It doesn't exist. If NSA wants to read your mail, they're reading it. If Google wants to read your mail, they're reading it. They are probably doing so by means more devious than a brute-force decrypt -- but the "never decodable in finite time" stuff is a happy fantasy, playing off the manner in which we are easily impressed by large numbers.
The Phone Company has been in bed with the Feds since there was Federal regulation of 'em. They rat people out as a matter of routine. The Telegraph Company was doing so even before there was regulation! ...Mind you, there's a whole alternate to AT&T for the Internet backbone. It's a network built and operated by the Federal government. Ahem.
Communications security is like the Maginot Line: not nearly as impregnable as it appears.
If you have information that has really, truly got to be kept secret, don't put it on the phone. Don't put it on the Internet. Don't talk about it near a phone. Don't talk about it at all, if you can manage to, and don't write it down, either.
Two people can keep a secret -- if one of them is dead. (Cribbed from Heinlein but absolutely true.) Don't want your mail read? Tough.
T. R. MCELROY'S STREAMLINED TELEGRAPH KEYS
8 months ago