diff --git a/src/hex.rs b/src/hex.rs
new file mode 100644
index 0000000..badcdd1
--- /dev/null
+++ b/src/hex.rs
@@ -0,0 +1,48 @@
+use std::iter;
+use std::str::Chars;
+
+pub enum HexResult {
+    Byte(u8),
+    Invalid(char),
+}
+
+pub struct HexDecoder<'a> {
+    input: Chars<'a>,
+}
+
+impl<'a> HexDecoder<'a> {
+    fn new(input: &'a str) -> HexDecoder {
+        HexDecoder { input: input.chars() }
+    }
+}
+
+impl<'a> iter::Iterator for HexDecoder<'a> {
+    type Item = HexResult;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(c) = self.input.next() {
+            if let Some(hex) = c.to_digit(16) {
+                Some(HexResult::Byte(hex as u8))
+            } else {
+                Some(HexResult::Invalid(c))
+            }
+        } else {
+            None
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn simple() {
+        let decoder = HexDecoder::new("123");
+        let collected: Vec<u8> = decoder.map(|c| match c {
+            HexResult::Byte(c) => c,
+            _ => panic!(),
+        }).collect();
+        assert_eq!(collected, vec![1, 2, 3]);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 07c2538..0b4ae41 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,2 +1,3 @@
 pub mod b64;
 pub mod xor;
+pub mod hex;