diff --git a/src/lib.rs b/src/lib.rs
index 17ff39e..07c2538 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1 +1,2 @@
 pub mod b64;
+pub mod xor;
diff --git a/src/xor.rs b/src/xor.rs
new file mode 100644
index 0000000..beb6406
--- /dev/null
+++ b/src/xor.rs
@@ -0,0 +1,20 @@
+pub fn fixed(a: &str, b: &str) -> Result<String, String> {
+    let a_decoded = a.chars().map(|c| c.to_digit(16).unwrap() as u16);
+    let b_decoded = b.chars().map(|c| c.to_digit(16).unwrap() as u16);
+    let xor = a_decoded.zip(b_decoded).map(|(a, b)| a ^ b as char);
+    String::from_iter(xor)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::fixed;
+
+    #[test]
+    fn cryptopals() {
+        let a = "1c0111001f010100061a024b53535009181c";
+        let b = "686974207468652062756c6c277320657965";
+        let ex_output = "746865206b696420646f6e277420706c6179";
+        let output = fixed(a, b);
+        assert_eq!(output, ex_output);
+    }
+}