decompose key register LFSR

src/tea3.rs

@@ -1,3 +1,5 @@
+use crate::Lfsr;
+
 #[derive(Debug, Clone)]
 pub struct Tea3 {
     /// Cipher key register k0..k9
@@ -89,14 +91,48 @@ mod tests {
         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
-    // }
+    #[test]
+    fn test_decompose() {
+        fn p(x: u8) -> u8 {
+            TEA3_P[x as usize]
+        }
+
+        let k = vec![1u8, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+        let mut tea3 = Tea3::new(k, vec![0; 8]);
+
+        let mut full_outputs = Vec::new();
+        for _ in 0..20 {
+            let kout = tea3.step_key_register();
+            full_outputs.push(kout);
+        }
+
+        let s = &full_outputs[0..10];
+
+        let g_orig = [s[0], s[1], s[2], s[3], s[4]];
+        let h_orig = [
+            s[5] ^ s[0],
+            s[6] ^ s[1],
+            s[7] ^ s[2],
+            s[8] ^ s[3],
+            s[9] ^ s[4],
+        ];
+
+        let g_rev: Vec<u8> = g_orig.iter().rev().copied().collect();
+        let h_rev: Vec<u8> = h_orig.iter().rev().copied().collect();
+
+        let mut g_lfsr = Lfsr::new(5, vec![], g_rev);
+        let mut h_lfsr = Lfsr::new(5, vec![], h_rev);
+
+        let mut decomposed_outputs = Vec::new();
+        for _ in 0..20 {
+            let h_out = h_lfsr.next_custom([4, 2], |[h0, h2]| h0 ^ p(h2));
+            let g_out = g_lfsr.next_custom([4], |[g0]| g0 ^ h_out);
+            decomposed_outputs.push(g_out);
+        }
+
+        assert_eq!(
+            decomposed_outputs, full_outputs,
+            "g + h LFSRs must reproduce the exact output sequence"
+        );
+    }
 }