More secure Signed implementation (#2963)

* Remove signature verification in backing.

`SignedFullStatement` now signals that the signature has already been
checked.

* Remove unused check_payload function.

* Introduced unchecked signed variants.

* Fix inclusion to use unchecked variant.

* More unchecked variants.

* Use unchecked variants in protocols.

* Start fixing statement-distribution.

* Fixup statement distribution.

* Fix inclusion.

* Fix warning.

* Fix backing properly.

* Fix bitfield distribution.

* Make crypto store optional for `RuntimeInfo`.

* Factor out utility functions.

* get_group_rotation_info

* WIP: Collator cleanup + check signatures.

* Convenience signature checking functions.

* Check signature on collator-side.

* Fix warnings.

* Fix collator side tests.

* Get rid of warnings.

* Better Signed/UncheckedSigned implementation.

Also get rid of Encode/Decode for Signed! *party*

* Get rid of dead code.

* Move Signed in its own module.

* into_checked -> try_into_checked

* Fix merge.

polkadot/node/core/backing/src/lib.rs

@@ -195,7 +195,6 @@ const fn group_quorum(n_validators: usize) -> usize {
 
 #[derive(Default)]
 struct TableContext {
-	signing_context: SigningContext,
 	validator: Option<Validator>,
 	groups: HashMap<ParaId, Vec<ValidatorIndex>>,
 	validators: Vec<ValidatorId>,
@@ -870,7 +869,6 @@ impl CandidateBackingJob {
 					.with_candidate(statement.payload().candidate_hash())
 					.with_relay_parent(_relay_parent);
 
-				self.check_statement_signature(&statement)?;
 				match self.maybe_validate_and_import(&root_span, sender, statement).await {
 					Err(Error::ValidationFailed(_)) => return Ok(()),
 					Err(e) => return Err(e),
@@ -1028,22 +1026,6 @@ impl CandidateBackingJob {
 		Some(signed)
 	}
 
-	#[tracing::instrument(level = "trace", skip(self), fields(subsystem = LOG_TARGET))]
-	fn check_statement_signature(&self, statement: &SignedFullStatement) -> Result<(), Error> {
-		let idx = statement.validator_index().0 as usize;
-
-		if self.table_context.validators.len() > idx {
-			statement.check_signature(
-				&self.table_context.signing_context,
-				&self.table_context.validators[idx],
-			).map_err(|_| Error::InvalidSignature)?;
-		} else {
-			return Err(Error::InvalidSignature);
-		}
-
-		Ok(())
-	}
-
 	/// Insert or get the unbacked-span for the given candidate hash.
 	fn insert_or_get_unbacked_span(
 		&mut self,
@@ -1204,7 +1186,6 @@ impl util::JobTrait for CandidateBackingJob {
 			let table_context = TableContext {
 				groups,
 				validators,
-				signing_context,
 				validator,
 			};
 
@@ -1658,14 +1639,9 @@ mod tests {
 				AllMessages::StatementDistribution(
 					StatementDistributionMessage::Share(
 						parent_hash,
-						signed_statement,
+						_signed_statement,
 					)
-				) if parent_hash == test_state.relay_parent => {
-					signed_statement.check_signature(
-						&test_state.signing_context,
-						&test_state.validator_public[0],
-					).unwrap();
-				}
+				) if parent_hash == test_state.relay_parent => {}
 			);
 
 			assert_matches!(
@@ -1708,11 +1684,6 @@ mod tests {
 			}.build();
 
 			let candidate_a_hash = candidate_a.hash();
-			let public0 = CryptoStore::sr25519_generate_new(
-				&*test_state.keystore,
-				ValidatorId::ID,
-				Some(&test_state.validators[0].to_seed()),
-			).await.expect("Insert key into keystore");
 			let public1 = CryptoStore::sr25519_generate_new(
 				&*test_state.keystore,
 				ValidatorId::ID,
@@ -1795,10 +1766,9 @@ mod tests {
 			assert_matches!(
 				virtual_overseer.recv().await,
 				AllMessages::StatementDistribution(
-					StatementDistributionMessage::Share(hash, stmt)
+					StatementDistributionMessage::Share(hash, _stmt)
 				) => {
 					assert_eq!(test_state.relay_parent, hash);
-					stmt.check_signature(&test_state.signing_context, &public0.into()).expect("Is signed correctly");
 				}
 			);
 
@@ -2092,11 +2062,6 @@ mod tests {
 						signed_statement,
 					)
 				) if relay_parent == test_state.relay_parent => {
-					signed_statement.check_signature(
-						&test_state.signing_context,
-						&test_state.validator_public[0],
-					).unwrap();
-
 					assert_eq!(*signed_statement.payload(), Statement::Valid(candidate_a_hash));
 				}
 			);
@@ -2257,11 +2222,6 @@ mod tests {
 						signed_statement,
 					)
 				) if parent_hash == test_state.relay_parent => {
-					signed_statement.check_signature(
-						&test_state.signing_context,
-						&test_state.validator_public[0],
-					).unwrap();
-
 					assert_eq!(*signed_statement.payload(), Statement::Seconded(candidate_b));
 				}
 			);
@@ -2593,10 +2553,7 @@ mod tests {
 		use sp_core::Encode;
 		use std::convert::TryFrom;
 
-		let relay_parent = [1; 32].into();
 		let para_id = ParaId::from(10);
-		let session_index = 5;
-		let signing_context = SigningContext { parent_hash: relay_parent, session_index };
 		let validators = vec![
 			Sr25519Keyring::Alice,
 			Sr25519Keyring::Bob,
@@ -2614,7 +2571,6 @@ mod tests {
 		};
 
 		let table_context = TableContext {
-			signing_context,
 			validator: None,
 			groups: validator_groups,
 			validators: validator_public.clone(),