Why does vim complain about weak crypto when using blowfish2?

Question

I opened up a document from 2015 I saved using the built in crypto in vim (:X). That was saves using blowfish at the time.

The message I got now (2017) was this:

Warning: Using a weak encryption method; see :help 'cm'
Enter encryption key: *********************************************
Enter same key again: *********************************************

That was weird, I thought, but I assumed it had to do with some flaw, and after reading up on the crypto methods I set cryptomethod=blowfish2. This works after opening it, showing cryptomethod as blowfish 2. The problem is that I still get the message about using weak encryption! Why? Are there remnants of the old blowfish1 crypto signature? Do I have to export the text and resave it in order to fix it?

Answer 1

The cryptmethod is stored in the file itself, and isn't automatically updated.

You can see this with less:

$ less a
VimCrypt~02!!<C7>\<E1>wZzm
^E^X^Z<92>5<C9>I6<9C>S

VimCrypt~01! is cm=zip, VimCrypt~02! is cm=blowfish, and I think you can deduce what VimCrypt~03! stands for :-)

To upgrade a file, open it, explicitly set cm=blowfish2, and write it:

$ vim a
Need encryption key for "a"
Warning: Using a weak encryption method; see :help 'cm'
Enter encryption key: [enter-key]

:set cm?
  cryptmethod=blowfish
:set cm=blowfish2
:w
"a" [blowfish2] 1L, 3C written
:q

$ less a
VimCrypt~03!O[P<D5>1<D1><DA>^Zis<88>q<AB><99><8C><A6><C4><FB>6
a lines 1-1/1 (END)