feat: finish p18, implment aes ctr mode

problems/p18/src/main.rs

@@ -1,3 +1,42 @@
-fn main() {
-    println!("Hello, world!");
+use anyhow::Result;
+use base64::{Engine as _, engine::general_purpose::STANDARD};
+use rayon::prelude::*;
+
+use common::aes_ecb_enc;
+
+fn aes_ctr_enc(input: &[u8], key: &[u8; 16], nonce: u64) -> Result<Vec<u8>> {
+    let mut key_stream = Vec::new();
+    for round in 0..=(input.len() / 16) as u64 {
+        let input: Vec<u8> = nonce
+            .to_le_bytes()
+            .into_iter()
+            .chain(round.to_le_bytes())
+            .collect();
+        let stream_block = aes_ecb_enc(&input, key)?;
+        key_stream.extend(stream_block);
+    }
+    let output: Vec<u8> = input
+        .par_iter()
+        .zip(key_stream)
+        .map(|(&a, b)| a ^ b)
+        .collect();
+
+    Ok(output)
+}
+
+fn aes_ctr_dec(input: &[u8], key: &[u8; 16], nonce: u64) -> Result<Vec<u8>> {
+    aes_ctr_enc(input, key, nonce)
+}
+
+fn main() -> Result<()> {
+    let cipher = STANDARD
+        .decode("L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==")?;
+
+    let key: [u8; 16] = "YELLOW SUBMARINE".as_bytes().try_into()?;
+
+    let plain = aes_ctr_dec(&cipher, &key, 0)?;
+    let plain = String::from_utf8(plain)?;
+    println!("{plain}");
+
+    Ok(())
 }