# IMPOST

Yet Another Pow capTCHA.

## What is this

IMPOST is a proof of work based challenge-response system that is designed to
ward off spam and abuse.

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## Basic configuration notes

Leading zeroes takes in a difficulty from 1 to 64, this indicates the number of
leading hexidecimal zeroes that are required for a problem to be solved. The
probability for a digit to be a zero is 1/16^$D$ where $D$ is the difficulty.

The following chart is provided for a baseline of the theoretical expected
average iterations it would take to solve challenge of difficulty $D$.

| $D$ | expected average iterations |
| --- | --------------------------- |
| 1   | 16                          |
| 2   | 256                         |
| 3   | 4,096                       |
| 4   | 65,536                      |
| 5   | 1,048,576                   |
| 6   | 16,777,216                  |
| 7   | 268,435,456                 |
| 8   | 4,294,967,296               |

Target Number takes in a max number of iterations, $M$, to be solved. The
probability that a solution is solved will be 1/targetNumber. This provides a
more preciese way of determining the difficulty of a challenge. With leading
zeroes, there is only a theoretical chance that a solution is found within a
certain number of iterations. With target number, there is a gurantee that a
solution will be found in, at most, $M$ iterations.

## Interesting links and blogs I used while writing this

- [Anubis](https://github.com/TecharoHQ/anubis) for inspiration. (once again, I
  have some strong opinions on Anubis, but thats is still neithehrere nor there)
- ["Proof of Mutex: Outspeeding Anubis with Valid PoW" by yumechi](https://yumechi.jp/en/blog/2025/proof-of-mutex-outspeeding-anubis-with-valid-pow/)
- https://www.arkoselabs.com/blog/proof-of-work-invisible-security-visible-results/
  for giving me hope that PoW is not a fruitless endeavor for captchas.
- [ALTCHA](https://github.com/altcha-org/altcha) purely to see how another
  option in the space works. (I have some choice words about the implementation
  of ALTCHA, but I digress. **No code from ALTCHA was used in this project.**)
- [The monero project](https://www.getmonero.org/) for its list of algorithms
  they use in xmrig, and some inspiration.