PIN Block Formats - The Basics

I had a call yesterday about approved TG-3 (Which is now called TR-39 ) ANSI PIN Block Formats. The TR-39 Audit Procedures state that ISO 9564–1 Format 0 (ISO-0) and Format 3 (ISO-3) are the only approved formats:

4.1.3 X9 Approved PIN Block Formats

Documented procedures exist and are followed that ensure any cleartext PIN-block format combined with a PIN encryption process has the characteristic that, for different accounts, encryption of the same PIN value under a given encryption key does not predictably produce the same encrypted result. (Note: any cleartext PIN block, formats 0 and 3 meet this requirement, as specified in X9.8-1).

Reference X9.8-1 - Sec. 4(c), Sec. 6.2, Sec. 8.3.1, Sec.8.3.2, and Sec. 8.3.5

In case you are curious here are Visa’s PIN Security Requirements

Requirement 3: For online interchange transactions, PINs are only encrypted using ISO 9564–1 PIN block formats 0, 1 or 3. Format 2 must be used for PINs that are submitted from the IC card reader to the IC card. Other ISO approved formats may be used.

This requirement further states:

PINs enciphered using ISO format 0 or ISO format 3 must not be translated into any other PIN block format other than ISO format 0 or ISO format 3. PINs enciphered using ISO format 1 may be translated into ISO format 0 or ISO format 3, but must not be translated back into ISO format 1.

(This last paragraph addresses an attack on Pin Blocks that can be translated in to format 1 on a HSM which would expose the clear PIN)

Let’s take a look at a few Pin Block formats:

For our examples: P - PIN Number F - Hex 0xF A- Last 12 digits of PAN not including check digit R - Random Hex Character (0-9, A-F) Let us use the account number 4111111111111111 and PIN Number 1234 (examples use a PIN Length of 4 but could be 4-12 digits)

“Pin Pad” format or IBM 3624

PPPP FFFF FFFF FFFF our Pin Block 1234 FFFF FFFF FFFF Notes: Not allowed and is an old legacy method - not approved to be used.

ISO-0

04PP PPFF FFFF FFFF (0 = ISO-0 Format, 4 = length of PIN) XOR with 0000 AAAA AAAA AAAA (Formatted PAN) our Pin Block: 0412 34FF FFFF FFFF XOR 0000 1111 1111 1111 = 0412 25EE EEEE EEEE Notes: Introduces variability in the PIN block by XOR’ing with a Formatted PAN - Best practice is to use ISO-3 instead of ISO-0 as there are attacks against ISO-0

ISO-1

1412 34RR RRRR RRRR (1 = ISO-0 Format, 4 = length of PIN) our Pin Block: 1412 348D 665A C5A3 Notes: Introduces variability in the PIN block by using Random padding chars - Best practice is not to allow HSM’s to accept or use this PIN Block format. Not allowed by TR-39 but is VISA.

ISO-3

34PP PPRR RRRR RRRR (3 = ISO-3 Format, 4 = length of PIN) XOR with 0000 AAAA AAAA AAAA (Formatted PAN) our Pin Block: 3412 34C8 CBA4 285C XOR 0000 1111 1111 1111 = 3412 25D9 dAB5 394D Notes: Introduces variability in the PIN block by using Random padding chars and by XOR’ing with a Formatted PAN - Best practice is to use this format.

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