diff --git a/data/20.txt b/data/20.txt
new file mode 100644
index 0000000..d601f0b
--- /dev/null
+++ b/data/20.txt
@@ -0,0 +1,60 @@
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diff --git a/src/bytes.rs b/src/bytes.rs
index 587b2ab..440ddcf 100644
--- a/src/bytes.rs
+++ b/src/bytes.rs
@@ -93,6 +93,16 @@ impl Bytes {
         r
     }
 
+    pub fn guess_key(&self) -> u8 {
+        // Assuming all bytes have been xor-encrypted by the same u8 key, find that key
+        // by trying all u8 values and compute an ascii score for the resulting "plaintext".
+        // The u8 key for the "plaintext" with the highest score is returned as the guessed
+        // key.
+        let mut h: Vec<(Bytes, u8)> = (0..=255).map(|i| (Bytes::xor_byte(self, i), i)).collect();
+        h.sort_by(|a, b| a.0.ascii_score().partial_cmp(&b.0.ascii_score()).unwrap());
+        h.last().unwrap().1
+    }
+
     pub fn pad_pkcs7(&mut self, block_size: usize) -> () {
         let Bytes(v) = self;
         let padding_value = (block_size - v.len() % block_size) as u8;
diff --git a/src/main.rs b/src/main.rs
index 5fb88ed..62cfcf0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -31,7 +31,8 @@ fn main() {
         set3::challenge18();
         set3::challenge19();
         set3::challenge20();
+        set3::challenge21();
     } else {
-        set3::challenge20();
+        set3::challenge21();
     }
 }
diff --git a/src/set1.rs b/src/set1.rs
index 3fbcbc0..7fd0418 100644
--- a/src/set1.rs
+++ b/src/set1.rs
@@ -138,17 +138,16 @@ pub fn challenge6() {
         result
     }
 
-    fn guess_key(bytes: &Bytes) -> u8 {
-        let mut h: Vec<(Bytes, u8)> = (0..=255).map(|i| (Bytes::xor_byte(bytes, i), i)).collect();
-        h.sort_by(|a, b| a.0.ascii_score().partial_cmp(&b.0.ascii_score()).unwrap());
-        h.last().unwrap().1
-    }
-
     _test_roundtrip();
     let bytes = read("data/6.txt");
     let key_size = guess_key_size(&bytes);
     let bytes_tranposed = transpose(&bytes, key_size);
-    let key = Bytes(bytes_tranposed.iter().map(|b| guess_key(&b)).collect());
+    let key = Bytes(
+        bytes_tranposed
+            .iter()
+            .map(|b| Bytes::guess_key(&b))
+            .collect(),
+    );
     let msg = Bytes::xor_cycle(&bytes, &key).to_utf8();
 
     let partial_msg = msg[..20].to_string();
diff --git a/src/set3.rs b/src/set3.rs
index 5e9c857..840ebbc 100644
--- a/src/set3.rs
+++ b/src/set3.rs
@@ -268,5 +268,38 @@ pub fn challenge19() {
 }
 
 pub fn challenge20() {
-    println!("[xxxx] Challenge 20: TBD");
+    fn read(path: &str) -> Vec<Bytes> {
+        let file = std::fs::File::open(path).unwrap();
+        let br = BufReader::new(file);
+        br.lines()
+            .map(|line| BytesBase64::from_base64(&line.unwrap()).to_bytes())
+            .collect()
+    }
+
+    fn attack(ciphers: Vec<Bytes>) -> Vec<Bytes> {
+        let min_cipher_len = ciphers.iter().map(|c| c.len()).min().unwrap_or(0);
+        let mut key: Vec<u8> = vec![];
+        for byte_index in 0..min_cipher_len {
+            let bytes = Bytes(ciphers.iter().map(|c| c.0[byte_index]).collect());
+            let key_char = Bytes::guess_key(&bytes);
+            key.push(key_char);
+        }
+        let key = Bytes(key);
+        ciphers
+            .iter()
+            .map(|cipher| Bytes::xor(&key, cipher))
+            .collect()
+    }
+
+    let plaintexts = read("data/20.txt");
+    let key = Bytes::from_utf8("YELLOW SUBMARINE");
+    let encrypt = |plaintext: &Bytes| -> Bytes { ctr::encrypt(&key, 0, plaintext) };
+    let ciphers: Vec<Bytes> = plaintexts.iter().map(|ct| encrypt(&ct)).collect();
+    let plaintexts = attack(ciphers);
+
+    println!("[okay] Challenge 20: {}", plaintexts[0].to_utf8());
+}
+
+pub fn challenge21() {
+    println!("[xxxx] Challenge 21: TBD");
 }