MAC Verification on Transaction Response

The 16-character alphanumeric MessageAuthenticationCode should be Encrypted using the MAC Encryption Key. The value is calculated and sent in the Debit request message. The Debit response contains a MessageAuthenticationCode value; the value in the response must be verified as follows:

Transaction Response	Rules
Approval	Verify the value.
Timeout/No Response	No value to verify. Send a reversal with ReversalReasonCode set to TIMEOUT.
Response Indicates MAC Verification Failed	No value to verify. Send DebitReversal with ReversalReasonCode set to MACFAILURE.
Host Out of Sync	If the POSSequenceNbr value in the POS and on the Host get out of sync, then the host will return a decline message with Bank Response Code = 899; when this occurs, the MAC value will need to be recalculated before the next Debit request message is sent. After POSSequenceNbr is reset, recalculate the MessageAuthenticationCode. If a MACKey was returned in the response, use that key in the MAC calculation. If the POS automatically resends the transaction without re-initializing from the beginning where cardholder enters PIN, the request will be declined with BankResponseCode 877 (Invalid PIN BLOCK). A response with 877 would necessitate a Key Exchange, which must be performed before the Debit transaction request is sent (see InteracDeviceKeys).
Response Received without BankResponseCode	This scenario is treated as "Transaction Not Completed." Do not verify the value.
Response Received with BankResponseCode = 877	Key Exchange required. Perform InteracDeviceKeys request to reset the value.
Response Received with BankResponseCode = 898	Key Exchange required. Perform InteracDeviceKeys request to reset the value.