Finish challenge 16 and problem set 2.

src/set2.rs

@@ -411,5 +411,37 @@ pub fn challenge15() {
 }
 
 pub fn challenge16() {
-    println!("[xxxx] Challenge 16: TBD");
+    fn encrypt(input: &str, key: &Bytes, iv: &Bytes) -> Bytes {
+        let mut r = String::new();
+        for c in input.chars() {
+            if c == ';' || c == '=' {
+                panic!("encrypt: invalid char {}", c);
+            }
+        }
+        r.push_str("comment1=cooking%20MCs;userdata=");
+        r.push_str(input);
+        r.push_str(";comment2=%20like%20a%20pound%20of%20bacon");
+        let mut cleartext = Bytes(r.as_bytes().to_vec());
+        cleartext.pad_pkcs7(16);
+        cbc::encrypt(&key, &iv, &cleartext)
+    }
+
+    let iv = Bytes::random(16);
+    let key = Bytes::random(16);
+
+    // 0               16              32              48              64
+    // 0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f
+    // comment1=cooking%20MCs;userdata=xxx=x;admin=true;comment2=%20like%20a%20pound%20of%20bacon
+    // flip_bit(2, '9') = '='; flip_bit(1, '9') = ';'
+    let mut cipher = encrypt("xxx9x9admin9true", &key, &iv);
+    cipher.flip_bit(19, 2); // results in flipping same bit in byte of the following block
+    cipher.flip_bit(21, 1);
+    cipher.flip_bit(27, 2);
+    let mut cleartext = cbc::decrypt(&key, &iv, &cipher);
+    cleartext.remove_pkcs7(16);
+    let cleartext_stripped = Bytes(cleartext.0[32..].to_vec());
+    let dict = parser::parse_key_value(&cleartext_stripped.to_utf8());
+    let admin_status = dict.get("admin").unwrap();
+    assert_eq!(admin_status, "true");
+    println!("[okay] Challenge 16: admin={}", admin_status);
 }