PIN blocks are the standard format for encrypting and transmitting Personal Identification Numbers (PINs) in financial systems. This guide covers all ISO 9564 formats.

What is a PIN Block?

A PIN block is a formatted data block that contains a PIN in a specific format, designed to be encrypted and transmitted securely. The format combines:

The PIN itself
A "fill" pattern (random or fixed data)
Often the Primary Account Number (PAN)

PIN blocks are encrypted using a PIN Encryption Key (PEK), which is a working key in the HSM key hierarchy. For high-volume deployments, the PEK is typically a DUKPT-derived key, unique per transaction.

ISO 9564 PIN Block Formats

Format 0 (ISO-0)

The most widely used format, requires PAN for encoding:

PIN Block = PIN Field XOR PAN Field

PIN Field Structure:

Byte 0: 0x0 (format identifier) | PIN length (4 bits)
Bytes 1-7: PIN digits followed by 0xF padding

PAN Field Structure:

First 4 nibbles: 0x0000
Next 12 nibbles: Rightmost 12 digits of PAN (excluding check digit)

Encryption: Typically 3DES in ECB mode

Format 1 (ISO-1)

Does NOT require PAN, uses random fill:

Byte 0: 0x1 | PIN length
PIN digits followed by random fill

Advantage: Works without PAN, more secure random padding
Disadvantage: Cannot verify PIN block integrity without decryption

Format 2 (ISO-2)

ICC (chip card) format, never transmitted:

Byte 0: 0x2 | PIN length
PIN digits followed by 0xF fill
Used only for PIN verification within chip cards

Format 3 (ISO-3)

Similar to Format 0 but with random fill:

Byte 0: 0x3 | PIN length
PIN digits followed by random fill (0xA-0xE range)
XORed with PAN field

More secure than Format 0 due to randomness.

Format 4 (ISO-4)

The newest and most secure format, designed for AES encryption:

16 bytes (128 bits) instead of 8 bytes
Uses AES-128 encryption
Includes additional security features

Structure:

Control field with format indicator
PIN length
PIN digits
Random fill
PAN hash

Format 4 is part of the migration from 3DES to AES in payment systems, alongside DUKPT AES and TR-31 Version D.

Security Comparison

Format	PAN Required	Fill Type	Block Size	Recommended
0	Yes	Fixed (0xF)	8 bytes	Legacy
1	No	Random	8 bytes	Good
2	N/A	Fixed	8 bytes	Chip only
3	Yes	Random	8 bytes	Better
4	Yes	Random	16 bytes	Best

When to Use Each Format

Format 0: Legacy systems, ATM networks (most common today)
Format 1: Systems without PAN access
Format 3: Modern TDES-based systems
Format 4: New AES-based implementations

PIN Block in the Transaction Flow

Customer enters PIN at ATM/POS
Terminal formats PIN as PIN block (Format 0 or 3)
PIN block encrypted with PEK (or DUKPT-derived key)
Encrypted PIN block placed in ISO 8583 Field 52
Message with MAC sent to acquirer
Acquirer re-encrypts PIN block under interchange key
Issuer decrypts and verifies PIN

The MAC in the ISO 8583 message protects the entire transaction from tampering.

PIN Verification Methods

IBM 3624 (PIN Offset)

Issuer derives a "natural PIN" from the PAN using a PIN derivation key
PIN offset = customer PIN - natural PIN (mod 10)
Offset stored on card or in database
Use our PIN Offset Calculator

VISA PVV (PIN Verification Value)

4-digit value derived from PIN, PAN, and PVK
Stored on magnetic stripe
Use our PIN PVV Calculator

Try It Yourself

Use our PIN Block tools to encode and decode PIN blocks in all formats:

PIN Block General Tool — All ISO 9564 formats (0, 1, 2, 3)
PIN Block AES Tool — Format 4 (AES-based)
PIN Offset Calculator — IBM 3624 PIN offset
PIN PVV Calculator — VISA PIN Verification Value

Step-by-step visualization of the encoding/decoding process is included.

Understanding PIN Block Formats (ISO 9564)