diff --git a/examples/main_pod_points.rs b/examples/main_pod_points.rs
index 4312f2ca..d7679f68 100644
--- a/examples/main_pod_points.rs
+++ b/examples/main_pod_points.rs
@@ -46,14 +46,14 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let game_sk = SecretKey::new_rand();
     let game_pk = game_sk.public_key();
 
-    let mut game_signer = Signer(game_sk);
+    let game_signer = Signer(game_sk);
 
     // Build 2 signed pods where the game assigns points to a player that has completed a level.
     let mut builder = SignedPodBuilder::new(&params);
     builder.insert("player", "Alice");
     builder.insert("level", 1);
     builder.insert("points", 3512);
-    let pod_points_lvl_1 = builder.sign(&mut game_signer)?;
+    let pod_points_lvl_1 = builder.sign(&game_signer)?;
     pod_points_lvl_1.verify()?;
     println!("# pod_points_lvl_1:\n{}", pod_points_lvl_1);
 
@@ -61,7 +61,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     builder.insert("player", "Alice");
     builder.insert("level", 2);
     builder.insert("points", 5771);
-    let pod_points_lvl_2 = builder.sign(&mut game_signer)?;
+    let pod_points_lvl_2 = builder.sign(&game_signer)?;
     pod_points_lvl_2.verify()?;
     println!("# pod_points_lvl_2:\n{}", pod_points_lvl_2);
 
diff --git a/examples/signed_pod.rs b/examples/signed_pod.rs
index 03db74cd..ec84b4d0 100644
--- a/examples/signed_pod.rs
+++ b/examples/signed_pod.rs
@@ -17,7 +17,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let pk = sk.public_key();
     println!("Public key: {}\n", pk);
 
-    let mut signer = Signer(sk);
+    let signer = Signer(sk);
 
     // Build the signed pod
     let mut builder = SignedPodBuilder::new(&params);
@@ -35,7 +35,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     );
 
     // Sign the pod and verify it
-    let pod = builder.sign(&mut signer)?;
+    let pod = builder.sign(&signer)?;
     pod.verify()?;
 
     println!("{}", pod);
diff --git a/src/backends/plonky2/circuits/signedpod.rs b/src/backends/plonky2/circuits/signedpod.rs
index ee5a1e2d..e803cdcf 100644
--- a/src/backends/plonky2/circuits/signedpod.rs
+++ b/src/backends/plonky2/circuits/signedpod.rs
@@ -219,8 +219,8 @@ pub mod tests {
         pod.insert("dateOfBirth", 1169909384);
         pod.insert("socialSecurityNumber", "G2121210");
         let sk = SecretKey::new_rand();
-        let mut signer = Signer(sk);
-        let pod = pod.sign(&mut signer).unwrap();
+        let signer = Signer(sk);
+        let pod = pod.sign(&signer).unwrap();
         let signed_pod = (pod.pod as Box<dyn Any>).downcast::<SignedPod>().unwrap();
 
         // use the pod in the circuit
diff --git a/src/backends/plonky2/mainpod/mod.rs b/src/backends/plonky2/mainpod/mod.rs
index a962467d..db0eb34c 100644
--- a/src/backends/plonky2/mainpod/mod.rs
+++ b/src/backends/plonky2/mainpod/mod.rs
@@ -739,12 +739,12 @@ pub mod tests {
 
         let (gov_id_builder, pay_stub_builder, sanction_list_builder) =
             zu_kyc_sign_pod_builders(&params);
-        let mut signer = Signer(SecretKey(BigUint::one()));
-        let gov_id_pod = gov_id_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(2u64.into()));
-        let pay_stub_pod = pay_stub_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(3u64.into()));
-        let sanction_list_pod = sanction_list_builder.sign(&mut signer)?;
+        let signer = Signer(SecretKey(BigUint::one()));
+        let gov_id_pod = gov_id_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(2u64.into()));
+        let pay_stub_pod = pay_stub_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(3u64.into()));
+        let sanction_list_pod = sanction_list_builder.sign(&signer)?;
         let kyc_builder = zu_kyc_pod_builder(
             &params,
             &vd_set,
@@ -753,8 +753,8 @@ pub mod tests {
             &sanction_list_pod,
         )?;
 
