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# Sudocrypto

Those puzzles might be ubiquitous for a non-obvious reason
 (+2, -2) [vote for, against]

A big problem with sending encrypted messages is managing the keys required to decrypt them. There are various solutions to this problem, here’s a fun one.

A solved sudoku puzzle consists of 81 decimal digits. Why not use this as a crypto key? You could even send the unsolved puzzle right along with the encrypted message. The recipient solves the puzzle, punches the digits into a program and voila.

A big limitation, of course, is anyone who can solve the puzzle can read the message. As long as your arch-enemy is stupid or lazy, this might well be enough.

Can we improve this? Sure…

First, the number of possible sudoku puzzles is roughly equivalent to the key space of a 72 bit crypto key. That is not good at all. Instead of using the solution as a key directly, we could use it as a numeric index into a large database of pre-calculated keys. The keys themselves could be as large as necessary to resist direct cryptographic attack. The key database could be transmitted ahead of time, perhaps on a portable disk, and thus acts somewhat like a “one time pad.”

Second, don’t transmit the puzzle with the message. Pre-arrange to find the correct puzzle in a known public location. Perhaps if we could get a series of these published somewhere, we could use a different one for each day you transmit a message...

...hey, wait a sec….I wonder if this is baked?

 — krelnik, May 02 2007

WIkipedia: Mathematics of Sudoku http://en.wikipedia...thematics_of_Sudoku
[jutta, May 02 2007]

I'm not sure I get your point, bigsleep. I'm not proposing anything new cryptographically, just a novel way to manage keys that involves puzzles. This method would work equally well (if at all) for any chosen crypto algorithm.
 — krelnik, May 02 2007

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