[dm-crypt] How can a passphrase be incorrect even after `luksHeaderBackup` and `luksHeaderRestore`?

[Paul Menzel]

Dear dm-crypt folks,


trying to save my data [1][2][3] I do not understand the following.

The partitions of two drives `/dev/sd{a,b}2` start at exactly the same point.

------- 8< --- partition table --- >8 -------
# partition table of /dev/sda
unit: sectors

/dev/sda1 : start=       63, size=   995967, Id=fd, bootable
/dev/sda2 : start=   996030, size=3906028035, Id=fd
/dev/sda3 : start=        0, size=        0, Id= 0
/dev/sda4 : start=        0, size=        0, Id= 0

# partition table of /dev/sdb
unit: sectors

/dev/sdb1 : start=       63, size=   995967, Id=fd, bootable
/dev/sdb2 : start=   996030, size=975772035, Id=fd
/dev/sdb3 : start=        0, size=        0, Id= 0
/dev/sdb4 : start=        0, size=        0, Id= 0
------- 8< --- partition table --- >8 -------

Doing `cryptsetup luksHeaderRestore /dev/sda2 --header-backup-file
sdb.luksHeaderBackup` with `sdb.luksHeaderBackup` obtained from
`/dev/sdb2` the passphrase, which works on sdb, should definitely work
on sda although the data might be read as garbage.


Thanks,

Paul


[1] http://www.saout.de/pipermail/dm-crypt/2011-August/001858.html
[2] http://www.saout.de/pipermail/dm-crypt/2011-August/001858.html
[3] http://marc.info/?l=linux-raid&m=131248606026407&w=2