diff --git a/src/main.rs b/src/main.rs
index 69ec6ee..5ec909a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,13 +4,16 @@ mod cbc;
 mod ctr;
 mod ecb;
 mod mt19937;
+mod mtcipher;
 mod parser;
 mod set1;
 mod set2;
 mod set3;
+mod set4;
+mod utils;
 
 fn main() {
-    const RUN_ALL: bool = true;
+    const RUN_ALL: bool = false;
     if RUN_ALL {
         set1::challenge1();
         set1::challenge2();
@@ -36,7 +39,8 @@ fn main() {
         set3::challenge22();
         set3::challenge23();
         set3::challenge24();
+        set4::challenge25();
     } else {
-        set3::challenge24();
+        set4::challenge25();
     }
 }
diff --git a/src/mt19937.rs b/src/mt19937.rs
index c5ca01a..8c55827 100644
--- a/src/mt19937.rs
+++ b/src/mt19937.rs
@@ -34,6 +34,11 @@ impl MT19937 {
         self.twist();
     }
 
+    pub fn extract_bytes(&mut self) -> [u8; 4] {
+        let n = self.extract_number();
+        n.to_ne_bytes()
+    }
+
     pub fn extract_number(&mut self) -> u32 {
         if self.index == N {
             self.twist();
@@ -60,10 +65,9 @@ impl MT19937 {
 
     fn twist(&mut self) {
         const M: usize = 397;
-        const R: u32 = 31;
-        const A: u32 = 0x9908B0DF;
-        const LOWER_MASK: u32 = (1 << R) - 1; // 0x7fffffff
-        const UPPER_MASK: u32 = !LOWER_MASK; // 0x80000000
+        const A: u32 = 0x9908_B0DF;
+        const LOWER_MASK: u32 = 0x7fff_ffff;
+        const UPPER_MASK: u32 = 0x8000_0000;
         const FIRST_HALF: usize = N - M;
 
         for i in 0..FIRST_HALF {
diff --git a/src/mtcipher.rs b/src/mtcipher.rs
new file mode 100644
index 0000000..b11cdac
--- /dev/null
+++ b/src/mtcipher.rs
@@ -0,0 +1,26 @@
+use crate::{bytes::Bytes, mt19937};
+
+pub fn decrypt(key: u16, data: &Bytes) -> Bytes {
+    encrypt(key, data)
+}
+
+pub fn encrypt(key: u16, Bytes(data): &Bytes) -> Bytes {
+    // You can create a trivial stream cipher out of any PRNG; use it to
+    // generate a sequence of 8 bit outputs and call those outputs a keystream.
+    // XOR each byte of plaintext with each successive byte of keystream.
+    // Write the function that does this for MT19937 using a 16-bit seed.
+
+    let mut mt = mt19937::MT19937::new();
+    mt.seed(key as u32);
+    let mut result: Vec<u8> = vec![];
+
+    for chunk in data.chunks(4) {
+        let key = mt.extract_bytes();
+        for i in 0..chunk.len() {
+            let cipher_char = chunk[i] ^ key[i];
+            result.push(cipher_char);
+        }
+    }
+
+    Bytes(result)
+}
diff --git a/src/set3.rs b/src/set3.rs
index 2148b7f..adcedc6 100644
--- a/src/set3.rs
+++ b/src/set3.rs
@@ -3,12 +3,13 @@ use crate::bytes_base64::BytesBase64;
 use crate::cbc;
 use crate::ctr;
 use crate::mt19937;
+use crate::mtcipher;
+use crate::utils;
 use rand::Rng;
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::io::{BufRead, BufReader};
-use std::time::{SystemTime, UNIX_EPOCH};
 
 pub fn challenge17() {
     fn read(path: &str) -> Vec<Bytes> {
@@ -317,17 +318,8 @@ pub fn challenge21() {
 }
 
 pub fn challenge22() {
-    // let mut mt = mt19937::MT19937::new();
-    fn unix_timestamp() -> u32 {
-        let start = SystemTime::now();
-        let since_the_epoch = start
-            .duration_since(UNIX_EPOCH)
-            .expect("Time went backwards");
-        since_the_epoch.as_secs() as u32
-    }
-
     // Wait a random number of seconds between, I don't know, 40 and 1000.
-    let now = unix_timestamp();
+    let now = utils::unix_timestamp();
     let wait_time: u32 = rand::thread_rng().gen_range(40..1000);
     let seed = now + wait_time;
 
