moth/docs/tokens.md

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Tokens
======
Tokens are good for a single point in a single category. They are
formed by prepending the category and a colon to the bubblebabble digest
of 3 random octets. A token for the "merfing" category might look like
this:
merfing:xunap-motex
Entropy
-------
3 octets provides 24 bits of entropy. This gives 16777216 possible
tokens in each category. The longest contest yet run lasted 24 hours,
which would give 2^24/24/60 = 11650 tokens per category per minute. I
think this is a large enough pool to discourage brute-force attacks.
Assuming /dev/urandom is as good as is claimed, brute-force would be the
only way to attack it.
Token server
------------
Sometimes it's a good idea to have certain puzzles run on a different
machine than the server. For instance, something that loads down the
CPU, or something that carries a high risk of local exploit. The token
server listens on TCP port 1, issuing tokens encrypted with ARC4
(symmetric encryption). Here's how the transaction goes:
C: category
S: nonce (4 bytes)
C: nonce encrypted with symmetric key
S: token encrypted with symmetric key
Token client
------------
The token client (in package "tokencli") runs as a daemon, requesting a
new token every minute for each puzzle. Because we want you to have
multiple puzzles within a category, and the server only knows about
categories, each puzzle needs to be associated with a category.
Additionally, tokens are encrypted before being written to the local
filesystem, with a different key for each puzzle.
The token client thus needs a 4-tuple for each puzzle:
(puzzle name, puzzle key, category, category key)
In the interest of making things easy to administer and code, this
4-tuple is stored in files and directories:
/packages/packagename/tokencli/puzzle_name/enc.key
/packages/packagename/tokencli/puzzle_name/category.key
/packages/packagename/tokencli/puzzle_name/category
And puzzles are stored in:
/state/tokens/puzzle_name
Using this scheme, the token client has only to iterate over
/packages/*/tokencli/* instead of implementing some sort of parser.