Michael,

> I just want to ask if there is any slightly effective
> possibility to do this better than:
> 0.upto(7){|i| a[i] = (a[i].to_i ^ s[i+ofs].to_i).chr}

    One trivial improvement: String#[](aFixnum) returns a Fixnum, not a
one-character String (String#[]= behaves similarly).  So:

0.upto(7) {|i| a[i] ^= s[i+ofs] }

    I realize this isn't what you were looking for, but every little bit
helps.

    - Warren Brown