library/hmac_drbg.c

/*
 *  HMAC_DRBG implementation (NIST SP 800-90)
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 *
 *  This file is provided under the Apache License 2.0, or the
 *  GNU General Public License v2.0 or later.
 *
 *  **********
 *  Apache License 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.
 *
 *  **********
 *
 *  **********
 *  GNU General Public License v2.0 or later:
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 *  **********
 */

/*
 *  The NIST SP 800-90A DRBGs are described in the following publication.
 *  http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf
 *  References below are based on rev. 1 (January 2012).
 */

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

#if defined(MBEDTLS_HMAC_DRBG_C)

#include "mbedtls/hmac_drbg.h"

#include <string.h>

#if defined(MBEDTLS_FS_IO)
#include <stdio.h>
#endif

#if defined(MBEDTLS_SELF_TEST)
#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdio.h>
#define mbedtls_printf printf
#endif /* MBEDTLS_SELF_TEST */
#endif /* MBEDTLS_PLATFORM_C */

/* 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;
}

/*
 * HMAC_DRBG context initialization
 */
void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx )
{
    memset( ctx, 0, sizeof( mbedtls_hmac_drbg_context ) );

    ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init( &ctx->mutex );
#endif
}

/*
 * HMAC_DRBG update, using optional additional data (10.1.2.2)
 */
int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
                                  const unsigned char *additional,
                                  size_t add_len )
{
    size_t md_len = mbedtls_md_get_size( ctx->md_ctx.md_info );
    unsigned char rounds = ( additional != NULL && add_len != 0 ) ? 2 : 1;
    unsigned char sep[1];
    unsigned char K[MBEDTLS_MD_MAX_SIZE];
    int ret;

    for( sep[0] = 0; sep[0] < rounds; sep[0]++ )
    {
        /* Step 1 or 4 */
        if( ( ret = mbedtls_md_hmac_reset( &ctx->md_ctx ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_update( &ctx->md_ctx,
                                            ctx->V, md_len ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_update( &ctx->md_ctx,
                                            sep, 1 ) ) != 0 )
            goto exit;
        if( rounds == 2 )
        {
            if( ( ret = mbedtls_md_hmac_update( &ctx->md_ctx,
                                                additional, add_len ) ) != 0 )
            goto exit;
        }
        if( ( ret = mbedtls_md_hmac_finish( &ctx->md_ctx, K ) ) != 0 )
            goto exit;

        /* Step 2 or 5 */
        if( ( ret = mbedtls_md_hmac_starts( &ctx->md_ctx, K, md_len ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_update( &ctx->md_ctx,
                                            ctx->V, md_len ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_finish( &ctx->md_ctx, ctx->V ) ) != 0 )
            goto exit;
    }

exit:
    mbedtls_zeroize( K, sizeof( K ) );
    return( ret );
}

void mbedtls_hmac_drbg_update( mbedtls_hmac_drbg_context *ctx,
                               const unsigned char *additional,
                               size_t add_len )
{
    (void) mbedtls_hmac_drbg_update_ret( ctx, additional, add_len );
}

/*
 * Simplified HMAC_DRBG initialisation (for use with deterministic ECDSA)
 */
int mbedtls_hmac_drbg_seed_buf( mbedtls_hmac_drbg_context *ctx,
                        const mbedtls_md_info_t * md_info,
                        const unsigned char *data, size_t data_len )
{
    int ret;

    if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
        return( ret );

    /*
     * Set initial working state.
     * Use the V memory location, which is currently all 0, to initialize the
     * MD context with an all-zero key. Then set V to its initial value.
     */
    if( ( ret = mbedtls_md_hmac_starts( &ctx->md_ctx, ctx->V,
                                        mbedtls_md_get_size( md_info ) ) ) != 0 )
        return( ret );
    memset( ctx->V, 0x01, mbedtls_md_get_size( md_info ) );

    if( ( ret = mbedtls_hmac_drbg_update_ret( ctx, data, data_len ) ) != 0 )
        return( ret );

    return( 0 );
}

/*
 * Internal function used both for seeding and reseeding the DRBG.
 * Comments starting with arabic numbers refer to section 10.1.2.4
 * of SP800-90A, while roman numbers refer to section 9.2.
 */
static int hmac_drbg_reseed_core( mbedtls_hmac_drbg_context *ctx,
                                  const unsigned char *additional, size_t len,
                                  int use_nonce )
{
    unsigned char seed[MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT];
    size_t seedlen = 0;
    int ret;

