Purpose

Proof-of-work (PoW) in Dscuss is used for resistance against flooding. PoW is just a uint64 number, which is sent along with users' public key. Hash of user's public key plus PoW must has a determined number of leading zero bits (see below). It should be hard to find such a PoW. Hence the selected hashing function is scrypt. Peers are forced to do this work before participating in discussion, otherwise their messages will be dropped.

Math

Average time to find a PoW is

Tavg = C * 2 ^ n

Where C is a constant, which depends on peer's CPU performance and n is the number of leading zeros in the resulting hash.

The time to find PoW (T) is actually a random variable, which has a geometric distribution. Tavg is the expected value of this variable. The real distribution of time to find an 8-bit (as an example) PoW is shown on the histogram below. The histogram is drawn for 10,000 values.

The number of required leading zero bits is set to 16 for the proof-of-concept version. In this case, Tavg is about 20 minutes on Intel(R) Core(TM) i7-4790k CPU @ 4.0GHz or 2 hours on Intel(R) Core(TM) 2 Duo P8600 @ 2.40GHz.

Benchmarking

How to benchmark proof-of-work on your system:

1. Disable debug in dscuss.go.
2. Run utils/benchmark_pow.sh [bit_num] [count]
   where,
   bit_num is the number of leading zero bits in PoW (can be 8 or 16);
   count specifies how many times to run the test.

for example:

utils/benchmark_pow.sh 16 50

This test takes about 12 hours on Intel Core i7-4790k.

When the benchmark is over, you will see the results: the Tavg and the histogram:

The benchmark is over.
The average time to find proof is: 22.2161
The histogram is ready: pow_histogram.png.

In benchmark.log you will find details about each test run.