diff --git a/src/main.rs b/src/main.rs
index 53f91ad..0f8bebb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 mod lfsr;
+mod tea3;
 
 use lfsr::Lfsr;
 
diff --git a/src/tea3.rs b/src/tea3.rs
new file mode 100644
index 0000000..081cdd2
--- /dev/null
+++ b/src/tea3.rs
@@ -0,0 +1,102 @@
+#[derive(Debug, Clone)]
+pub struct Tea3 {
+    /// Cipher key register k0..k9
+    key_register: Vec<u8>,
+    /// State register
+    state_register: Vec<u8>,
+}
+
+impl Tea3 {
+    pub fn new(key_register: Vec<u8>, state_register: Vec<u8>) -> Self {
+        assert_eq!(key_register.len(), 10, "TEA3 key register is 10B");
+        assert_eq!(state_register.len(), 8, "TEA3 state register is 8B");
+
+        Self {
+            key_register,
+            state_register,
+        }
+    }
+
+    /// TEA-3 S box (called permutation)
+    pub fn p(x: u8) -> u8 {
+        TEA3_P[x as usize]
+    }
+
+    /// key-register step
+    /// k0' = k9 ^ P(k7 ^ k2)
+    /// shift the other bytes
+    ///
+    /// Returns k-out = k0'.
+    pub fn step_key_register(&mut self) -> u8 {
+        let k2 = self.key_register[2];
+        let k7 = self.key_register[7];
+        let k9 = self.key_register[9];
+
+        let kout = k9 ^ Self::p(k7 ^ k2);
+
+        for i in (1..10).rev() {
+            self.key_register[i] = self.key_register[i - 1];
+        }
+        self.key_register[0] = kout;
+
+        kout
+    }
+
+    pub fn key_register(&self) -> &[u8] {
+        &self.key_register
+    }
+
+    pub fn state_register(&self) -> &[u8] {
+        &self.state_register
+    }
+}
+
+/// TEA-3 S box (called permutation)
+/// ETSI TS 104 053-1 Figure 15
+const TEA3_P: [u8; 256] = [
+    0x7D, 0xBF, 0x7B, 0x92, 0xAE, 0x7C, 0xF2, 0x10, 0x5A, 0x0F, 0x61, 0x7A, 0x98, 0x76, 0x07, 0x64,
+    0xEE, 0x89, 0xF7, 0xBA, 0xC2, 0x02, 0x0D, 0xE8, 0x56, 0x2E, 0xCA, 0x58, 0xC0, 0xFA, 0x2A, 0x01,
+    0x57, 0x6E, 0x3F, 0x4B, 0x9C, 0xDA, 0xA6, 0x5B, 0x41, 0x26, 0x50, 0x24, 0x3E, 0xF8, 0x0A, 0x86,
+    0xB6, 0x5C, 0x34, 0xE9, 0x06, 0x88, 0x1F, 0x39, 0x33, 0xDF, 0xD9, 0x78, 0xD8, 0xA8, 0x51, 0xB2,
+    0x09, 0xCD, 0xA1, 0xDD, 0x8E, 0x62, 0x69, 0x4D, 0x23, 0x2B, 0xA9, 0xE1, 0x53, 0x94, 0x90, 0x1E,
+    0xB4, 0x3B, 0xF9, 0x4E, 0x36, 0xFE, 0xB5, 0xD1, 0xA2, 0x8D, 0x66, 0xCE, 0xB7, 0xC4, 0x60, 0xED,
+    0x96, 0x4F, 0x31, 0x79, 0x35, 0xEB, 0x8F, 0xBB, 0x54, 0x14, 0xCB, 0xDE, 0x6B, 0x2D, 0x19, 0x82,
+    0x80, 0xAC, 0x17, 0x05, 0xFF, 0xA4, 0xCF, 0xC6, 0x6F, 0x65, 0xE6, 0x74, 0xC8, 0x93, 0xF4, 0x7E,
+    0xF3, 0x43, 0x9F, 0x71, 0xAB, 0x9A, 0x0B, 0x87, 0x55, 0x70, 0x0C, 0xAD, 0xCC, 0xA5, 0x44, 0xE7,
+    0x46, 0x45, 0x03, 0x30, 0x1A, 0xEA, 0x67, 0x99, 0xDB, 0x4A, 0x42, 0xD7, 0xAA, 0xE4, 0xC2, 0xD5,
+    0xF0, 0x77, 0x20, 0xC3, 0x3C, 0x16, 0xB9, 0xE2, 0xEF, 0x6C, 0x3D, 0x1B, 0x22, 0x84, 0x2F, 0x81,
+    0x1D, 0xB1, 0x3A, 0xE5, 0x73, 0x40, 0xD0, 0x18, 0xC7, 0x6A, 0x9E, 0x91, 0x48, 0x27, 0x95, 0x72,
+    0x68, 0x0E, 0x00, 0xFC, 0xC5, 0x5F, 0xF1, 0xF5, 0x38, 0x11, 0x7F, 0xE3, 0x5E, 0x13, 0xAF, 0x37,
+    0xE0, 0x8A, 0x49, 0x1C, 0x21, 0x47, 0xD4, 0xDC, 0xB0, 0xEC, 0x83, 0x28, 0xB8, 0xF6, 0xA7, 0xC9,
+    0x63, 0x59, 0xBD, 0x32, 0x85, 0x08, 0xBE, 0xD3, 0xFD, 0x4C, 0x2C, 0xFB, 0xA0, 0xC1, 0x9D, 0xB3,
+    0x52, 0x8C, 0x5D, 0x29, 0x6D, 0x04, 0xBC, 0x25, 0x15, 0x8B, 0x12, 0x9B, 0xD6, 0x75, 0xA3, 0x97,
+];
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_tea3_key_register_step() {
+        let mut tea3 = Tea3::new(vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10], vec![0; 8]);
+
+        let kout = tea3.step_key_register();
+
+        // k7 ^ k2 = 8 ^ 3 = 11 = 0x0B
+        // P(0x0B) = 0x7A
+        // k-out = 10 ^ 0x7A = 0x70
+        assert_eq!(kout, 0x70);
+        assert_eq!(tea3.key_register(), &[0x70, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+    }
+
+    // #[test]
+    // fn test_decompose() {
+    //     // key register should be able to be decomposed into 2 registers g and h,
+    //     // both register of 5 bytes.
+    //     // let's note the bytes of g, a0 to a4
+    //     // and the bytes of h, r0 to r4
+    //     // h feedback function should be r0 ^ s(r2)
+    //     // g feedback function should be a0 ^ h.next_custom
+    //     // then the byte a0 should be outputed
+    // }
+}