Adding Bridges code as git subtree. (#2515)

* Add instructions. * Squashed 'bridges/' content from commit 345e84a21 git-subtree-dir: bridges git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9 * Remove bridges workspace file to avoid confusing Cargo. * Add some bridges primitives to Polkadot workspace. * Improve docs.
2026-05-30 18:41:03 +00:00 · 2021-03-01 22:33:16 +01:00
parent 7a2c7aa3fe
commit 5169155f94
291 changed files with 64249 additions and 0 deletions
@@ -0,0 +1,48 @@
+[package]
+name = "ethereum-poa-relay"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+ansi_term = "0.12"
+async-std = "1.9.0"
+async-trait = "0.1.42"
+clap = { version = "2.33.3", features = ["yaml"] }
+codec = { package = "parity-scale-codec", version = "2.0.0" }
+env_logger = "0.8.3"
+ethabi = { git = "https://github.com/paritytech/ethabi.git", branch = "td-eth-types-11" }
+ethabi-contract = { git = "https://github.com/paritytech/ethabi.git", branch = "td-eth-types-11" }
+ethabi-derive = { git = "https://github.com/paritytech/ethabi.git", branch = "td-eth-types-11" }
+futures = "0.3.12"
+hex = "0.4"
+hex-literal = "0.3"
+libsecp256k1 = { version = "0.3.4", default-features = false, features = ["hmac"] }
+log = "0.4.14"
+num-traits = "0.2"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0.62"
+time = "0.2"
+
+# Bridge dependencies
+
+bp-currency-exchange = { path = "../../primitives/currency-exchange" }
+bp-eth-poa = { path = "../../primitives/ethereum-poa" }
+exchange-relay = { path = "../exchange-relay" }
+headers-relay = { path = "../headers-relay" }
+messages-relay = { path = "../messages-relay" }
+relay-ethereum-client = { path = "../ethereum-client" }
+relay-rialto-client = { path = "../rialto-client" }
+relay-substrate-client = { path = "../substrate-client" }
+relay-utils = { path = "../utils" }
+rialto-runtime = { path = "../../bin/rialto/runtime" }
+
+# Substrate Dependencies
+
+frame-system = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+pallet-transaction-payment = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-core = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-keyring = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-runtime = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+substrate-prometheus-endpoint = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
@@ -0,0 +1,7 @@
+# PoA <> Substrate Bridge
+
+**DISCLAIMER:** *we recommend not using the bridge in "production" (to bridge significant amounts) just yet.
+it's missing a code audit and should still be considered alpha. we can't rule out that there are bugs that might result in loss of the bridged amounts.
+we'll update this disclaimer once that changes*
+
+These docs are very incomplete yet. Describe high-level goals here in the (near) future.
@@ -0,0 +1,167 @@
+[
+	{
+		"inputs": [
+			{
+				"internalType": "bytes",
+				"name": "rawInitialHeader",
+				"type": "bytes"
+			},
+			{
+				"internalType": "uint64",
+				"name": "initialValidatorsSetId",
+				"type": "uint64"
+			},
+			{
+				"internalType": "bytes",
+				"name": "initialValidatorsSet",
+				"type": "bytes"
+			}
+		],
+		"stateMutability": "nonpayable",
+		"type": "constructor"
+	},
+	{
+		"stateMutability": "nonpayable",
+		"type": "fallback"
+	},
+	{
+		"inputs": [],
+		"name": "bestKnownHeader",
+		"outputs": [
+			{
+				"internalType": "uint256",
+				"name": "",
+				"type": "uint256"
+			},
+			{
+				"internalType": "bytes32",
+				"name": "",
+				"type": "bytes32"
+			}
+		],
+		"stateMutability": "view",
+		"type": "function"
+	},
+	{
+		"inputs": [
+			{
+				"internalType": "uint256",
+				"name": "finalityTargetNumber",
+				"type": "uint256"
+			},
+			{
+				"internalType": "bytes32",
+				"name": "finalityTargetHash",
+				"type": "bytes32"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawFinalityProof",
+				"type": "bytes"
+			}
+		],
+		"name": "importFinalityProof",
+		"outputs": [],
+		"stateMutability": "nonpayable",
+		"type": "function"
+	},
+	{
+		"inputs": [
+			{
+				"internalType": "bytes",
+				"name": "rawHeader1",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader2",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader3",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader4",
+				"type": "bytes"
+			}
+		],
+		"name": "importHeaders",
+		"outputs": [],
+		"stateMutability": "nonpayable",
+		"type": "function"
+	},
+	{
+		"inputs": [],
+		"name": "incompleteHeaders",
+		"outputs": [
+			{
+				"internalType": "uint256[]",
+				"name": "",
+				"type": "uint256[]"
+			},
+			{
+				"internalType": "bytes32[]",
+				"name": "",
+				"type": "bytes32[]"
+			}
+		],
+		"stateMutability": "view",
+		"type": "function"
+	},
+	{
+		"inputs": [
+			{
+				"internalType": "bytes",
+				"name": "rawHeader1",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader2",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader3",
+				"type": "bytes"
+			},
+			{
+				"internalType": "bytes",
+				"name": "rawHeader4",
+				"type": "bytes"
+			}
+		],
+		"name": "isIncompleteHeaders",
+		"outputs": [
+			{
+				"internalType": "uint256",
+				"name": "",
+				"type": "uint256"
+			}
+		],
+		"stateMutability": "view",
+		"type": "function"
+	},
+	{
+		"inputs": [
+			{
+				"internalType": "bytes32",
+				"name": "headerHash",
+				"type": "bytes32"
+			}
+		],
+		"name": "isKnownHeader",
+		"outputs": [
+			{
+				"internalType": "bool",
+				"name": "",
+				"type": "bool"
+			}
+		],
+		"stateMutability": "view",
+		"type": "function"
+	}
+]
@@ -0,0 +1,5 @@
+Last Change Date: 2020-07-30
+Solc version: 0.6.6+commit.6c089d02.Linux.g++
+Source hash (keccak256): 0xea5d6d744f69157adc2857166792aca139c0b5b186ba89c1011358fbcad90d7e
+Source gist: https://github.com/svyatonik/substrate-bridge-sol/blob/6456d3e016c95cd5e6d5e817c23e9e69e739aa78/substrate-bridge.sol
+Compiler flags used (command to produce the file): `docker run -i ethereum/solc:0.6.6 --optimize --bin - < substrate-bridge.sol`
@@ -0,0 +1,166 @@
+name: ethsub-bridge
+version: "0.1.0"
+author: Parity Technologies <admin@parity.io>
+about: Parity Ethereum (PoA) <-> Substrate bridge
+subcommands:
+    - eth-to-sub:
+        about: Synchronize headers from Ethereum node to Substrate node.
+        args:
+            - eth-host: &eth-host
+                long: eth-host
+                value_name: ETH_HOST
+                help: Connect to Ethereum node at given host.
+                takes_value: true
+            - eth-port: &eth-port
+                long: eth-port
+                value_name: ETH_PORT
+                help: Connect to Ethereum node at given port.
+                takes_value: true
+            - sub-host: &sub-host
+                long: sub-host
+                value_name: SUB_HOST
+                help: Connect to Substrate node at given host.
+                takes_value: true
+            - sub-port: &sub-port
+                long: sub-port
+                value_name: SUB_PORT
+                help: Connect to Substrate node websocket server at given port.
+                takes_value: true
+            - sub-tx-mode:
+                long: sub-tx-mode
+                value_name: MODE
+                help: Submit headers using signed (default) or unsigned transactions. Third mode - backup - submits signed transactions only when we believe that sync has stalled.
+                takes_value: true
+                possible_values:
+                    - signed
+                    - unsigned
+                    - backup
+            - sub-signer: &sub-signer
+                long: sub-signer
+                value_name: SUB_SIGNER
+                help: The SURI of secret key to use when transactions are submitted to the Substrate node.
+            - sub-signer-password: &sub-signer-password
+                long: sub-signer-password
+                value_name: SUB_SIGNER_PASSWORD
+                help: The password for the SURI of secret key to use when transactions are submitted to the Substrate node.
+            - sub-pallet-instance: &sub-pallet-instance
+                long: instance
+                short: i
+                value_name: PALLET_INSTANCE
+                help: The instance of the bridge pallet the relay should follow.
+                takes_value: true
+                case_insensitive: true
+                possible_values:
+                  - Rialto
+                  - Kovan
+                default_value: Rialto
+            - no-prometheus: &no-prometheus
+                long: no-prometheus
+                help: Do not expose a Prometheus metric endpoint.
+            - prometheus-host: &prometheus-host
+                long: prometheus-host
+                value_name: PROMETHEUS_HOST
+                help: Expose Prometheus endpoint at given interface.
+            - prometheus-port: &prometheus-port
+                long: prometheus-port
+                value_name: PROMETHEUS_PORT
+                help: Expose Prometheus endpoint at given port.
+    - sub-to-eth:
+        about: Synchronize headers from Substrate node to Ethereum node.
+        args:
+            - eth-host: *eth-host
+            - eth-port: *eth-port
+            - eth-contract:
+                long: eth-contract
+                value_name: ETH_CONTRACT
+                help: Address of deployed bridge contract.
+                takes_value: true
+            - eth-chain-id: &eth-chain-id
+                long: eth-chain-id
+                value_name: ETH_CHAIN_ID
+                help: Chain ID to use for signing.
+            - eth-signer: &eth-signer
+                long: eth-signer
+                value_name: ETH_SIGNER
+                help: Hex-encoded secret to use when transactions are submitted to the Ethereum node.
+            - sub-host: *sub-host
+            - sub-port: *sub-port
+            - no-prometheus: *no-prometheus
+            - prometheus-host: *prometheus-host
+            - prometheus-port: *prometheus-port
+    - eth-deploy-contract:
+        about: Deploy Bridge contract on Ethereum node.
+        args:
+            - eth-host: *eth-host
+            - eth-port: *eth-port
+            - eth-signer: *eth-signer
+            - eth-chain-id: *eth-chain-id
+            - eth-contract-code:
+                long: eth-contract-code
+                value_name: ETH_CONTRACT_CODE
+                help: Bytecode of bridge contract.
+                takes_value: true
+            - sub-host: *sub-host
+            - sub-port: *sub-port
+            - sub-authorities-set-id:
+                long: sub-authorities-set-id
+                value_name: SUB_AUTHORITIES_SET_ID
+                help: ID of initial GRANDPA authorities set.
