Checksommen

Checksommen, in computing, are short fixed-length values derived from a block of data to help detect errors in transmission or storage. They are used to verify data integrity by comparing a previously known value with one calculated from the received or stored data. The term checksomme is the Dutch plural form; in English this concept is commonly called checksums.

There are two broad kinds: non-cryptographic checksums and cryptographic hash-based digests. Non-cryptographic checksums aim to catch common errors such as single bit flips or byte inversions and are typically fast and simple. Examples include basic additive checksums and parity bits. Cyclic redundancy checks (CRCs) use polynomial division and are widely used in network protocols and file formats because they detect many common error patterns. Other lightweight checksums include Fletcher and Adler variants, which offer improved error detection for certain data patterns.

Cryptographic checksums refer to digests produced by cryptographic hash functions or MACs. These are designed to resist intentional tampering and collisions and are used when stronger integrity guarantees are required. Common examples include MD5 and the SHA family (SHA-1, SHA-256). While these are often termed checksums in informal contexts, they are technically hash functions; MD5 and SHA-1 are now considered insecure for security-critical purposes, whereas SHA-256 remains widely used.

In practice, checksums are generated for files or messages and distributed with them. The recipient re-computes the checksum and compares it to the provided value to detect alterations during transfer or storage. Limitations include the possibility of collisions and tampering in non-cryptographic checksums; for security-sensitive tasks, cryptographic hashes or MACs should be used.