VeriFone MX800 series Programmer's Manual page 223

The resultant MAC value is placed in the users result buffer and is formatted as
follows:
Process code field One character, indicates status
"0" = no error and MAC follows
MAC field 16 character MAC value
The message parameter is blocked into 32-byte blocks and a running calculation
is performed on each block. Maximum number of blocks = 100, so maximum
message length is 3200 bytes.
If the second_key parameter is non-zero then MAC generation starts with the
master_key and if the message is longer than 1 32-byte block, the final block will
be processed using the second_key for enhanced security.
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ipp_mac()
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