-        let mut prover = Prover {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params)?;
+        let prover = Prover {};
+        let kyc_pod = kyc_builder.prove(&prover, &params)?;
         crate::measure_gates_print!();
         let pod = (kyc_pod.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
 
@@ -778,8 +778,8 @@ pub mod tests {
         gov_id_builder.insert("idNumber", "4242424242");
         gov_id_builder.insert("dateOfBirth", 1169909384);
         gov_id_builder.insert("socialSecurityNumber", "G2121210");
-        let mut signer = Signer(SecretKey(42u64.into()));
-        let gov_id = gov_id_builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(42u64.into()));
+        let gov_id = gov_id_builder.sign(&signer).unwrap();
         let now_minus_18y: i64 = 1169909388;
         let mut kyc_builder = frontend::MainPodBuilder::new(&params, &vd_set);
         kyc_builder.add_signed_pod(&gov_id);
@@ -791,8 +791,8 @@ pub mod tests {
         println!();
 
         // Mock
-        let mut prover = MockProver {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let kyc_pod = kyc_builder.prove(&prover, &params).unwrap();
         let pod = (kyc_pod.pod as Box<dyn Any>)
             .downcast::<MockMainPod>()
             .unwrap();
@@ -800,8 +800,8 @@ pub mod tests {
         println!("{:#}", pod);
 
         // Real
-        let mut prover = Prover {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params).unwrap();
+        let prover = Prover {};
+        let kyc_pod = kyc_builder.prove(&prover, &params).unwrap();
         let pod = (kyc_pod.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
         pod.verify().unwrap()
     }
@@ -832,8 +832,8 @@ pub mod tests {
         let pod_builder = frontend::MainPodBuilder::new(&params, &vd_set);
 
         // Mock
-        let mut prover = MockProver {};
-        let kyc_pod = pod_builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let kyc_pod = pod_builder.prove(&prover, &params).unwrap();
         let pod = (kyc_pod.pod as Box<dyn Any>)
             .downcast::<MockMainPod>()
             .unwrap();
@@ -841,8 +841,8 @@ pub mod tests {
         println!("{:#}", pod);
 
         // Real
-        let mut prover = Prover {};
-        let kyc_pod = pod_builder.prove(&mut prover, &params).unwrap();
+        let prover = Prover {};
+        let kyc_pod = pod_builder.prove(&prover, &params).unwrap();
         let pod = (kyc_pod.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
         pod.verify().unwrap()
     }
@@ -853,27 +853,23 @@ pub mod tests {
         println!("{:#?}", params);
         let vd_set = &*DEFAULT_VD_SET;
 
-        let mut alice = Signer(SecretKey(1u32.into()));
-        let mut bob = Signer(SecretKey(2u32.into()));
+        let alice = Signer(SecretKey(1u32.into()));
+        let bob = Signer(SecretKey(2u32.into()));
         let charlie = Signer(SecretKey(3u32.into()));
 
         // Alice attests that she is ETH friends with Bob and Bob
         // attests that he is ETH friends with Charlie.
-        let alice_attestation =
-            attest_eth_friend(&params, &mut alice, Value::from(bob.public_key()));
-        let bob_attestation =
-            attest_eth_friend(&params, &mut bob, Value::from(charlie.public_key()));
+        let alice_attestation = attest_eth_friend(&params, &alice, Value::from(bob.public_key()));
+        let bob_attestation = attest_eth_friend(&params, &bob, Value::from(charlie.public_key()));
 
         let helper = EthDosHelper::new(&params, vd_set, false, Value::from(alice.public_key()))?;
-        let mut prover = Prover {};
-        let dist_1 = helper
-            .dist_1(&alice_attestation)?
-            .prove(&mut prover, &params)?;
+        let prover = Prover {};
+        let dist_1 = helper.dist_1(&alice_attestation)?.prove(&prover, &params)?;
         crate::measure_gates_print!();
         dist_1.pod.verify()?;
         let dist_2 = helper
             .dist_n_plus_1(&dist_1, &bob_attestation)?
-            .prove(&mut prover, &params)?;
+            .prove(&prover, &params)?;
         Ok(dist_2.pod.verify()?)
     }
 
