Accessing an old file encrypted with the serpent cipher

Many years ago I created an encrypted file to store some very personal stuff I didn't want to be accessible to anyone: at that time I was running slackware-1x on a 32 bit laptop with no disk encryption. A few years later, with slackware-14.x I moved to x86_64 and I started encrypting my whole disk, so I opened that old file and transferred its content (all of it?) to my file system.

Today I was cleaning up my home directory and, finding that old file still sitting there, I thought to open it to check if everything was actually saved. To open it I used to use these commands:

Code:losetup -e serpent /dev/loop0 crypto.serpent
mount -t ext2 /dev/loop0 /mnt/tmp/(crypto.serpent is the file... I named it so to remember the cipher)

Now, losetup -e has gone but, if I'm reading the documentation correctly, cryptsetup should be backward compatible, and so something like this should do the job:

Code:cryptsetup plainOpen crypto.serpent mysecrets -c serpent
mount -t ext2 /dev/mapper/mysecrets /mnt/tmp/The problem is that I'm now getting:

Code:mount: /mnt/tmp: wrong fs type, bad option, bad superblock on /dev/mapper/mysecrets, missing codepage or helper program, or other errorNow, serpent support, in slackware, should be built into the kernel so I do not know what I'm missing.

As I said, I do not think I'm facing a data loss problem, but I'm puzzled because when I created that file I thought I would not have problems accessing it in the (also far) future. Am I doing something wrong?

Best,
andrea

ps: obviously I remember the passphrase, and I know because I already opened it a few years after creating it.