SB nº SU-28 : Format 1 Secure Messaging Chaining (Spec Change)

Specification Update: Bulletin No. 28 April 2004 First Edition Format 1 Secure Messaging Chaining1 This bulletin applies to EMV 2000 Integrated Circuit Card Specifications for Payment Systems Version 4.0 Book 2, Section 9.2.3 MAC Computation. For Format 1 secure messaging, it specifies a recommended method for chaining the MACs from one command to the next when chaining of the MACs is supported. Please make the following change to EMV 2000 Integrated Circuit Card Specifications for Payment Systems Version 4.0, Book 2, Section 9.2.3 MAC Computation: Add the following to the end of the section: If secure messaging is according to Format 1 and chaining of MACs from one command to the next is supported, the recommended method for chaining the MACs is as follows: An 8-byte value is inserted at the beginning of the message to be protected2. This 8-byte value is

• for the first or only script command, the Application Cryptogram generated by the card for the first GENERATE AC command;
• for subsequent script commands, the full MAC of the preceding script command (this is the full 8-byte block computed by the MAC algorithm prior to any truncation that occurs if shorter MACs are transmitted). Note: Issuers should be aware that when multiple issuer scripts in a single response are supported, the failure of a command in one script may result in a gap in the MAC chain. This gap will cause MAC failures for commands in subsequent scripts. 1 Note that ‘MAC chaining’ is not the same as ISO/IEC 7816-4 ‘command chaining’ and so does not require that bit 5 of the Class byte of the current command APDU be set to '1'. 2 In the terms of ISO/IEC 7816-4 this is equivalent to using an auxiliary block in the initial stage where this auxiliary block is the single DES encryption of either the Application Cryptogram or the MAC of the preceding command. This document contains proprietary and confidential information of EMVCo LLC. Copyright © EMVCo LLC 2004