library/ctr_drbg.c

/*
 *  CTR_DRBG implementation based on AES-256 (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 800-90 DRBGs are described in the following publication.
 *
 *  http://csrc.nist.gov/publications/nistpubs/800-90/SP800-90revised_March2007.pdf
 */

#include "common.h"

#if defined(MBEDTLS_CTR_DRBG_C)

#include "mbedtls/ctr_drbg.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/error.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_PLATFORM_C */
#endif /* MBEDTLS_SELF_TEST */

/*
 * CTR_DRBG context initialization
 */
void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx )
{
    memset( ctx, 0, sizeof( mbedtls_ctr_drbg_context ) );
    /* Indicate that the entropy nonce length is not set explicitly.
     * See mbedtls_ctr_drbg_set_nonce_len(). */
    ctx->reseed_counter = -1;

    ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;

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

/*
 *  This function resets CTR_DRBG context to the state immediately
 *  after initial call of mbedtls_ctr_drbg_init().
 */
void mbedtls_ctr_drbg_free( mbedtls_ctr_drbg_context *ctx )
{
    if( ctx == NULL )
        return;

#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_free( &ctx->mutex );
#endif
    mbedtls_aes_free( &ctx->aes_ctx );
    mbedtls_platform_zeroize( ctx, sizeof( mbedtls_ctr_drbg_context ) );
    ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
    ctx->reseed_counter = -1;
#if defined(MBEDTLS_THREADING_C)
    mbedtls_mutex_init( &ctx->mutex );
#endif
}

void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
                                                 int resistance )
{
    ctx->prediction_resistance = resistance;
}

void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx,
                                       size_t len )
{
    ctx->entropy_len = len;
}

int mbedtls_ctr_drbg_set_nonce_len( mbedtls_ctr_drbg_context *ctx,
                                    size_t len )
{
    /* If mbedtls_ctr_drbg_seed() has already been called, it's
     * too late. Return the error code that's closest to making sense. */
    if( ctx->f_entropy != NULL )
        return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );

    if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
#if SIZE_MAX > INT_MAX
    /* This shouldn't be an issue because
     * MBEDTLS_CTR_DRBG_MAX_SEED_INPUT < INT_MAX in any sensible
     * configuration, but make sure anyway. */
    if( len > INT_MAX )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
#endif

    /* For backward compatibility with Mbed TLS <= 2.19, store the
     * entropy nonce length in a field that already exists, but isn't
     * used until after the initial seeding. */
    /* Due to the capping of len above, the value fits in an int. */
    ctx->reseed_counter = (int) len;
    return( 0 );
}

void mbedtls_ctr_drbg_set_reseed_interval( mbedtls_ctr_drbg_context *ctx,
                                           int interval )
{
    ctx->reseed_interval = interval;
}

static int block_cipher_df( unsigned char *output,
                            const unsigned char *data, size_t data_len )
{
    unsigned char buf[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT +
                      MBEDTLS_CTR_DRBG_BLOCKSIZE + 16];
    unsigned char tmp[MBEDTLS_CTR_DRBG_SEEDLEN];
    unsigned char key[MBEDTLS_CTR_DRBG_KEYSIZE];
    unsigned char chain[MBEDTLS_CTR_DRBG_BLOCKSIZE];
    unsigned char *p, *iv;
    mbedtls_aes_context aes_ctx;
    int ret = 0;

    int i, j;
    size_t buf_len, use_len;

    if( data_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );

    memset( buf, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT +
            MBEDTLS_CTR_DRBG_BLOCKSIZE + 16 );
    mbedtls_aes_init( &aes_ctx );

