[boris1]

There's no difference for the MITM how I pick the Short Authentication String (SAS) (the check digits).

But there's a difference in terms of strength of the encryption key, if you are planning to use the full password as input to Key Derivation Function (KDF). If you make public the first 5 letters of a 44 letter password, you've just made lost some of the entropy.

By the way, based on a comment in this thread, I added a SHA-256 stage. I now hash the full password, and sum the bytes of the hash to generate the check digits.