    {
        size_t total_entropy_len;

        if( use_nonce == 0 )
            total_entropy_len = ctx->entropy_len;
        else
            total_entropy_len = ctx->entropy_len * 3 / 2;

        /* III. Check input length */
        if( len > MBEDTLS_HMAC_DRBG_MAX_INPUT ||
            total_entropy_len + len > MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT )
        {
            return( MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG );
        }
    }

    memset( seed, 0, MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT );

    /* IV. Gather entropy_len bytes of entropy for the seed */
    if( ( ret = ctx->f_entropy( ctx->p_entropy,
                                seed, ctx->entropy_len ) ) != 0 )
    {
        return( MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED );
    }
    seedlen += ctx->entropy_len;

    /* For initial seeding, allow adding of nonce generated
     * from the entropy source. See Sect 8.6.7 in SP800-90A. */
    if( use_nonce )
    {
        /* Note: We don't merge the two calls to f_entropy() in order
         *       to avoid requesting too much entropy from f_entropy()
         *       at once. Specifically, if the underlying digest is not
         *       SHA-1, 3 / 2 * entropy_len is at least 36 Bytes, which
         *       is larger than the maximum of 32 Bytes that our own
         *       entropy source implementation can emit in a single
         *       call in configurations disabling SHA-512. */
        if( ( ret = ctx->f_entropy( ctx->p_entropy,
                                    seed + seedlen,
                                    ctx->entropy_len / 2 ) ) != 0 )
        {
            return( MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED );
        }

        seedlen += ctx->entropy_len / 2;
    }


    /* 1. Concatenate entropy and additional data if any */
    if( additional != NULL && len != 0 )
    {
        memcpy( seed + seedlen, additional, len );
        seedlen += len;
    }

    /* 2. Update state */
    if( ( ret = mbedtls_hmac_drbg_update_ret( ctx, seed, seedlen ) ) != 0 )
        goto exit;

    /* 3. Reset reseed_counter */
    ctx->reseed_counter = 1;

exit:
    /* 4. Done */
    mbedtls_zeroize( seed, seedlen );
    return( ret );
}

/*
 * HMAC_DRBG reseeding: 10.1.2.4 + 9.2
 */
int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
                      const unsigned char *additional, size_t len )
{
    return( hmac_drbg_reseed_core( ctx, additional, len, 0 ) );
}

/*
 * HMAC_DRBG initialisation (10.1.2.3 + 9.1)
 *
 * The nonce is not passed as a separate parameter but extracted
 * from the entropy source as suggested in 8.6.7.
 */
int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
                    const mbedtls_md_info_t * md_info,
                    int (*f_entropy)(void *, unsigned char *, size_t),
                    void *p_entropy,
                    const unsigned char *custom,
                    size_t len )
{
    int ret;
    size_t md_size;

    if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
        return( ret );

    md_size = mbedtls_md_get_size( md_info );

    /*
     * Set initial working state.
     * Use the V memory location, which is currently all 0, to initialize the
     * MD context with an all-zero key. Then set V to its initial value.
     */
    if( ( ret = mbedtls_md_hmac_starts( &ctx->md_ctx, ctx->V, md_size ) ) != 0 )
        return( ret );
    memset( ctx->V, 0x01, md_size );

    ctx->f_entropy = f_entropy;
    ctx->p_entropy = p_entropy;

    if( ctx->entropy_len == 0 )
    {
        /*
         * See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
         * each hash function, then according to SP800-90A rev1 10.1 table 2,
         * min_entropy_len (in bits) is security_strength.
         *
         * (This also matches the sizes used in the NIST test vectors.)
         */
        ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
                           md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
                           32;  /* better (256+) -> 256 bits */
    }

    if( ( ret = hmac_drbg_reseed_core( ctx, custom, len,
                                       1 /* add nonce */ ) ) != 0 )
    {
        return( ret );
    }

    return( 0 );
}

/*
 * Set prediction resistance
 */
void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx,
                                          int resistance )
{
    ctx->prediction_resistance = resistance;
}

