How do I detect I've entered a wrong password when using cryptmethod (and make Vim ask for it again)?

Question

If I encrypt a file with :set cryptmethod=blowfish or :set cryptmethod=blowfish2, and open the file in Vim, it asks for my password on startup. I type in the correct password and continue happily.

However, if I enter the wrong password, Vim will show me something like this:

<1f>íW><00>©?<11>­·Á<91>´!<98>!<0c>C ¦

This is because Vim used the wrong key (and thus gets the wrong decryption).

This behaviour is not just annoying, but also dangerous (if you accidentally save it like this, you've lost your file).

Vim is the only program I've ever used that does this. All other programs simply ask the password again. Can I make Vim do this?

Answer 1

The best I've managed to do is use a wrapper script, I've named this svim and put it in ~/bin:

#!/bin/sh

[ -z "$1" ] && (echo "We need a file"; exit 1)
[ -f "$1" ] || (echo "File needs to exist"; exit 1)
[ "$(head -c8 "$1")" = "VimCrypt" ] || (echo "File needs to be encrypted"; exit 1)

while :; do
    vim -c "source ~/.vim/password.vim" "$1"

    [ $? -eq 0 ] && break

    clear
    echo "That seems to be the wrong password; press enter to try again, or ^C to abort"

    # Make sure we don't echo anything to the screen, in the case the user
    # enters the password here (as I did a few times...)
    # TODO: We really want to solve this better by allowing password input here
    stty -echo
    read x
    stty echo
done

~/.vim/password.vim looks like:

" If there are less than 3 + (bytes / 100) newlines, we assume the password
" is incorrect, and we're displaying a bunch of gibberish. Quit, and try
" again
if getline(1) != '' && line("$") < 3 + (line2byte(line("$")) / 100)
    " User pressed ^C
    if strpart(getline("."), 0, 12) == "VimCrypt~02!"
        quit!
    else
        cquit!
    endif
endif

This isn't perfect. It won't work for small files, for example, and perhaps some other scenarios as well. But it's the best I could think of ...

PS. I asked Bram about this, and his reply was "just type the correct password", and that it's a 'feature'... So I don't expect a fix in Vim.

Answer 2

Is this closer to what you want? It doesn't improve on the testing for key correctness, but it does remove the need for the wrapper script.

augroup check_enc
    autocmd!
    autocmd BufRead * call Check_Enc()
augroup END

function! Check_Enc()
    if &l:cm != ""
        if getline(1) != '' && line("$") < 3 + (line2byte(line("$")) / 100)
            set key=
            edit
            call Check_Enc()
        endif
    endif
endfunction

When a buffer is read it checks if a cryptmethod is set, and if so is uses your check to see if it assumes the key is incorrect. If so, it sets the key to blank and re-edits the file causing it to prompt you for the password again.