Comment Port knocking (Score 1) 56
This reminds me of "port knocking", but using small financial transactions instead of socket connection requests to various ports. Send a transaction of $1.23, then a transaction of $3.95 within 60 seconds, and that means a specific thing, etc. The cool thing about this is you can merely scan the transactions database looking for the pattern any time after the fact to see if the message had been sent.
If they can also include reversals, and can use up to, say, $9,999 transactions, then they can encode a decent amount of information.
Another cool thing about this is they can test their algorithm - that is the specific formula / numeric pattern that indicates a message - against many years of transaction history to make sure it is unique enough to avoid collisions that could happen randomly with the real-world data. Their trigger could be as naive as just using two specific transaction dollar amounts happening within so many seconds, or something far more advanced where two transaction amounts combined in some mathematical way. Think of a public / private key encryption just with much smaller numbers. For example two transactions happening within 30 seconds that are bitwise NOT (bitwise complement ~ operator in languages like C++ and java) of one another.