int mbedtls_aes_crypt_xts()

in Reference/RIoT/RIoTCrypt/aes.c [1171:1275]

• 66 lines of code
• 18 McCabe index (conditional complexity)

int mbedtls_aes_crypt_xts( mbedtls_aes_xts_context *ctx,
                           int mode,
                           size_t length,
                           const unsigned char data_unit[16],
                           const unsigned char *input,
                           unsigned char *output )
{
    int ret;
    size_t blocks = length / 16;
    size_t leftover = length % 16;
    unsigned char tweak[16];
    unsigned char prev_tweak[16];
    unsigned char tmp[16];

    AES_VALIDATE_RET( ctx != NULL );
    AES_VALIDATE_RET( mode == MBEDTLS_AES_ENCRYPT ||
                      mode == MBEDTLS_AES_DECRYPT );
    AES_VALIDATE_RET( data_unit != NULL );
    AES_VALIDATE_RET( input != NULL );
    AES_VALIDATE_RET( output != NULL );

    /* Data units must be at least 16 bytes long. */
    if( length < 16 )
        return MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH;

    /* NIST SP 800-38E disallows data units larger than 2**20 blocks. */
    if( length > ( 1 << 20 ) * 16 )
        return MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH;

    /* Compute the tweak. */
    ret = mbedtls_aes_crypt_ecb( &ctx->tweak, MBEDTLS_AES_ENCRYPT,
                                 data_unit, tweak );
    if( ret != 0 )
        return( ret );

    while( blocks-- )
    {
        size_t i;

        if( leftover && ( mode == MBEDTLS_AES_DECRYPT ) && blocks == 0 )
        {
            /* We are on the last block in a decrypt operation that has
             * leftover bytes, so we need to use the next tweak for this block,
             * and this tweak for the lefover bytes. Save the current tweak for
             * the leftovers and then update the current tweak for use on this,
             * the last full block. */
            memcpy( prev_tweak, tweak, sizeof( tweak ) );
            mbedtls_gf128mul_x_ble( tweak, tweak );
        }

        for( i = 0; i < 16; i++ )
            tmp[i] = input[i] ^ tweak[i];

        ret = mbedtls_aes_crypt_ecb( &ctx->crypt, mode, tmp, tmp );
        if( ret != 0 )
            return( ret );

        for( i = 0; i < 16; i++ )
            output[i] = tmp[i] ^ tweak[i];

        /* Update the tweak for the next block. */
        mbedtls_gf128mul_x_ble( tweak, tweak );

        output += 16;
        input += 16;
    }

    if( leftover )
    {
        /* If we are on the leftover bytes in a decrypt operation, we need to
         * use the previous tweak for these bytes (as saved in prev_tweak). */
        unsigned char *t = mode == MBEDTLS_AES_DECRYPT ? prev_tweak : tweak;

        /* We are now on the final part of the data unit, which doesn't divide
         * evenly by 16. It's time for ciphertext stealing. */
        size_t i;
        unsigned char *prev_output = output - 16;

        /* Copy ciphertext bytes from the previous block to our output for each
         * byte of cyphertext we won't steal. At the same time, copy the
         * remainder of the input for this final round (since the loop bounds
         * are the same). */
        for( i = 0; i < leftover; i++ )
        {
            output[i] = prev_output[i];
            tmp[i] = input[i] ^ t[i];
        }

        /* Copy ciphertext bytes from the previous block for input in this
         * round. */
        for( ; i < 16; i++ )
            tmp[i] = prev_output[i] ^ t[i];

        ret = mbedtls_aes_crypt_ecb( &ctx->crypt, mode, tmp, tmp );
        if( ret != 0 )
            return ret;

        /* Write the result back to the previous block, overriding the previous
         * output we copied. */
        for( i = 0; i < 16; i++ )
            prev_output[i] = tmp[i] ^ t[i];
    }

    return( 0 );
}