+                takes_value: true
+            - sub-authorities-set:
+                long: sub-authorities-set
+                value_name: SUB_AUTHORITIES_SET
+                help: Encoded initial GRANDPA authorities set.
+                takes_value: true
+            - sub-initial-header:
+                long: sub-initial-header
+                value_name: SUB_INITIAL_HEADER
+                help: Encoded initial Substrate header.
+                takes_value: true
+    - eth-submit-exchange-tx:
+        about: Submit lock funds transaction to Ethereum node.
+        args:
+            - eth-host: *eth-host
+            - eth-port: *eth-port
+            - eth-nonce:
+                long: eth-nonce
+                value_name: ETH_NONCE
+                help: Nonce that have to be used when building transaction. If not specified, read from PoA node.
+                takes_value: true
+            - eth-signer: *eth-signer
+            - eth-chain-id: *eth-chain-id
+            - eth-amount:
+                long: eth-amount
+                value_name: ETH_AMOUNT
+                help: Amount of ETH to lock (in wei).
+                takes_value: true
+            - sub-recipient:
+                long: sub-recipient
+                value_name: SUB_RECIPIENT
+                help: Hex-encoded Public key of funds recipient in Substrate chain.
+                takes_value: true
+    - eth-exchange-sub:
+        about: Submit proof of PoA lock funds transaction to Substrate node.
+        args:
+            - eth-host: *eth-host
+            - eth-port: *eth-port
+            - eth-start-with-block:
+                long: eth-start-with-block
+                value_name: ETH_START_WITH_BLOCK
+                help: Auto-relay transactions starting with given block number. If not specified, starts with best finalized Ethereum block (known to Substrate node) transactions.
+                takes_value: true
+                conflicts_with:
+                    - eth-tx-hash
+            - eth-tx-hash:
+                long: eth-tx-hash
+                value_name: ETH_TX_HASH
+                help: Hash of the lock funds transaction.
+                takes_value: true
+            - sub-host: *sub-host
+            - sub-port: *sub-port
+            - sub-signer: *sub-signer
+            - sub-signer-password: *sub-signer-password
+            - sub-pallet-instance: *sub-pallet-instance
+            - no-prometheus: *no-prometheus
+            - prometheus-host: *prometheus-host
+            - prometheus-port: *prometheus-port
@@ -0,0 +1,653 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::rpc_errors::RpcError;
+use crate::substrate_sync_loop::QueuedRialtoHeader;
+
+use async_trait::async_trait;
+use bp_eth_poa::signatures::secret_to_address;
+use codec::{Decode, Encode};
+use ethabi::FunctionOutputDecoder;
+use headers_relay::sync_types::SubmittedHeaders;
+use relay_ethereum_client::{
+	sign_and_submit_transaction,
+	types::{Address, CallRequest, HeaderId as EthereumHeaderId, Receipt, H256, U256},
+	Client as EthereumClient, Error as EthereumNodeError, SigningParams as EthereumSigningParams,
+};
+use relay_rialto_client::HeaderId as RialtoHeaderId;
+use relay_utils::{HeaderId, MaybeConnectionError};
+use sp_runtime::Justification;
+use std::collections::HashSet;
+
+// to encode/decode contract calls
+ethabi_contract::use_contract!(bridge_contract, "res/substrate-bridge-abi.json");
+
+type RpcResult<T> = std::result::Result<T, RpcError>;
+
+/// A trait which contains methods that work by using multiple low-level RPCs, or more complicated
+/// interactions involving, for example, an Ethereum contract.
+#[async_trait]
+pub trait EthereumHighLevelRpc {
+	/// Returns best Substrate block that PoA chain knows of.
+	async fn best_substrate_block(&self, contract_address: Address) -> RpcResult<RialtoHeaderId>;
+
+	/// Returns true if Substrate header is known to Ethereum node.
+	async fn substrate_header_known(
+		&self,
+		contract_address: Address,
+		id: RialtoHeaderId,
+	) -> RpcResult<(RialtoHeaderId, bool)>;
+
+	/// Submits Substrate headers to Ethereum contract.
+	async fn submit_substrate_headers(
+		&self,
+		params: EthereumSigningParams,
+		contract_address: Address,
+		headers: Vec<QueuedRialtoHeader>,
+	) -> SubmittedHeaders<RialtoHeaderId, RpcError>;
+
+	/// Returns ids of incomplete Substrate headers.
+	async fn incomplete_substrate_headers(&self, contract_address: Address) -> RpcResult<HashSet<RialtoHeaderId>>;
+
+	/// Complete Substrate header.
+	async fn complete_substrate_header(
+		&self,
+		params: EthereumSigningParams,
+		contract_address: Address,
+		id: RialtoHeaderId,
+		justification: Justification,
+	) -> RpcResult<RialtoHeaderId>;
+
+	/// Submit ethereum transaction.
+	async fn submit_ethereum_transaction(
+		&self,
+		params: &EthereumSigningParams,
+		contract_address: Option<Address>,
+		nonce: Option<U256>,
+		double_gas: bool,
+		encoded_call: Vec<u8>,
+	) -> RpcResult<()>;
+
+	/// Retrieve transactions receipts for given block.
+	async fn transaction_receipts(
+		&self,
+		id: EthereumHeaderId,
+		transactions: Vec<H256>,
+	) -> RpcResult<(EthereumHeaderId, Vec<Receipt>)>;
+}
+
+#[async_trait]
+impl EthereumHighLevelRpc for EthereumClient {
+	async fn best_substrate_block(&self, contract_address: Address) -> RpcResult<RialtoHeaderId> {
+		let (encoded_call, call_decoder) = bridge_contract::functions::best_known_header::call();
+		let call_request = CallRequest {
+			to: Some(contract_address),
+			data: Some(encoded_call.into()),
+			..Default::default()
+		};
+
+		let call_result = self.eth_call(call_request).await?;
+		let (number, raw_hash) = call_decoder.decode(&call_result.0)?;
+		let hash = rialto_runtime::Hash::decode(&mut &raw_hash[..])?;
+
+		if number != number.low_u32().into() {
+			return Err(RpcError::Ethereum(EthereumNodeError::InvalidSubstrateBlockNumber));
+		}
+
+		Ok(HeaderId(number.low_u32(), hash))
+	}
+
+	async fn substrate_header_known(
+		&self,
+		contract_address: Address,
+		id: RialtoHeaderId,
+	) -> RpcResult<(RialtoHeaderId, bool)> {
+		let (encoded_call, call_decoder) = bridge_contract::functions::is_known_header::call(id.1);
+		let call_request = CallRequest {
+			to: Some(contract_address),
+			data: Some(encoded_call.into()),
+			..Default::default()
+		};
+
+		let call_result = self.eth_call(call_request).await?;
+		let is_known_block = call_decoder.decode(&call_result.0)?;
+
+		Ok((id, is_known_block))
+	}
+
+	async fn submit_substrate_headers(
+		&self,
+		params: EthereumSigningParams,
+		contract_address: Address,
+		headers: Vec<QueuedRialtoHeader>,
+	) -> SubmittedHeaders<RialtoHeaderId, RpcError> {
+		// read nonce of signer
+		let address: Address = secret_to_address(&params.signer);
+		let nonce = match self.account_nonce(address).await {
+			Ok(nonce) => nonce,
+			Err(error) => {
+				return SubmittedHeaders {
+					submitted: Vec::new(),
+					incomplete: Vec::new(),
+					rejected: headers.iter().rev().map(|header| header.id()).collect(),
+					fatal_error: Some(error.into()),
+				}
+			}
+		};
+
+		// submit headers. Note that we're cloning self here. It is ok, because
+		// cloning `jsonrpsee::Client` only clones reference to background threads
+		submit_substrate_headers(
+			EthereumHeadersSubmitter {
+				client: self.clone(),
+				params,
+				contract_address,
+				nonce,
+			},
+			headers,
+		)
+		.await
+	}
+
+	async fn incomplete_substrate_headers(&self, contract_address: Address) -> RpcResult<HashSet<RialtoHeaderId>> {
+		let (encoded_call, call_decoder) = bridge_contract::functions::incomplete_headers::call();
+		let call_request = CallRequest {
+			to: Some(contract_address),
+			data: Some(encoded_call.into()),
+			..Default::default()
+		};
+
+		let call_result = self.eth_call(call_request).await?;
+
+		// Q: Is is correct to call these "incomplete_ids"?
+		let (incomplete_headers_numbers, incomplete_headers_hashes) = call_decoder.decode(&call_result.0)?;
+		let incomplete_ids = incomplete_headers_numbers
+			.into_iter()
+			.zip(incomplete_headers_hashes)
+			.filter_map(|(number, hash)| {
+				if number != number.low_u32().into() {
+					return None;
+				}
+
+				Some(HeaderId(number.low_u32(), hash))
+			})
+			.collect();
+
+		Ok(incomplete_ids)
+	}
+
+	async fn complete_substrate_header(
+		&self,
+		params: EthereumSigningParams,
+		contract_address: Address,
+		id: RialtoHeaderId,
+		justification: Justification,
+	) -> RpcResult<RialtoHeaderId> {
+		let _ = self
+			.submit_ethereum_transaction(
+				&params,
+				Some(contract_address),
+				None,
+				false,
+				bridge_contract::functions::import_finality_proof::encode_input(id.0, id.1, justification),
+			)
+			.await?;
+
+		Ok(id)
+	}
+
+	async fn submit_ethereum_transaction(
+		&self,
+		params: &EthereumSigningParams,
+		contract_address: Option<Address>,
+		nonce: Option<U256>,
+		double_gas: bool,
+		encoded_call: Vec<u8>,
+	) -> RpcResult<()> {
+		sign_and_submit_transaction(self, params, contract_address, nonce, double_gas, encoded_call)
+			.await
+			.map_err(Into::into)
+	}
+
+	async fn transaction_receipts(
+		&self,
+		id: EthereumHeaderId,
+		transactions: Vec<H256>,
+	) -> RpcResult<(EthereumHeaderId, Vec<Receipt>)> {
+		let mut transaction_receipts = Vec::with_capacity(transactions.len());
+		for transaction in transactions {
+			let transaction_receipt = self.transaction_receipt(transaction).await?;
+			transaction_receipts.push(transaction_receipt);
+		}
+		Ok((id, transaction_receipts))
+	}
+}
+
+/// Max number of headers which can be sent to Solidity contract.
