Fletcher-64 checksum
Fletcher-64 checksum. A fast error-detection checksum, not a cryptographic hash.
Runs on your device. The file is never uploaded.
Fletcher-64 checksum reads your file as 32-bit little-endian words and keeps two running sums, packing them into 16 hex digits beneath the tool name and the file name. Bytes past the end of the final word count as zero. The modulus is 4294967295 and not 2 to the 32, so a tool rounding it up reports another value.
Questions
How wide is Fletcher-64?
Sixteen hex digits. It reads your bytes as 32-bit little-endian words, missing bytes at the end treated as zero, and keeps both running sums modulo 4294967295 before packing them into one 64-bit value.
Is this a cryptographic hash?
No. It is an error-detection checksum. Producing a second input with the same value takes ordinary arithmetic, so it protects against accidents only. Use SHA-256 or BLAKE3 when the input might be hostile.
Why is my value different from another implementation?
Check the word order and the modulus. Implementations differ over endianness and over whether they use 4294967295 or 4294967296 as the modulus. This one uses little-endian words and modulo 4294967295.
What is it good for?
Spotting corruption in large blocks cheaply, which is why it turns up in storage and filesystem code. If you need speed on modern hardware, xxHash64 is faster and equally non-cryptographic.