This stores a file mtime, a size, and a cryptographically secure content checksum in an xattr, so that you can verify that your files aren't corrupted on filesystems that are dumb and don't include data checksums (e.g. apfs)

background

You can dd a few random bytes into the middle of an hfs+ or apfs filesystem, and, if they land in file data, an fsck/Disk First Aid on the filesystem will pass with flying colors. There is no file content checksum.

author

Jeffrey Paul sneak@sneak.berlin