diff --git a/src/hex.rs b/src/hex.rs
index 0a0078c..f772fe2 100644
--- a/src/hex.rs
+++ b/src/hex.rs
@@ -93,6 +93,12 @@ impl HexDigest for Vec<u8> {
     }
 }
 
+impl<'a> HexDigest for &'a [u8] {
+    fn hex_digest(self) -> String {
+        self.into_iter().map(|x| char::from_digit(*x as u32, 16).unwrap()).collect()
+    }
+}
+
 /*
  * AsHexbytes --
  */
diff --git a/src/xor.rs b/src/xor.rs
index 4548d75..9a81d75 100644
--- a/src/xor.rs
+++ b/src/xor.rs
@@ -14,6 +14,8 @@ pub fn fixed(a: &str, b: &str) -> Result<String, FixedXORError> {
 
 #[cfg(test)]
 mod tests {
+    use hex::AsHexBytes;
+    use std::char;
     use super::fixed;
 
     #[test]
@@ -24,4 +26,24 @@ mod tests {
         let output = fixed(a, b);
         assert_eq!(output.unwrap(), ex_output);
     }
+
+    static ENGLISH_LETTER_FREQ: &'static str = "EARIOTNSLCUDPMHGBFYWKVXZJQ";
+
+    fn letter_freq_score(input: &str) -> f32 {
+        let mut score: f32 = 0.0;
+        score
+    }
+
+    #[test]
+    fn cryptopals13() {
+        let input = "1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736";
+        for key in 32u32..127 {
+            let possible_output = input.hex_bytes().valid()
+                .map(|c| char::from_u32(c as u32 ^ key))
+                .take_while(|c| c.is_some())
+                .map(|c| c.unwrap())
+                .collect::<String>();
+            println!("{}: {}", key, possible_output);
+        }
+    }
 }