int unhexify(unsigned char *obuf, const char *ibuf)
{
    unsigned char c, c2;
    int len = strlen(ibuf) / 2;
    assert(!(strlen(ibuf) %1)); // must be even number of bytes

    while (*ibuf != 0)
    {
        c = *ibuf++;
        if( c >= '0' && c <= '9' )
            c -= '0';
        else if( c >= 'a' && c <= 'f' )
            c -= 'a' - 10;
        else if( c >= 'A' && c <= 'F' )
            c -= 'A' - 10;
        else
            assert( 0 );

        c2 = *ibuf++;
        if( c2 >= '0' && c2 <= '9' )
            c2 -= '0';
        else if( c2 >= 'a' && c2 <= 'f' )
            c2 -= 'a' - 10;
        else if( c2 >= 'A' && c2 <= 'F' )
            c2 -= 'A' - 10;
        else
            assert( 0 );

        *obuf++ = ( c << 4 ) | c2;
    }

    return len;
}

void hexify(unsigned char *obuf, const unsigned char *ibuf, int len)
{
    unsigned char l, h;

    while (len != 0)
    {
        h = (*ibuf) / 16;
        l = (*ibuf) % 16;

        if( h < 10 )
            *obuf++ = '0' + h;
        else
            *obuf++ = 'a' + h - 10;

        if( l < 10 )
            *obuf++ = '0' + l;
        else
            *obuf++ = 'a' + l - 10;

        ++ibuf;
        len--;
    }
}

/**
 * This function just returns data from rand().
 *
 * rng_state shall be NULL.
 */
static int rnd_std_rand( void *rng_state )
{
    if( rng_state != NULL )
        rng_state  = NULL;

    return( rand() );
}

/**
 * This function only returns zeros
 *
 * rng_state shall be NULL.
 */
static int rnd_zero_rand( void *rng_state )
{
    if( rng_state != NULL )
        rng_state  = NULL;

    return( 0 );
}

typedef struct
{
    unsigned char *buf;
    int length;
    int per_call;
} rnd_info;

/**
 * This function returns random based on a buffer it receives.
 *
 * rng_state shall be a pointer to a rnd_buf structure.
 *
 * After the buffer is empty it will return rand();
 */
static int rnd_buffer_rand( void *rng_state )
{
    rnd_info *info = (rnd_info *) rng_state;
    int res;

    if( rng_state == NULL )
        return( rand() );

    res = rand();

    if( info->length >= info->per_call )
    {
        memcpy( &res, info->buf, info->per_call );
        info->buf += info->per_call;
        info->length -= info->per_call;
    }
    else if( info->length > 0 )
    {
        memcpy( &res, info->buf, info->length );
        info->length = 0;
    }

    return( res );
}