Checksum

A checksum is a short value computed from source data to verify that the data has been transmitted or stored correctly. The sender includes the checksum alongside the data, and the receiver performs the same calculation to check for a match. A mismatch indicates data corruption.

QR codes employ checksum concepts at multiple layers. The format information (15 bits encoding the error correction level and mask pattern) includes a BCH (Bose-Chaudhuri-Hocquenghem) checksum: 10 error correction bits appended to 5 data bits. The main data payload uses Reed-Solomon error correction codewords, which function as a checksum for the entire data stream.

The critical difference between simple checksums and Reed-Solomon codes is repair capability. Basic checksums (CRC, parity bits) can only detect errors, while Reed-Solomon codes can locate and repair them. This is why QR codes tolerate dirt and damage - they use error-correcting codes, not mere checksums. At error correction level H, up to 30% of codewords can be damaged and the original data is still recoverable.