@@ -920,12 +916,12 @@ pub mod tests {
 
         let _st3 = pod_builder.priv_op(op!(custom, cpb_and.clone(), st0, st2))?;
 
-        let mut prover = MockProver {};
-        let pod = pod_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let pod = pod_builder.prove(&prover, &params)?;
         assert!(pod.pod.verify().is_ok());
 
-        let mut prover = Prover {};
-        let pod = pod_builder.prove(&mut prover, &params)?;
+        let prover = Prover {};
+        let pod = pod_builder.prove(&prover, &params)?;
         crate::measure_gates_print!();
 
         let pod = (pod.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
@@ -948,8 +944,8 @@ pub mod tests {
 
         builder.pub_op(op!(set_contains, st, 1))?;
 
-        let mut prover = Prover {};
-        let proof = builder.prove(&mut prover, &params).unwrap();
+        let prover = Prover {};
+        let proof = builder.prove(&prover, &params).unwrap();
         let pod = (proof.pod as Box<dyn Any>).downcast::<MainPod>().unwrap();
         Ok(pod.verify()?)
     }
diff --git a/src/backends/plonky2/mock/mainpod.rs b/src/backends/plonky2/mock/mainpod.rs
index 7d7b7608..fa9f8cf3 100644
--- a/src/backends/plonky2/mock/mainpod.rs
+++ b/src/backends/plonky2/mock/mainpod.rs
@@ -441,12 +441,12 @@ pub mod tests {
         let vd_set = &*MOCK_VD_SET;
         let (gov_id_builder, pay_stub_builder, sanction_list_builder) =
             zu_kyc_sign_pod_builders(&params);
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let gov_id_pod = gov_id_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(2u32.into()));
-        let pay_stub_pod = pay_stub_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(3u32.into()));
-        let sanction_list_pod = sanction_list_builder.sign(&mut signer)?;
+        let signer = Signer(SecretKey(1u32.into()));
+        let gov_id_pod = gov_id_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(2u32.into()));
+        let pay_stub_pod = pay_stub_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(3u32.into()));
+        let sanction_list_pod = sanction_list_builder.sign(&signer)?;
         let kyc_builder = zu_kyc_pod_builder(
             &params,
             &vd_set,
@@ -455,8 +455,8 @@ pub mod tests {
             &sanction_list_pod,
         )?;
 
-        let mut prover = MockProver {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let kyc_pod = kyc_builder.prove(&prover, &params)?;
         let pod = (kyc_pod.pod as Box<dyn Any>)
             .downcast::<MockMainPod>()
             .unwrap();
@@ -471,8 +471,8 @@ pub mod tests {
     fn test_mock_main_great_boy() -> frontend::Result<()> {
         let (params, great_boy_builder) = great_boy_pod_full_flow()?;
 
-        let mut prover = MockProver {};
-        let great_boy_pod = great_boy_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let great_boy_pod = great_boy_builder.prove(&prover, &params)?;
         let pod = (great_boy_pod.pod as Box<dyn Any>)
             .downcast::<MockMainPod>()
             .unwrap();
@@ -488,8 +488,8 @@ pub mod tests {
     fn test_mock_main_tickets() -> frontend::Result<()> {
         let params = middleware::Params::default();
         let tickets_builder = tickets_pod_full_flow()?;
-        let mut prover = MockProver {};
-        let proof_pod = tickets_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let proof_pod = tickets_builder.prove(&prover, &params)?;
         let pod = (proof_pod.pod as Box<dyn Any>)
             .downcast::<MockMainPod>()
             .unwrap();
diff --git a/src/backends/plonky2/signedpod.rs b/src/backends/plonky2/signedpod.rs
index 684d3cc2..8d890e05 100644
--- a/src/backends/plonky2/signedpod.rs
+++ b/src/backends/plonky2/signedpod.rs
@@ -27,7 +27,7 @@ pub struct Signer(pub SecretKey);
 