    /*
     * Construct IV (16 bytes) and S in buffer
     * IV = Counter (in 32-bits) padded to 16 with zeroes
     * S = Length input string (in 32-bits) || Length of output (in 32-bits) ||
     *     data || 0x80
     *     (Total is padded to a multiple of 16-bytes with zeroes)
     */
    p = buf + MBEDTLS_CTR_DRBG_BLOCKSIZE;
    *p++ = ( data_len >> 24 ) & 0xff;
    *p++ = ( data_len >> 16 ) & 0xff;
    *p++ = ( data_len >> 8  ) & 0xff;
    *p++ = ( data_len       ) & 0xff;
    p += 3;
    *p++ = MBEDTLS_CTR_DRBG_SEEDLEN;
    memcpy( p, data, data_len );
    p[data_len] = 0x80;

    buf_len = MBEDTLS_CTR_DRBG_BLOCKSIZE + 8 + data_len + 1;

    for( i = 0; i < MBEDTLS_CTR_DRBG_KEYSIZE; i++ )
        key[i] = i;

    if( ( ret = mbedtls_aes_setkey_enc( &aes_ctx, key,
                                        MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
    {
        goto exit;
    }

    /*
     * Reduce data to MBEDTLS_CTR_DRBG_SEEDLEN bytes of data
     */
    for( j = 0; j < MBEDTLS_CTR_DRBG_SEEDLEN; j += MBEDTLS_CTR_DRBG_BLOCKSIZE )
    {
        p = buf;
        memset( chain, 0, MBEDTLS_CTR_DRBG_BLOCKSIZE );
        use_len = buf_len;

        while( use_len > 0 )
        {
            for( i = 0; i < MBEDTLS_CTR_DRBG_BLOCKSIZE; i++ )
                chain[i] ^= p[i];
            p += MBEDTLS_CTR_DRBG_BLOCKSIZE;
            use_len -= ( use_len >= MBEDTLS_CTR_DRBG_BLOCKSIZE ) ?
                       MBEDTLS_CTR_DRBG_BLOCKSIZE : use_len;

            if( ( ret = mbedtls_aes_crypt_ecb( &aes_ctx, MBEDTLS_AES_ENCRYPT,
                                               chain, chain ) ) != 0 )
            {
                goto exit;
            }
        }

        memcpy( tmp + j, chain, MBEDTLS_CTR_DRBG_BLOCKSIZE );

        /*
         * Update IV
         */
        buf[3]++;
    }

    /*
     * Do final encryption with reduced data
     */
    if( ( ret = mbedtls_aes_setkey_enc( &aes_ctx, tmp,
                                        MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
    {
        goto exit;
    }
    iv = tmp + MBEDTLS_CTR_DRBG_KEYSIZE;
    p = output;

    for( j = 0; j < MBEDTLS_CTR_DRBG_SEEDLEN; j += MBEDTLS_CTR_DRBG_BLOCKSIZE )
    {
        if( ( ret = mbedtls_aes_crypt_ecb( &aes_ctx, MBEDTLS_AES_ENCRYPT,
                                           iv, iv ) ) != 0 )
        {
            goto exit;
        }
        memcpy( p, iv, MBEDTLS_CTR_DRBG_BLOCKSIZE );
        p += MBEDTLS_CTR_DRBG_BLOCKSIZE;
    }
exit:
    mbedtls_aes_free( &aes_ctx );
    /*
    * tidy up the stack
    */
    mbedtls_platform_zeroize( buf, sizeof( buf ) );
    mbedtls_platform_zeroize( tmp, sizeof( tmp ) );
    mbedtls_platform_zeroize( key, sizeof( key ) );
    mbedtls_platform_zeroize( chain, sizeof( chain ) );
    if( 0 != ret )
    {
        /*
        * wipe partial seed from memory
        */
        mbedtls_platform_zeroize( output, MBEDTLS_CTR_DRBG_SEEDLEN );
    }

    return( ret );
}

/* CTR_DRBG_Update (SP 800-90A &sect;10.2.1.2)
 * ctr_drbg_update_internal(ctx, provided_data)
 * implements
 * CTR_DRBG_Update(provided_data, Key, V)
 * with inputs and outputs
 *   ctx->aes_ctx = Key
 *   ctx->counter = V
 */
static int ctr_drbg_update_internal( mbedtls_ctr_drbg_context *ctx,
                          const unsigned char data[MBEDTLS_CTR_DRBG_SEEDLEN] )
{
    unsigned char tmp[MBEDTLS_CTR_DRBG_SEEDLEN];
    unsigned char *p = tmp;
    int i, j;
    int ret = 0;

