MODE_DECRYPT
mode = MODE_DECRYPT;
inverse = (alg == ALG_CBC || alg == ALG_ECB) && mode == MODE_DECRYPT;
case MODE_DECRYPT: /* decrypt using CBC mode */
case MODE_DECRYPT: /* decrypt using CFB mode */
case MODE_DECRYPT: /* decrypt using CFBA mode */
case MODE_DECRYPT: /* decrypt using ECB mode */
case MODE_DECRYPT: /* decrypt using OFB mode */