+pub const HEADERS_BATCH: usize = 4;
+
+/// Substrate headers to send to the Ethereum light client.
+///
+/// The Solidity contract can only accept a fixed number of headers in one go.
+/// This struct is meant to encapsulate this limitation.
+#[derive(Debug)]
+#[cfg_attr(test, derive(Clone))]
+pub struct HeadersBatch {
+	pub header1: QueuedRialtoHeader,
+	pub header2: Option<QueuedRialtoHeader>,
+	pub header3: Option<QueuedRialtoHeader>,
+	pub header4: Option<QueuedRialtoHeader>,
+}
+
+impl HeadersBatch {
+	/// Create new headers from given header & ids collections.
+	///
+	/// This method will pop `HEADERS_BATCH` items from both collections
+	/// and construct `Headers` object and a vector of `RialtoHeaderId`s.
+	pub fn pop_from(
+		headers: &mut Vec<QueuedRialtoHeader>,
+		ids: &mut Vec<RialtoHeaderId>,
+	) -> Result<(Self, Vec<RialtoHeaderId>), ()> {
+		if headers.len() != ids.len() {
+			log::error!(target: "bridge", "Collection size mismatch ({} vs {})", headers.len(), ids.len());
+			return Err(());
+		}
+
+		let header1 = headers.pop().ok_or(())?;
+		let header2 = headers.pop();
+		let header3 = headers.pop();
+		let header4 = headers.pop();
+
+		let mut submitting_ids = Vec::with_capacity(HEADERS_BATCH);
+		for _ in 0..HEADERS_BATCH {
+			submitting_ids.extend(ids.pop().iter());
+		}
+
+		Ok((
+			Self {
+				header1,
+				header2,
+				header3,
+				header4,
+			},
+			submitting_ids,
+		))
+	}
+
+	/// Returns unified array of headers.
+	///
+	/// The first element is always `Some`.
+	fn headers(&self) -> [Option<&QueuedRialtoHeader>; HEADERS_BATCH] {
+		[
+			Some(&self.header1),
+			self.header2.as_ref(),
+			self.header3.as_ref(),
+			self.header4.as_ref(),
+		]
+	}
+
+	/// Encodes all headers. If header is not present an empty vector will be returned.
+	pub fn encode(&self) -> [Vec<u8>; HEADERS_BATCH] {
+		let encode = |h: &QueuedRialtoHeader| h.header().encode();
+		let headers = self.headers();
+		[
+			headers[0].map(encode).unwrap_or_default(),
+			headers[1].map(encode).unwrap_or_default(),
+			headers[2].map(encode).unwrap_or_default(),
+			headers[3].map(encode).unwrap_or_default(),
+		]
+	}
+	/// Returns number of contained headers.
+	pub fn len(&self) -> usize {
+		let is_set = |h: &Option<&QueuedRialtoHeader>| if h.is_some() { 1 } else { 0 };
+		self.headers().iter().map(is_set).sum()
+	}
+
+	/// Remove headers starting from `idx` (0-based) from this collection.
+	///
+	/// The collection will be left with `[0, idx)` headers.
+	/// Returns `Err` when `idx == 0`, since `Headers` must contain at least one header,
+	/// or when `idx > HEADERS_BATCH`.
+	pub fn split_off(&mut self, idx: usize) -> Result<(), ()> {
+		if idx == 0 || idx > HEADERS_BATCH {
+			return Err(());
+		}
+		let mut vals: [_; HEADERS_BATCH] = [&mut None, &mut self.header2, &mut self.header3, &mut self.header4];
+		for val in vals.iter_mut().skip(idx) {
+			**val = None;
+		}
+		Ok(())
+	}
+}
+
+/// Substrate headers submitter API.
+#[async_trait]
+trait HeadersSubmitter {
+	/// Returns Ok(0) if all given not-yet-imported headers are complete.
+	/// Returns Ok(index != 0) where index is 1-based index of first header that is incomplete.
+	///
+	/// Returns Err(()) if contract has rejected headers. This means that the contract is
+	/// unable to import first header (e.g. it may already be imported).
+	async fn is_headers_incomplete(&self, headers: &HeadersBatch) -> RpcResult<usize>;
+
+	/// Submit given headers to Ethereum node.
+	async fn submit_headers(&mut self, headers: HeadersBatch) -> RpcResult<()>;
+}
+
+/// Implementation of Substrate headers submitter that sends headers to running Ethereum node.
+struct EthereumHeadersSubmitter {
+	client: EthereumClient,
+	params: EthereumSigningParams,
+	contract_address: Address,
+	nonce: U256,
+}
+
+#[async_trait]
+impl HeadersSubmitter for EthereumHeadersSubmitter {
+	async fn is_headers_incomplete(&self, headers: &HeadersBatch) -> RpcResult<usize> {
+		let [h1, h2, h3, h4] = headers.encode();
+		let (encoded_call, call_decoder) = bridge_contract::functions::is_incomplete_headers::call(h1, h2, h3, h4);
+		let call_request = CallRequest {
+			to: Some(self.contract_address),
+			data: Some(encoded_call.into()),
+			..Default::default()
+		};
+
+		let call_result = self.client.eth_call(call_request).await?;
+		let incomplete_index: U256 = call_decoder.decode(&call_result.0)?;
+		if incomplete_index > HEADERS_BATCH.into() {
+			return Err(RpcError::Ethereum(EthereumNodeError::InvalidIncompleteIndex));
+		}
+
+		Ok(incomplete_index.low_u32() as _)
+	}
+
+	async fn submit_headers(&mut self, headers: HeadersBatch) -> RpcResult<()> {
+		let [h1, h2, h3, h4] = headers.encode();
+		let result = self
+			.client
+			.submit_ethereum_transaction(
+				&self.params,
+				Some(self.contract_address),
+				Some(self.nonce),
+				false,
+				bridge_contract::functions::import_headers::encode_input(h1, h2, h3, h4),
+			)
+			.await;
+
+		if result.is_ok() {
+			self.nonce += U256::one();
+		}
+
+		result
+	}
+}
+
+/// Submit multiple Substrate headers.
+async fn submit_substrate_headers(
+	mut header_submitter: impl HeadersSubmitter,
+	mut headers: Vec<QueuedRialtoHeader>,
+) -> SubmittedHeaders<RialtoHeaderId, RpcError> {
+	let mut submitted_headers = SubmittedHeaders::default();
+
+	let mut ids = headers.iter().map(|header| header.id()).rev().collect::<Vec<_>>();
+	headers.reverse();
+
+	while !headers.is_empty() {
+		let (headers, submitting_ids) =
+			HeadersBatch::pop_from(&mut headers, &mut ids).expect("Headers and ids are not empty; qed");
+
+		submitted_headers.fatal_error =
+			submit_substrate_headers_batch(&mut header_submitter, &mut submitted_headers, submitting_ids, headers)
+				.await;
+
+		if submitted_headers.fatal_error.is_some() {
+			ids.reverse();
+			submitted_headers.rejected.extend(ids);
+			break;
+		}
+	}
+
+	submitted_headers
+}
+
+/// Submit 4 Substrate headers in single PoA transaction.
+async fn submit_substrate_headers_batch(
+	header_submitter: &mut impl HeadersSubmitter,
+	submitted_headers: &mut SubmittedHeaders<RialtoHeaderId, RpcError>,
+	mut ids: Vec<RialtoHeaderId>,
+	mut headers: HeadersBatch,
+) -> Option<RpcError> {
+	debug_assert_eq!(ids.len(), headers.len(),);
+
+	// if parent of first header is either incomplete, or rejected, we assume that contract
+	// will reject this header as well
+	let parent_id = headers.header1.parent_id();
+	if submitted_headers.rejected.contains(&parent_id) || submitted_headers.incomplete.contains(&parent_id) {
+		submitted_headers.rejected.extend(ids);
+		return None;
+	}
+
+	// check if headers are incomplete
+	let incomplete_header_index = match header_submitter.is_headers_incomplete(&headers).await {
+		// All headers valid
+		Ok(0) => None,
+		Ok(incomplete_header_index) => Some(incomplete_header_index),
+		Err(error) => {
+			// contract has rejected all headers => we do not want to submit it
+			submitted_headers.rejected.extend(ids);
+			if error.is_connection_error() {
+				return Some(error);
+			} else {
+				return None;
+			}
+		}
+	};
+
+	// Modify `ids` and `headers` to only contain values that are going to be accepted.