    memset( tmp, 0, MBEDTLS_CTR_DRBG_SEEDLEN );

    for( j = 0; j < MBEDTLS_CTR_DRBG_SEEDLEN; j += MBEDTLS_CTR_DRBG_BLOCKSIZE )
    {
        /*
         * Increase counter
         */
        for( i = MBEDTLS_CTR_DRBG_BLOCKSIZE; i > 0; i-- )
            if( ++ctx->counter[i - 1] != 0 )
                break;

        /*
         * Crypt counter block
         */
        if( ( ret = mbedtls_aes_crypt_ecb( &ctx->aes_ctx, MBEDTLS_AES_ENCRYPT,
                                           ctx->counter, p ) ) != 0 )
        {
            goto exit;
        }

        p += MBEDTLS_CTR_DRBG_BLOCKSIZE;
    }

    for( i = 0; i < MBEDTLS_CTR_DRBG_SEEDLEN; i++ )
        tmp[i] ^= data[i];

    /*
     * Update key and counter
     */
    if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, tmp,
                                        MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
    {
        goto exit;
    }
    memcpy( ctx->counter, tmp + MBEDTLS_CTR_DRBG_KEYSIZE,
            MBEDTLS_CTR_DRBG_BLOCKSIZE );

exit:
    mbedtls_platform_zeroize( tmp, sizeof( tmp ) );
    return( ret );
}

/* CTR_DRBG_Instantiate with derivation function (SP 800-90A &sect;10.2.1.3.2)
 * mbedtls_ctr_drbg_update(ctx, additional, add_len)
 * implements
 * CTR_DRBG_Instantiate(entropy_input, nonce, personalization_string,
 *                      security_strength) -> initial_working_state
 * with inputs
 *   ctx->counter = all-bits-0
 *   ctx->aes_ctx = context from all-bits-0 key
 *   additional[:add_len] = entropy_input || nonce || personalization_string
 * and with outputs
 *   ctx = initial_working_state
 */
int mbedtls_ctr_drbg_update_ret( mbedtls_ctr_drbg_context *ctx,
                                 const unsigned char *additional,
                                 size_t add_len )
{
    unsigned char add_input[MBEDTLS_CTR_DRBG_SEEDLEN];
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( add_len == 0 )
        return( 0 );

    if( ( ret = block_cipher_df( add_input, additional, add_len ) ) != 0 )
        goto exit;
    if( ( ret = ctr_drbg_update_internal( ctx, add_input ) ) != 0 )
        goto exit;

exit:
    mbedtls_platform_zeroize( add_input, sizeof( add_input ) );
    return( ret );
}

#if !defined(MBEDTLS_DEPRECATED_REMOVED)
void mbedtls_ctr_drbg_update( mbedtls_ctr_drbg_context *ctx,
                              const unsigned char *additional,
                              size_t add_len )
{
    /* MAX_INPUT would be more logical here, but we have to match
     * block_cipher_df()'s limits since we can't propagate errors */
    if( add_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
        add_len = MBEDTLS_CTR_DRBG_MAX_SEED_INPUT;
    (void) mbedtls_ctr_drbg_update_ret( ctx, additional, add_len );
}
#endif /* MBEDTLS_DEPRECATED_REMOVED */

