diff --git a/src/main.rs b/src/main.rs
index 1bb6886..5f58c13 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,8 +1,10 @@
 mod lfsr;
+mod period;
 mod tea3;
 
 use lfsr::Lfsr;
-use tea3::{Tea3, period_floyd};
+use period::{period_bruteforce, period_floyd, period_paper};
+use tea3::Tea3;
 
 fn main() {
     let mut lfsr = Lfsr::new(4, vec![0, 3], vec![0x12, 0x34, 0x56, 0x78]);
@@ -47,5 +49,8 @@ fn main() {
 
     println!("Decrypted : {:?}", decrypted);
 
-    // println!("\nperiod: {}", period_floyd(vec![0, 0, 0, 0, 0, 0, 0, 0, 0, 0], vec![0; 8]));
+    println!(
+        "\nPaper period: {}",
+        period_paper(vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
+    );
 }
diff --git a/src/period.rs b/src/period.rs
new file mode 100644
index 0000000..675095f
--- /dev/null
+++ b/src/period.rs
@@ -0,0 +1,163 @@
+use std::collections::HashMap;
+
+use crate::tea3::Tea3;
+
+/// period brute force with hashmap
+pub fn period_bruteforce(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;
+    }
+}
+
+/// period brute force with Floyd tortoise and hare algorithm
+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();
+
+    tortoise.step();
+    hare.step();
+    hare.step();
+
+    while (tortoise.key_register(), tortoise.state_register())
+        != (hare.key_register(), hare.state_register())
+    {
+        tortoise.step();
+        hare.step();
+        hare.step();
+    }
+
+    let mut lambda = 1;
+    hare.step();
+
+    while (tortoise.key_register(), tortoise.state_register())
+        != (hare.key_register(), hare.state_register())
+    {
+        hare.step();
+        lambda += 1;
+    }
+
+    lambda
+}
+
+// paper method, decomposition key register
+// (Observations on TETRA Encryption Algorithm TEA-3)
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct Reg5([u8; 5]);
+
+impl Reg5 {
+    fn step_h(&mut self) -> u8 {
+        let out = self.0[0];
+        let feedback = out ^ Tea3::p(self.0[2]);
+
+        self.0.copy_within(1..5, 0);
+        self.0[4] = feedback;
+
+        out
+    }
+
+    fn step_g(&mut self, h_out: u8) -> u8 {
+        let out = self.0[0];
+        let feedback = out ^ h_out;
+
+        self.0.copy_within(1..5, 0);
+        self.0[4] = feedback;
+
+        out
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct Decomposed {
+    g: Reg5,
+    h: Reg5,
+}
+
+impl Decomposed {
+    fn from_key(key: &[u8]) -> Self {
+        Self {
+            g: Reg5([key[0], key[1], key[2], key[3], key[4]]),
+            h: Reg5([
+                key[0] ^ key[5],
+                key[1] ^ key[6],
+                key[2] ^ key[7],
+                key[3] ^ key[8],
+                key[4] ^ key[9],
+            ]),
+        }
+    }
+
+    fn step(&mut self) -> u8 {
+        let h_out = self.h.step_h();
+        self.g.step_g(h_out)
+    }
+}
+
+fn period_h(start: Reg5) -> u64 {
+    let mut tortoise = start;
+    let mut hare = start;
+
+    tortoise.step_h();
+    hare.step_h();
+    hare.step_h();
+
+    while tortoise != hare {
+        tortoise.step_h();
+        hare.step_h();
+        hare.step_h();
+    }
+
+    let mut lambda = 1;
+    hare.step_h();
+
+    while tortoise != hare {
+        hare.step_h();
+        lambda += 1;
+    }
+
+    lambda
+}
+
+/// paper method, decomposition key register
+/// (Observations on TETRA Encryption Algorithm TEA-3)
+pub fn period_paper(key: Vec<u8>) -> u64 {
+    let start = Decomposed::from_key(&key);
+
+    let p = period_h(start.h);
+
+    for mult in [1u64, 2, 5, 10] {
+        let target = p * mult;
+
+        let mut cur = start;
+
+        for _ in 0..target {
+            cur.step();
+        }
+
+        if cur == start {
+            return target;
+        }
+    }
+
+    panic!("No valid period found");
+}
diff --git a/src/tea3.rs b/src/tea3.rs
index 307e129..a2e4558 100644
--- a/src/tea3.rs
+++ b/src/tea3.rs
@@ -1,5 +1,3 @@
-use std::collections::HashMap;
-
 #[derive(Debug, Clone)]
 pub struct Tea3 {
     /// Cipher key register k0..k9
@@ -207,65 +205,6 @@ 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::*;
@@ -470,7 +409,6 @@ mod tests {
 
         let poly = Lfsr::berlekamp_massey(&keystream);
 
-
         let history = &keystream[keystream.len() - (poly.len() - 1)..];
         let predicted = Lfsr::predict_next_from_poly(history, &poly);