diff --git a/common/blockchain/src/block.rs b/common/blockchain/src/block.rs
index b4c92012..b826c613 100644
--- a/common/blockchain/src/block.rs
+++ b/common/blockchain/src/block.rs
@@ -69,17 +69,19 @@ impl<PK: crate::PubKey> Block<PK> {
 
 
     /// Signs the block.
-    pub fn sign<S>(&self, signer: &S) -> PK::Signature
-    where
-        S: crate::validators::Signer<Signature = PK::Signature>,
-    {
+    pub fn sign(&self, signer: &impl crate::Signer<PK>) -> PK::Signature {
         signer.sign(self.calc_hash().as_slice())
     }
 
     /// Verifies signature for the block.
     #[inline]
-    pub fn verify(&self, pk: &PK, signature: &PK::Signature) -> bool {
-        pk.verify(self.calc_hash().as_slice(), signature)
+    pub fn verify(
+        &self,
+        pubkey: &PK,
+        signature: &PK::Signature,
+        verifier: &impl crate::Verifier<PK>,
+    ) -> bool {
+        verifier.verify(self.calc_hash().as_slice(), pubkey, signature)
     }
 
     /// Constructs next block.
@@ -179,14 +181,14 @@ fn test_block_generation() {
     let signer = MockSigner(pk);
     let signature = genesis.sign(&signer);
     assert_eq!(MockSignature(1722674425, pk), signature);
-    assert!(genesis.verify(&pk, &signature));
-    assert!(!genesis.verify(&MockPubKey(88), &signature));
-    assert!(!genesis.verify(&pk, &MockSignature(0, pk)));
+    assert!(genesis.verify(&pk, &signature, &()));
+    assert!(!genesis.verify(&MockPubKey(88), &signature, &()));
+    assert!(!genesis.verify(&pk, &MockSignature(0, pk), &()));
 
     let mut block = genesis.clone();
     block.host_timestamp += 1;
     assert_ne!(genesis_hash, block.calc_hash());
-    assert!(!block.verify(&pk, &signature));
+    assert!(!block.verify(&pk, &signature, &()));
 
     // Try creating invalid next block.
     assert_eq!(
diff --git a/common/blockchain/src/lib.rs b/common/blockchain/src/lib.rs
index 1161cf3c..4a68e12a 100644
--- a/common/blockchain/src/lib.rs
+++ b/common/blockchain/src/lib.rs
@@ -19,4 +19,4 @@ pub use config::Config;
 pub use epoch::Epoch;
 pub use height::{BlockDelta, BlockHeight, HostDelta, HostHeight};
 pub use manager::ChainManager;
-pub use validators::{PubKey, Signer, Validator};
+pub use validators::{PubKey, Signer, Validator, Verifier};
diff --git a/common/blockchain/src/manager.rs b/common/blockchain/src/manager.rs
index d612784d..3d26cb04 100644
--- a/common/blockchain/src/manager.rs
+++ b/common/blockchain/src/manager.rs
@@ -199,26 +199,27 @@ impl<PK: crate::PubKey> ChainManager<PK> {
         &mut self,
         pubkey: PK,
         signature: &PK::Signature,
+        verifier: &impl crate::Verifier<PK>,
     ) -> Result<bool, AddSignatureError> {
         let pending = self
             .pending_block
             .as_mut()
             .ok_or(AddSignatureError::NoPendingBlock)?;
-        if pending.signers.contains(&pubkey) {
-            return Ok(false);
-        }
-        if !pubkey.verify(pending.hash.as_slice(), signature) {
-            return Err(AddSignatureError::BadSignature);
-        }
-
-        pending.signing_stake += self
+        let validator_stake = self
             .next_epoch
             .validator(&pubkey)
             .ok_or(AddSignatureError::BadValidator)?
             .stake()
             .get();
-        assert!(pending.signers.insert(pubkey));
+        if !verifier.verify(pending.hash.as_slice(), &pubkey, signature) {
+            return Err(AddSignatureError::BadSignature);
+        }
+
+        if !pending.signers.insert(pubkey) {
+            return Ok(false);
+        }
 
+        pending.signing_stake += validator_stake;
         if pending.signing_stake < self.next_epoch.quorum_stake().get() {
             return Ok(false);
         }
@@ -304,7 +305,7 @@ fn test_generate() {
         validator: &crate::validators::Validator<MockPubKey>,
     ) -> Result<bool, AddSignatureError> {
         let signature = mgr.head().1.sign(&validator.pubkey().make_signer());
-        mgr.add_signature(validator.pubkey().clone(), &signature)
+        mgr.add_signature(validator.pubkey().clone(), &signature, &())
     }
 
     // The head hasn’t been fully signed yet.
@@ -329,11 +330,11 @@ fn test_generate() {
     let signature = mgr.head().1.sign(&pubkey.make_signer());
     assert_eq!(
         Err(AddSignatureError::BadValidator),
-        mgr.add_signature(pubkey, &signature)
+        mgr.add_signature(pubkey, &signature, &())
     );
     assert_eq!(
         Err(AddSignatureError::BadSignature),
-        mgr.add_signature(bob.pubkey().clone(), &signature)
+        mgr.add_signature(bob.pubkey().clone(), &signature, &())
     );
 
     assert_eq!(
diff --git a/common/blockchain/src/validators.rs b/common/blockchain/src/validators.rs
index 80f18d70..11ee6c79 100644
--- a/common/blockchain/src/validators.rs
+++ b/common/blockchain/src/validators.rs
@@ -12,17 +12,23 @@ pub trait PubKey:
 {
     /// Signature corresponding to this public key type.
     type Signature: Clone + borsh::BorshSerialize + borsh::BorshDeserialize;
-
-    /// Verifies the signature for given message.
-    fn verify(&self, message: &[u8], signature: &Self::Signature) -> bool;
 }
 
-pub trait Signer {
-    /// Signature created by this signer.
-    type Signature: Clone + borsh::BorshSerialize + borsh::BorshDeserialize;
+/// Function verifying a signature.
+pub trait Verifier<PK: PubKey> {
+    /// Verify signature for given message.
+    fn verify(
+        &self,
+        message: &[u8],
+        pubkey: &PK,
+        signature: &PK::Signature,
+    ) -> bool;
+}
 
+/// Function generating signatures.
+pub trait Signer<PK: PubKey> {
     /// Signs given message.
-    fn sign(&self, message: &[u8]) -> Self::Signature;
+    fn sign(&self, message: &[u8]) -> PK::Signature;
 }
 
 /// A validator
@@ -115,16 +121,24 @@ pub(crate) mod test_utils {
 
     impl super::PubKey for MockPubKey {
         type Signature = MockSignature;
+    }
 
-        fn verify(&self, message: &[u8], signature: &Self::Signature) -> bool {
-            signature.0 == short_hash(message) && &signature.1 == self
+    impl super::Verifier<MockPubKey> for () {
+        fn verify(
+            &self,
+            message: &[u8],
+            pubkey: &MockPubKey,
+            signature: &<MockPubKey as super::PubKey>::Signature,
+        ) -> bool {
+            signature.0 == short_hash(message) && &signature.1 == pubkey
         }
     }
 
-    impl super::Signer for MockSigner {
-        type Signature = MockSignature;
-
-        fn sign(&self, message: &[u8]) -> Self::Signature {
+    impl super::Signer<MockPubKey> for MockSigner {
+        fn sign(
+            &self,
+            message: &[u8],
+        ) -> <MockPubKey as super::PubKey>::Signature {
             MockSignature(short_hash(message), self.0)
         }
     }