+	let rejected = if let Some(idx) = incomplete_header_index {
+		let len = std::cmp::min(idx, ids.len());
+		headers
+			.split_off(len)
+			.expect("len > 0, the case where all headers are valid is converted to None; qed");
+		ids.split_off(len)
+	} else {
+		Vec::new()
+	};
+	let submitted = ids;
+	let submit_result = header_submitter.submit_headers(headers).await;
+	match submit_result {
+		Ok(_) => {
+			if incomplete_header_index.is_some() {
+				submitted_headers.incomplete.extend(submitted.iter().last().cloned());
+			}
+			submitted_headers.submitted.extend(submitted);
+			submitted_headers.rejected.extend(rejected);
+			None
+		}
+		Err(error) => {
+			submitted_headers.rejected.extend(submitted);
+			submitted_headers.rejected.extend(rejected);
+			Some(error)
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use sp_runtime::traits::Header;
+
+	struct TestHeadersSubmitter {
+		incomplete: Vec<RialtoHeaderId>,
+		failed: Vec<RialtoHeaderId>,
+	}
+
+	#[async_trait]
+	impl HeadersSubmitter for TestHeadersSubmitter {
+		async fn is_headers_incomplete(&self, headers: &HeadersBatch) -> RpcResult<usize> {
+			if self.incomplete.iter().any(|i| i.0 == headers.header1.id().0) {
+				Ok(1)
+			} else {
+				Ok(0)
+			}
+		}
+
+		async fn submit_headers(&mut self, headers: HeadersBatch) -> RpcResult<()> {
+			if self.failed.iter().any(|i| i.0 == headers.header1.id().0) {
+				Err(RpcError::Ethereum(EthereumNodeError::InvalidSubstrateBlockNumber))
+			} else {
+				Ok(())
+			}
+		}
+	}
+
+	fn header(number: rialto_runtime::BlockNumber) -> QueuedRialtoHeader {
+		QueuedRialtoHeader::new(
+			rialto_runtime::Header::new(
+				number,
+				Default::default(),
+				Default::default(),
+				if number == 0 {
+					Default::default()
+				} else {
+					header(number - 1).id().1
+				},
+				Default::default(),
+			)
+			.into(),
+		)
+	}
+
+	#[test]
+	fn descendants_of_incomplete_headers_are_not_submitted() {
+		let submitted_headers = async_std::task::block_on(submit_substrate_headers(
+			TestHeadersSubmitter {
+				incomplete: vec![header(5).id()],
+				failed: vec![],
+			},
+			vec![header(5), header(6)],
+		));
+		assert_eq!(submitted_headers.submitted, vec![header(5).id()]);
+		assert_eq!(submitted_headers.incomplete, vec![header(5).id()]);
+		assert_eq!(submitted_headers.rejected, vec![header(6).id()]);
+		assert!(submitted_headers.fatal_error.is_none());
+	}
+
+	#[test]
+	fn headers_after_fatal_error_are_not_submitted() {
+		let submitted_headers = async_std::task::block_on(submit_substrate_headers(
+			TestHeadersSubmitter {
+				incomplete: vec![],
+				failed: vec![header(9).id()],
+			},
+			vec![
+				header(5),
+				header(6),
+				header(7),
+				header(8),
+				header(9),
+				header(10),
+				header(11),
+			],
+		));
+		assert_eq!(
+			submitted_headers.submitted,
+			vec![header(5).id(), header(6).id(), header(7).id(), header(8).id()]
+		);
+		assert_eq!(submitted_headers.incomplete, vec![]);
+		assert_eq!(
+			submitted_headers.rejected,
+			vec![header(9).id(), header(10).id(), header(11).id(),]
+		);
+		assert!(submitted_headers.fatal_error.is_some());
+	}
+
+	fn headers_batch() -> HeadersBatch {
+		let mut init_headers = vec![header(1), header(2), header(3), header(4), header(5)];
+		init_headers.reverse();
+		let mut init_ids = init_headers.iter().map(|h| h.id()).collect();
+		let (headers, ids) = HeadersBatch::pop_from(&mut init_headers, &mut init_ids).unwrap();
+		assert_eq!(init_headers, vec![header(5)]);
+		assert_eq!(init_ids, vec![header(5).id()]);
+		assert_eq!(
+			ids,
+			vec![header(1).id(), header(2).id(), header(3).id(), header(4).id()]
+		);
+		headers
+	}
+
+	#[test]
+	fn headers_batch_len() {
+		let headers = headers_batch();
+		assert_eq!(headers.len(), 4);
+	}
+
+	#[test]
+	fn headers_batch_encode() {
+		let headers = headers_batch();
+		assert_eq!(
+			headers.encode(),
+			[
+				header(1).header().encode(),
+				header(2).header().encode(),
+				header(3).header().encode(),
+				header(4).header().encode(),
+			]
+		);
+	}
+
+	#[test]
+	fn headers_batch_split_off() {
+		// given
+		let mut headers = headers_batch();
+
+		// when
+		assert!(headers.split_off(0).is_err());
+		assert_eq!(headers.header1, header(1));
+		assert!(headers.header2.is_some());
+		assert!(headers.header3.is_some());
+		assert!(headers.header4.is_some());
+
+		// when
+		let mut h = headers.clone();
+		h.split_off(1).unwrap();
+		assert!(h.header2.is_none());
+		assert!(h.header3.is_none());
+		assert!(h.header4.is_none());
+
+		// when
+		let mut h = headers.clone();
+		h.split_off(2).unwrap();
+		assert!(h.header2.is_some());
+		assert!(h.header3.is_none());
+		assert!(h.header4.is_none());
+
+		// when
+		let mut h = headers.clone();
+		h.split_off(3).unwrap();
+		assert!(h.header2.is_some());
+		assert!(h.header3.is_some());
+		assert!(h.header4.is_none());
+
+		// when
+		let mut h = headers;
+		h.split_off(4).unwrap();
+		assert!(h.header2.is_some());
+		assert!(h.header3.is_some());
+		assert!(h.header4.is_some());
+	}
+}
@@ -0,0 +1,156 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::ethereum_client::{bridge_contract, EthereumHighLevelRpc};
+use crate::rpc_errors::RpcError;
+
+use codec::{Decode, Encode};
+use num_traits::Zero;
+use relay_ethereum_client::{
+	Client as EthereumClient, ConnectionParams as EthereumConnectionParams, SigningParams as EthereumSigningParams,
+};
+use relay_rialto_client::{HeaderId as RialtoHeaderId, Rialto};
+use relay_substrate_client::{
+	Client as SubstrateClient, ConnectionParams as SubstrateConnectionParams, OpaqueGrandpaAuthoritiesSet,
+};
+use relay_utils::HeaderId;
+
+/// Ethereum synchronization parameters.
+#[derive(Debug)]
+pub struct EthereumDeployContractParams {
+	/// Ethereum connection params.
+	pub eth_params: EthereumConnectionParams,
+	/// Ethereum signing params.
+	pub eth_sign: EthereumSigningParams,
+	/// Ethereum contract bytecode.
+	pub eth_contract_code: Vec<u8>,
+	/// Substrate connection params.
+	pub sub_params: SubstrateConnectionParams,
+	/// Initial authorities set id.
+	pub sub_initial_authorities_set_id: Option<u64>,
+	/// Initial authorities set.
+	pub sub_initial_authorities_set: Option<Vec<u8>>,
+	/// Initial header.
+	pub sub_initial_header: Option<Vec<u8>>,
+}
+
+/// Deploy Bridge contract on Ethereum chain.
+pub fn run(params: EthereumDeployContractParams) {
+	let mut local_pool = futures::executor::LocalPool::new();
+
+	let EthereumDeployContractParams {
+		eth_params,
+		eth_sign,
+		sub_params,
+		sub_initial_authorities_set_id,
+		sub_initial_authorities_set,
+		sub_initial_header,
+		eth_contract_code,
+	} = params;
+
+	let result = local_pool.run_until(async move {
+		let eth_client = EthereumClient::new(eth_params);
+		let sub_client = SubstrateClient::<Rialto>::new(sub_params).await.map_err(RpcError::Substrate)?;
+
+		let (initial_header_id, initial_header) = prepare_initial_header(&sub_client, sub_initial_header).await?;
+		let initial_set_id = sub_initial_authorities_set_id.unwrap_or(0);
+		let initial_set = prepare_initial_authorities_set(
+			&sub_client,
+			initial_header_id.1,
+			sub_initial_authorities_set,
+		).await?;
+
+		log::info!(
+			target: "bridge",
+			"Deploying Ethereum contract.\r\n\tInitial header: {:?}\r\n\tInitial header id: {:?}\r\n\tInitial header encoded: {}\r\n\tInitial authorities set ID: {}\r\n\tInitial authorities set: {}",
+			initial_header,
+			initial_header_id,
+			hex::encode(&initial_header),
+			initial_set_id,
+			hex::encode(&initial_set),
+		);
+
+		deploy_bridge_contract(
+			&eth_client,
+			&eth_sign,
+			eth_contract_code,
+			initial_header,
+			initial_set_id,
+			initial_set,
+		).await
+	});
+
+	if let Err(error) = result {
+		log::error!(target: "bridge", "{}", error);
+	}
+}
+
+/// Prepare initial header.
+async fn prepare_initial_header(
+	sub_client: &SubstrateClient<Rialto>,
+	sub_initial_header: Option<Vec<u8>>,
+) -> Result<(RialtoHeaderId, Vec<u8>), String> {
+	match sub_initial_header {
+		Some(raw_initial_header) => match rialto_runtime::Header::decode(&mut &raw_initial_header[..]) {
+			Ok(initial_header) => Ok((
+				HeaderId(initial_header.number, initial_header.hash()),
+				raw_initial_header,
+			)),
+			Err(error) => Err(format!("Error decoding initial header: {}", error)),
+		},
+		None => {
+			let initial_header = sub_client.header_by_number(Zero::zero()).await;
+			initial_header
+				.map(|header| (HeaderId(Zero::zero(), header.hash()), header.encode()))
+				.map_err(|error| format!("Error reading Substrate genesis header: {:?}", error))
+		}
+	}
+}
+
+/// Prepare initial GRANDPA authorities set.
+async fn prepare_initial_authorities_set(
+	sub_client: &SubstrateClient<Rialto>,
+	sub_initial_header_hash: rialto_runtime::Hash,
+	sub_initial_authorities_set: Option<Vec<u8>>,
+) -> Result<OpaqueGrandpaAuthoritiesSet, String> {
+	let initial_authorities_set = match sub_initial_authorities_set {
+		Some(initial_authorities_set) => Ok(initial_authorities_set),
+		None => sub_client.grandpa_authorities_set(sub_initial_header_hash).await,
+	};
+
+	initial_authorities_set.map_err(|error| format!("Error reading GRANDPA authorities set: {:?}", error))
+}
+
+/// Deploy bridge contract to Ethereum chain.
+async fn deploy_bridge_contract(
+	eth_client: &EthereumClient,
+	params: &EthereumSigningParams,
+	contract_code: Vec<u8>,
+	initial_header: Vec<u8>,
+	initial_set_id: u64,
+	initial_authorities: Vec<u8>,
+) -> Result<(), String> {
+	eth_client
+		.submit_ethereum_transaction(
+			params,
+			None,
+			None,
+			false,
+			bridge_contract::constructor(contract_code, initial_header, initial_set_id, initial_authorities),
+		)
+		.await
+		.map_err(|error| format!("Error deploying contract: {:?}", error))
+}
@@ -0,0 +1,394 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Relaying proofs of PoA -> Substrate exchange transactions.
