Vim Encryption

VIM can encrypt text files transparently. I wrote a python script which can decrypt all three types given the password, or do a simple dictionary search.

Never use the old methods ( zip and blowfish ) the way these are used in VIM is broken. The latest blowfish2 method is somewhat better, though there is room for improvement.

How to use VIM encryption

Select the mode using 'set cryptmethod={zip, blowfish, blowfish2}' and set the key using 'set key='

Beware when entering the wrong password, VIM wil happily open the file for you. And display nonsense content. When you now save this again, it will be quite difficult to recover the original file.

You can retry the password by either quitting and reloading vim, or by typing:

:bdel | edit #

in VIM. (from)

The encryption methods

command: set cryptmethod=zip
header: VimCrypt~01!

Uses the same algorithm as the old PKZIP program. There is a tool called pkcrack which does a known plaintext attack on zip files encrypted using this algorithm.

By wrapping the VimCrypt file in a .zip file you can crack this using PKCRACK. The -w option of vimdecrypt.py creates a PKCRACKable .zip archive from a given VimCrypt file.

Note: there exists a tool vimzipper.c by Richard Jones, which can also do this.

command: set cryptmethod=blowfish
header: VimCrypt~02!

Uses blowfish in little-endian mode, using Cipher Feedback Mode, but with a bug because of which the first 8 blocks all use the same IV.

command: set cryptmethod=blowfish2
header: VimCrypt~03!

Uses blowfish in little-endian mode, this time with a correct Cipher Feedback Mode implementation.

Both blowfish methods use 1000 iterations of a salted sha256 of the password. The undo and swap are also encrypted when editing an encrypted file.

Security problems

ZIP

The zip method is very weak, you need 13 bytes of plaintext to find the key.

Blowfish / bf1

The bf1 method is problematic for short files. The problem is that the first 8 blocks all use the same IV, so:

enc(block1) XOR enc(block2) == block1 XOR block2

This leaks lots of information which can be used to guess the contents of the first 64 bytes.

Blowfish2 / bf2

The bf2 method does not have the broken CFB problem, but since it is using CFB without any checksum, an attacker can modify the last block of the encrypted text without the user noticing.

Blowfish though not really insecure, is quite old. Better ciphers, like AES, or Twofish have been designed since 1993.

password hashing

The ZIP cipher uses it's own weak hashing algorithm. With the bf1 and bf2 methods, the user password is hashed 1000 times using sha256. This does make bruteforcing a bit more difficult, but still, this method is easily accelerated using FPGA or GPU crackers. Better would be to use a hashing algorithm which is difficult in both time and space, like PBKDF2, or scrypt.

Password cracking

vimdecrypt.py can do some simple password cracking, either by dictionary, or bruteforce. Note that this all done in python, and not very fast:

algorithm speed notes
zip 650 pw/sec
bf2 300 pw/sec python2, pycrypto
bf2 180 pw/sec python3, pycrypto

You can also use a word generator like John the Ripper, and pipe the wordlist to stdin of vimdecrypt.py, and specify - for the wordlist.

For bruteforce cracking you need some kind of heuristic to tell if the decryption was successful. Since encrypted data will generally compress really badly, while text compresses very well, this is what i test against in vimdecrypt.

VimDecrypt

Tool for decrypting VIM encrypted files.

Dependencies: * pycrypto

vimdecrypt should work with both python2 and python3.

Usage:

python vimdecrypt.py -p PASSWORD yourfile.txt

option description
--test run vim selftest
--verbose print details about keys etc.
--password PASSWD use PASSWD to decrypt the specified files
--encoding ENC use an alternate encoding ( default = utf-8, example: latin-1, hex )
--writezip create PKCRACKable .zip file from VimCrypt file
--dictionary DICT try all words from DICT as password
--bruteforce try all lowercase passwords

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