It’s about time I encrypt my home directory. So far, I’ve found
two tools for the job: dmcrypt and
encfs.
dmcrypt links in between the VFS layer and the data
store. For an encrypted partition or image, it creates a device
under /dev/mapper, which you then mount. Think
cryptoloop. Encryption covers the whole scale of
symmetric encryption, but I didn’t find anything related to public
key crypto. I am also not entirely clear yet on why a damaged block
on the storage medium doesn’t screw the encryption block
(partially) stored on it, as well as all following ones, given a
chaining mode like CBC for instance. However, as this
thread shows, I am wrong and technology is right (and works),
and I still just don’t get it. It feels like another Monty Hall
problem to me (which I also never really understood to the point of
being able to convince someone else thinking 50/50 is the answer of
the alleged 2/3).
So then there’s encfs, a FUSE-filesystem that seems (I
have not tried it) to create a virtual directory for an existing
one, transparently encrypting filenames and contents as they are
written to the existing directory (via the virtual one), and
decrypting them when requested through the virtual one. So it’s a
1-to-1 mapping, which has its pro’s and con’s. encfs
seems equally strong on the symmetric crypto side, but seems to
lack public key support.
For each, there’s a proper PAM module: libpam-mount
can mount a cryptoloop filesystem upon login (though
the README says it cannot do so for OpenSSH logins. If
that’s right, well, one candidate less…). On the other hand,
there’s libpam-encfs, which creates the virtual
directory on login (and removes it on logout).
I can well imagine how any of the PAM modules would do their job
just fine on a machine where xdm is the primary/only
login method. However, what about SSH? What happens if I log in
through xdm, then SSH, then quit X?
How do you solve the problem? I am curious and will blog about the things I hear…
Update: After rereading my thread, I am understanding, I think. We’re talking encrypted block devices (or loop devices). It’s not difficult at this stages to tie IV chains in with storage blocks. I am still curious why my 8M file was still all zeroes after changing the cryptotext.
Update2: Sesse points out that CBC uses the ciphertext of the previous block as an IV, not the plaintext. So if some block gets corrupted, that messes up the next one’s plaintext, but not its ciphertext, so the one after that is OK. Thanks.