/* CTR_DRBG_Reseed with derivation function (SP 800-90A &sect;10.2.1.4.2)
 * mbedtls_ctr_drbg_reseed(ctx, additional, len, nonce_len)
 * implements
 * CTR_DRBG_Reseed(working_state, entropy_input, additional_input)
 *                -> new_working_state
 * with inputs
 *   ctx contains working_state
 *   additional[:len] = additional_input
 * and entropy_input comes from calling ctx->f_entropy
 *                              for (ctx->entropy_len + nonce_len) bytes
 * and with output
 *   ctx contains new_working_state
 */
static int mbedtls_ctr_drbg_reseed_internal( mbedtls_ctr_drbg_context *ctx,
                                             const unsigned char *additional,
                                             size_t len,
                                             size_t nonce_len )
{
    unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT];
    size_t seedlen = 0;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
    if( nonce_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
    if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len - nonce_len )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );

    memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT );

    /* Gather entropy_len bytes of entropy to seed state. */
    if( 0 != ctx->f_entropy( ctx->p_entropy, seed, ctx->entropy_len ) )
    {
        return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );
    }
    seedlen += ctx->entropy_len;

    /* Gather entropy for a nonce if requested. */
    if( nonce_len != 0 )
    {
        if( 0 != ctx->f_entropy( ctx->p_entropy, seed, nonce_len ) )
        {
            return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );
        }
        seedlen += nonce_len;
    }

    /* Add additional data if provided. */
    if( additional != NULL && len != 0 )
    {
        memcpy( seed + seedlen, additional, len );
        seedlen += len;
    }

    /* Reduce to 384 bits. */
    if( ( ret = block_cipher_df( seed, seed, seedlen ) ) != 0 )
        goto exit;

    /* Update state. */
    if( ( ret = ctr_drbg_update_internal( ctx, seed ) ) != 0 )
        goto exit;
    ctx->reseed_counter = 1;

exit:
    mbedtls_platform_zeroize( seed, sizeof( seed ) );
    return( ret );
}

int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
                             const unsigned char *additional, size_t len )
{
    return( mbedtls_ctr_drbg_reseed_internal( ctx, additional, len, 0 ) );
}

/* Return a "good" nonce length for CTR_DRBG. The chosen nonce length
 * is sufficient to achieve the maximum security strength given the key
 * size and entropy length. If there is enough entropy in the initial
 * call to the entropy function to serve as both the entropy input and
 * the nonce, don't make a second call to get a nonce. */
static size_t good_nonce_len( size_t entropy_len )
{
    if( entropy_len >= MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2 )
        return( 0 );
    else
        return( ( entropy_len + 1 ) / 2 );
}

/* CTR_DRBG_Instantiate with derivation function (SP 800-90A &sect;10.2.1.3.2)
 * mbedtls_ctr_drbg_seed(ctx, f_entropy, p_entropy, custom, len)
 * implements
 * CTR_DRBG_Instantiate(entropy_input, nonce, personalization_string,
 *                      security_strength) -> initial_working_state
 * with inputs
 *   custom[:len] = nonce || personalization_string
 * where entropy_input comes from f_entropy for ctx->entropy_len bytes
 * and with outputs
 *   ctx = initial_working_state
 */
int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
                           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;
    unsigned char key[MBEDTLS_CTR_DRBG_KEYSIZE];
    size_t nonce_len;

    memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );

    mbedtls_aes_init( &ctx->aes_ctx );

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

    if( ctx->entropy_len == 0 )
        ctx->entropy_len = MBEDTLS_CTR_DRBG_ENTROPY_LEN;
    /* ctx->reseed_counter contains the desired amount of entropy to
     * grab for a nonce (see mbedtls_ctr_drbg_set_nonce_len()).
     * If it's -1, indicating that the entropy nonce length was not set
     * explicitly, use a sufficiently large nonce for security. */
    nonce_len = ( ctx->reseed_counter >= 0 ?
                  (size_t) ctx->reseed_counter :
                  good_nonce_len( ctx->entropy_len ) );

    /* Initialize with an empty key. */
    if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, key,
                                        MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
    {
        return( ret );
    }

    /* Do the initial seeding. */
    if( ( ret = mbedtls_ctr_drbg_reseed_internal( ctx, custom, len,
                                                  nonce_len ) ) != 0 )
    {
        return( ret );
    }
    return( 0 );
}

