[Tutor] Iterate over letters in a word

[Danny Yoo]

> As a side note, remember that that xor-ing a key with a message is
> trivial to break (it's just a variation on the Vigenere cipher first
> published in 1568). So don't use if for any real applications.

Hi Matthew,

Counterpoint: think of "one-time pads".

    http://en.wikipedia.org/wiki/One-time_pad

XOR itself is just a technique --- it's just a binary operation between
two bit patterns --- but the use of one-time pads provides unbreakable
encryption.  As long as the xor-ing key is protected, and as long as the
encrypting key pattern is as long as the message, it's theoretically
unbreakable.

If we reuse the same encrypting bit-pattern over and over, or make the
XORing key less than random, then all bets are off, of course.  *grin*

There are different encryption schemes that, at its heart, use XOR.  But
XOR itself is not inherently insecure: it's our use of it that determines
the quality of the result.