/*
 * Set entropy length grabbed for seeding
 */
void mbedtls_hmac_drbg_set_entropy_len( mbedtls_hmac_drbg_context *ctx, size_t len )
{
    ctx->entropy_len = len;
}

/*
 * Set reseed interval
 */
void mbedtls_hmac_drbg_set_reseed_interval( mbedtls_hmac_drbg_context *ctx, int interval )
{
    ctx->reseed_interval = interval;
}

/*
 * HMAC_DRBG random function with optional additional data:
 * 10.1.2.5 (arabic) + 9.3 (Roman)
 */
int mbedtls_hmac_drbg_random_with_add( void *p_rng,
                               unsigned char *output, size_t out_len,
                               const unsigned char *additional, size_t add_len )
{
    int ret;
    mbedtls_hmac_drbg_context *ctx = (mbedtls_hmac_drbg_context *) p_rng;
    size_t md_len = mbedtls_md_get_size( ctx->md_ctx.md_info );
    size_t left = out_len;
    unsigned char *out = output;

    /* II. Check request length */
    if( out_len > MBEDTLS_HMAC_DRBG_MAX_REQUEST )
        return( MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG );

    /* III. Check input length */
    if( add_len > MBEDTLS_HMAC_DRBG_MAX_INPUT )
        return( MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG );

    /* 1. (aka VII and IX) Check reseed counter and PR */
    if( ctx->f_entropy != NULL && /* For no-reseeding instances */
        ( ctx->prediction_resistance == MBEDTLS_HMAC_DRBG_PR_ON ||
          ctx->reseed_counter > ctx->reseed_interval ) )
    {
        if( ( ret = mbedtls_hmac_drbg_reseed( ctx, additional, add_len ) ) != 0 )
            return( ret );

        add_len = 0; /* VII.4 */
    }

    /* 2. Use additional data if any */
    if( additional != NULL && add_len != 0 )
    {
        if( ( ret = mbedtls_hmac_drbg_update_ret( ctx,
                                                  additional, add_len ) ) != 0 )
            goto exit;
    }

    /* 3, 4, 5. Generate bytes */
    while( left != 0 )
    {
        size_t use_len = left > md_len ? md_len : left;

        if( ( ret = mbedtls_md_hmac_reset( &ctx->md_ctx ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_update( &ctx->md_ctx,
                                            ctx->V, md_len ) ) != 0 )
            goto exit;
        if( ( ret = mbedtls_md_hmac_finish( &ctx->md_ctx, ctx->V ) ) != 0 )
            goto exit;

        memcpy( out, ctx->V, use_len );
        out += use_len;
        left -= use_len;
    }

    /* 6. Update */
    if( ( ret = mbedtls_hmac_drbg_update_ret( ctx,
                                              additional, add_len ) ) != 0 )
        goto exit;

    /* 7. Update reseed counter */
    ctx->reseed_counter++;

exit:
    /* 8. Done */
    return( ret );
}

/*
 * HMAC_DRBG random function
 */
int mbedtls_hmac_drbg_random( void *p_rng, unsigned char *output, size_t out_len )
{
    int ret;
    mbedtls_hmac_drbg_context *ctx = (mbedtls_hmac_drbg_context *) p_rng;

#if defined(MBEDTLS_THREADING_C)
    if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
        return( ret );
#endif

    ret = mbedtls_hmac_drbg_random_with_add( ctx, output, out_len, NULL, 0 );

#if defined(MBEDTLS_THREADING_C)
    if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
        return( MBEDTLS_ERR_THREADING_MUTEX_ERROR );
#endif

    return( ret );
}

/*
 *  This function resets HMAC_DRBG context to the state immediately
 *  after initial call of mbedtls_hmac_drbg_init().
 */
void mbedtls_hmac_drbg_free( mbedtls_hmac_drbg_context *ctx )
{
    if( ctx == NULL )
        return;

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_free( &ctx->mutex );
#endif
    mbedtls_md_free( &ctx->md_ctx );
    mbedtls_zeroize( ctx, sizeof( mbedtls_hmac_drbg_context ) );
    ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init( &ctx->mutex );
#endif
}