+
+use crate::instances::BridgeInstance;
+use crate::rialto_client::{SubmitEthereumExchangeTransactionProof, SubstrateHighLevelRpc};
+use crate::rpc_errors::RpcError;
+use crate::substrate_types::into_substrate_ethereum_receipt;
+
+use async_trait::async_trait;
+use bp_currency_exchange::MaybeLockFundsTransaction;
+use exchange_relay::exchange::{
+	relay_single_transaction_proof, SourceBlock, SourceClient, SourceTransaction, TargetClient,
+	TransactionProofPipeline,
+};
+use exchange_relay::exchange_loop::{run as run_loop, InMemoryStorage};
+use relay_ethereum_client::{
+	types::{
+		HeaderId as EthereumHeaderId, HeaderWithTransactions as EthereumHeaderWithTransactions,
+		Transaction as EthereumTransaction, TransactionHash as EthereumTransactionHash, H256, HEADER_ID_PROOF,
+	},
+	Client as EthereumClient, ConnectionParams as EthereumConnectionParams,
+};
+use relay_rialto_client::{Rialto, SigningParams as RialtoSigningParams};
+use relay_substrate_client::{
+	Chain as SubstrateChain, Client as SubstrateClient, ConnectionParams as SubstrateConnectionParams,
+};
+use relay_utils::{metrics::MetricsParams, relay_loop::Client as RelayClient, HeaderId};
+use rialto_runtime::exchange::EthereumTransactionInclusionProof;
+use std::{sync::Arc, time::Duration};
+
+/// Interval at which we ask Ethereum node for updates.
+const ETHEREUM_TICK_INTERVAL: Duration = Duration::from_secs(10);
+
+/// Exchange relay mode.
+#[derive(Debug)]
+pub enum ExchangeRelayMode {
+	/// Relay single transaction and quit.
+	Single(EthereumTransactionHash),
+	/// Auto-relay transactions starting with given block.
+	Auto(Option<u64>),
+}
+
+/// PoA exchange transaction relay params.
+#[derive(Debug)]
+pub struct EthereumExchangeParams {
+	/// Ethereum connection params.
+	pub eth_params: EthereumConnectionParams,
+	/// Substrate connection params.
+	pub sub_params: SubstrateConnectionParams,
+	/// Substrate signing params.
+	pub sub_sign: RialtoSigningParams,
+	/// Relay working mode.
+	pub mode: ExchangeRelayMode,
+	/// Metrics parameters.
+	pub metrics_params: Option<MetricsParams>,
+	/// Instance of the bridge pallet being synchronized.
+	pub instance: Arc<dyn BridgeInstance>,
+}
+
+/// Ethereum to Substrate exchange pipeline.
+struct EthereumToSubstrateExchange;
+
+impl TransactionProofPipeline for EthereumToSubstrateExchange {
+	const SOURCE_NAME: &'static str = "Ethereum";
+	const TARGET_NAME: &'static str = "Substrate";
+
+	type Block = EthereumSourceBlock;
+	type TransactionProof = EthereumTransactionInclusionProof;
+}
+
+/// Ethereum source block.
+struct EthereumSourceBlock(EthereumHeaderWithTransactions);
+
+impl SourceBlock for EthereumSourceBlock {
+	type Hash = H256;
+	type Number = u64;
+	type Transaction = EthereumSourceTransaction;
+
+	fn id(&self) -> EthereumHeaderId {
+		HeaderId(
+			self.0.number.expect(HEADER_ID_PROOF).as_u64(),
+			self.0.hash.expect(HEADER_ID_PROOF),
+		)
+	}
+
+	fn transactions(&self) -> Vec<Self::Transaction> {
+		self.0
+			.transactions
+			.iter()
+			.cloned()
+			.map(EthereumSourceTransaction)
+			.collect()
+	}
+}
+
+/// Ethereum source transaction.
+struct EthereumSourceTransaction(EthereumTransaction);
+
+impl SourceTransaction for EthereumSourceTransaction {
+	type Hash = EthereumTransactionHash;
+
+	fn hash(&self) -> Self::Hash {
+		self.0.hash
+	}
+}
+
+/// Ethereum node as transactions proof source.
+#[derive(Clone)]
+struct EthereumTransactionsSource {
+	client: EthereumClient,
+}
+
+#[async_trait]
+impl RelayClient for EthereumTransactionsSource {
+	type Error = RpcError;
+
+	async fn reconnect(&mut self) -> Result<(), RpcError> {
+		self.client.reconnect();
+		Ok(())
+	}
+}
+
+#[async_trait]
+impl SourceClient<EthereumToSubstrateExchange> for EthereumTransactionsSource {
+	async fn tick(&self) {
+		async_std::task::sleep(ETHEREUM_TICK_INTERVAL).await;
+	}
+
+	async fn block_by_hash(&self, hash: H256) -> Result<EthereumSourceBlock, RpcError> {
+		self.client
+			.header_by_hash_with_transactions(hash)
+			.await
+			.map(EthereumSourceBlock)
+			.map_err(Into::into)
+	}
+
+	async fn block_by_number(&self, number: u64) -> Result<EthereumSourceBlock, RpcError> {
+		self.client
+			.header_by_number_with_transactions(number)
+			.await
+			.map(EthereumSourceBlock)
+			.map_err(Into::into)
+	}
+
+	async fn transaction_block(
+		&self,
+		hash: &EthereumTransactionHash,
+	) -> Result<Option<(EthereumHeaderId, usize)>, RpcError> {
+		let eth_tx = match self.client.transaction_by_hash(*hash).await? {
+			Some(eth_tx) => eth_tx,
+			None => return Ok(None),
+		};
+
+		// we need transaction to be mined => check if it is included in the block
+		let (eth_header_id, eth_tx_index) = match (eth_tx.block_number, eth_tx.block_hash, eth_tx.transaction_index) {
+			(Some(block_number), Some(block_hash), Some(transaction_index)) => (
+				HeaderId(block_number.as_u64(), block_hash),
+				transaction_index.as_u64() as _,
+			),
+			_ => return Ok(None),
+		};
+
+		Ok(Some((eth_header_id, eth_tx_index)))
+	}
+
+	async fn transaction_proof(
+		&self,
+		block: &EthereumSourceBlock,
+		tx_index: usize,
+	) -> Result<EthereumTransactionInclusionProof, RpcError> {
+		const TRANSACTION_HAS_RAW_FIELD_PROOF: &str = "RPC level checks that transactions from Ethereum\
+			node are having `raw` field; qed";
+		const BLOCK_HAS_HASH_FIELD_PROOF: &str = "RPC level checks that block has `hash` field; qed";
+
+		let mut transaction_proof = Vec::with_capacity(block.0.transactions.len());
+		for tx in &block.0.transactions {
+			let raw_tx_receipt = self
+				.client
+				.transaction_receipt(tx.hash)
+				.await
+				.map(|receipt| into_substrate_ethereum_receipt(&receipt))
+				.map(|receipt| receipt.rlp())?;
+			let raw_tx = tx.raw.clone().expect(TRANSACTION_HAS_RAW_FIELD_PROOF).0;
+			transaction_proof.push((raw_tx, raw_tx_receipt));
+		}
+
+		Ok(EthereumTransactionInclusionProof {
+			block: block.0.hash.expect(BLOCK_HAS_HASH_FIELD_PROOF),
+			index: tx_index as _,
+			proof: transaction_proof,
+		})
+	}
+}
+
+/// Substrate node as transactions proof target.
+#[derive(Clone)]
+struct SubstrateTransactionsTarget {
+	client: SubstrateClient<Rialto>,
+	sign_params: RialtoSigningParams,
+	bridge_instance: Arc<dyn BridgeInstance>,
+}
+
+#[async_trait]
+impl RelayClient for SubstrateTransactionsTarget {
+	type Error = RpcError;
+
+	async fn reconnect(&mut self) -> Result<(), RpcError> {
+		Ok(self.client.reconnect().await?)
+	}
+}
+
+#[async_trait]
+impl TargetClient<EthereumToSubstrateExchange> for SubstrateTransactionsTarget {
+	async fn tick(&self) {
+		async_std::task::sleep(Rialto::AVERAGE_BLOCK_INTERVAL).await;
+	}
+
+	async fn is_header_known(&self, id: &EthereumHeaderId) -> Result<bool, RpcError> {
+		self.client.ethereum_header_known(*id).await
+	}
+
+	async fn is_header_finalized(&self, id: &EthereumHeaderId) -> Result<bool, RpcError> {
+		// we check if header is finalized by simple comparison of the header number and
+		// number of best finalized PoA header known to Substrate node.
+		//
+		// this may lead to failure in tx proof import if PoA reorganization has happened
+		// after we have checked that our tx has been included into given block
+		//
+		// the fix is easy, but since this code is mostly developed for demonstration purposes,
+		// I'm leaving this KISS-based design here
+		let best_finalized_ethereum_block = self.client.best_ethereum_finalized_block().await?;
+		Ok(id.0 <= best_finalized_ethereum_block.0)
+	}
+
+	async fn best_finalized_header_id(&self) -> Result<EthereumHeaderId, RpcError> {
+		// we can't continue to relay exchange proofs if Substrate node is out of sync, because
+		// it may have already received (some of) proofs that we're going to relay
+		self.client.ensure_synced().await?;
+
+		self.client.best_ethereum_finalized_block().await
+	}
+
+	async fn filter_transaction_proof(&self, proof: &EthereumTransactionInclusionProof) -> Result<bool, RpcError> {
+		// let's try to parse transaction locally
+		let (raw_tx, raw_tx_receipt) = &proof.proof[proof.index as usize];
+		let parse_result = rialto_runtime::exchange::EthTransaction::parse(raw_tx);
+		if parse_result.is_err() {
+			return Ok(false);
+		}
+
+		// now let's check if transaction is successful
+		match bp_eth_poa::Receipt::is_successful_raw_receipt(raw_tx_receipt) {
+			Ok(true) => (),
+			_ => return Ok(false),
+		}
+
+		// seems that transaction is relayable - let's check if runtime is able to import it
+		// (we can't if e.g. header is pruned or there's some issue with tx data)
+		self.client.verify_exchange_transaction_proof(proof.clone()).await
+	}
+
+	async fn submit_transaction_proof(&self, proof: EthereumTransactionInclusionProof) -> Result<(), RpcError> {
+		let (sign_params, bridge_instance) = (self.sign_params.clone(), self.bridge_instance.clone());
+		self.client
+			.submit_exchange_transaction_proof(sign_params, bridge_instance, proof)
+			.await
+	}
+}
+
+/// Relay exchange transaction proof(s) to Substrate node.
