From 7a2a5c567e956aa84085f62637751aa2d6a1e165 Mon Sep 17 00:00:00 2001
From: Sam Hadow <sam.hadow@inbox.lv>
Date: Fri, 26 Apr 2024 18:56:56 +0200
Subject: [PATCH] decrypt + test

---
 src/aes.rs | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 78 insertions(+), 1 deletion(-)

diff --git a/src/aes.rs b/src/aes.rs
index eadefab..0966947 100644
--- a/src/aes.rs
+++ b/src/aes.rs
@@ -143,7 +143,6 @@ static RC: [u8; 11] = [
     0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1B, 0x36,
 ];
 
-
 fn clone_into_array<A, T>(slice: &[T]) -> A
 where
     A: Default + AsMut<[T]>,
@@ -273,6 +272,32 @@ fn mix_columns(state: &mut [[u8; 4]; 4]) {
     }
 }
 
+fn inverse_mix_columns(state: &mut [[u8; 4]; 4]) {
+    for i in 0..4 {
+        let mut temp = [0u8; 4];
+        for j in 0..4 {
+            temp[j] = state[j][i];
+        }
+
+        state[0][i] = galois_multiplication(temp[0], 14)
+            ^ galois_multiplication(temp[3], 9)
+            ^ galois_multiplication(temp[2], 13)
+            ^ galois_multiplication(temp[1], 11);
+        state[1][i] = galois_multiplication(temp[1], 14)
+            ^ galois_multiplication(temp[0], 9)
+            ^ galois_multiplication(temp[3], 13)
+            ^ galois_multiplication(temp[2], 11);
+        state[2][i] = galois_multiplication(temp[2], 14)
+            ^ galois_multiplication(temp[1], 9)
+            ^ galois_multiplication(temp[0], 13)
+            ^ galois_multiplication(temp[3], 11);
+        state[3][i] = galois_multiplication(temp[3], 14)
+            ^ galois_multiplication(temp[2], 9)
+            ^ galois_multiplication(temp[1], 13)
+            ^ galois_multiplication(temp[0], 11);
+    }
+}
+
 fn add_round_key(state: &mut [[u8; 4]; 4], key: &[[u8; 4]; 4]) {
     // for i in 0..4 {
     //     for j in 0..4 {
@@ -333,6 +358,39 @@ impl Aes {
         result
     }
 
+    fn decrypt_block(&self, block: &[u8; 16]) -> [u8; 16] {
+        let mut result = [0u8; 16];
+
+        let mut state = [[0u8; 4]; 4];
+        for i in 0..16 {
+            state[i % 4][i / 4] = block[i];
+        }
+
+        add_round_key(&mut state, &clone_into_array(&self.expanded_key[40..44]));
+        inverse_shift_rows(&mut state);
+        inverse_substitute_state(&mut state);
+
+        for i in (1..self.n_turn).rev() {
+            add_round_key(
+                &mut state,
+                &clone_into_array(&self.expanded_key[i * 4..(i + 1) * 4]),
+            );
+            inverse_mix_columns(&mut state);
+            inverse_shift_rows(&mut state);
+            inverse_substitute_state(&mut state);
+        }
+
+        add_round_key(&mut state, &clone_into_array(&self.expanded_key[0..4]));
+
+        for i in 0..4 {
+            for j in 0..4 {
+                result[4 * j + i] = state[i][j]
+            }
+        }
+
+        result
+    }
+
     pub fn key_schedule(key_bytes: &[u8; 16]) -> [[u8; 4]; 44] {
         let mut original_key = [[0u8; 4]; 4];
         let mut expanded_key = [[0u8; 4]; 44];
@@ -438,4 +496,23 @@ mod tests {
         ];
         assert_eq!(ciphertext, expected_ciphertext);
     }
+    #[test]
+    fn decrypt_test() {
+        let ciphertext: [u8; 16] = [
+            0x39, 0x25, 0x84, 0x1d, 0x02, 0xdc, 0x09, 0xfb, 0xdc, 0x11, 0x85, 0x97, 0x19, 0x6a,
+            0x0b, 0x32,
+        ];
+        let key: [u8; 16] = [
+            0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6, 0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf,
+            0x4f, 0x3c,
+        ];
+        let nturn = 10;
+        let aescipher = Aes::new(&key, &nturn);
+        let cleartext: [u8; 16] = aescipher.decrypt_block(&ciphertext);
+        let expected_cleartext: [u8; 16] = [
+            0x32, 0x43, 0xf6, 0xa8, 0x88, 0x5a, 0x30, 0x8d, 0x31, 0x31, 0x98, 0xa2, 0xe0, 0x37,
+            0x07, 0x34,
+        ];
+        assert_eq!(cleartext, expected_cleartext);
+    }
 }