 impl Signer {
     fn sign_with_nonce(
-        &mut self,
+        &self,
         params: &Params,
         nonce: BigUint,
         kvs: &HashMap<Key, Value>,
@@ -48,7 +48,7 @@ impl Signer {
             dict,
         })
     }
-    fn _sign(&mut self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<SignedPod> {
+    fn _sign(&self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<SignedPod> {
         let nonce = OsRng.gen_biguint_below(&GROUP_ORDER);
         self.sign_with_nonce(params, nonce, kvs)
     }
@@ -59,7 +59,7 @@ impl Signer {
 }
 
 impl PodSigner for Signer {
-    fn sign(&mut self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<Box<dyn Pod>> {
+    fn sign(&self, params: &Params, kvs: &HashMap<Key, Value>) -> Result<Box<dyn Pod>> {
         Ok(self._sign(params, kvs).map(Box::new)?)
     }
 }
@@ -200,8 +200,8 @@ pub mod tests {
         pod.insert("socialSecurityNumber", "G2121210");
 
         let sk = SecretKey(123u64.into());
-        let mut signer = Signer(sk);
-        let pod = pod.sign(&mut signer).unwrap();
+        let signer = Signer(sk);
+        let pod = pod.sign(&signer).unwrap();
         let pod = (pod.pod as Box<dyn Any>).downcast::<SignedPod>().unwrap();
 
         pod.verify()?;
diff --git a/src/examples/mod.rs b/src/examples/mod.rs
index 54faf2a4..9dd73761 100644
--- a/src/examples/mod.rs
+++ b/src/examples/mod.rs
@@ -73,7 +73,7 @@ pub fn zu_kyc_pod_builder(
 
 // ETHDoS
 
-pub fn attest_eth_friend(params: &Params, src: &mut impl PodSigner, dst: Value) -> SignedPod {
+pub fn attest_eth_friend(params: &Params, src: &impl PodSigner, dst: Value) -> SignedPod {
     let mut attestation = SignedPodBuilder::new(params);
     attestation.insert("attestation", dst);
     attestation.sign(src).unwrap()
@@ -334,13 +334,13 @@ pub fn great_boy_pod_full_flow() -> Result<(Params, MainPodBuilder)> {
     };
     let vd_set = &*MOCK_VD_SET;
 
-    let mut giggles_signer = Signer(SecretKey(1u32.into()));
-    let mut macrosoft_signer = Signer(SecretKey(2u32.into()));
-    let mut faebook_signer = Signer(SecretKey(3u32.into()));
+    let giggles_signer = Signer(SecretKey(1u32.into()));
+    let macrosoft_signer = Signer(SecretKey(2u32.into()));
+    let faebook_signer = Signer(SecretKey(3u32.into()));
     let good_boy_issuers =
         [&giggles_signer, &macrosoft_signer, &faebook_signer].map(|s| s.0.public_key());
-    let mut bob_signer = Signer(SecretKey(11u32.into()));
-    let mut charlie_signer = Signer(SecretKey(12u32.into()));
+    let bob_signer = Signer(SecretKey(11u32.into()));
+    let charlie_signer = Signer(SecretKey(12u32.into()));
     let alice_signer = Signer(SecretKey(13u32.into()));
     let bob = bob_signer.public_key();
     let charlie = charlie_signer.public_key();
@@ -351,23 +351,23 @@ pub fn great_boy_pod_full_flow() -> Result<(Params, MainPodBuilder)> {
     let mut bob_good_boys = Vec::new();
 
     let good_boy = good_boy_sign_pod_builder(&params, &bob, 36);
-    bob_good_boys.push(good_boy.sign(&mut giggles_signer).unwrap());
-    bob_good_boys.push(good_boy.sign(&mut macrosoft_signer).unwrap());
+    bob_good_boys.push(good_boy.sign(&giggles_signer).unwrap());
+    bob_good_boys.push(good_boy.sign(&macrosoft_signer).unwrap());
 