#if defined(MBEDTLS_FS_IO)
int mbedtls_hmac_drbg_write_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path )
{
    int ret;
    FILE *f;
    unsigned char buf[ MBEDTLS_HMAC_DRBG_MAX_INPUT ];

    if( ( f = fopen( path, "wb" ) ) == NULL )
        return( MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR );

    if( ( ret = mbedtls_hmac_drbg_random( ctx, buf, sizeof( buf ) ) ) != 0 )
        goto exit;

    if( fwrite( buf, 1, sizeof( buf ), f ) != sizeof( buf ) )
    {
        ret = MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR;
        goto exit;
    }

    ret = 0;

exit:
    fclose( f );
    mbedtls_zeroize( buf, sizeof( buf ) );

    return( ret );
}

int mbedtls_hmac_drbg_update_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path )
{
    int ret = 0;
    FILE *f;
    size_t n;
    unsigned char buf[ MBEDTLS_HMAC_DRBG_MAX_INPUT ];

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

    fseek( f, 0, SEEK_END );
    n = (size_t) ftell( f );
    fseek( f, 0, SEEK_SET );

    if( n > MBEDTLS_HMAC_DRBG_MAX_INPUT )
    {
        fclose( f );
        return( MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG );
    }

    if( fread( buf, 1, n, f ) != n )
        ret = MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR;
    else
        ret = mbedtls_hmac_drbg_update_ret( ctx, buf, n );

    fclose( f );

    mbedtls_zeroize( buf, sizeof( buf ) );

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

    return( mbedtls_hmac_drbg_write_seed_file( ctx, path ) );
}
#endif /* MBEDTLS_FS_IO */


#if defined(MBEDTLS_SELF_TEST)

#if !defined(MBEDTLS_SHA1_C)
/* Dummy checkup routine */
int mbedtls_hmac_drbg_self_test( int verbose )
{
    (void) verbose;
    return( 0 );
}
#else

#define OUTPUT_LEN  80

/* From a NIST PR=true test vector */
static const unsigned char entropy_pr[] = {
    0xa0, 0xc9, 0xab, 0x58, 0xf1, 0xe2, 0xe5, 0xa4, 0xde, 0x3e, 0xbd, 0x4f,
    0xf7, 0x3e, 0x9c, 0x5b, 0x64, 0xef, 0xd8, 0xca, 0x02, 0x8c, 0xf8, 0x11,
    0x48, 0xa5, 0x84, 0xfe, 0x69, 0xab, 0x5a, 0xee, 0x42, 0xaa, 0x4d, 0x42,
    0x17, 0x60, 0x99, 0xd4, 0x5e, 0x13, 0x97, 0xdc, 0x40, 0x4d, 0x86, 0xa3,
    0x7b, 0xf5, 0x59, 0x54, 0x75, 0x69, 0x51, 0xe4 };
static const unsigned char result_pr[OUTPUT_LEN] = {
    0x9a, 0x00, 0xa2, 0xd0, 0x0e, 0xd5, 0x9b, 0xfe, 0x31, 0xec, 0xb1, 0x39,
    0x9b, 0x60, 0x81, 0x48, 0xd1, 0x96, 0x9d, 0x25, 0x0d, 0x3c, 0x1e, 0x94,
    0x10, 0x10, 0x98, 0x12, 0x93, 0x25, 0xca, 0xb8, 0xfc, 0xcc, 0x2d, 0x54,
    0x73, 0x19, 0x70, 0xc0, 0x10, 0x7a, 0xa4, 0x89, 0x25, 0x19, 0x95, 0x5e,
    0x4b, 0xc6, 0x00, 0x1d, 0x7f, 0x4e, 0x6a, 0x2b, 0xf8, 0xa3, 0x01, 0xab,
    0x46, 0x05, 0x5c, 0x09, 0xa6, 0x71, 0x88, 0xf1, 0xa7, 0x40, 0xee, 0xf3,
    0xe1, 0x5c, 0x02, 0x9b, 0x44, 0xaf, 0x03, 0x44 };

