
        if( verbose != 0 )
            mbedtls_printf( "failed\n" );

        ret = 1;
        goto exit;
    }

    if( verbose != 0 )
        mbedtls_printf( "passed\n\n" );

exit:
    mbedtls_dhm_free( &dhm );

    return( ret );
}

#endif /* MBEDTLS_SELF_TEST */

#endif /* MBEDTLS_DHM_C */