+pub fn run(params: EthereumExchangeParams) {
+	match params.mode {
+		ExchangeRelayMode::Single(eth_tx_hash) => run_single_transaction_relay(params, eth_tx_hash),
+		ExchangeRelayMode::Auto(eth_start_with_block_number) => {
+			run_auto_transactions_relay_loop(params, eth_start_with_block_number)
+		}
+	};
+}
+
+/// Run single transaction proof relay and stop.
+fn run_single_transaction_relay(params: EthereumExchangeParams, eth_tx_hash: H256) {
+	let mut local_pool = futures::executor::LocalPool::new();
+
+	let EthereumExchangeParams {
+		eth_params,
+		sub_params,
+		sub_sign,
+		instance,
+		..
+	} = params;
+
+	let result = local_pool.run_until(async move {
+		let eth_client = EthereumClient::new(eth_params);
+		let sub_client = SubstrateClient::<Rialto>::new(sub_params)
+			.await
+			.map_err(RpcError::Substrate)?;
+
+		let source = EthereumTransactionsSource { client: eth_client };
+		let target = SubstrateTransactionsTarget {
+			client: sub_client,
+			sign_params: sub_sign,
+			bridge_instance: instance,
+		};
+
+		relay_single_transaction_proof(&source, &target, eth_tx_hash).await
+	});
+
+	match result {
+		Ok(_) => {
+			log::info!(
+				target: "bridge",
+				"Ethereum transaction {} proof has been successfully submitted to Substrate node",
+				eth_tx_hash,
+			);
+		}
+		Err(err) => {
+			log::error!(
+				target: "bridge",
+				"Error submitting Ethereum transaction {} proof to Substrate node: {}",
+				eth_tx_hash,
+				err,
+			);
+		}
+	}
+}
+
+/// Run auto-relay loop.
+fn run_auto_transactions_relay_loop(params: EthereumExchangeParams, eth_start_with_block_number: Option<u64>) {
+	let EthereumExchangeParams {
+		eth_params,
+		sub_params,
+		sub_sign,
+		metrics_params,
+		instance,
+		..
+	} = params;
+
+	let do_run_loop = move || -> Result<(), String> {
+		let eth_client = EthereumClient::new(eth_params);
+		let sub_client = async_std::task::block_on(SubstrateClient::<Rialto>::new(sub_params))
+			.map_err(|err| format!("Error starting Substrate client: {:?}", err))?;
+
+		let eth_start_with_block_number = match eth_start_with_block_number {
+			Some(eth_start_with_block_number) => eth_start_with_block_number,
+			None => {
+				async_std::task::block_on(sub_client.best_ethereum_finalized_block())
+					.map_err(|err| {
+						format!(
+							"Error retrieving best finalized Ethereum block from Substrate node: {:?}",
+							err
+						)
+					})?
+					.0
+			}
+		};
+
+		run_loop(
+			InMemoryStorage::new(eth_start_with_block_number),
+			EthereumTransactionsSource { client: eth_client },
+			SubstrateTransactionsTarget {
+				client: sub_client,
+				sign_params: sub_sign,
+				bridge_instance: instance,
+			},
+			metrics_params,
+			futures::future::pending(),
+		);
+
+		Ok(())
+	};
+
+	if let Err(err) = do_run_loop() {
+		log::error!(
+			target: "bridge",
+			"Error auto-relaying Ethereum transactions proofs to Substrate node: {}",
+			err,
+		);
+	}
+}
@@ -0,0 +1,113 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Submitting Ethereum -> Substrate exchange transactions.
+
+use bp_eth_poa::{
+	signatures::{secret_to_address, SignTransaction},
+	UnsignedTransaction,
+};
+use relay_ethereum_client::{
+	types::{CallRequest, U256},
+	Client as EthereumClient, ConnectionParams as EthereumConnectionParams, SigningParams as EthereumSigningParams,
+};
+use rialto_runtime::exchange::LOCK_FUNDS_ADDRESS;
+
+/// Ethereum exchange transaction params.
+#[derive(Debug)]
+pub struct EthereumExchangeSubmitParams {
+	/// Ethereum connection params.
+	pub eth_params: EthereumConnectionParams,
+	/// Ethereum signing params.
+	pub eth_sign: EthereumSigningParams,
+	/// Ethereum signer nonce.
+	pub eth_nonce: Option<U256>,
+	/// Amount of Ethereum tokens to lock.
+	pub eth_amount: U256,
+	/// Funds recipient on Substrate side.
+	pub sub_recipient: [u8; 32],
+}
+
+/// Submit single Ethereum -> Substrate exchange transaction.
+pub fn run(params: EthereumExchangeSubmitParams) {
+	let mut local_pool = futures::executor::LocalPool::new();
+
+	let EthereumExchangeSubmitParams {
+		eth_params,
+		eth_sign,
+		eth_nonce,
+		eth_amount,
+		sub_recipient,
+	} = params;
+
+	let result: Result<_, String> = local_pool.run_until(async move {
+		let eth_client = EthereumClient::new(eth_params);
+
+		let eth_signer_address = secret_to_address(&eth_sign.signer);
+		let sub_recipient_encoded = sub_recipient;
+		let nonce = match eth_nonce {
+			Some(eth_nonce) => eth_nonce,
+			None => eth_client
+				.account_nonce(eth_signer_address)
+				.await
+				.map_err(|err| format!("error fetching acount nonce: {:?}", err))?,
+		};
+		let gas = eth_client
+			.estimate_gas(CallRequest {
+				from: Some(eth_signer_address),
+				to: Some(LOCK_FUNDS_ADDRESS.into()),
+				value: Some(eth_amount),
+				data: Some(sub_recipient_encoded.to_vec().into()),
+				..Default::default()
+			})
+			.await
+			.map_err(|err| format!("error estimating gas requirements: {:?}", err))?;
+		let eth_tx_unsigned = UnsignedTransaction {
+			nonce,
+			gas_price: eth_sign.gas_price,
+			gas,
+			to: Some(LOCK_FUNDS_ADDRESS.into()),
+			value: eth_amount,
+			payload: sub_recipient_encoded.to_vec(),
+		};
+		let eth_tx_signed = eth_tx_unsigned
+			.clone()
+			.sign_by(&eth_sign.signer, Some(eth_sign.chain_id));
+		eth_client
+			.submit_transaction(eth_tx_signed)
+			.await
+			.map_err(|err| format!("error submitting transaction: {:?}", err))?;
+
+		Ok(eth_tx_unsigned)
+	});
+
+	match result {
+		Ok(eth_tx_unsigned) => {
+			log::info!(
+				target: "bridge",
+				"Exchange transaction has been submitted to Ethereum node: {:?}",
+				eth_tx_unsigned,
+			);
+		}
+		Err(err) => {
+			log::error!(
+				target: "bridge",
+				"Error submitting exchange transaction to Ethereum node: {}",
+				err,
+			);
+		}
+	}
+}
@@ -0,0 +1,285 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Ethereum PoA -> Rialto-Substrate synchronization.
+
+use crate::ethereum_client::EthereumHighLevelRpc;
+use crate::instances::BridgeInstance;
+use crate::rialto_client::{SubmitEthereumHeaders, SubstrateHighLevelRpc};
+use crate::rpc_errors::RpcError;
+use crate::substrate_types::{into_substrate_ethereum_header, into_substrate_ethereum_receipts};
+
+use async_trait::async_trait;
+use codec::Encode;
+use headers_relay::{
+	sync::{HeadersSyncParams, TargetTransactionMode},
+	sync_loop::{SourceClient, TargetClient},
+	sync_types::{HeadersSyncPipeline, QueuedHeader, SourceHeader, SubmittedHeaders},
+};
+use relay_ethereum_client::{
+	types::{HeaderHash, HeaderId as EthereumHeaderId, Receipt, SyncHeader as Header},
+	Client as EthereumClient, ConnectionParams as EthereumConnectionParams,
+};
+use relay_rialto_client::{Rialto, SigningParams as RialtoSigningParams};
+use relay_substrate_client::{
+	Chain as SubstrateChain, Client as SubstrateClient, ConnectionParams as SubstrateConnectionParams,
+};
+use relay_utils::{metrics::MetricsParams, relay_loop::Client as RelayClient};
+
+use std::fmt::Debug;
+use std::{collections::HashSet, sync::Arc, time::Duration};
+
+pub mod consts {
+	use super::*;
+
+	/// Interval at which we check new Ethereum headers when we are synced/almost synced.
+	pub const ETHEREUM_TICK_INTERVAL: Duration = Duration::from_secs(10);
+	/// Max number of headers in single submit transaction.
+	pub const MAX_HEADERS_IN_SINGLE_SUBMIT: usize = 32;
+	/// Max total size of headers in single submit transaction. This only affects signed
+	/// submissions, when several headers are submitted at once. 4096 is the maximal **expected**
+	/// size of the Ethereum header + transactions receipts (if they're required).
+	pub const MAX_HEADERS_SIZE_IN_SINGLE_SUBMIT: usize = MAX_HEADERS_IN_SINGLE_SUBMIT * 4096;
+	/// Max Ethereum headers we want to have in all 'before-submitted' states.
+	pub const MAX_FUTURE_HEADERS_TO_DOWNLOAD: usize = 128;
+	/// Max Ethereum headers count we want to have in 'submitted' state.
+	pub const MAX_SUBMITTED_HEADERS: usize = 128;
+	/// Max depth of in-memory headers in all states. Past this depth they will be forgotten (pruned).
+	pub const PRUNE_DEPTH: u32 = 4096;
+}
+
+/// Ethereum synchronization parameters.
+#[derive(Debug)]
+pub struct EthereumSyncParams {
+	/// Ethereum connection params.
+	pub eth_params: EthereumConnectionParams,
+	/// Substrate connection params.
+	pub sub_params: SubstrateConnectionParams,
+	/// Substrate signing params.
+	pub sub_sign: RialtoSigningParams,
+	/// Synchronization parameters.
+	pub sync_params: HeadersSyncParams,
+	/// Metrics parameters.
+	pub metrics_params: Option<MetricsParams>,
+	/// Instance of the bridge pallet being synchronized.
+	pub instance: Arc<dyn BridgeInstance>,
+}
+
+/// Ethereum synchronization pipeline.
