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I know that sixes_and_sevens probably wants a software solution here, but if they are willing to put up with sending lots of extra data they could just declare the third letter of every sixth word or use some other similarly arbitrary rule to determine which letters were pieces of data in the secret message.
There's also lots of tools to steganographically embed data in images.
So, my actual motivation is solving a specific problem I set myself: if I have two or more processes running on different machines, with access to the public internet, but no guarantee of locally open ports beyond 80 and 443, no knowledge of any existing sibling processes, and no guarantee that any previously-established communication channel (like a mail server, message queue or coordinating service) is still active, how do they form a peer group?
The first not-completely-terrible solution I hit upon was for them to monitor existing chatter-heavy online se... (read more)