pad
Eventually, you decide to send the message “HELLOWORLD”. Using mod-26 (wraparound) arithmetic, you add this string to the key. H (the first letter of the message) + F (the first letter of the key) becomes 8 (H is the eighth letter) + 6 (F is the sixth) = 14, becomes N. Similarly, E + V becomes 5 + 22 = 27, which wraps around to 1 and becomes A. In this manner, you translate HELLOWORLD to NALSZYFALO. What you do next depends on the scheme: you can use the entire rest of the pad as-is, add a “EOM” to the end of your message, save the rest of the pad for later (and make it clear that you only sent 10 letters), or whatever else.The point of this scheme is that, from the point of view of somebody who intercepts the message, NALSZYFALO could translate to literally any 10-character message depending on the pad. It is completely impossible to break, even in principal, because with a different key it could have been ADIOSAMIGO, or EMBEZZLEIT, or anything else of the right length. As long as you don’t use the one-time pad more than once, it’s impossible to break.
Historically, some people used their one-time pads more than once, leading to their unbreakable crypto being broken.
¹ This is a simplification, but not much of one. You can often infer spaces, punctuation, etc. from just a block of letters if you try hard enough.
