mbedtls/library/hmac_drbg.c

/*
 *  HMAC_DRBG implementation (NIST SP 800-90)
 *
 *  Copyright The Mbed TLS Contributors
 *  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.
 */

/*
 *  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).
 */

#include "common.h"

#if defined(MBEDTLS_HMAC_DRBG_C)

#include "mbedtls/hmac_drbg.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/error.h"

#include <string.h>

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

#include "mbedtls/platform.h"

/*
 * 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;
}

/*
 * HMAC_DRBG update, using optional additional data (10.1.2.2)
 */
int mbedtls_hmac_drbg_update(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 = MBEDTLS_ERR_MD_BAD_INPUT_DATA;

    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_platform_zeroize(K, sizeof(K));
    return ret;
}

/*
 * 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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

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

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

    /*
     * 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(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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    {
        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(ctx, seed, seedlen)) != 0) {
        goto exit;
    }

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

exit:
    /* 4. Done */
    mbedtls_platform_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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    size_t md_size;

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

    /* The mutex is initialized iff the md context is set up. */
#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init(&ctx->mutex);
#endif

    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_ERR_ERROR_CORRUPTION_DETECTED;
    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(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(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_ERR_ERROR_CORRUPTION_DETECTED;
    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)
    /* The mutex is initialized iff the md context is set up. */
    if (ctx->md_ctx.md_info != NULL) {
        mbedtls_mutex_free(&ctx->mutex);
    }
#endif
    mbedtls_md_free(&ctx->md_ctx);
    mbedtls_platform_zeroize(ctx, sizeof(mbedtls_hmac_drbg_context));
    ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
}

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

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

    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(f, NULL);

    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_platform_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 = NULL;
    size_t n;
    unsigned char buf[MBEDTLS_HMAC_DRBG_MAX_INPUT];
    unsigned char c;

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

    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(f, NULL);

    n = fread(buf, 1, sizeof(buf), f);
    if (fread(&c, 1, 1, f) != 0) {
        ret = MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG;
        goto exit;
    }
    if (n == 0 || ferror(f)) {
        ret = MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR;
        goto exit;
    }
    fclose(f);
    f = NULL;

    ret = mbedtls_hmac_drbg_update(ctx, buf, n);

exit:
    mbedtls_platform_zeroize(buf, sizeof(buf));
    if (f != NULL) {
        fclose(f);
    }
    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_MD_CAN_SHA1)
/* 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_MD_CAN_SHA1 */
#endif /* MBEDTLS_SELF_TEST */

#endif /* MBEDTLS_HMAC_DRBG_C */