period finder, brute force

src/main.rs

@@ -2,7 +2,7 @@ mod lfsr;
 mod tea3;
 
 use lfsr::Lfsr;
-use tea3::Tea3;
+use tea3::{Tea3, period_floyd};
 
 fn main() {
     let mut lfsr = Lfsr::new(4, vec![0, 3], vec![0x12, 0x34, 0x56, 0x78]);
@@ -46,4 +46,6 @@ fn main() {
         .collect();
 
     println!("Decrypted : {:?}", decrypted);
+
+    // println!("\nperiod: {}", period_floyd(vec![0, 0, 0, 0, 0, 0, 0, 0, 0, 0], vec![0; 8]));
 }

src/tea3.rs

@@ -1,4 +1,4 @@
-use crate::Lfsr;
+use std::collections::HashMap;
 
 #[derive(Debug, Clone)]
 pub struct Tea3 {
@@ -207,9 +207,69 @@ fn bp(x: u8) -> u8 {
         | bit(5)
 }
 
+pub fn period(key: Vec<u8>, iv: Vec<u8>) -> u64 {
+    let mut cipher = Tea3::new(key, iv);
+    cipher.init();
+
+    let mut seen = HashMap::new();
+
+    let mut step: u64 = 0;
+
+    loop {
+        let state = (
+            cipher.key_register().to_vec(),
+            cipher.state_register().to_vec(),
+        );
+
+        if let Some(prev) = seen.get(&state) {
+            return step - prev;
+        }
+
+        seen.insert(state, step);
+
+        cipher.step();
+        step += 1;
+    }
+}
+
+pub fn period_floyd(key: Vec<u8>, iv: Vec<u8>) -> u64 {
+    let mut tortoise = Tea3::new(key.clone(), iv.clone());
+    let mut hare = Tea3::new(key, iv);
+
+    tortoise.init();
+    hare.init();
+
+    // Move tortoise by 1, hare by 2
+    tortoise.step();
+    hare.step();
+    hare.step();
+
+    while tortoise.key_register() != hare.key_register()
+        || tortoise.state_register() != hare.state_register()
+    {
+        tortoise.step();
+
+        hare.step();
+        hare.step();
+    }
+
+    let mut lambda = 1;
+    hare.step();
+
+    while tortoise.key_register() != hare.key_register()
+        || tortoise.state_register() != hare.state_register()
+    {
+        hare.step();
+        lambda += 1;
+    }
+
+    lambda
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::Lfsr;
 
     #[test]
     fn test_tea3_key_register_step() {