Fletcher-16 checksum
Fletcher-16 checksum. A fast error-detection checksum, not a cryptographic hash.
Runs on your device. The file is never uploaded.
Fletcher-16 checksum adds each byte into a running sum modulo 255, then adds that sum into a second one. The pair is packed into four hex digits, under a line naming the tool and your file. Because the modulus is 255, a 0x00 byte and a 0xFF byte count alike.
Questions
How does Fletcher-16 work?
It keeps two running sums over your bytes modulo 255: the first adds each byte, the second adds the first sum each step. The two are packed into one 16-bit value, printed as four hex digits.
Why two sums instead of one?
Because a plain sum cannot see reordering. The second sum weights each byte by its position, so swapping two bytes changes the result. That positional sensitivity is the point of the Fletcher family.
Is it as good as CRC?
Not quite. Fletcher is cheaper to compute and catches most single-bit and burst errors, while CRC detects some error patterns that Fletcher misses. Fletcher was chosen in protocols where the arithmetic was cheaper than a CRC table.
Can I use it to check a download?
Only against accidental damage, and only if the source published a Fletcher-16 value. It offers nothing against deliberate tampering, since forging a match is trivial. Use SHA-256 for that.