diff --git a/src/main.rs b/src/main.rs
index 62cfcf0..d5c3fec 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -3,6 +3,7 @@ mod bytes_base64;
 mod cbc;
 mod ctr;
 mod ecb;
+mod mt19937;
 mod parser;
 mod set1;
 mod set2;
@@ -32,7 +33,8 @@ fn main() {
         set3::challenge19();
         set3::challenge20();
         set3::challenge21();
+        set3::challenge22();
     } else {
-        set3::challenge21();
+        set3::challenge22();
     }
 }
diff --git a/src/mt19937.rs b/src/mt19937.rs
new file mode 100644
index 0000000..ffa8170
--- /dev/null
+++ b/src/mt19937.rs
@@ -0,0 +1,80 @@
+use std::num::Wrapping;
+
+#[derive(Debug)]
+pub struct MT19937 {
+    mt: Vec<u32>,
+    index: usize,
+}
+
+const N: usize = 624;
+
+impl MT19937 {
+    pub fn new() -> MT19937 {
+        let mut mt = MT19937 {
+            mt: vec![0; N],
+            index: N + 1,
+        };
+        mt.seed(5489);
+        mt
+    }
+
+    pub fn seed(&mut self, seed: u32) {
+        self.index = N;
+        const F: u32 = 1812433253;
+        self.mt[0] = seed;
+        for i in 1..N {
+            self.mt[i] = (Wrapping(F) * Wrapping(self.mt[i - 1] ^ (self.mt[i - 1] >> 30))
+                + Wrapping(i as u32))
+            .0;
+        }
+    }
+
+    pub fn extract_number(&mut self) -> u32 {
+        if self.index >= N {
+            if self.index > N {
+                panic!("Generator was never seeded");
+            }
+            self.twist();
+        }
+
+        const S: u32 = 7;
+        const T: u32 = 15;
+        const U: u32 = 11;
+        const B: u32 = 0x9D2C5680;
+        const C: u32 = 0xEFC60000;
+        const D: u32 = 0xFFFFFFFF;
+        const L: u32 = 18;
+
+        let mut y = self.mt[self.index];
+        y = y ^ ((y >> U) & D);
+        y = y ^ ((y << S) & B);
+        y = y ^ ((y << T) & C);
+        y = y ^ (y >> L);
+        self.index += 1;
+        y
+    }
+
+    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 FIRST_HALF: usize = N - M;
+
+        for i in 0..FIRST_HALF {
+            let bits = self.mt[i] & UPPER_MASK | self.mt[i + 1] & LOWER_MASK;
+            self.mt[i] = self.mt[i + M] ^ (bits >> 1) ^ ((bits & 1) * A);
+        }
+
+        for i in FIRST_HALF..(N - 1) {
+            let bits = self.mt[i] & UPPER_MASK | self.mt[i + 1] & LOWER_MASK;
+            self.mt[i] = self.mt[i - FIRST_HALF] ^ (bits >> 1) ^ ((bits & 1) * A);
+        }
+
+        let i = N - 1;
+        let bits = self.mt[i] & UPPER_MASK | self.mt[0] & LOWER_MASK;
+        self.mt[i] = self.mt[M - 1] ^ (bits >> 1) ^ ((bits & 1) * A);
+        self.index = 0;
+    }
+}
diff --git a/src/set3.rs b/src/set3.rs
index 840ebbc..5eeebe0 100644
--- a/src/set3.rs
+++ b/src/set3.rs
@@ -2,6 +2,7 @@ use crate::bytes::Bytes;
 use crate::bytes_base64::BytesBase64;
 use crate::cbc;
 use crate::ctr;
+use crate::mt19937;
 use rand::Rng;
 use std::cell::RefCell;
 use std::collections::HashMap;
@@ -301,5 +302,20 @@ pub fn challenge20() {
 }
 
 pub fn challenge21() {
-    println!("[xxxx] Challenge 21: TBD");
+    // Implement the MT19937 Mersenne Twister RNG
+
+    let expected: Vec<u32> = vec![
+        0xD091BB5C, 0x22AE9EF6, 0xE7E1FAEE, 0xD5C31F79, 0x2082352C, 0xF807B7DF, 0xE9D30005,
+        0x3895AFE1, 0xA1E24BBA, 0x4EE4092B,
+    ];
+    let mut mt = mt19937::MT19937::new();
+    mt.seed(5489);
+    for i in 0..expected.len() {
+        assert_eq!(mt.extract_number(), expected[i]);
+    }
+    println!("[okay] Challenge 21: Implemented MT19937");
+}
+
+pub fn challenge22() {
+    println!("[xxxx] Challenge 22: TBD!");
 }