SB nº AN-41 : Recommendations for CDA Terminals (Clarification)

Application Note Bulletin No. 41 2nd Edition October 2010 Recommendations for CDA Terminals (revised) This bulletin clarifies the CDA modes introduced in SU Bulletin 44, now incorporated into EMV v4.2, and provides implementation recommendations.

Applicability

This Application Note Bulletin applies to:

• EMV Integrated Circuit Card Specifications for Payment Systems Version 4.2 Book 2 Security and Key Management

Related Documents

• None

Description

Following publication of Specification Update Bulletin 44, EMV CDA permits flexible terminal behaviour. This Application Note bulletin adds an informative Annex to EMV that introduces a classification of terminal behaviours, so-called CDA 'modes', and provides related recommendations. This second edition of the bulletin revises recommendations from the first edition. EMVCo Terminal Type Approval identifies these different terminal behaviours within the ICS (Implementation Conformance Statement). New Annex to Book 2 The following will be added to Annex D Implementation Considerations of EMV Integrated Circuit Card Specifications for Payment Systems Version 4.2 Book 2 Security and Key Management:

Annex D.4 Following publication of EMV Specification Update Bulletin 44 (SU44), now incorporated into EMV v4.2, EMV permits flexible terminal CDA behaviour that can potentially improve transaction performance. These include the selective use of CDA for online authorisations and public key retrieval relative to Terminal Action Analysis (TAA). CDA for online authorisations Terminals supporting CDA have the following options

• Request or not request CDA on ARQCs
• Request or not request CDA on 2nd GENERATE AC (TC) after an approved online authorisation As part of the EMV type approval, a terminal kernel configuration supporting CDA must now identify which of the above options the terminal supports. Thus an EMV terminal configuration supporting CDA will operate in one of four modes: Mode Request CDA on ARQC 1 Yes 2 Yes 3 No 4 No Request CDA on 2nd GEN AC (TC) after approved online authorisation Yes No No Yes Public Key retrieval Before publication of SU44, terminals experiencing CDA failure prior to TAA decline the transaction. Following publication of SU44, terminals that comply with SU44 that experience CDA failure prior to TAA shall proceed with TAA to decide whether to decline or send the transaction online. One possible reason for CDA failing is a problem retrieving the public keys. According to SU44, terminals that find this problem before TAA will proceed with TAA to decide whether to decline or send the transaction online. Thus an online authorisation (without CDA) is possible, rather than the decline that was previously required. SU44 also clarifies that keys can be retrieved before or after TAA which can lead to performance improvements for terminals operating predominantly online. This is because if the TAA results in an online authorisation and if the terminal requests an ARQC without CDA (i.e. it is operating in Mode 3 or 4), then the retrieval of the issuer and ICC public keys need not be completed, saving the RSA processing.1 1 If Offline Enciphered PIN is performed then this will force the retrieval of the issuer and ICC public keys to happen before PIN verification is performed. Recommendations The following recommendations apply to terminals supporting CDA. All terminals should verify before TAA that they contain the Certification Authority Public Key identified by the card. Such verification does not involve time-consuming cryptographic processing. If the correct key is not present, then online terminals have an opportunity to send the transaction online without requesting CDA in the GENERATE AC. For online capable terminals that are able to perform certificate verification quickly, it is recommended that the terminal retrieve the issuer and ICC public keys before TAA. This is to ensure that in the unlikely event that key retrieval fails then the terminal can request an ARQC without CDA rather than decline the transaction. Exceptions to this recommendation might be slower terminals which gain efficiencies by overlapping terminal certificate verification with card signature generation or Mode 3 terminals that normally send transactions online (i.e. request an ARQC rather than a TC at the first GENERATE AC) and for which a fast transaction is critical. As Mode 4 does not provide significant benefit, terminal vendors are recommended not to implement Mode 4. If CDA is needed on all 2nd GENERATE AC commands requesting a TC, then Mode 1 can be used.