diff --git a/beacon_node/beacon_chain/src/state_transition.rs b/beacon_node/beacon_chain/src/state_transition.rs
index 5b614234f8..0537ad1959 100644
--- a/beacon_node/beacon_chain/src/state_transition.rs
+++ b/beacon_node/beacon_chain/src/state_transition.rs
@@ -265,18 +265,11 @@ where
                         .as_raw(),
                 )
             }
-            let attestation_message = {
-                let attestation_data_and_custody_bit = AttestationDataAndCustodyBit {
-                    data: attestation.data.clone(),
-                    custody_bit: false,
-                };
-                &attestation_data_and_custody_bit.hash_tree_root()
-            };
             // Signature verification.
             ensure!(
                 bls_verify_aggregate(
                     &group_public_key,
-                    &attestation_message[..],
+                    &attestation.signable_message(),
                     &attestation.aggregate_signature,
                     get_domain(&state.fork_data, attestation.data.slot, DOMAIN_ATTESTATION)
                 ),
diff --git a/eth2/types/src/attestation.rs b/eth2/types/src/attestation/mod.rs
similarity index 99%
rename from eth2/types/src/attestation.rs
rename to eth2/types/src/attestation/mod.rs
index d6738bd6ee..3b278ef8ce 100644
--- a/eth2/types/src/attestation.rs
+++ b/eth2/types/src/attestation/mod.rs
@@ -5,6 +5,8 @@ use rand::RngCore;
 use serde_derive::Serialize;
 use ssz::{hash, Decodable, DecodeError, Encodable, SszStream, TreeHash};
 
+mod signing;
+
 #[derive(Debug, Clone, PartialEq, Serialize)]
 pub struct Attestation {
     pub data: AttestationData,
diff --git a/eth2/types/src/attestation/signing.rs b/eth2/types/src/attestation/signing.rs
new file mode 100644
index 0000000000..a2ef13d2f8
--- /dev/null
+++ b/eth2/types/src/attestation/signing.rs
@@ -0,0 +1,12 @@
+use crate::{Attestation, AttestationDataAndCustodyBit};
+use ssz::TreeHash;
+
+impl Attestation {
+    pub fn signable_message(&self) -> Vec<u8> {
+        let attestation_data_and_custody_bit = AttestationDataAndCustodyBit {
+            data: self.data.clone(),
+            custody_bit: false,
+        };
+        attestation_data_and_custody_bit.hash_tree_root()
+    }
+}