“This telegram must be closely paraphrased before being communicated to anyone”

The term to google for more information about this would be Known plaintext attack.

For people interested in these kinds of things, there is a very interesting military manual on the internet archives which goes though all the various pre computer pen and paper ciphers and how to crack them.

1. https://archive.org/details/Fm3440.2BasicCryptAnalysis/mode/...

This is a familiar concept from reading about WW2 spy stuff (Between Silk and Cyanide, for example, which I highly recommend). But what REALLY intrigues me is the typeface of the letter with its upper-case 'E' used in place of 'e'. What's up with that?

Interesting. I liked the explanations in the accepted answer. This rule especially,“Never repeat in the clear the identical text of a message once sent in cryptographic form, or repeat in cryptographic form the text of a message once sent in the clear.”

As a child I learned about codes from a library book. Fascinated with one-time pads, I convinced a friend to try a correspondence. We exchanged a few messages, and then got bored, because the juice wasn’t worth the squeeze.

Which makes me wonder about people who work in secrets. Encrypted communications seem opposite of scientific communications. Secrets peeps seem prolly aligned to politics.

Does this also apply if someone were to do the following: Receive encrypted transmission -> unencrypt it -> need to pass it on, so re-encrypt it and pass it on?

I would imagine that the paraphrasing wouldn't be necessary in this case because it isn't quite as useful to compare two encrypted versions of the text versus an encrypted version and an unencrypted version (also I feel like there is some risk of a game of 'telephone' in that the meaning would change bit by bit to the point of having a different meaning over time, even if not intentionally)