https://www.freebsd.org/doc/handbook/security-intro.html the mtree(8) example at section 13.2.6 Binary Verification is not working correctly. The output from the verification step is empty as expected but the checksum printed differs from the one calculated at the specification creation step.
# mtree -s 3483151339707503 -c -K cksum,sha256digest -p /bin > /root/.bin_chksum_mtree
mtree: /bin checksum: 1817933408
# mtree -s 3483151339707503 -p /bin < /root/.bin_chksum_mtree >> /root/.bin_chksum_output
mtree: /bin checksum: 1510745247
# file .bin_chksum_output
The system I'm using is:
# uname -a
FreeBSD firewall 11.2-RELEASE-p2 FreeBSD 11.2-RELEASE-p2 #3 r338073: Mon Aug 20 16:44:39 EEST 2018 root@firewall:/usr/obj/usr/src/sys/GENERIC amd64
Filesystems used are ZFS all around except tmpfs /tmp and /var/run.