/* CTR_DRBG_Generate with derivation function (SP 800-90A &sect;10.2.1.5.2)
 * mbedtls_ctr_drbg_random_with_add(ctx, output, output_len, additional, add_len)
 * implements
 * CTR_DRBG_Reseed(working_state, entropy_input, additional[:add_len])
 *                -> working_state_after_reseed
 *                if required, then
 * CTR_DRBG_Generate(working_state_after_reseed,
 *                   requested_number_of_bits, additional_input)
 *                -> status, returned_bits, new_working_state
 * with inputs
 *   ctx contains working_state
 *   requested_number_of_bits = 8 * output_len
 *   additional[:add_len] = additional_input
 * and entropy_input comes from calling ctx->f_entropy
 * and with outputs
 *   status = SUCCESS (this function does the reseed internally)
 *   returned_bits = output[:output_len]
 *   ctx contains new_working_state
 */
int mbedtls_ctr_drbg_random_with_add( void *p_rng,
                              unsigned char *output, size_t output_len,
                              const unsigned char *additional, size_t add_len )
{
    int ret = 0;
    mbedtls_ctr_drbg_context *ctx = (mbedtls_ctr_drbg_context *) p_rng;
    unsigned char add_input[MBEDTLS_CTR_DRBG_SEEDLEN];
    unsigned char *p = output;
    unsigned char tmp[MBEDTLS_CTR_DRBG_BLOCKSIZE];
    int i;
    size_t use_len;

    if( output_len > MBEDTLS_CTR_DRBG_MAX_REQUEST )
        return( MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG );

    if( add_len > MBEDTLS_CTR_DRBG_MAX_INPUT )
        return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );

    memset( add_input, 0, MBEDTLS_CTR_DRBG_SEEDLEN );

    if( ctx->reseed_counter > ctx->reseed_interval ||
        ctx->prediction_resistance )
    {
        if( ( ret = mbedtls_ctr_drbg_reseed( ctx, additional, add_len ) ) != 0 )
        {
            return( ret );
        }
        add_len = 0;
    }

    if( add_len > 0 )
    {
        if( ( ret = block_cipher_df( add_input, additional, add_len ) ) != 0 )
            goto exit;
        if( ( ret = ctr_drbg_update_internal( ctx, add_input ) ) != 0 )
            goto exit;
    }

    while( output_len > 0 )
    {
        /*
         * Increase counter
         */
        for( i = MBEDTLS_CTR_DRBG_BLOCKSIZE; i > 0; i-- )
            if( ++ctx->counter[i - 1] != 0 )
                break;

        /*
         * Crypt counter block
         */
        if( ( ret = mbedtls_aes_crypt_ecb( &ctx->aes_ctx, MBEDTLS_AES_ENCRYPT,
                                           ctx->counter, tmp ) ) != 0 )
        {
            goto exit;
        }

        use_len = ( output_len > MBEDTLS_CTR_DRBG_BLOCKSIZE )
            ? MBEDTLS_CTR_DRBG_BLOCKSIZE : output_len;
        /*
         * Copy random block to destination
         */
        memcpy( p, tmp, use_len );
        p += use_len;
        output_len -= use_len;
    }

    if( ( ret = ctr_drbg_update_internal( ctx, add_input ) ) != 0 )
        goto exit;

    ctx->reseed_counter++;

exit:
    mbedtls_platform_zeroize( add_input, sizeof( add_input ) );
    mbedtls_platform_zeroize( tmp, sizeof( tmp ) );
    return( ret );
}

int mbedtls_ctr_drbg_random( void *p_rng, unsigned char *output,
                             size_t output_len )
{
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    mbedtls_ctr_drbg_context *ctx = (mbedtls_ctr_drbg_context *) p_rng;

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

    ret = mbedtls_ctr_drbg_random_with_add( ctx, output, output_len, NULL, 0 );

