diff --git a/src/ctr.rs b/src/ctr.rs
index a26c967..603b0d4 100644
--- a/src/ctr.rs
+++ b/src/ctr.rs
@@ -1,37 +1,29 @@
 use crate::bytes::Bytes;
-// use crate::cbc;
-// use openssl::symm;
+use crate::cbc;
+use openssl::symm;
 
-fn _xor(a: &[u8], b: &[u8]) -> Vec<u8> {
+fn xor(a: &Vec<u8>, b: &Vec<u8>) -> Vec<u8> {
     Iterator::zip(a.iter(), b.iter())
         .map(|z| *(z.0) ^ *(z.1))
         .collect()
 }
 
-pub fn _encrypt(Bytes(_key): &Bytes, Bytes(_data): &Bytes) -> Bytes {
-    // let block_size = 16;
-    let result: Vec<u8> = vec![];
-    // let mut prev_cipher: Vec<u8> = iv.to_vec(); // first xor input is IV
-    // for data in data.chunks(block_size) {
-    //     let xored = xor(&prev_cipher, data);
-    //     let mut cipher = cbc::enrypt_aes_128_ecb_block(key, &xored);
-    //     prev_cipher = cipher.to_vec(); // cipher is input for next xor
-    //     result.append(&mut cipher);
-    // }
-    Bytes(result)
+pub fn encrypt(key: &Bytes, nonce: u64, data: &Bytes) -> Bytes {
+    decrypt(key, nonce, data)
 }
 
-pub fn decrypt(Bytes(_key): &Bytes, Bytes(_data): &Bytes) -> Bytes {
-    // XXX: does not handle padding for last block
-    // let cipher_type = symm::Cipher::aes_128_ecb();
-    // let block_size = cipher_type.block_size();
-    let result: Vec<u8> = vec![];
-    // let mut prev_cipher: Vec<u8> = iv.to_vec(); // first xor input is IV
-    // for cipher in data.chunks(block_size) {
-    //     let xored = cbc::decrypt_aes_128_ecb_block(key, &cipher);
-    //     let data = xor(&xored, &prev_cipher);
-    //     result.extend(data.to_vec());
-    //     prev_cipher = cipher.to_vec();
-    // }
+pub fn decrypt(Bytes(key): &Bytes, nonce: u64, Bytes(data): &Bytes) -> Bytes {
+    let mut counter: u64 = 0;
+    let cipher_type = symm::Cipher::aes_128_ecb();
+    let block_size = cipher_type.block_size();
+    let mut result: Vec<u8> = vec![];
+    for cipher in data.chunks(block_size) {
+        let mut keyinput = nonce.to_le_bytes().to_vec();
+        keyinput.append(&mut counter.to_le_bytes().to_vec());
+        let keystream = cbc::enrypt_aes_128_ecb_block(key, &keyinput);
+        let mut data = xor(&keystream, &cipher.to_vec());
+        result.append(&mut data);
+        counter += 1;
+    }
     Bytes(result)
 }
diff --git a/src/main.rs b/src/main.rs
index edb91b3..70dd938 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -27,4 +27,5 @@ fn main() {
     set2::challenge16();
     set3::challenge17();
     set3::challenge18();
+    set3::challenge19();
 }
diff --git a/src/set3.rs b/src/set3.rs
index ba46f66..02e7f3b 100644
--- a/src/set3.rs
+++ b/src/set3.rs
@@ -97,10 +97,20 @@ pub fn challenge17() {
 
 pub fn challenge18() {
     let key = Bytes::from_utf8("YELLOW SUBMARINE");
+
+    let cleartext = Bytes::from_utf8("Let's see if we can get the party started hard my friends.");
+    let cipher = ctr::encrypt(&key, 42351234, &cleartext);
+    let roundtrip = ctr::encrypt(&key, 42351234, &cipher);
+    assert_eq!(cleartext, roundtrip);
+
     let cipher = BytesBase64::from_base64(
         "L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==",
     )
     .to_bytes();
-    let cleartext = ctr::decrypt(&key, &cipher).to_utf8();
-    println!("[xxxx] Challenge 18: {cleartext}");
+    let cleartext = ctr::decrypt(&key, 0, &cipher).to_utf8();
+    println!("[okay] Challenge 18: {cleartext}");
+}
+
+pub fn challenge19() {
+    println!("[xxxx] Challenge 19: TBD");
 }