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One-time pad is extremely simple and also provable to be mathematically secure against any brute force attack.

A universe full of quantum computers is no match for one-time-pad.

It does however re-introduce the key-exchange problem.

And the key is also at least as long as the message itself...

It requires a true random number source.... but this is easy using something like diode noise.

The old German Enigma was actually a type of one-time-pad ... but it had a horribly flawed mechanical random number generator. (and some doofus thought it a great idea to make it so no letter could ever be encrypted as itself i.e. A = A ... lol)

> One-time pad ... the key-exchange problem... It requires a true random number source... but this is easy using something like diode noise.

There's an easier way, assuming one initialization.

Regularly exchanging secret keys is risky; that's the whole point of public-key encryption. One-time pads should never have to be exchanged, but rather they should be independently derived at each end.

A good one-time pad (like your suggestion of diode noise) can be derived from a commercial live broadcast which is available to both the sending and receiving parties. For example (this is a very weak way to use it, for explanation purposes only):

Compile a list of the songs played on a certain music radio station between (say) 9PM and 10PM on a certain evening. This merely requires listening to the radio for an hour and noting the song titles.
For each song, take the first two letters of each word in the first two lines of the lyrics.
Concatenate all those letters. An hour's worth is about 15 songs; each line is perhaps 8 words, so that's 32 letters times 15 songs = 480 characters, or about 2880 bits of key.
Need a longer key, take more letters, or more lines, etc.

A better way to do it is to digitize the audio of a live broadcast (suitably low-pass filtered first, of course) and use that digital stream as the key.

The idea is to derive the key from something agreed-upon in one initialization, and which never has to be exchanged again. Subsequent broadcast times, stations, etc. can be encoded into messages, or perhaps posted in an innocuous third-party forum.