     // Charlie receives two good_boy pods from Macrosoft and Faebook
 
     let mut charlie_good_boys = Vec::new();
 
     let good_boy = good_boy_sign_pod_builder(&params, &charlie, 27);
-    charlie_good_boys.push(good_boy.sign(&mut macrosoft_signer).unwrap());
-    charlie_good_boys.push(good_boy.sign(&mut faebook_signer).unwrap());
+    charlie_good_boys.push(good_boy.sign(&macrosoft_signer).unwrap());
+    charlie_good_boys.push(good_boy.sign(&faebook_signer).unwrap());
 
     // Bob and Charlie send Alice a Friend POD
 
     let mut alice_friend_pods = Vec::new();
     let friend = friend_sign_pod_builder(&params, &alice);
-    alice_friend_pods.push(friend.sign(&mut bob_signer).unwrap());
-    alice_friend_pods.push(friend.sign(&mut charlie_signer).unwrap());
+    alice_friend_pods.push(friend.sign(&bob_signer).unwrap());
+    alice_friend_pods.push(friend.sign(&charlie_signer).unwrap());
 
     let good_boy_issuers = Value::from(Set::new(
         params.max_depth_mt_containers,
@@ -433,7 +433,7 @@ pub fn tickets_pod_full_flow() -> Result<MainPodBuilder> {
     let vd_set = &*MOCK_VD_SET;
     let builder = tickets_sign_pod_builder(&params);
 
-    let signed_pod = builder.sign(&mut Signer(SecretKey(1u32.into()))).unwrap();
+    let signed_pod = builder.sign(&Signer(SecretKey(1u32.into()))).unwrap();
     tickets_pod_builder(
         &params,
         vd_set,
diff --git a/src/frontend/custom.rs b/src/frontend/custom.rs
index 1efcae13..2c7e898e 100644
--- a/src/frontend/custom.rs
+++ b/src/frontend/custom.rs
@@ -311,8 +311,8 @@ mod tests {
         mp_builder.pub_op(op!(custom, gt_custom_pred, desugared_gt))?;
 
         // Check that the POD builds
-        let mut prover = MockProver {};
-        let proof = mp_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let proof = mp_builder.prove(&prover, &params)?;
 
         Ok(())
     }
@@ -360,8 +360,8 @@ mod tests {
         let set_contains_custom_pred = CustomPredicateRef::new(batch, 0);
         mp_builder.pub_op(op!(custom, set_contains_custom_pred, set_contains))?;
 
-        let mut prover = MockProver {};
-        let proof = mp_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let proof = mp_builder.prove(&prover, &params)?;
 
         Ok(())
     }
diff --git a/src/frontend/mod.rs b/src/frontend/mod.rs
index ab7e0ca8..180bf12b 100644
--- a/src/frontend/mod.rs
+++ b/src/frontend/mod.rs
@@ -49,9 +49,7 @@ impl SignedPodBuilder {
         self.kvs.insert(key.into(), value.into());
     }
 
-    // TODO: Remove mut because Schnorr signature doesn't need any mutability of the signer, the
-    // nonces are sourced from OS randomness.
-    pub fn sign<S: PodSigner>(&self, signer: &mut S) -> Result<SignedPod> {
+    pub fn sign<S: PodSigner>(&self, signer: &S) -> Result<SignedPod> {
         // Sign POD with committed KV store.
         let pod = signer.sign(&self.params, &self.kvs)?;
 
@@ -879,18 +877,18 @@ pub mod tests {
         println!("{}", gov_id);
         println!("{}", pay_stub);
 
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let gov_id = gov_id.sign(&mut signer)?;
+        let signer = Signer(SecretKey(1u32.into()));
+        let gov_id = gov_id.sign(&signer)?;
         check_kvs(&gov_id)?;
         println!("{}", gov_id);
 
