diff --git a/src/dsa.rs b/src/dsa.rs
new file mode 100644
index 0000000..3ea730c
--- /dev/null
+++ b/src/dsa.rs
@@ -0,0 +1,105 @@
+use crate::bytes::Bytes;
+use crate::rsa;
+use crate::sha1;
+use openssl::bn::BigNum;
+use openssl::bn::BigNumContext;
+use openssl::error::ErrorStack;
+
+#[derive(Debug)]
+pub struct DsaParameters {
+    pub p: BigNum,
+    pub q: BigNum,
+    pub g: BigNum,
+}
+
+#[derive(Debug)]
+pub struct DsaKeys {
+    pub x: BigNum,
+    pub y: BigNum,
+}
+
+#[derive(Debug)]
+pub struct DsaSig {
+    pub r: BigNum,
+    pub s: BigNum,
+}
+
+impl DsaParameters {
+    pub fn new() -> Result<Self, ErrorStack> {
+        Ok(Self {
+            p: BigNum::from_hex_str("800000000000000089e1855218a0e7dac38136ffafa72eda7859f2171e25e65eac698c1702578b07dc2a1076da241c76c62d374d8389ea5aeffd3226a0530cc565f3bf6b50929139ebeac04f48c3c84afb796d61e5a4f9a8fda812ab59494232c7d2b4deb50aa18ee9e132bfa85ac4374d7f9091abc3d015efc871a584471bb1")?,
+            q: BigNum::from_hex_str("f4f47f05794b256174bba6e9b396a7707e563c5b")?,
+            g: BigNum::from_hex_str("5958c9d3898b224b12672c0b98e06c60df923cb8bc999d119458fef538b8fa4046c8db53039db620c094c9fa077ef389b5322a559946a71903f990f1f7e0e025e2d7f7cf494aff1a0470f5b64c36b625a097f1651fe775323556fe00b3608c887892878480e99041be601a62166ca6894bdd41a7054ec89f756ba9fc95302291")?,
+        })
+    }
+}
+
+impl DsaKeys {
+    pub fn new(params: &DsaParameters) -> Result<Self, ErrorStack> {
+        let mut ctx = BigNumContext::new()?;
+        let mut x = BigNum::new()?;
+        let mut y = BigNum::new()?;
+        // Choose an integer x randomly from { 1 … q − 1 }
+        params.q.rand_range(&mut x)?;
+        // Compute y := g^x mod p
+        y.mod_exp(&params.g, &x, &params.p, &mut ctx)?;
+        Ok(Self { x: x, y: y })
+    }
+
+    pub fn sign(&self, params: &DsaParameters, msg: &Bytes) -> Result<DsaSig, ErrorStack> {
+        let mut ctx = BigNumContext::new()?;
+        let mut k = BigNum::new()?;
+        let mut r = BigNum::from_u32(0)?;
+        let zero = BigNum::from_u32(0)?;
+        while r == zero {
+            // Choose an integer k randomly from { 1 … q − 1 }
+            params.q.rand_range(&mut k)?;
+            // Compute r := (g^k mod p) mod q. In the unlikely case that r=0, start again.
+            r.mod_exp(&params.g, &k, &params.p, &mut ctx)?;
+            r = &r % &params.q;
+        }
+
+        // Compute s := (k^(−1) * ( H ( m ) + x r ) ) mod q.
+        let s = &(&rsa::invmod(&k, &params.q)? * &(&h(msg)? + &(&self.x * &r))) % &params.q;
+
+        // In the unlikely case that s=0, crash.
