Hi,

The blowfish encryption mode is vulnerable (not to revelation of the
plaintext), but the encryption is not checked for integrity or
authenticity. This means that someone might corrupt the encrypted file
(hexedit or similar), and vim will decrypt it without notice of error or
warning.

This attack allows someone to modfiy encrypted files so that the owner
doesn't notice. With sufficient tries or skill it might be possible to
change a file's values in a predictable way at a certain offset.

The solution is an authenticated encryption mode. The common way to do
it is 'Encrypt-then-MAC' where a message authentication code is formed
from the ciphertext and the key. This code when matching will prove that
the document is unchanged and was produced by someone with access to the
key. This code will detect the previous attack case, and additionally it
allows vim to detect that the wrong password was entered. Security
practise says that Vim must fail with an error if the MAC does not match.

HTH,
ulrik

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