-        let mut signer = Signer(SecretKey(2u32.into()));
-        let pay_stub = pay_stub.sign(&mut signer)?;
+        let signer = Signer(SecretKey(2u32.into()));
+        let pay_stub = pay_stub.sign(&signer)?;
         check_kvs(&pay_stub)?;
         println!("{}", pay_stub);
 
-        let mut signer = Signer(SecretKey(3u32.into()));
-        let sanction_list = sanction_list.sign(&mut signer)?;
+        let signer = Signer(SecretKey(3u32.into()));
+        let sanction_list = sanction_list.sign(&signer)?;
         check_kvs(&sanction_list)?;
         println!("{}", sanction_list);
 
@@ -898,8 +896,8 @@ pub mod tests {
         println!("{}", kyc_builder);
 
         // prove kyc with MockProver and print it
-        let mut prover = MockProver {};
-        let kyc = kyc_builder.prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let kyc = kyc_builder.prove(&prover, &params)?;
 
         println!("{}", kyc);
 
@@ -916,31 +914,29 @@ pub mod tests {
         };
         let vd_set = &*MOCK_VD_SET;
 
-        let mut alice = Signer(SecretKey(1u32.into()));
-        let mut bob = Signer(SecretKey(2u32.into()));
-        let mut charlie = Signer(SecretKey(3u32.into()));
+        let alice = Signer(SecretKey(1u32.into()));
+        let bob = Signer(SecretKey(2u32.into()));
+        let charlie = Signer(SecretKey(3u32.into()));
         let david = Signer(SecretKey(4u32.into()));
 
         let helper = EthDosHelper::new(&params, vd_set, true, alice.public_key())?;
 
-        let mut prover = MockProver {};
+        let prover = MockProver {};
 
-        let alice_attestation = attest_eth_friend(&params, &mut alice, bob.public_key());
-        let dist_1 = helper
-            .dist_1(&alice_attestation)?
-            .prove(&mut prover, &params)?;
+        let alice_attestation = attest_eth_friend(&params, &alice, bob.public_key());
+        let dist_1 = helper.dist_1(&alice_attestation)?.prove(&prover, &params)?;
         dist_1.pod.verify()?;
 
-        let bob_attestation = attest_eth_friend(&params, &mut bob, charlie.public_key());
+        let bob_attestation = attest_eth_friend(&params, &bob, charlie.public_key());
         let dist_2 = helper
             .dist_n_plus_1(&dist_1, &bob_attestation)?
-            .prove(&mut prover, &params)?;
+            .prove(&prover, &params)?;
         dist_2.pod.verify()?;
 
-        let charlie_attestation = attest_eth_friend(&params, &mut charlie, david.public_key());
+        let charlie_attestation = attest_eth_friend(&params, &charlie, david.public_key());
         let dist_3 = helper
             .dist_n_plus_1(&dist_2, &charlie_attestation)?
-            .prove(&mut prover, &params)?;
+            .prove(&prover, &params)?;
         dist_3.pod.verify()?;
 
         Ok(())
@@ -974,8 +970,8 @@ pub mod tests {
         let mut signed_builder = SignedPodBuilder::new(&params);
         signed_builder.insert("a", 1);
         signed_builder.insert("b", 1);
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let signed_pod = signed_builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let signed_pod = signed_builder.sign(&signer).unwrap();
 
         let mut builder = MainPodBuilder::new(&params, &vd_set);
         builder.add_signed_pod(&signed_pod);
@@ -1011,8 +1007,8 @@ pub mod tests {
         );
         builder.op(true, op_eq3).unwrap();
 
-        let mut prover = MockProver {};
-        let pod = builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let pod = builder.prove(&prover, &params).unwrap();
 
         println!("{}", pod);
     }
@@ -1025,8 +1021,8 @@ pub mod tests {
         let mut builder = SignedPodBuilder::new(&params);
 
         builder.insert("num", 2);
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let pod = builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let pod = builder.sign(&signer).unwrap();
 
         println!("{}", pod);
 
