Reversibly encrypting filenames so that result is valid filename - encryption

Question

How do I reversibly (symmetrically) encrypt a filename (with or
without directory path, I'm OK w/ either) so that the result is also a
valid filename (less than 64 characters [or whatever the limit is], no
funny characters, ideally no spaces [but not a requirement], etc)?
Googling finds only filename encryption algorithms where the result is
a long string of binary characters (using MIME64, converting to
non-binary is easy, but this just makes the filename longer) and/or
non-symmetric one-way encrption schemes (eg, salted MD5, SHA1, DES,
etc). I don't want to store a table of hashes: I want to decrypt the
filename with a simple key I've memorized.
My own attempts with things like "mcrypt -b" failed too: the resulting
output (even before converting to ASCII) grows in size very rapidly as
the filename and key length increase.
Reasoning: I plan to use an "infinite backup" service (like mozy,
blazebackup, etc), but none encrypt filenames (just file
content). I'll create a directory that consists of encrypted filenames
with symlinks (or even hard links) to the real file. I'll back up only
that directory (and choose my own private key), and have
filename-encrypted and filecontent-encrypted backups.
EDIT: Petey's method worked like a charm!
# "-b 512" yields "Bits has bad value 512 (too small)"
ssh-keygen -t rsa -b 768 -f /tmp/test.rsa
echo "thisisareallylongfilenameknightswhosayniioratleastusedto" |\
openssl rsautl -inkey /tmp/test.rsa -encrypt | base64 |\
perl -0777 -pnle 's/\//-/isg;s/\n//isg'
yields a 130 character result that should always be a filename!

Answer 1

You could use an RSA key pair to do this. Generate an rsa key pair plus certificate, then import that into your cert store. Use the public key to encrypt your file name, then base64 encode the result. The maximum file name length for ntfs is 255 characters, so a 1024 bit RSA key should be fine, if you need shorter file names, use a 512 bit key. When you want to decrypt the file name: base64 decode the encrypted file name, then use the private key to decrypt back to the actual file name.
Not sure if there is any freeware available to do this. If you don't want to write the program yourself, I'll do it in .Net for you (for a small fee ;).

Answer 2

How strong do you need your encryption to be? You could use one of the classical alphabet based cyphers, such as Vigenère which will produce strictly alphabetical output, though it won't handle non-alphanumeric characters well if you want a valid filename as output. The result will still have "/" and "." where they were before.

Answer 3

If you are very concerned about security, it's worth noting that high entropy filenames may get more attention than just "file000001", ... "fileNNNNNNN". A separate file that maps from fileNNNN to the correct name can be encrypted separately and stored in duplicate in multiple locations. Both methods leak zero information about the original filenames. Alternatively, you could add a short header to every unencrypted file that encrypts the filename, thereby dispensing with a separate index.
What's more, the ability to do error detection of corrupted filenames is easier when the list of names is both known and in a pre-determined order.
[Posting as CW, because this is really more of a comment than an answer.]