ANSI X9.19 Optional Procedure 1 MAC
The Financial Institution (Retail) Message Authentication Standard, ANSI X9.19 Optional Procedure 1 (ISO/IEC 9797-1, Algorithm 3) specifies additional processing of the 64-bit On MAC value.
The CCA X9.19OPT process employs a double-length DES key. After calculating the 64-bit MAC as above with the left half of the double-length key, the result is decrypted using the right half of the double-length key. This result is then encrypted with the left half of the double-length key. The resulting MAC value is processed according to other specifications supplied to the verb call.