add pollard-rho

This commit is contained in:
theBreadCompany 2024-12-03 23:27:00 +01:00
commit b8775f3675
5 changed files with 216 additions and 0 deletions

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mod pollard_rho;
fn main() {
let n = rand::random::<u8>();
let n_primediv = pollard_rho::pollard_rho(n.into());
eprintln!("Generated random number {}, got prime divisor {}", n, n_primediv);
}

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use gcd::Gcd;
/**
* Calculate the primedivisor for some number `n`
*/
pub fn pollard_rho(n: u16) -> u16 {
if n == 1 { return 1; } // 1 only has 1 as prime divisor
if n % 2 == 0 { return 2; } // even numbers have at least 2 as prime divisor
if is_prime(n) { return n; }
let mut x = rand::random::<u16>() % 2;
let mut y = x;
let c = rand::random::<u16>() % 2;
let mut div = 1;
while div == 1 {
x = (mod_pow(x, 2, n) + c + n) % n;
y = (mod_pow(y, 2, n) + c + n) % n;
y = (mod_pow(y, 2, n) + c + n) % n;
div = u16::try_from((i16::try_from(x).unwrap()-i16::try_from(y).unwrap()).abs()).unwrap().gcd(n);
eprintln!("Got div {}", div);
if div == n { return pollard_rho(n); }
}
return div;
}
/**
* Discrete/Modular exponentiation
*
* Highly memory efficient because the full result is never stored, but shortened by defined modulo instead.
* We can use that because the prime divisor required for our algorithm is guarenteed to be smaller
* than n.
*
* Counterpart function to the discrete logarithm.
*/
fn mod_pow(base: u16, exp: u16, r#mod: u16) -> u16 {
let mut result = 1;
for _ in 0..exp-1 {
result = (result * base) % r#mod;
}
result
}
fn is_prime(n: u16) -> bool {
for i in (3..=(n as f32).sqrt() as u16).step_by(2) {
if n % i == 0 { return false; }
}
true
}