@@ -1034,8 +1030,8 @@ pub mod tests {
         builder.add_signed_pod(&pod);
         builder.pub_op(op!(gt, (&pod, "num"), 5)).unwrap();
 
-        let mut prover = MockProver {};
-        let false_pod = builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let false_pod = builder.prove(&prover, &params).unwrap();
 
         println!("{}", builder);
         println!("{}", false_pod);
@@ -1057,8 +1053,8 @@ pub mod tests {
         let dict_root = Value::from(dict.clone());
         builder.insert("dict", dict_root);
 
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let pod = builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let pod = builder.sign(&signer).unwrap();
 
         let mut builder = MainPodBuilder::new(&params, &vd_set);
         builder.add_signed_pod(&pod);
@@ -1079,8 +1075,8 @@ pub mod tests {
                 OperationAux::MerkleProof(dict.prove(&Key::from("a")).unwrap().1),
             ))
             .unwrap();
-        let mut main_prover = MockProver {};
-        let main_pod = builder.prove(&mut main_prover, &params).unwrap();
+        let main_prover = MockProver {};
+        let main_pod = builder.prove(&main_prover, &params).unwrap();
 
         println!("{}", main_pod);
 
@@ -1109,8 +1105,8 @@ pub mod tests {
         let st = Statement::equal(AnchoredKey::from((SELF, "a")), Value::from(28));
         builder.insert(false, (st, op_new_entry.clone()));
 
-        let mut prover = MockProver {};
-        let pod = builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let pod = builder.prove(&prover, &params).unwrap();
         pod.pod.verify().unwrap();
     }
 
@@ -1145,8 +1141,8 @@ pub mod tests {
         );
         builder.insert(false, (st, op));
 
-        let mut prover = MockProver {};
-        let pod = builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let pod = builder.prove(&prover, &params).unwrap();
         pod.pod.verify().unwrap();
     }
 }
diff --git a/src/frontend/serialization.rs b/src/frontend/serialization.rs
index 92180022..e1e8c948 100644
--- a/src/frontend/serialization.rs
+++ b/src/frontend/serialization.rs
@@ -222,8 +222,8 @@ mod tests {
     #[test]
     fn test_signed_pod_serialization() {
         let builder = signed_pod_builder();
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let pod = builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let pod = builder.sign(&signer).unwrap();
 
         let serialized = serde_json::to_string_pretty(&pod).unwrap();
         println!("serialized: {}", serialized);
@@ -240,8 +240,8 @@ mod tests {
     #[test]
     fn test_mock_signed_pod_serialization() {
         let builder = signed_pod_builder();
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let pod = builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let pod = builder.sign(&signer).unwrap();
 
         let serialized = serde_json::to_string_pretty(&pod).unwrap();
         println!("serialized: {}", serialized);
@@ -261,12 +261,12 @@ mod tests {
 
         let (gov_id_builder, pay_stub_builder, sanction_list_builder) =
             zu_kyc_sign_pod_builders(&params);
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let gov_id_pod = gov_id_builder.sign(&mut signer).unwrap();
-        let mut signer = Signer(SecretKey(2u32.into()));
-        let pay_stub_pod = pay_stub_builder.sign(&mut signer).unwrap();
-        let mut signer = Signer(SecretKey(3u32.into()));
-        let sanction_list_pod = sanction_list_builder.sign(&mut signer).unwrap();
+        let signer = Signer(SecretKey(1u32.into()));
+        let gov_id_pod = gov_id_builder.sign(&signer).unwrap();
+        let signer = Signer(SecretKey(2u32.into()));
+        let pay_stub_pod = pay_stub_builder.sign(&signer).unwrap();
+        let signer = Signer(SecretKey(3u32.into()));
+        let sanction_list_pod = sanction_list_builder.sign(&signer).unwrap();
         let kyc_builder = zu_kyc_pod_builder(
             &params,
             &vd_set,
@@ -276,8 +276,8 @@ mod tests {
         )
         .unwrap();
 
