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I want to make an ISO Message with a field 64 message authentication code (MAC). I want to know what to make mac with, the binary of the ISO message without field 64? or the binary of the iso message with noting set on Field 64 but a 1 in the end of bitmap showing that there is something in Field 64?

Panagiotis Giannakis
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Saeed
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1 Answers1

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You're supposed to determine the fields you wish to use in the MAC calculation. Select specific fields and apply your MAC-ing algorithm.

Generally, you can go by the following guidlines:

  1. Do not use the either of the MAC fields (F64/F128) in the calculation of the MAC. Those fields are supposed to contain the results of the calculation of MAC; including them in the calculation will guarantee that the MAC value will always be inconsistent

  2. Try to use mandatory fields, i.e. fields that you (or ISO) have designated as mandatory for the message type you're looking to MAC. For some vendors (like ACI, Base24), the Message Header, Message Type Identifier (MTI) and primary bitmap are all available to be included in the MAC calculation.

Ultimately, you're supposed to just select a handful of guaranteed fields and apply your MAC-ing algorithm. What would be the point of flagging F64 as enabled without populating it?

kolossus
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