+#[derive(Clone, Copy, Debug)]
+#[cfg_attr(test, derive(PartialEq))]
+pub struct EthereumHeadersSyncPipeline;
+
+impl HeadersSyncPipeline for EthereumHeadersSyncPipeline {
+	const SOURCE_NAME: &'static str = "Ethereum";
+	const TARGET_NAME: &'static str = "Substrate";
+
+	type Hash = HeaderHash;
+	type Number = u64;
+	type Header = Header;
+	type Extra = Vec<Receipt>;
+	type Completion = ();
+
+	fn estimate_size(source: &QueuedHeader<Self>) -> usize {
+		into_substrate_ethereum_header(source.header()).encode().len()
+			+ into_substrate_ethereum_receipts(source.extra())
+				.map(|extra| extra.encode().len())
+				.unwrap_or(0)
+	}
+}
+
+/// Queued ethereum header ID.
+pub type QueuedEthereumHeader = QueuedHeader<EthereumHeadersSyncPipeline>;
+
+/// Ethereum client as headers source.
+#[derive(Clone)]
+struct EthereumHeadersSource {
+	/// Ethereum node client.
+	client: EthereumClient,
+}
+
+impl EthereumHeadersSource {
+	fn new(client: EthereumClient) -> Self {
+		Self { client }
+	}
+}
+
+#[async_trait]
+impl RelayClient for EthereumHeadersSource {
+	type Error = RpcError;
+
+	async fn reconnect(&mut self) -> Result<(), RpcError> {
+		self.client.reconnect();
+		Ok(())
+	}
+}
+
+#[async_trait]
+impl SourceClient<EthereumHeadersSyncPipeline> for EthereumHeadersSource {
+	async fn best_block_number(&self) -> Result<u64, RpcError> {
+		// we **CAN** continue to relay headers if Ethereum node is out of sync, because
+		// Substrate node may be missing headers that are already available at the Ethereum
+
+		self.client.best_block_number().await.map_err(Into::into)
+	}
+
+	async fn header_by_hash(&self, hash: HeaderHash) -> Result<Header, RpcError> {
+		self.client
+			.header_by_hash(hash)
+			.await
+			.map(Into::into)
+			.map_err(Into::into)
+	}
+
+	async fn header_by_number(&self, number: u64) -> Result<Header, RpcError> {
+		self.client
+			.header_by_number(number)
+			.await
+			.map(Into::into)
+			.map_err(Into::into)
+	}
+
+	async fn header_completion(&self, id: EthereumHeaderId) -> Result<(EthereumHeaderId, Option<()>), RpcError> {
+		Ok((id, None))
+	}
+
+	async fn header_extra(
+		&self,
+		id: EthereumHeaderId,
+		header: QueuedEthereumHeader,
+	) -> Result<(EthereumHeaderId, Vec<Receipt>), RpcError> {
+		self.client
+			.transaction_receipts(id, header.header().transactions.clone())
+			.await
+	}
+}
+
+#[derive(Clone)]
+struct SubstrateHeadersTarget {
+	/// Substrate node client.
+	client: SubstrateClient<Rialto>,
+	/// Whether we want to submit signed (true), or unsigned (false) transactions.
+	sign_transactions: bool,
+	/// Substrate signing params.
+	sign_params: RialtoSigningParams,
+	/// Bridge instance used in Ethereum to Substrate sync.
+	bridge_instance: Arc<dyn BridgeInstance>,
+}
+
+impl SubstrateHeadersTarget {
+	fn new(
+		client: SubstrateClient<Rialto>,
+		sign_transactions: bool,
+		sign_params: RialtoSigningParams,
+		bridge_instance: Arc<dyn BridgeInstance>,
+	) -> Self {
+		Self {
+			client,
+			sign_transactions,
+			sign_params,
+			bridge_instance,
+		}
+	}
+}
+
+#[async_trait]
+impl RelayClient for SubstrateHeadersTarget {
+	type Error = RpcError;
+
+	async fn reconnect(&mut self) -> Result<(), RpcError> {
+		Ok(self.client.reconnect().await?)
+	}
+}
+
+#[async_trait]
+impl TargetClient<EthereumHeadersSyncPipeline> for SubstrateHeadersTarget {
+	async fn best_header_id(&self) -> Result<EthereumHeaderId, RpcError> {
+		// we can't continue to relay headers if Substrate node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
+		self.client.best_ethereum_block().await
+	}
+
+	async fn is_known_header(&self, id: EthereumHeaderId) -> Result<(EthereumHeaderId, bool), RpcError> {
+		Ok((id, self.client.ethereum_header_known(id).await?))
+	}
+
+	async fn submit_headers(&self, headers: Vec<QueuedEthereumHeader>) -> SubmittedHeaders<EthereumHeaderId, RpcError> {
+		let (sign_params, bridge_instance, sign_transactions) = (
+			self.sign_params.clone(),
+			self.bridge_instance.clone(),
+			self.sign_transactions,
+		);
+		self.client
+			.submit_ethereum_headers(sign_params, bridge_instance, headers, sign_transactions)
+			.await
+	}
+
+	async fn incomplete_headers_ids(&self) -> Result<HashSet<EthereumHeaderId>, RpcError> {
+		Ok(HashSet::new())
+	}
+
+	#[allow(clippy::unit_arg)]
+	async fn complete_header(&self, id: EthereumHeaderId, _completion: ()) -> Result<EthereumHeaderId, RpcError> {
+		Ok(id)
+	}
+
+	async fn requires_extra(&self, header: QueuedEthereumHeader) -> Result<(EthereumHeaderId, bool), RpcError> {
+		// we can minimize number of receipts_check calls by checking header
+		// logs bloom here, but it may give us false positives (when authorities
+		// source is contract, we never need any logs)
+		let id = header.header().id();
+		let sub_eth_header = into_substrate_ethereum_header(header.header());
+		Ok((id, self.client.ethereum_receipts_required(sub_eth_header).await?))
+	}
+}
+
+/// Run Ethereum headers synchronization.
+pub fn run(params: EthereumSyncParams) -> Result<(), RpcError> {
+	let EthereumSyncParams {
+		eth_params,
+		sub_params,
+		sub_sign,
+		sync_params,
+		metrics_params,
+		instance,
+	} = params;
+
+	let eth_client = EthereumClient::new(eth_params);
+	let sub_client = async_std::task::block_on(async { SubstrateClient::<Rialto>::new(sub_params).await })?;
+
+	let sign_sub_transactions = match sync_params.target_tx_mode {
+		TargetTransactionMode::Signed | TargetTransactionMode::Backup => true,
+		TargetTransactionMode::Unsigned => false,
+	};
+
+	let source = EthereumHeadersSource::new(eth_client);
+	let target = SubstrateHeadersTarget::new(sub_client, sign_sub_transactions, sub_sign, instance);
+
+	headers_relay::sync_loop::run(
+		source,
+		consts::ETHEREUM_TICK_INTERVAL,
+		target,
+		Rialto::AVERAGE_BLOCK_INTERVAL,
+		(),
+		sync_params,
+		metrics_params,
+		futures::future::pending(),
+	);
+
+	Ok(())
+}
@@ -0,0 +1,115 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! The PoA Bridge Pallet provides a way to include multiple instances of itself in a runtime. When
+//! synchronizing a Substrate chain which can include multiple instances of the bridge pallet we
+//! must somehow decide which of the instances to sync.
+//!
+//! Note that each instance of the bridge pallet is coupled with an instance of the currency exchange
+//! pallet. We must also have a way to create `Call`s for the correct currency exchange instance.
+//!
+//! This module helps by preparing the correct `Call`s for each of the different pallet instances.
+
+use crate::ethereum_sync_loop::QueuedEthereumHeader;
+use crate::substrate_types::{into_substrate_ethereum_header, into_substrate_ethereum_receipts};
+
+use rialto_runtime::exchange::EthereumTransactionInclusionProof as Proof;
+use rialto_runtime::Call;
+
+/// Interface for `Calls` which are needed to correctly sync the bridge.
+///
+/// Each instance of the bridge and currency exchange pallets in the bridge runtime requires similar
+/// but slightly different `Call` in order to be synchronized.
+pub trait BridgeInstance: Send + Sync + std::fmt::Debug {
+	/// Used to build a `Call` for importing signed headers to a Substrate runtime.
+	fn build_signed_header_call(&self, headers: Vec<QueuedEthereumHeader>) -> Call;
+	/// Used to build a `Call` for importing an unsigned header to a Substrate runtime.
+	fn build_unsigned_header_call(&self, header: QueuedEthereumHeader) -> Call;
+	/// Used to build a `Call` for importing peer transactions to a Substrate runtime.
+	fn build_currency_exchange_call(&self, proof: Proof) -> Call;
+}
+
+/// Corresponds to the Rialto instance used in the bridge runtime.
+#[derive(Default, Clone, Debug)]
+pub struct RialtoPoA;
+
+impl BridgeInstance for RialtoPoA {
+	fn build_signed_header_call(&self, headers: Vec<QueuedEthereumHeader>) -> Call {
+		let pallet_call = rialto_runtime::BridgeEthPoACall::import_signed_headers(
+			headers
+				.into_iter()
+				.map(|header| {
+					(
+						into_substrate_ethereum_header(&header.header()),
+						into_substrate_ethereum_receipts(header.extra()),
+					)
+				})
+				.collect(),
+		);
+
+		rialto_runtime::Call::BridgeRialtoPoA(pallet_call)
+	}
+
+	fn build_unsigned_header_call(&self, header: QueuedEthereumHeader) -> Call {
+		let pallet_call = rialto_runtime::BridgeEthPoACall::import_unsigned_header(
+			into_substrate_ethereum_header(&header.header()),
+			into_substrate_ethereum_receipts(header.extra()),
+		);
+
+		rialto_runtime::Call::BridgeRialtoPoA(pallet_call)
+	}
+
+	fn build_currency_exchange_call(&self, proof: Proof) -> Call {
+		let pallet_call = rialto_runtime::BridgeCurrencyExchangeCall::import_peer_transaction(proof);
+		rialto_runtime::Call::BridgeRialtoCurrencyExchange(pallet_call)
+	}
+}
+
+/// Corresponds to the Kovan instance used in the bridge runtime.
