# ANSI X9.9 Cryptography Standards

I'm trying to create a ISO8583 Rev93 message. What is the standard way of generating MAC key in ANSI X9.9
DES-CBC encryption algorithm is used to encrypt the message and generate the MAC.

• How many bytes should I take from the encrypted byte array based on ANSI X9.9 standard? 8 or 16? I think I should take 4 bytes (8 ASCII characters) and place them on field 128 (MAC). But very doubtful about that.

• The MAC key that the customer has provided me is 16 characters. (like 1234567890123456). Should I convert it to an 8 byte key? or a 16 byte key? (Based on ISO8583 Rev93 and ANSI X9.9)

Thank you.

• I suspect your best path forward is going to be reading the relevant standard, or hiring a consultant who knows the standard. Given that this is a non-public standard, I don't know whether anyone here is going to be able to answer your questions off the top of their head.
– D.W.
Sep 4, 2011 at 4:20
• I think the standard that the OP is referring to must be ISO 8583, second edition, 1993; perhaps with corrigenda 1 to the second edition, 1999. It is listed on the ISO site but is no longer for sale.
– fgrieu
Sep 4, 2011 at 16:44

ANSI X9.9 uses DES for its crypto - these keys are 56 bits long, or 112 bits for 3DES. The financial world mostly moved to 3DES a few years back so keys will generally be 112 bits. Normally they have parity bits added (1 per 7 bits of key), so are transmitted as 64 bit (single DES) or 128 bit (triple DES) values.

When presented in human-readable form, they are almost always shown as hexadecimal, so a typical 3DES key would show as 32 characters, from 0-9 and A-F. Whether you need to convert it back to binary depends on the crypto API you are using - read the documentation.

With ISO8583, usual practice is to calculate the full 64 bit MAC using 3DES in CBC mode, then put the first 4 bytes of the result into field 64 or 128 of the ISO8583 message - field 128 if any of the fields from 65-127 are present, field 64 if only fields 1-63 are in use. The final 4 bytes of the field are normally just padded with zeros.

In some legacy systems the full 64 bit MAC value may be transmitted.

I have also seen ancient systems (20+ years ago) which use ECB mode rather than CBC - basically just XOR every 8 byte block of the message together, then apply a single DES encryption to the result. This is highly insecure.

without doing your homework for you: did you check the open-source implementations of ISO 8583? is the answer somewhere in there?

Retrieved from wikipedia (strangely, the Spanish page, a good tip for finding more external references is changing the language tab)