Use SignatureBytes and PublicKeyBytes for deposits (#472)

* Replace deposit signatures with SignatureBytes, a struct which lazyly parsers signatures only on demand.

* check byte length when parsing SignatureBytes

* add comment to struct

* distinguish BadSignature and BadSignatureBytes in verify_deposit_signature

* add test for valid signature

* Implements TryInto<Signature> for &SignatureBytes and From<Signature> for &SignatureBytes

* add and use PublicKeyBytes + fix formatting

* fix compiler warning + docs for macro generated structs

* adds tests to ensure correct byte lengths

* small style improvement as suggested by michaelsproul

eth2/state_processing/src/per_block_processing/errors.rs

@@ -349,6 +349,8 @@ pub enum DepositInvalid {
     BadIndex { state: u64, deposit: u64 },
     /// The signature (proof-of-possession) does not match the given pubkey.
     BadSignature,
+    /// The signature does not represent a valid BLS signature.
+    BadSignatureBytes,
     /// The specified `branch` and `index` did not form a valid proof that the deposit is included
     /// in the eth1 deposit root.
     BadMerkleProof,

eth2/state_processing/src/per_block_processing/verify_deposit.rs

@@ -1,5 +1,6 @@
 use super::errors::{DepositInvalid as Invalid, DepositValidationError as Error};
 use merkle_proof::verify_merkle_proof;
+use std::convert::TryInto;
 use tree_hash::{SignedRoot, TreeHash};
 use types::*;
 
@@ -10,15 +11,17 @@ pub fn verify_deposit_signature<T: EthSpec>(
     state: &BeaconState<T>,
     deposit: &Deposit,
     spec: &ChainSpec,
+    pubkey: &PublicKey,
 ) -> Result<(), Error> {
     // Note: Deposits are valid across forks, thus the deposit domain is computed
     // with the fork zeroed.
     let domain = spec.get_domain(state.current_epoch(), Domain::Deposit, &Fork::default());
+    let signature: Signature = (&deposit.data.signature)
+        .try_into()
+        .map_err(|_| Error::Invalid(Invalid::BadSignatureBytes))?;
+
     verify!(
-        deposit
-            .data
-            .signature
-            .verify(&deposit.data.signed_root(), domain, &deposit.data.pubkey,),
+        signature.verify(&deposit.data.signed_root(), domain, pubkey),
         Invalid::BadSignature
     );
 
@@ -33,9 +36,9 @@ pub fn verify_deposit_signature<T: EthSpec>(
 /// Errors if the state's `pubkey_cache` is not current.
 pub fn get_existing_validator_index<T: EthSpec>(
     state: &BeaconState<T>,
-    deposit: &Deposit,
+    pub_key: &PublicKey,
 ) -> Result<Option<u64>, Error> {
-    let validator_index = state.get_validator_index(&deposit.data.pubkey)?;
+    let validator_index = state.get_validator_index(pub_key)?;
     Ok(validator_index.map(|idx| idx as u64))
 }