diff --git a/src/main.rs b/src/main.rs
index 4657ce5..265938f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,10 @@
+#![warn(clippy::pedantic)]
+#![allow(clippy::cast_precision_loss)]
+#![allow(clippy::items_after_statements)]
+#![allow(clippy::many_single_char_names)]
+#![allow(clippy::module_name_repetitions)]
+#![allow(clippy::cast_possible_truncation)]
+#![allow(clippy::unnested_or_patterns)]
 mod bytes;
 mod bytes_base64;
 mod cbc;
@@ -15,7 +22,7 @@ mod sha1;
 mod utils;
 
 fn main() {
-    const RUN_ALL: bool = true;
+    const RUN_ALL: bool = false;
     if RUN_ALL {
         set1::challenge1();
         set1::challenge2();
diff --git a/src/set4.rs b/src/set4.rs
index 31d0553..ec1a715 100644
--- a/src/set4.rs
+++ b/src/set4.rs
@@ -1,3 +1,6 @@
+use std::path::Path;
+use std::fs;
+use std::{thread, time};
 use crate::{bytes::Bytes, cbc, ctr, ecb, md4, parser, sha1, utils};
 
 pub fn challenge25() {
@@ -299,5 +302,65 @@ pub fn challenge30() {
 }
 
 pub fn challenge31() {
-    println!("[xxxx] Challenge 31: tbd");
+    fn insecure_compare(a: &[u8], b: &[u8]) -> bool {
+        // Write a function, call it "insecure_compare", that implements the == operation by doing
+        // byte-at-a-time comparisons with early exit (ie, return false at the first non-matching
+        // byte).
+        let delay = time::Duration::from_millis(8);
+        if a.len() != b.len() {
+            return false;
+        }
+        for (a, b) in a.iter().zip(b.iter()) {
+            if a != b {
+                return false;
+            }
+            // In the loop for "insecure_compare", add a 50ms sleep (sleep 50ms after each byte).
+            thread::sleep(delay);
+        }
+        true
+    }
+
+    fn verify(file: &Path, signature: &[u8]) -> bool {
+        // Have the server generate an HMAC key, and then verify that the "signature" on incoming
+        // requests is valid for "file", using the "==" operator to compare the valid MAC for a
+        // file with the "signature" parameter (in other words, verify the HMAC the way any normal
+        // programmer would verify it).
+        let key = Bytes::from_utf8("sosecretbb");
+        let contents = fs::read_to_string(file);
+        assert!(contents.is_ok(), "Could not read: {}", file.display());
+        let contents = Bytes(contents.unwrap().as_bytes().to_vec());
+        insecure_compare(&sha1::hmac_sha1(&key, &contents).0, signature)
+    }
+
+    fn attack(file: &Path) -> Bytes {
+        const BLOCK_SIZE: usize = 20;
+        let mut sig = vec![0x0; BLOCK_SIZE];
+        for i in 0..BLOCK_SIZE {
+            let mut max_tuple: (u128, u8) = (u128::MIN, 0);
+            for c in 0_u8..=255_u8 {
+                let now = time::Instant::now();
+                sig[i] = c;
+                verify(file, &sig);
+                let elapsed = now.elapsed().as_micros();
+                if elapsed > max_tuple.0 {
+                    max_tuple = (elapsed, c);
+                }
+            }
+            sig[i] = max_tuple.1;
+        }
+        Bytes(sig)
+    }
+
+    let key = Bytes::from_utf8("YELLOW SUBMARINE");
+    let message = Bytes::from_utf8("Attact at dawn after tomorrow when it's cold inside.");
+    assert_eq!(sha1::hmac_sha1(&key, &message), Bytes::from_hex("8232f3d05afb6bce7e09fe764885cc158e435e36"));
+
+    let path = Path::new("data/12.txt");
+    let expected_sig = Bytes::from_hex("62f4527ea6cb716d0ad1ca0fc69135a49bc2d138");
+    assert!(verify(path, &expected_sig.0), "Invalid signature");
+
+    let signature = attack(path);
+    assert_eq!(expected_sig.0, signature.0, "Recovery of HMAC-SHA1 failed");
+
+    println!("[okay] Challenge 31: recoverd HMAC-SHA1 via timing attack");
 }
diff --git a/src/sha1.rs b/src/sha1.rs
index cbff060..443e9ef 100644
--- a/src/sha1.rs
+++ b/src/sha1.rs
@@ -264,3 +264,33 @@ pub fn authenticate(message: &Bytes, key: &Bytes) -> Bytes {
 pub fn verify(message: &Bytes, key: &Bytes, mac: &Bytes) -> bool {
     authenticate(message, key) == *mac
 }
+
+pub fn hmac_sha1(key: &Bytes, message: &Bytes) -> Bytes {
+    fn hash(message: Vec<u8>) -> Vec<u8> {
+        let mut sha1 = Sha1::default();
+        sha1.hash(&Bytes(message)).0
+    }
+
+    fn compute_block_sized_key(key: &Bytes, block_size: usize) -> Bytes {
+        let mut key = key.0.clone();
+        if key.len() > block_size {
+            key = hash(key);
+        }
+
+        while key.len() < block_size {
+            key.push(0x0);
+        }
+        Bytes(key)
+    }
+
+    const BLOCK_SIZE: usize = 64;
+    let block_sized_key = compute_block_sized_key(key, BLOCK_SIZE);
+    let mut o_key_pad = Bytes::xor(&block_sized_key, &Bytes(vec![0x5c; BLOCK_SIZE])).0;
+    let mut i_key_pad = Bytes::xor(&block_sized_key, &Bytes(vec![0x36; BLOCK_SIZE])).0;
+
+    // hash(o_key_pad ∥ hash(i_key_pad ∥ message))
+    i_key_pad.append(&mut message.0.clone());
+    let mut inner_hashed = hash(i_key_pad);
+    o_key_pad.append(&mut inner_hashed);
+    Bytes(hash(o_key_pad))
+}