diff --git a/sachy-crypto/src/lib.rs b/sachy-crypto/src/lib.rs
--- a/sachy-crypto/src/lib.rs
+++ b/sachy-crypto/src/lib.rs
@@ -153,12 +153,12 @@
 }
 
 impl SendingState<'_> {
-    pub fn encrypt(&mut self, msg: &mut alloc::vec::Vec<u8>) -> Result<(), ProtoError> {
+    pub fn encrypt(&mut self, msg: &mut alloc::vec::Vec<u8>, associated_data: &[u8]) -> Result<(), ProtoError> {
         let counter = self.counter.to_be_bytes();
 
         self.transport.encrypt_in_place(
             &self.transport.mix_nonce(&counter, Role::Client),
-            &counter,
+            associated_data,
             msg,
         )?;
 
@@ -180,12 +180,12 @@
 }
 
 impl ReceivingState<'_> {
-    pub fn decrypt(&mut self, msg: &mut alloc::vec::Vec<u8>) -> Result<(), ProtoError> {
+    pub fn decrypt(&mut self, msg: &mut alloc::vec::Vec<u8>, associated_data: &[u8]) -> Result<(), ProtoError> {
         let counter = self.counter.to_be_bytes();
 
         self.transport.decrypt_in_place(
             &self.transport.mix_nonce(&counter, Role::Server),
-            &counter,
+            associated_data,
             msg,
         )?;
 
@@ -387,33 +387,35 @@
 
         let orig = b"Test Message, Please ignore.";
 
+        let ad = b"random";
+
         let mut msg = orig.to_vec();
 
         // a -> b
-        alice_send.encrypt(&mut msg)?;
+        alice_send.encrypt(&mut msg, ad)?;
 
         assert_ne!(orig.as_slice(), msg.as_slice());
         let ct1 = msg.clone();
 
-        bob_recv.decrypt(&mut msg)?;
+        bob_recv.decrypt(&mut msg, ad)?;
 
         // a -> b
-        alice_send.encrypt(&mut msg)?;
+        alice_send.encrypt(&mut msg, b"")?;
 
         assert_ne!(msg.as_slice(), ct1.as_slice());
         let ct2 = msg.clone();
 
-        bob_recv.decrypt(&mut msg)?;
+        bob_recv.decrypt(&mut msg, b"")?;
 
         // b -> a
-        bob_send.encrypt(&mut msg)?;
+        bob_send.encrypt(&mut msg, ad)?;
 
         // None of the ciphertexts should match each other
         assert_ne!(msg.as_slice(), ct1.as_slice());
         assert_ne!(msg.as_slice(), ct2.as_slice());
         assert_ne!(ct1.as_slice(), ct2.as_slice());
 
-        alice_recv.decrypt(&mut msg)?;
+        alice_recv.decrypt(&mut msg, ad)?;
 
         assert_eq!(orig.as_slice(), msg.as_slice());