diff --git a/Cargo.lock b/Cargo.lock
index 6077800..a0d2b78 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1813,6 +1813,7 @@ dependencies = [
  "ethlambda-types",
  "prometheus",
  "spawned-concurrency",
+ "thiserror 2.0.17",
  "tracing",
 ]
 
diff --git a/crates/blockchain/Cargo.toml b/crates/blockchain/Cargo.toml
index fd0a09a..0926640 100644
--- a/crates/blockchain/Cargo.toml
+++ b/crates/blockchain/Cargo.toml
@@ -17,5 +17,6 @@ ethlambda-types.workspace = true
 spawned-concurrency.workspace = true
 
 tracing.workspace = true
+thiserror.workspace = true
 
 prometheus.workspace = true
diff --git a/crates/blockchain/src/lib.rs b/crates/blockchain/src/lib.rs
index 3316194..5cfa86d 100644
--- a/crates/blockchain/src/lib.rs
+++ b/crates/blockchain/src/lib.rs
@@ -1,5 +1,5 @@
 use ethlambda_storage::Store;
-use ethlambda_types::{block::SignedBlockWithAttestation, primitives::TreeHash};
+use ethlambda_types::{block::{SignedBlockWithAttestation, VerifySignatureError}, primitives::TreeHash};
 use spawned_concurrency::tasks::{CallResponse, CastResponse, GenServer, GenServerHandle};
 use tracing::{error, warn};
 
@@ -7,6 +7,13 @@ pub struct BlockChain {
     handle: GenServerHandle<BlockChainServer>,
 }
 
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("Signed block signature verification failed")]
+    VerifySignatureError(#[from] VerifySignatureError),
+}
+
 impl BlockChain {
     pub fn spawn(store: Store) -> BlockChain {
         BlockChain {
@@ -35,9 +42,9 @@ impl BlockChainServer {
         let slot = signed_block.message.block.slot;
         update_head_slot(slot);
 
-        let block = signed_block.message.block;
-        let proposer_attestation = signed_block.message.proposer_attestation;
-        let signatures = signed_block.signature;
+        let block = &signed_block.message.block;
+        let proposer_attestation = &signed_block.message.proposer_attestation;
+        let signatures = &signed_block.signature;
 
         let block_root = block.tree_hash_root();
 
@@ -51,7 +58,10 @@ impl BlockChainServer {
             return;
         };
 
-        // TODO: validate block signatures
+        if let Err(err) = signed_block.verify_signatures(&pre_state) {
+            error!(%slot, %block_root, parent=%block.parent_root, "Signed block signature verification failed: {}", err);
+            return;
+        }
 
         let state_changes = ethlambda_state_transition::state_transition(&pre_state, &block);
     }
diff --git a/crates/common/types/src/block.rs b/crates/common/types/src/block.rs
index 8e8678f..5030a7e 100644
--- a/crates/common/types/src/block.rs
+++ b/crates/common/types/src/block.rs
@@ -1,12 +1,14 @@
+use leansig::{signature::SignatureScheme, serialization::Serializable};
 use ssz_derive::{Decode, Encode};
 use ssz_types::typenum::{Diff, U488, U3600, U4096};
+use tree_hash::TreeHash;
 use tree_hash_derive::TreeHash;
-
+use thiserror::Error;
 use crate::{
     attestation::{Attestation, Attestations},
     primitives::H256,
-    signature::{Signature, SignatureSize},
-    state::ValidatorRegistryLimit,
+    signature::{LeanPublicKey, LeanSignatureScheme, Signature, SignatureSize},
+    state::{State, ValidatorRegistryLimit},
 };
 
 /// Envelope carrying a block, an attestation from proposer, and aggregated signatures.
@@ -27,6 +29,65 @@ pub struct SignedBlockWithAttestation {
     pub signature: BlockSignatures,
 }
 
+#[derive(Error, Debug)]
+pub enum VerifySignatureError {
+    #[error("Number of signatures {0} does not match number of attestations {1}")]
+    NumberOfSignaturesMismatch(usize, usize),
+    #[error("Validator {0} index out of range")]
+    ValidatorOutOfRange(u64),
+    #[error("Failed to deserialize public key")]
+    PublicKeyDeserializationError,
+    #[error("Failed to deserialize signature")]
+    SignatureDeserializationError,
+    #[error("Signature verification failed")]
+    SignatureVerificationFailed,
+}
+
+impl SignedBlockWithAttestation {
+    /// Verify all XMSS signatures in this signed block.
+
+    /// This function ensures that every attestation included in the block
+    /// (both on-chain attestations from the block body and the proposer's
+    /// own attestation) is properly signed by the claimed validator using
+    /// their registered XMSS public key.
+    pub fn verify_signatures(&self, parent_state: &State) -> Result<bool, VerifySignatureError> {
+        let block = &self.message.block;
+        let signature = &self.signature;
+        
+        let all_attestations: Vec<_> = block.body.attestations
+            .iter()
+            .chain(std::iter::once(&self.message.proposer_attestation))
+            .collect();
+        
+        let validators = &parent_state.validators;
+        
+        if signature.len() != all_attestations.len() {
+            return Err(VerifySignatureError::NumberOfSignaturesMismatch(signature.len(), all_attestations.len()));
+        }
+        
+        for (attestation, signature) in all_attestations.iter().zip(signature.iter()) {
+            let validator_id = attestation.validator_id;
+            
+            // Ensure validator is in the active set
+            if validator_id < validators.len() as u64 {
+                return Err(VerifySignatureError::ValidatorOutOfRange(validator_id));
+            }
+            
+            let validator = &validators[validator_id as usize];
+            
+            // Verify the XMSS signature
+            let lean_public_key = LeanPublicKey::from_bytes(&validator.pubkey).map_err(|_| VerifySignatureError::PublicKeyDeserializationError)?;
+            let lean_signature = Signature::from_bytes(signature).map_err(|_| VerifySignatureError::SignatureDeserializationError)?;
+            if !LeanSignatureScheme::verify(&lean_public_key, attestation.data.slot as u32, &attestation.tree_hash_root(), &lean_signature) {
+                return Err(VerifySignatureError::SignatureVerificationFailed);
+            }
+        }
+        
+        
+        Ok(true)
+    }
+}
+
 // Manual Debug impl because leanSig signatures don't implement Debug.
 impl core::fmt::Debug for SignedBlockWithAttestation {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
diff --git a/crates/common/types/src/signature.rs b/crates/common/types/src/signature.rs
index 4b91bb6..43b7112 100644
--- a/crates/common/types/src/signature.rs
+++ b/crates/common/types/src/signature.rs
@@ -1,10 +1,12 @@
 use leansig::signature::SignatureScheme;
 use ssz_types::typenum::{Diff, U488, U3600};
 
-type LeanSignatureScheme = leansig::signature::generalized_xmss::instantiations_poseidon_top_level::lifetime_2_to_the_32::hashing_optimized::SIGTopLevelTargetSumLifetime32Dim64Base8;
+pub type LeanSignatureScheme = leansig::signature::generalized_xmss::instantiations_poseidon_top_level::lifetime_2_to_the_32::hashing_optimized::SIGTopLevelTargetSumLifetime32Dim64Base8;
 
 type LeanSigSignature = <LeanSignatureScheme as SignatureScheme>::Signature;
 
 pub type Signature = LeanSigSignature;
 
 pub type SignatureSize = Diff<U3600, U488>;
+
+pub type LeanPublicKey = <LeanSignatureScheme as SignatureScheme>::PublicKey;