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

    return( ret );
}

#if defined(MBEDTLS_FS_IO)
int mbedtls_ctr_drbg_write_seed_file( mbedtls_ctr_drbg_context *ctx,
                                      const char *path )
{
    int ret = MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR;
    FILE *f;
    unsigned char buf[ MBEDTLS_CTR_DRBG_MAX_INPUT ];

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

    if( ( ret = mbedtls_ctr_drbg_random( ctx, buf,
                                         MBEDTLS_CTR_DRBG_MAX_INPUT ) ) != 0 )
        goto exit;

    if( fwrite( buf, 1, MBEDTLS_CTR_DRBG_MAX_INPUT, f ) !=
        MBEDTLS_CTR_DRBG_MAX_INPUT )
    {
        ret = MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR;
    }
    else
    {
        ret = 0;
    }

exit:
    mbedtls_platform_zeroize( buf, sizeof( buf ) );

    fclose( f );
    return( ret );
}

int mbedtls_ctr_drbg_update_seed_file( mbedtls_ctr_drbg_context *ctx,
                                       const char *path )
{
    int ret = 0;
    FILE *f = NULL;
    size_t n;
    unsigned char buf[ MBEDTLS_CTR_DRBG_MAX_INPUT ];
    unsigned char c;

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

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

    ret = mbedtls_ctr_drbg_update_ret( ctx, buf, n );

exit:
    mbedtls_platform_zeroize( buf, sizeof( buf ) );
    if( f != NULL )
        fclose( f );
    if( ret != 0 )
        return( ret );
    return( mbedtls_ctr_drbg_write_seed_file( ctx, path ) );
}
#endif /* MBEDTLS_FS_IO */

#if defined(MBEDTLS_SELF_TEST)

static const unsigned char entropy_source_pr[96] =
    { 0xc1, 0x80, 0x81, 0xa6, 0x5d, 0x44, 0x02, 0x16,
      0x19, 0xb3, 0xf1, 0x80, 0xb1, 0xc9, 0x20, 0x02,
      0x6a, 0x54, 0x6f, 0x0c, 0x70, 0x81, 0x49, 0x8b,
      0x6e, 0xa6, 0x62, 0x52, 0x6d, 0x51, 0xb1, 0xcb,
      0x58, 0x3b, 0xfa, 0xd5, 0x37, 0x5f, 0xfb, 0xc9,
      0xff, 0x46, 0xd2, 0x19, 0xc7, 0x22, 0x3e, 0x95,
      0x45, 0x9d, 0x82, 0xe1, 0xe7, 0x22, 0x9f, 0x63,
      0x31, 0x69, 0xd2, 0x6b, 0x57, 0x47, 0x4f, 0xa3,
      0x37, 0xc9, 0x98, 0x1c, 0x0b, 0xfb, 0x91, 0x31,
      0x4d, 0x55, 0xb9, 0xe9, 0x1c, 0x5a, 0x5e, 0xe4,
      0x93, 0x92, 0xcf, 0xc5, 0x23, 0x12, 0xd5, 0x56,
      0x2c, 0x4a, 0x6e, 0xff, 0xdc, 0x10, 0xd0, 0x68 };

static const unsigned char entropy_source_nopr[64] =
    { 0x5a, 0x19, 0x4d, 0x5e, 0x2b, 0x31, 0x58, 0x14,
      0x54, 0xde, 0xf6, 0x75, 0xfb, 0x79, 0x58, 0xfe,
      0xc7, 0xdb, 0x87, 0x3e, 0x56, 0x89, 0xfc, 0x9d,
      0x03, 0x21, 0x7c, 0x68, 0xd8, 0x03, 0x38, 0x20,
      0xf9, 0xe6, 0x5e, 0x04, 0xd8, 0x56, 0xf3, 0xa9,
      0xc4, 0x4a, 0x4c, 0xbd, 0xc1, 0xd0, 0x08, 0x46,
      0xf5, 0x98, 0x3d, 0x77, 0x1c, 0x1b, 0x13, 0x7e,
      0x4e, 0x0f, 0x9d, 0x8e, 0xf4, 0x09, 0xf9, 0x2e };

