A solution for those who use vhcs2 debian packages
I used vhcs2 debian packages from apt.scunc.it. As I wanted to upgrade a vhcs2 installation via apt-get upgrade or apt-get dist-upgrade I only got this:
The script responsible for this was said to be vhcs.postinstall accoarding to apt. The message beat me to key size issues. Takink a closer look i found out that the installed vhcs package used the same cipher but a key size of 56 for the db pass key while the new package wanted to do key size of 32 in its postinstall. The cipher package however don't allow this.
If specified by -literal_key, then the key length must be equal to the chosen cipher's key length of 56 bytes in ... line ...
So my solution is simple: force that postinstall to use a key size of 56 instead.
Open up vhcs.postinstall (the error message tells you the exact path to it) and go down to the line it reports the error in. Then go some lines backowards and search for $keylen => 32. Replace the 32 with 56 and save it.
Then do apt-get (dist-)upgrade again. It then worked here without any more error :)
Prolly an issue that should be reported to the package maintainers...