Weak encryption keys

[joefresh at gmx.us]

> Interestingly, when I tried searching the latest GnuPG code base (cloned from github)
> for the "Weak encryption key" error message, nothing showed up.
>
> $ "grep -iRl "Weak encryption key" gnupg
> <no matches>
>

It appears that the problem lies in libgcrypt, which refuses to set a key for this
cipher that's considered weak.

libgcrypt/cipher/blowfish.c

static gcry_err_code_t
do_bf_setkey (BLOWFISH_context *c, const byte *key, unsigned keylen)
...

  /* Check for weak key.  A weak key is a key in which a value in
     the P-array (here c) occurs more than once per table.  */
  if (weak)
    return GPG_ERR_WEAK_KEY;