static const unsigned char nonce_pers_pr[16] =
    { 0xd2, 0x54, 0xfc, 0xff, 0x02, 0x1e, 0x69, 0xd2,
      0x29, 0xc9, 0xcf, 0xad, 0x85, 0xfa, 0x48, 0x6c };

static const unsigned char nonce_pers_nopr[16] =
    { 0x1b, 0x54, 0xb8, 0xff, 0x06, 0x42, 0xbf, 0xf5,
      0x21, 0xf1, 0x5c, 0x1c, 0x0b, 0x66, 0x5f, 0x3f };

#if defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY)
static const unsigned char result_pr[16] =
    { 0x95, 0x3c, 0xa5, 0xbd, 0x44, 0x1, 0x34, 0xb7,
      0x13, 0x58, 0x3e, 0x6a, 0x6c, 0x7e, 0x88, 0x8a };

static const unsigned char result_nopr[16] =
    { 0x6c, 0x25, 0x27, 0x95, 0xa3, 0x62, 0xd6, 0xdb,
      0x90, 0xfd, 0x69, 0xb5, 0x42, 0x9, 0x4b, 0x84 };
#else /* MBEDTLS_CTR_DRBG_USE_128_BIT_KEY */
static const unsigned char result_pr[16] =
    { 0x34, 0x01, 0x16, 0x56, 0xb4, 0x29, 0x00, 0x8f,
      0x35, 0x63, 0xec, 0xb5, 0xf2, 0x59, 0x07, 0x23 };

static const unsigned char result_nopr[16] =
    { 0xa0, 0x54, 0x30, 0x3d, 0x8a, 0x7e, 0xa9, 0x88,
      0x9d, 0x90, 0x3e, 0x07, 0x7c, 0x6f, 0x21, 0x8f };
#endif /* MBEDTLS_CTR_DRBG_USE_128_BIT_KEY */

static size_t test_offset;
static int ctr_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
 */
int mbedtls_ctr_drbg_self_test( int verbose )
{
    mbedtls_ctr_drbg_context ctx;
    unsigned char buf[16];

    mbedtls_ctr_drbg_init( &ctx );

    /*
     * Based on a NIST CTR_DRBG test vector (PR = True)
     */
    if( verbose != 0 )
        mbedtls_printf( "  CTR_DRBG (PR = TRUE) : " );

    test_offset = 0;
    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
    mbedtls_ctr_drbg_set_nonce_len( &ctx, 0 );
    CHK( mbedtls_ctr_drbg_seed( &ctx,
                                ctr_drbg_self_test_entropy,
                                (void *) entropy_source_pr,
                                nonce_pers_pr, 16 ) );
    mbedtls_ctr_drbg_set_prediction_resistance( &ctx, MBEDTLS_CTR_DRBG_PR_ON );
    CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
    CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
    CHK( memcmp( buf, result_pr, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );

    mbedtls_ctr_drbg_free( &ctx );

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

    /*
     * Based on a NIST CTR_DRBG test vector (PR = FALSE)
     */
    if( verbose != 0 )
        mbedtls_printf( "  CTR_DRBG (PR = FALSE): " );

    mbedtls_ctr_drbg_init( &ctx );

    test_offset = 0;
    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
    mbedtls_ctr_drbg_set_nonce_len( &ctx, 0 );
    CHK( mbedtls_ctr_drbg_seed( &ctx,
                                ctr_drbg_self_test_entropy,
                                (void *) entropy_source_nopr,
                                nonce_pers_nopr, 16 ) );
    CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
    CHK( mbedtls_ctr_drbg_reseed( &ctx, NULL, 0 ) );
    CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
    CHK( memcmp( buf, result_nopr, 16 ) );

    mbedtls_ctr_drbg_free( &ctx );

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

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

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

#endif /* MBEDTLS_CTR_DRBG_C */