CRC

CRC

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.

Informations

• First published - 1961
• Series - CRC-(number)
• Digest sizes - 8, 16, 32, 64 bits
• Structure - cyclic codes

Authors

W. Wesley Peterson

Implementations

• crc16.c
• crc32.c

Materials

• Reversing CRC – Theory and Practice
• The Great CRC Mystery
• CRC Cyclic Redundancy Check Analysing and Correcting Errors
• Reverse-Engineering a CRC Algorithm

Pseudocode

Since a CRC is fundamentally bit-oriented, the order that you assign to bits in bytes matters. You can do it big-endian.

function crc(bit array bitString[1..len], int polynomial) {
    shiftRegister := initial value // commonly all 0 bits or all 1 bits
    for i from 1 to len {
        if most significant bit of shiftRegister xor bitString[i] = 1 {
            shiftRegister := (shiftRegister left shift 1) xor polynomial
        } else {
            shiftRegister := (shiftRegister left shift 1)
        }
    }
    return shiftRegister
}