Why it matters
Bytes get damaged on serial lines, dial-up links, radio links and in files. Checksums let the receiver notice:
- an LRC on a terminal or ECR link;
- a CRC in a contactless frame, an EMV QR code or a file;
- the Luhn digit at the end of every PAN.
They are simple, but a checksum computed over the wrong bytes rejects every message. And a checksum mistaken for security protects nothing.