Track F · Foundations

You will be able to

• Convert between hex, binary, decimal, packed and unpacked BCD, ASCII and EBCDIC
• XOR two values
• Read bit numbers the ISO 8583 and EMV way (bit 1 is the leftmost)

The lesson

1. Why it matters
2. The concept
3. Step by step
4. In the real world
5. Common mistakes
6. What to test

In the module

• The lesson, with diagrams, common mistakes and what to test
• A worked example you can change: One value, many encodings
• Practice: 5 kinds of generated exercise, each with a step-by-step solution
• A quiz of 8 questions

Key terms

ASCII

The common 7-bit character set: digits are 30–39 hex, 'A' is 41. Many ISO 8583 implementations send numeric and text fields in ASCII.

Base64

A text encoding of binary data with 64 characters. Common in APIs and certificates (PEM), rare inside ISO 8583.

BCD (Binary-coded decimal, packed decimal)

Decimal digits stored one per nibble, two per byte: 1234 is stored as the bytes 12 34. ISO 8583 numeric fields are often BCD on binary hosts.

Big-endian

Most significant byte first. Length headers and binary counters in payment messages are usually big-endian.

EBCDIC

The IBM mainframe character set. Digits are F0–F9 and letters are in other ranges than ASCII, so EBCDIC hosts need conversion at the switch.

Hexadecimal (Hex)

Base 16 written with 0–9 and A–F. Each hex digit is 4 bits, so one byte is two hex digits; keys, blocks and MACs are usually written in hex.

Justification

Where a value sits in a fixed-length field: numbers are usually right-justified with leading zeros, text left-justified with trailing spaces.

Nibble

Half a byte: 4 bits, one hex digit.

XOR (Exclusive or)

The bit operation that gives 1 when the two bits differ. It is its own inverse, which is why it is used to combine key components, build PIN blocks and apply key variants.

Tools for this module: Code ConverterXOR / BitwiseBit Shift

Read the opening of the lesson →