+        assert!(s != zero, "s is zero");
+
+        Ok(DsaSig { r, s })
+    }
+
+    pub fn verify(
+        &self,
+        params: &DsaParameters,
+        msg: &Bytes,
+        sig: &DsaSig,
+    ) -> Result<bool, ErrorStack> {
+        let mut ctx = BigNumContext::new()?;
+        let zero = BigNum::from_u32(0)?;
+
+        // Verify that 0 < r < q  and 0 < s < q
+        if !(zero < sig.r && sig.r < params.q && zero < sig.s && sig.s < params.q) {
+            return Ok(false);
+        }
+
+        // Compute w := s^(−1) mod q
+        let w = rsa::invmod(&sig.s, &params.q)?;
+        let u1 = &(&h(&msg)? * &w) % &params.q;
+        let u2 = &(&sig.r * &w) % &params.q;
+
+        // v := ((g^u1 * y^u2) % p) % q)
+        let mut v1 = BigNum::new()?;
+        v1.mod_exp(&params.g, &u1, &params.p, &mut ctx)?;
+        let mut v2 = BigNum::new()?;
+        v2.mod_exp(&self.y, &u2, &params.p, &mut ctx)?;
+        let v = &(&(&v1 * &v2) % &params.p) % &params.q;
+
+        // The signature is valid if and only if v = r
+        Ok(v == sig.r)
+    }
+}
+
+fn h(message: &Bytes) -> Result<BigNum, ErrorStack> {
+    let mut s1 = sha1::Sha1::default();
+    BigNum::from_slice(&(s1.hash(message).0))
+}
diff --git a/src/main.rs b/src/main.rs
index cf5fc4f..01f9b96 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,6 +8,7 @@ mod bytes;
 mod bytes_base64;
 mod cbc;
 mod ctr;
+mod dsa;
 mod ecb;
 mod md4;
 mod mt19937;
@@ -62,12 +63,12 @@ fn main() {
         set5::challenge33();
         set5::challenge34();
         set5::challenge35();
+        set5::challenge36().unwrap_or_else(|| println!("[fail] challenge 36"));
+        set5::challenge37().unwrap_or_else(|| println!("[fail] challenge 37"));
+        set5::challenge38().unwrap_or_else(|| println!("[fail] challenge 38"));
+        set5::challenge39().unwrap_or_else(|| println!("[fail] challenge 39"));
+        set5::challenge40().unwrap_or_else(|| println!("[fail] challenge 40"));
     }
-    set5::challenge36().unwrap_or_else(|| println!("[fail] challenge 36"));
-    set5::challenge37().unwrap_or_else(|| println!("[fail] challenge 37"));
-    set5::challenge38().unwrap_or_else(|| println!("[fail] challenge 38"));
-    set5::challenge39().unwrap_or_else(|| println!("[fail] challenge 39"));
-    set5::challenge40().unwrap_or_else(|| println!("[fail] challenge 40"));
     set6::challenge41().unwrap_or_else(|| println!("[fail] challenge 41"));
     set6::challenge42().unwrap_or_else(|| println!("[fail] challenge 42"));
     set6::challenge43().unwrap_or_else(|| println!("[fail] challenge 43"));
diff --git a/src/set5.rs b/src/set5.rs
index ec1b84e..41dc340 100644
--- a/src/set5.rs
+++ b/src/set5.rs
@@ -583,6 +583,6 @@ pub fn challenge40() -> Option<()> {
     // let c = rsa::rsa_padding_remove_pkcs1(&c, n_0.num_bytes()).ok()?;
     assert_eq!(c, m, "cube root implementation did not work");
 
-    println!("[okay] Challenge 40: implement an E=3 RSA Broadcast attack");
+    println!("[okay] Challenge 40: implement an e=3 RSA Broadcast attack");
     Some(())
 }
diff --git a/src/set6.rs b/src/set6.rs
index b253616..be7952d 100644
--- a/src/set6.rs
+++ b/src/set6.rs
@@ -1,3 +1,5 @@
+use crate::bytes::Bytes;
+use crate::dsa;
 use crate::rsa;
 use num_bigint::BigUint;
 use openssl::bn::BigNum;
@@ -96,5 +98,12 @@ pub fn challenge42() -> Option<()> {
 
 pub fn challenge43() -> Option<()> {
     println!("[xxxx] Challenge 43: TBD");
+    let msg = Bytes::from_utf8("hello, world!");
+    let params = dsa::DsaParameters::new().ok()?;
+    let keys = dsa::DsaKeys::new(&params).ok()?;
+    let s = keys.sign(&params, &msg).ok()?;
+    let r = keys.verify(&params, &msg, &s).ok()?;
+    assert!(r, "verify failed unexpectedly");
+
     Some(())
 }