@@ -341,7 +333,7 @@ pub fn challenge22() {
     // From the 32 bit RNG output, discover the seed.
     fn find_seed(rngout: u32) -> Option<u32> {
         let mut mt = mt19937::MT19937::new();
-        let start = unix_timestamp() - 2000;
+        let start = utils::unix_timestamp() - 2000;
         for seed in start..(start + 4000) {
             mt.seed(seed);
             if rngout == mt.extract_number() {
@@ -448,5 +440,93 @@ pub fn challenge23() {
 }
 
 pub fn challenge24() {
-    println!("[xxxx] Challenge 24: xxx");
+    // Verify that you can encrypt and decrypt properly. This code should look
+    // similar to your CTR code.
+    let key: u16 = 111;
+    let cleartext = Bytes::from_utf8("Let's see if we can get the party started hard my friends.");
+    let cipher = mtcipher::encrypt(key, &cleartext);
+    let roundtrip = mtcipher::decrypt(key, &cipher);
+    assert_eq!(cleartext, roundtrip);
+
+    // Use your function to encrypt a known plaintext (say, 14 consecutive 'A'
+    // characters) prefixed by a random number of random characters.
+    fn get_plaintext() -> Bytes {
+        let length: usize = rand::thread_rng().gen_range(30..100);
+        let mut data = Bytes::random(length);
+        data.0.append(&mut Bytes(vec![b'A'; 14]).0);
+        data
+    }
+    let key: u16 = rand::thread_rng().gen::<u16>();
+    let plaintext = get_plaintext();
+    let cipher = mtcipher::encrypt(key, &plaintext);
+
+    // From the ciphertext, recover the "key" (the 16 bit seed).
+    fn recover_key(cipher: &Bytes) -> u16 {
+        let cipher_len = cipher.len();
+        // brute force bb!
+        for key in 0..u16::MAX {
+            let mut found_key = true;
+            let roundtrip = mtcipher::decrypt(key, &cipher);
+            // check if the last 14 chars are 'A' - if yes, we found the key
+            for i in (cipher_len - 14)..cipher_len {
+                if roundtrip.0[i] != b'A' {
+                    found_key = false;
+                    break;
+                }
+            }
+            if found_key {
+                return key;
+            }
+        }
+        0
+    }
+    let recovered_key = recover_key(&cipher);
+    assert_eq!(key, recovered_key);
+
+    // Use the same idea to generate a random "password reset token" using
+    // MT19937 seeded from the current time.
+    fn get_reset_token(time: Option<u32>) -> Bytes {
+        const TOKEN_LENGTH: usize = 16;
+        let time = match time {
+            Some(time) => time,
+            None => utils::unix_timestamp(),
+        };
+        let mut token = vec![];
+        let mut mt = mt19937::MT19937::new();
+        mt.seed(time);
+
+        while token.len() < (TOKEN_LENGTH - 1) {
+            for b in mt.extract_bytes() {
+                if token.len() >= TOKEN_LENGTH {
+                    break;
+                }
+                if b.is_ascii_alphanumeric() {
+                    token.push(b);
+                }
+            }
+        }
+        Bytes(token)
+    }
+
+    let token = get_reset_token(None);
+    // println!("{}", token.to_utf8());
+
+    // Write a function to check if any given password token is actually the
+    // product of an MT19937 PRNG seeded with the current time.
+    fn is_time_token(token: &Bytes) -> bool {
+        let current_time = utils::unix_timestamp();
+        for time in (current_time - 10)..(current_time + 10) {
+            let time_token = get_reset_token(Some(time));
+            if *token == time_token {
+                return true;
+            }
+        }
+        false
+    }
+
+    assert_eq!(is_time_token(&token), true);
+    let non_token = Bytes(vec![b'z', 16]);
+    assert_eq!(is_time_token(&non_token), false);
+
+    println!("[okay] Challenge 24: MT19937 stream cipher implemented and cracked");
 }
diff --git a/src/set4.rs b/src/set4.rs
new file mode 100644
index 0000000..4db2f3b
--- /dev/null
+++ b/src/set4.rs
@@ -0,0 +1,3 @@
+pub fn challenge25() {
+    println!("[xxxx] Challenge 25: TBD");
+}
diff --git a/src/utils.rs b/src/utils.rs
new file mode 100644
index 0000000..0ec0e51
--- /dev/null
+++ b/src/utils.rs
@@ -0,0 +1,9 @@
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub fn unix_timestamp() -> u32 {
+    let start = SystemTime::now();
+    let since_the_epoch = start
+        .duration_since(UNIX_EPOCH)
+        .expect("Time went backwards");
+    since_the_epoch.as_secs() as u32
+}