EMV PIN Block Format Guide

When you enter a PIN at a payment terminal, the PIN doesn't travel over the network in plain text. Instead, it's encoded into a PIN block, encrypted with a symmetric key (DES, 3DES, or AES), and sent to the issuing bank for online verification. This guide explains every PIN block format used in EMV and payment systems.

1. Overview of PIN Block Formats

2. ISO Format 0 (ANSI X9.8) — The Standard

Format 0 is the most widely used PIN block format globally. It XORs the PIN field with a PAN-derived field to prevent identical PINs on different cards from producing the same ciphertext.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|02|PL|P1|P2|P3|P4|F1|F2|F3|F4|F5|F6|F7|F8|F9|FA|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00|00|00|A1|A2|A3|A4|A5|A6|A7|A8|A9|AA|AB|AC|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

PIN block = PIN_field ⊕ PAN_field

3. ISO Format 1 — Random Fill

Format 1 uses random fill instead of PAN-derived data. It's simpler but requires the recipient to know the format indicator to extract the PIN correctly.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|01|PL|P1|P2|P3|P4|R1|R2|R3|R4|R5|R6|R7|R8|R9|RA|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

4. ISO Format 3 — DUKPT Variant

Format 3 is used with DUKPT (Derived Unique Key Per Transaction) key management, common in US retail environments. It's structurally similar to Format 1 but the fill is derived from the DUKPT process.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|03|PL|P1|P2|P3|P4|D1|D2|D3|D4|D5|D6|D7|D8|D9|DA|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

5. IBM 3624 Format

The IBM 3624 format is a legacy format still found in some banking HSMs. Unlike ISO formats, the PIN starts from the right side of the block.

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00|00|00|00|00|00|00|00|00|00|P1|P2|P3|P4|PL|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

6. PIN Block Encryption Keys

7. Online PIN Verification Flow

1. Cardholder enters PIN at terminal
2. Terminal constructs PIN block (Format 0/1/3)
3. Terminal encrypts PIN block with PEK (or DUKPT-derived key)
4. Terminal sends encrypted PIN block in EMV ARQC/ARPC or ISO 8583 field 52
5. Acquirer HSM decrypts PIN block
6. Acquirer re-encrypts PIN with ZPK (Zone PIN Key) for issuer
7. Issuer HSM decrypts, extracts PIN, compares with stored PIN offset (IBM 3624) or PVV (PIN Verification Value)
8. Issuer returns approval/decline (ISO 8583 field 39)

8. Code Examples

8.1 Format 0 PIN Block Construction (Python)

def construct_format0_pin_block(pin: str, pan: str) -> bytes:
    """Construct ISO Format 0 PIN block."""
    # Step 1: PIN field
    pin_len = len(pin)
    pin_field = bytes([(0x00 | pin_len)])  # format=0, length
    pin_field += bytes([int(d) for d in pin])  # PIN digits
    pin_field += b'\x0f' * (16 - 2 - pin_len)  # pad with 0F
    # pad to 8 bytes (16 nibbles)
    pin_hex = (0x00 | pin_len).to_bytes(1, 'big')
    for d in pin:
        pin_hex += int(d).to_bytes(1, 'big')
    # Better approach: construct as hex string
    pin_str = f"0{pin_len}" + pin + "F" * (16 - 1 - 1 - pin_len)
    # Actually let's be precise:
    pin_field_hex = "04" + pin + "F" * (16 - 2 - len(pin))
    # Step 2: PAN field
    pan_digits = pan[:-1]  # exclude check digit
    pan_right12 = pan_digits[-12:].zfill(12)
    pan_field_hex = "0000" + pan_right12
    # Step 3: XOR
    pin_int = int(pin_field_hex, 16)
    pan_int = int(pan_field_hex, 16)
    block_int = pin_int ^ pan_int
    return block_int.to_bytes(8, 'big')

# Example
block = construct_format0_pin_block("1234", "4111111111111111")
print(block.hex())  # prints the 16-hex-digit PIN block

8.2 Format 1 PIN Block (Python)

import os

def construct_format1_pin_block(pin: str) -> bytes:
    """Construct ISO Format 1 PIN block (random fill)."""
    pin_len = len(pin)
    hex_str = "01" + f"{pin_len:01X}" + pin
    # Fill remaining with random hex digits
    remaining = 16 - len(hex_str)
    for _ in range(remaining):
        hex_str += format(os.urandom(1)[0] % 16, 'X')
    block_int = int(hex_str, 16)
    return block_int.to_bytes(8, 'big')

block = construct_format1_pin_block("1234")
print(block.hex())

9. Common Issues & Debugging

10. Summary