Checksum

CRC-32

A 32-bit cyclic redundancy check , the classic polynomial-division-over-GF(2) family for detecting accidental data corruption. CRC-32 catches every single-bit error, every two-bit error within a span, all odd-bit-count errors, and burst errors up to 32 bits long. It is not cryptographic , an adversary can construct collisions in milliseconds.

At a glance

The two common variants

Why CRC32C is increasingly preferred

Castagnoli’s polynomial has slightly better error-detection properties for the message sizes typical in storage and network protocols, and Intel’s SSE4.2 CRC32 instruction implements it directly. Modern filesystems and protocol stacks pick CRC-32C for both reasons.

Where it shows up

• Ethernet FCS , every frame ends with a 4-byte CRC-32.
• ZIP, gzip, PNG, BZIP2 , per-record / per-chunk integrity.
• iSCSI, SCTP , CRC-32C as the payload checksum.
• ext4, Btrfs, F2FS , metadata block integrity.
• SD cards, USB Mass Storage , transfer integrity.

Not for adversaries

Given a target CRC-32, anyone can append 4 bytes to a message to force any desired CRC. This is by design , CRC is a linear function in GF(2). Never use CRC-32 to authenticate messages; use a real MAC like HMAC instead.

References

• Greg Cook’s CRC catalogue (the canonical list of CRC parameters)
• Intel , Fast CRC computation using CRC32 instruction (CRC-32C)
• Adler-32 · Fletcher · Internet checksum