-        let mut prover = MockProver {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params).unwrap();
+        let prover = MockProver {};
+        let kyc_pod = kyc_builder.prove(&prover, &params).unwrap();
         Ok(kyc_pod)
     }
 
@@ -292,12 +292,12 @@ mod tests {
 
         let (gov_id_builder, pay_stub_builder, sanction_list_builder) =
             zu_kyc_sign_pod_builders(&params);
-        let mut signer = Signer(SecretKey(1u32.into()));
-        let gov_id_pod = gov_id_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(2u32.into()));
-        let pay_stub_pod = pay_stub_builder.sign(&mut signer)?;
-        let mut signer = Signer(SecretKey(3u32.into()));
-        let sanction_list_pod = sanction_list_builder.sign(&mut signer)?;
+        let signer = Signer(SecretKey(1u32.into()));
+        let gov_id_pod = gov_id_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(2u32.into()));
+        let pay_stub_pod = pay_stub_builder.sign(&signer)?;
+        let signer = Signer(SecretKey(3u32.into()));
+        let sanction_list_pod = sanction_list_builder.sign(&signer)?;
         let kyc_builder = zu_kyc_pod_builder(
             &params,
             &vd_set,
@@ -306,8 +306,8 @@ mod tests {
             &sanction_list_pod,
         )?;
 
-        let mut prover = Prover {};
-        let kyc_pod = kyc_builder.prove(&mut prover, &params)?;
+        let prover = Prover {};
+        let kyc_pod = kyc_builder.prove(&prover, &params)?;
 
         Ok(kyc_pod)
     }
@@ -348,23 +348,21 @@ mod tests {
         };
         let vd_set = &*MOCK_VD_SET;
 
-        let mut alice = Signer(SecretKey(1u32.into()));
-        let mut bob = Signer(SecretKey(2u32.into()));
+        let alice = Signer(SecretKey(1u32.into()));
+        let bob = Signer(SecretKey(2u32.into()));
         let charlie = Signer(SecretKey(3u32.into()));
 
         // Alice attests that she is ETH friends with Bob and Bob
         // attests that he is ETH friends with Charlie.
-        let alice_attestation = attest_eth_friend(&params, &mut alice, bob.public_key());
-        let bob_attestation = attest_eth_friend(&params, &mut bob, charlie.public_key());
+        let alice_attestation = attest_eth_friend(&params, &alice, bob.public_key());
+        let bob_attestation = attest_eth_friend(&params, &bob, charlie.public_key());
 
         let helper = EthDosHelper::new(&params, vd_set, true, alice.public_key())?;
-        let mut prover = MockProver {};
-        let dist_1 = helper
-            .dist_1(&alice_attestation)?
-            .prove(&mut prover, &params)?;
+        let prover = MockProver {};
+        let dist_1 = helper.dist_1(&alice_attestation)?.prove(&prover, &params)?;
         let dist_2 = helper
             .dist_n_plus_1(&dist_1, &bob_attestation)?
-            .prove(&mut prover, &params)?;
+            .prove(&prover, &params)?;
 
         Ok(dist_2)
     }
@@ -380,7 +378,7 @@ mod tests {
 
         let kyc_pod = build_mock_zukyc_pod().unwrap();
         let signed_pod = signed_pod_builder()
-            .sign(&mut Signer(SecretKey(1u32.into())))
+            .sign(&Signer(SecretKey(1u32.into())))
             .unwrap();
         let ethdos_pod = build_ethdos_pod().unwrap();
         let mainpod_schema_value = serde_json::to_value(&mainpod_schema).unwrap();
diff --git a/src/middleware/mod.rs b/src/middleware/mod.rs
index 9936eda8..51df8a71 100644
--- a/src/middleware/mod.rs
+++ b/src/middleware/mod.rs
@@ -874,7 +874,7 @@ dyn_clone::clone_trait_object!(RecursivePod);
 
 pub trait PodSigner {
     fn sign(
-        &mut self,
+        &self,
         params: &Params,
         kvs: &HashMap<Key, Value>,
     ) -> Result<Box<dyn Pod>, BackendError>;