Implement challenge 24 and finish set 3.

src/mtcipher.rs

@@ -0,0 +1,26 @@
+use crate::{bytes::Bytes, mt19937};
+
+pub fn decrypt(key: u16, data: &Bytes) -> Bytes {
+    encrypt(key, data)
+}
+
+pub fn encrypt(key: u16, Bytes(data): &Bytes) -> Bytes {
+    // You can create a trivial stream cipher out of any PRNG; use it to
+    // generate a sequence of 8 bit outputs and call those outputs a keystream.
+    // XOR each byte of plaintext with each successive byte of keystream.
+    // Write the function that does this for MT19937 using a 16-bit seed.
+
+    let mut mt = mt19937::MT19937::new();
+    mt.seed(key as u32);
+    let mut result: Vec<u8> = vec![];
+
+    for chunk in data.chunks(4) {
+        let key = mt.extract_bytes();
+        for i in 0..chunk.len() {
+            let cipher_char = chunk[i] ^ key[i];
+            result.push(cipher_char);
+        }
+    }
+
+    Bytes(result)
+}