diff --git a/data/12.txt b/data/12.txt
new file mode 100644
index 0000000..3655bc4
--- /dev/null
+++ b/data/12.txt
@@ -0,0 +1,4 @@
+Um9sbGluJyBpbiBteSA1LjAKV2l0aCBteSByYWctdG9wIGRvd24gc28gbXkg
+aGFpciBjYW4gYmxvdwpUaGUgZ2lybGllcyBvbiBzdGFuZGJ5IHdhdmluZyBq
+dXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUg
+YnkK 
diff --git a/src/set2.rs b/src/set2.rs
index e6e7326..dd11ed3 100644
--- a/src/set2.rs
+++ b/src/set2.rs
@@ -102,15 +102,33 @@ pub fn challenge11() {
 }
 
 pub fn challenge12() {
-    let key = Bytes::random(16);
-    fn encrypt(Bytes(data): &Bytes, key: &Bytes) -> Bytes {
+    fn read(path: &str) -> Bytes {
+        let s = std::fs::read_to_string(path).unwrap();
+        BytesBase64::from_base64(&s).to_bytes()
+    }
+
+    fn encryption_oracle(key: &Bytes, Bytes(data): &Bytes) -> Bytes {
+        // Copy your oracle function to a new function that encrypts buffers under ECB mode using a consistent but unknown key
+        // Now take that same function and have it append to the plaintext, BEFORE ENCRYPTING, the following string (from 12.txt):
         let mut data = data.to_vec();
-        let mut b = Bytes::from_utf8("Um9sbGluJyBpbiBteSA1LjAKV2l0aCBteSByYWctdG9wIGRvd24gc28gbXkgaGFpciBjYW4gYmxvdwpUaGUgZ2lybGllcyBvbiBzdGFuZGJ5IHdhdmluZyBqdXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUgYnkK");
-        data.append(&mut b.0);
+        let mut string = read("data/12.txt");
+        data.append(&mut string.0);
         let cipher = ecb::encrypt(&key, &Bytes(data));
         cipher
     }
-    let text = Bytes::from_utf8("aaaabbbbccccddddaaaabbbbccccddddaaaabbbbccccdddd");
-    let cipher = encrypt(&text, &key);
-    println!("[xxxx] Challenge 12: {}", cipher.0.len());
+
+    let key = Bytes::random(16); // consistent but unknown key
+
+    // 1. Find key length
+    let mut v = vec![];
+    let initial_cipher_len = encryption_oracle(&key, &Bytes(v.to_vec())).0.len();
+    let mut new_cipher_len = initial_cipher_len;
+    while initial_cipher_len == new_cipher_len {
+        v.push(b'A');
+        let cipher = encryption_oracle(&key, &Bytes(v.to_vec()));
+        new_cipher_len = cipher.0.len();
+    }
+    let key_length = new_cipher_len - initial_cipher_len;
+
+    println!("[xxxx] Challenge 12: key_length={}", key_length);
 }