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COPYING

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+Copyright © 2017 Drew DeVault
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the “Software”), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README

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+				    mkproof
+
+mkproof is a small C program for generating proofs of work.
+
+				  Installation
+
+mkproof depends only on a POSIX-like environment and a C99 compiler.
+
+	$ ./configure
+	$ make
+
+This will produce three executables: mkchallenge, mkproof, and checkproof.
+
+				     Usage
+
+The situation: Bob wants Alice to do something, but Alice isn't sure if Bob is a
+robot.
+
+1. Alice runs `mkchallenge` and sends the challenge to Bob.
+2. Bob runs `mkproof <challenge>` and wastes some CPU time. After several
+   minutes of work, a proof is printed to stdout.
+3. Bob sends the proof to Alice.
+4. Alice runs `checkproof <challenge> <proof>` to verify the work.
+
+Now Alice can be reasonably confident that Bob is not a robot, and proceed with
+Bob's request.
+
+				   Algorithm
+
+To make a challenge, generate 16 random bytes. Choose the argon2 iterations and
+memory parameters, and the number of zeroed digits, to tune the difficulty. The
+challenge string is the terms "argon2id", the iterations, memory use, and zeroed
+digits as decimal integers, and the random bytes as hexadecimal, joined by ":".
+
+To make a proof, split the challenge by ":" and verify that the first token is
+"argon2id". Decode the iterations, memory, and digits parameters, and the
+challenge bytes.
+
+Repeat the following algorithm to generate proofs until an argon2id key is found
+whose first N hexadecimal digits are zero, where N is equal to the digits
+parameter:
+
+1. Generate 16 random bytes (seed) and concatenate the seed and challenge bytes
+   to form an argon2id salt.
+2. Run argon2id with the generated salt, and the memory and iteration parameters
+   provided by the challenge. The hash length and parallelism parameters shall
+   be respectively set to 32 and 1.
+3. Encode the argon2id hash as hexadecimal.
+
+When a suitable hash is found, encode the seed in hexadecimal. This is the proof
+which should be transmitted to the challenger.
+
+To verify the proof, simply run the proof algorithm with the original challenge
+parameters and the challengee's provided seed and verify that the resulting
+hexadecimal string is prefixed with the appropriate number of zeroes.