cryptsetup “No key available with this passphrase.” even though you are 100% certain its the right key (Debian Buster)

Something I stumbled upon today. I tried to unlock a LUKS container and was 100% certain that the passphrase is correct. Still, cryptsetup would not unlock the container:

# cryptsetup luksOpen /container_file container
Enter passphrase for /container_file: 
No key available with this passphrase.

Digging around, references can be found to either a problem with the character encoding of the shell or to a bug with cryptsetup and Samsung EVO SSDs with certain older 5.1 kernels. None of these were the case.

After some trial and error, I upgraded the cryptsetup packages from 2:2.1.0-5+deb10u2 (from buster/main repo) to 2:2.3.5-1~bpo10+1 (buster-backports/main repo). Suddenly the container would unlock.

I think most of the time people either get their passphrase wrong or their header was corrupted somehow, but in this case: Some bug in cryptsetup. (Not reported yet, no existing report found yet.)

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