diff --git a/src/main.rs b/src/main.rs
index ec6b617..4962ca6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -64,4 +64,5 @@ fn main() {
     }
     set5::challenge36();
     set5::challenge37();
+    set5::challenge38();
 }
diff --git a/src/set5.rs b/src/set5.rs
index b01494e..dc19384 100644
--- a/src/set5.rs
+++ b/src/set5.rs
@@ -350,23 +350,84 @@ pub fn challenge36() -> Option<()> {
     // I don't have HMAC-SHA256, so I will use HMAC-SHA1 instead.
     // C->S Send HMAC-SHA256(K, salt)
     let salt = Bytes(session_keys.salt.to_be_bytes().to_vec());
-    let mac_server = sha1::hmac_sha1(&Bytes(k_server.to_vec()), &salt);
-
-    // S->C Send "OK" if HMAC-SHA256(K, salt) validates
-    let mac_client = sha1::hmac_sha1(&Bytes(k_client.to_vec()), &salt);
-    assert_eq!(mac_server, mac_client, "HMAC verification failed");
+    let mac = sha1::hmac_sha1(&Bytes(k_client.to_vec()), &salt);
+    assert!(server.log_in(email, &mac)?);
 
     println!("[okay] Challenge 36: implement secure remote password");
     Some(())
 }
 
-pub fn challenge37() {
-    //  Get your SRP working in an actual client-server setting. "Log in" with a
-    //  valid password using the protocol.
+pub fn challenge37() -> Option<()> {
+    let mut rng = rand::thread_rng();
 
-    // Now log in without your password by having the client send 0 as its "A"
-    // value. What does this to the "S" value that both sides compute?
+    let mut server = srp::Server::default();
+    let email = "john1337@wayne.com";
+    let password = "horse planet carpet country";
+    let parameters = server.register(email, password)?;
 
-    // Now log in without your password by having the client send N, N*2, &c.
-    println!("[xxxx] Challenge 37: TBD");
+    let mut log_in_with_password = || -> Option<bool> {
+        //  Get your SRP working in an actual client-server setting. "Log in" with a
+        //  valid password using the protocol.
+        let n = &parameters.n;
+        let g = &parameters.g;
+        let a = rng.gen_biguint_below(n);
+        let a_public = g.modpow(&a, n);
+        let session_keys = server.exchange(email, &a_public)?;
+
+        // Client
+        let u = srp::compute_u(&a_public, &session_keys.b_public)?;
+        let x = srp::hash_password(session_keys.salt, password)?;
+        let b_public = &session_keys.b_public;
+        let k = &parameters.k;
+        let s_client = (b_public - ((k * g.modpow(&x, n)) % n)).modpow(&(a + &u * x), n);
+        let k_client = sha256(&s_client.to_bytes_be());
+
+        let salt = Bytes(session_keys.salt.to_be_bytes().to_vec());
+        let mac = sha1::hmac_sha1(&Bytes(k_client.to_vec()), &salt);
+        server.log_in(email, &mac)
+    };
+
+    assert!(log_in_with_password()?);
+
+    let mut log_in_without_password_a_zero = || -> Option<bool> {
+        // Now log in without your password by having the client send 0 as its "A"
+        // value.
+        let a_public = 0_u8.to_biguint()?;
+        let session_keys = server.exchange(email, &a_public)?;
+
+        // What does this to the "S" value that both sides compute? It becomes zero.
+        let s_client: BigUint = 0_u8.to_biguint()?;
+        let k_client = sha256(&s_client.to_bytes_be());
+
+        let salt = Bytes(session_keys.salt.to_be_bytes().to_vec());
+        let mac = sha1::hmac_sha1(&Bytes(k_client.to_vec()), &salt);
+        server.log_in(email, &mac)
+    };
+
+    assert!(log_in_without_password_a_zero()?);
+
+    let mut log_in_without_password_multiple_n = || -> Option<bool> {
+        // Now log in without your password by having the client send N, N*2, &c.
+        let n = &parameters.n;
+        let a_public = n * 2_u8.to_biguint()?;
+        let session_keys = server.exchange(email, &a_public)?;
+
+        // If A is a multiple of N, the term $(A * v**u) ** b % N$ also becomes zero.
+        let s_client: BigUint = 0_u8.to_biguint()?;
+        let k_client = sha256(&s_client.to_bytes_be());
+
+        let salt = Bytes(session_keys.salt.to_be_bytes().to_vec());
+        let mac = sha1::hmac_sha1(&Bytes(k_client.to_vec()), &salt);
+        server.log_in(email, &mac)
+    };
+
+    assert!(log_in_without_password_multiple_n()?);
+
+    println!("[okay] Challenge 37: break SRP with zero key");
+    Some(())
+}
+
+pub fn challenge38() -> Option<()> {
+    println!("[xxxx] Challenge 38: TBD");
+    Some(())
 }
diff --git a/src/srp.rs b/src/srp.rs
index 9e6d876..20c215c 100644
--- a/src/srp.rs
+++ b/src/srp.rs
@@ -1,4 +1,6 @@
+use crate::bytes::Bytes;
 use crate::set5;
+use crate::sha1;
 use num_bigint::BigUint;
 use num_bigint::RandBigInt;
 use num_bigint::ToBigUint;
@@ -122,4 +124,17 @@ impl Server {
         let k_server = sha256(&s_server.to_bytes_be());
         Some(k_server)
     }
+
+    pub fn log_in(&mut self, email: &str, mac_client: &Bytes) -> Option<bool> {
+        let k = self.get_k(email)?;
+        let password_entry = self.entries.get(email)?;
+        let salt = Bytes(password_entry.salt.to_be_bytes().to_vec());
+
+        // S->C Send "OK" if HMAC-SHA255(K, salt) validates
+        let mac_server = sha1::hmac_sha1(&Bytes(k.to_vec()), &salt);
+        if mac_server == *mac_client {
+            return Some(true);
+        }
+        Some(false)
+    }
 }