Add signable_message() to Attestation

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,

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()
+    }
+}