/* From a NIST PR=false test vector */
static const unsigned char entropy_nopr[] = {
    0x79, 0x34, 0x9b, 0xbf, 0x7c, 0xdd, 0xa5, 0x79, 0x95, 0x57, 0x86, 0x66,
    0x21, 0xc9, 0x13, 0x83, 0x11, 0x46, 0x73, 0x3a, 0xbf, 0x8c, 0x35, 0xc8,
    0xc7, 0x21, 0x5b, 0x5b, 0x96, 0xc4, 0x8e, 0x9b, 0x33, 0x8c, 0x74, 0xe3,
    0xe9, 0x9d, 0xfe, 0xdf };
static const unsigned char result_nopr[OUTPUT_LEN] = {
    0xc6, 0xa1, 0x6a, 0xb8, 0xd4, 0x20, 0x70, 0x6f, 0x0f, 0x34, 0xab, 0x7f,
    0xec, 0x5a, 0xdc, 0xa9, 0xd8, 0xca, 0x3a, 0x13, 0x3e, 0x15, 0x9c, 0xa6,
    0xac, 0x43, 0xc6, 0xf8, 0xa2, 0xbe, 0x22, 0x83, 0x4a, 0x4c, 0x0a, 0x0a,
    0xff, 0xb1, 0x0d, 0x71, 0x94, 0xf1, 0xc1, 0xa5, 0xcf, 0x73, 0x22, 0xec,
    0x1a, 0xe0, 0x96, 0x4e, 0xd4, 0xbf, 0x12, 0x27, 0x46, 0xe0, 0x87, 0xfd,
    0xb5, 0xb3, 0xe9, 0x1b, 0x34, 0x93, 0xd5, 0xbb, 0x98, 0xfa, 0xed, 0x49,
    0xe8, 0x5f, 0x13, 0x0f, 0xc8, 0xa4, 0x59, 0xb7 };

/* "Entropy" from buffer */
static size_t test_offset;
static int hmac_drbg_self_test_entropy( void *data,
                                        unsigned char *buf, size_t len )
{
    const unsigned char *p = data;
    memcpy( buf, p + test_offset, len );
    test_offset += len;
    return( 0 );
}

#define CHK( c )    if( (c) != 0 )                          \
                    {                                       \
                        if( verbose != 0 )                  \
                            mbedtls_printf( "failed\n" );  \
                        return( 1 );                        \
                    }

/*
 * Checkup routine for HMAC_DRBG with SHA-1
 */
int mbedtls_hmac_drbg_self_test( int verbose )
{
    mbedtls_hmac_drbg_context ctx;
    unsigned char buf[OUTPUT_LEN];
    const mbedtls_md_info_t *md_info = mbedtls_md_info_from_type( MBEDTLS_MD_SHA1 );

    mbedtls_hmac_drbg_init( &ctx );

    /*
     * PR = True
     */
    if( verbose != 0 )
        mbedtls_printf( "  HMAC_DRBG (PR = True) : " );

    test_offset = 0;
    CHK( mbedtls_hmac_drbg_seed( &ctx, md_info,
                         hmac_drbg_self_test_entropy, (void *) entropy_pr,
                         NULL, 0 ) );
    mbedtls_hmac_drbg_set_prediction_resistance( &ctx, MBEDTLS_HMAC_DRBG_PR_ON );
    CHK( mbedtls_hmac_drbg_random( &ctx, buf, OUTPUT_LEN ) );
    CHK( mbedtls_hmac_drbg_random( &ctx, buf, OUTPUT_LEN ) );
    CHK( memcmp( buf, result_pr, OUTPUT_LEN ) );
    mbedtls_hmac_drbg_free( &ctx );

    mbedtls_hmac_drbg_free( &ctx );

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

    /*
     * PR = False
     */
    if( verbose != 0 )
        mbedtls_printf( "  HMAC_DRBG (PR = False) : " );

    mbedtls_hmac_drbg_init( &ctx );

    test_offset = 0;
    CHK( mbedtls_hmac_drbg_seed( &ctx, md_info,
                         hmac_drbg_self_test_entropy, (void *) entropy_nopr,
                         NULL, 0 ) );
    CHK( mbedtls_hmac_drbg_reseed( &ctx, NULL, 0 ) );
    CHK( mbedtls_hmac_drbg_random( &ctx, buf, OUTPUT_LEN ) );
    CHK( mbedtls_hmac_drbg_random( &ctx, buf, OUTPUT_LEN ) );
    CHK( memcmp( buf, result_nopr, OUTPUT_LEN ) );
    mbedtls_hmac_drbg_free( &ctx );

    mbedtls_hmac_drbg_free( &ctx );

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

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

    return( 0 );
}
#endif /* MBEDTLS_SHA1_C */
#endif /* MBEDTLS_SELF_TEST */

#endif /* MBEDTLS_HMAC_DRBG_C */