+#[derive(Default, Clone, Debug)]
+pub struct Kovan;
+
+impl BridgeInstance for Kovan {
+	fn build_signed_header_call(&self, headers: Vec<QueuedEthereumHeader>) -> Call {
+		let pallet_call = rialto_runtime::BridgeEthPoACall::import_signed_headers(
+			headers
+				.into_iter()
+				.map(|header| {
+					(
+						into_substrate_ethereum_header(header.header()),
+						into_substrate_ethereum_receipts(header.extra()),
+					)
+				})
+				.collect(),
+		);
+
+		rialto_runtime::Call::BridgeKovan(pallet_call)
+	}
+
+	fn build_unsigned_header_call(&self, header: QueuedEthereumHeader) -> Call {
+		let pallet_call = rialto_runtime::BridgeEthPoACall::import_unsigned_header(
+			into_substrate_ethereum_header(header.header()),
+			into_substrate_ethereum_receipts(header.extra()),
+		);
+
+		rialto_runtime::Call::BridgeKovan(pallet_call)
+	}
+
+	fn build_currency_exchange_call(&self, proof: Proof) -> Call {
+		let pallet_call = rialto_runtime::BridgeCurrencyExchangeCall::import_peer_transaction(proof);
+		rialto_runtime::Call::BridgeKovanCurrencyExchange(pallet_call)
+	}
+}
@@ -0,0 +1,409 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+#![recursion_limit = "1024"]
+
+mod ethereum_client;
+mod ethereum_deploy_contract;
+mod ethereum_exchange;
+mod ethereum_exchange_submit;
+mod ethereum_sync_loop;
+mod instances;
+mod rialto_client;
+mod rpc_errors;
+mod substrate_sync_loop;
+mod substrate_types;
+
+use ethereum_deploy_contract::EthereumDeployContractParams;
+use ethereum_exchange::EthereumExchangeParams;
+use ethereum_exchange_submit::EthereumExchangeSubmitParams;
+use ethereum_sync_loop::EthereumSyncParams;
+use headers_relay::sync::TargetTransactionMode;
+use hex_literal::hex;
+use instances::{BridgeInstance, Kovan, RialtoPoA};
+use relay_utils::{initialize::initialize_relay, metrics::MetricsParams};
+use secp256k1::SecretKey;
+use sp_core::crypto::Pair;
+use substrate_sync_loop::SubstrateSyncParams;
+
+use headers_relay::sync::HeadersSyncParams;
+use relay_ethereum_client::{ConnectionParams as EthereumConnectionParams, SigningParams as EthereumSigningParams};
+use relay_rialto_client::SigningParams as RialtoSigningParams;
+use relay_substrate_client::ConnectionParams as SubstrateConnectionParams;
+use std::sync::Arc;
+
+fn main() {
+	initialize_relay();
+
+	let yaml = clap::load_yaml!("cli.yml");
+	let matches = clap::App::from_yaml(yaml).get_matches();
+	match matches.subcommand() {
+		("eth-to-sub", Some(eth_to_sub_matches)) => {
+			log::info!(target: "bridge", "Starting ETH ➡ SUB relay.");
+			if ethereum_sync_loop::run(match ethereum_sync_params(&eth_to_sub_matches) {
+				Ok(ethereum_sync_params) => ethereum_sync_params,
+				Err(err) => {
+					log::error!(target: "bridge", "Error parsing parameters: {}", err);
+					return;
+				}
+			})
+			.is_err()
+			{
+				log::error!(target: "bridge", "Unable to get Substrate genesis block for Ethereum sync.");
+			};
+		}
+		("sub-to-eth", Some(sub_to_eth_matches)) => {
+			log::info!(target: "bridge", "Starting SUB ➡ ETH relay.");
+			if substrate_sync_loop::run(match substrate_sync_params(&sub_to_eth_matches) {
+				Ok(substrate_sync_params) => substrate_sync_params,
+				Err(err) => {
+					log::error!(target: "bridge", "Error parsing parameters: {}", err);
+					return;
+				}
+			})
+			.is_err()
+			{
+				log::error!(target: "bridge", "Unable to get Substrate genesis block for Substrate sync.");
+			};
+		}
+		("eth-deploy-contract", Some(eth_deploy_matches)) => {
+			log::info!(target: "bridge", "Deploying ETH contracts.");
+			ethereum_deploy_contract::run(match ethereum_deploy_contract_params(&eth_deploy_matches) {
+				Ok(ethereum_deploy_params) => ethereum_deploy_params,
+				Err(err) => {
+					log::error!(target: "bridge", "Error during contract deployment: {}", err);
+					return;
+				}
+			});
+		}
+		("eth-submit-exchange-tx", Some(eth_exchange_submit_matches)) => {
+			log::info!(target: "bridge", "Submitting ETH ➡ SUB exchange transaction.");
+			ethereum_exchange_submit::run(match ethereum_exchange_submit_params(&eth_exchange_submit_matches) {
+				Ok(eth_exchange_submit_params) => eth_exchange_submit_params,
+				Err(err) => {
+					log::error!(target: "bridge", "Error submitting Eethereum exchange transaction: {}", err);
+					return;
+				}
+			});
+		}
+		("eth-exchange-sub", Some(eth_exchange_matches)) => {
+			log::info!(target: "bridge", "Starting ETH ➡ SUB exchange transactions relay.");
+			ethereum_exchange::run(match ethereum_exchange_params(&eth_exchange_matches) {
+				Ok(eth_exchange_params) => eth_exchange_params,
+				Err(err) => {
+					log::error!(target: "bridge", "Error relaying Ethereum transactions proofs: {}", err);
+					return;
+				}
+			});
+		}
+		("", _) => {
+			log::error!(target: "bridge", "No subcommand specified");
+		}
+		_ => unreachable!("all possible subcommands are checked above; qed"),
+	}
+}
+
+fn ethereum_connection_params(matches: &clap::ArgMatches) -> Result<EthereumConnectionParams, String> {
+	let mut params = EthereumConnectionParams::default();
+	if let Some(eth_host) = matches.value_of("eth-host") {
+		params.host = eth_host.into();
+	}
+	if let Some(eth_port) = matches.value_of("eth-port") {
+		params.port = eth_port
+			.parse()
+			.map_err(|e| format!("Failed to parse eth-port: {}", e))?;
+	}
+	Ok(params)
+}
+
+fn ethereum_signing_params(matches: &clap::ArgMatches) -> Result<EthereumSigningParams, String> {
+	let mut params = EthereumSigningParams::default();
+	if let Some(eth_signer) = matches.value_of("eth-signer") {
+		params.signer =
+			SecretKey::parse_slice(&hex::decode(eth_signer).map_err(|e| format!("Failed to parse eth-signer: {}", e))?)
+				.map_err(|e| format!("Invalid eth-signer: {}", e))?;
+	}
+	if let Some(eth_chain_id) = matches.value_of("eth-chain-id") {
+		params.chain_id = eth_chain_id
+			.parse::<u64>()
+			.map_err(|e| format!("Failed to parse eth-chain-id: {}", e))?;
+	}
+	Ok(params)
+}
+
+fn substrate_connection_params(matches: &clap::ArgMatches) -> Result<SubstrateConnectionParams, String> {
+	let mut params = SubstrateConnectionParams::default();
+	if let Some(sub_host) = matches.value_of("sub-host") {
+		params.host = sub_host.into();
+	}
+	if let Some(sub_port) = matches.value_of("sub-port") {
+		params.port = sub_port
+			.parse()
+			.map_err(|e| format!("Failed to parse sub-port: {}", e))?;
+	}
+	Ok(params)
+}
+
+fn rialto_signing_params(matches: &clap::ArgMatches) -> Result<RialtoSigningParams, String> {
+	let mut params = RialtoSigningParams::default();
+	if let Some(sub_signer) = matches.value_of("sub-signer") {
+		let sub_signer_password = matches.value_of("sub-signer-password");
+		params.signer = sp_core::sr25519::Pair::from_string(sub_signer, sub_signer_password)
+			.map_err(|e| format!("Failed to parse sub-signer: {:?}", e))?;
+	}
+	Ok(params)
+}
+
+fn ethereum_sync_params(matches: &clap::ArgMatches) -> Result<EthereumSyncParams, String> {
+	use crate::ethereum_sync_loop::consts::*;
+
+	let mut sync_params = HeadersSyncParams {
+		max_future_headers_to_download: MAX_FUTURE_HEADERS_TO_DOWNLOAD,
+		max_headers_in_submitted_status: MAX_SUBMITTED_HEADERS,
+		max_headers_in_single_submit: MAX_HEADERS_IN_SINGLE_SUBMIT,
+		max_headers_size_in_single_submit: MAX_HEADERS_SIZE_IN_SINGLE_SUBMIT,
+		prune_depth: PRUNE_DEPTH,
+		target_tx_mode: TargetTransactionMode::Signed,
+	};
+
+	match matches.value_of("sub-tx-mode") {
+		Some("signed") => sync_params.target_tx_mode = TargetTransactionMode::Signed,
+		Some("unsigned") => {
+			sync_params.target_tx_mode = TargetTransactionMode::Unsigned;
+
+			// tx pool won't accept too much unsigned transactions
+			sync_params.max_headers_in_submitted_status = 10;
+		}
+		Some("backup") => sync_params.target_tx_mode = TargetTransactionMode::Backup,
+		Some(mode) => return Err(format!("Invalid sub-tx-mode: {}", mode)),
+		None => sync_params.target_tx_mode = TargetTransactionMode::Signed,
+	}
+
+	let params = EthereumSyncParams {
+		eth_params: ethereum_connection_params(matches)?,
+		sub_params: substrate_connection_params(matches)?,
+		sub_sign: rialto_signing_params(matches)?,
+		metrics_params: metrics_params(matches)?,
+		instance: instance_params(matches)?,
+		sync_params,
+	};
+
+	log::debug!(target: "bridge", "Ethereum sync params: {:?}", params);
+
+	Ok(params)
+}
+
+fn substrate_sync_params(matches: &clap::ArgMatches) -> Result<SubstrateSyncParams, String> {
+	use crate::substrate_sync_loop::consts::*;
+
+	let eth_contract_address: relay_ethereum_client::types::Address =
+		if let Some(eth_contract) = matches.value_of("eth-contract") {
+			eth_contract.parse().map_err(|e| format!("{}", e))?
+		} else {
+			"731a10897d267e19b34503ad902d0a29173ba4b1"
+				.parse()
+				.expect("address is hardcoded, thus valid; qed")
+		};
+
+	let params = SubstrateSyncParams {
+		sub_params: substrate_connection_params(matches)?,
+		eth_params: ethereum_connection_params(matches)?,
+		eth_sign: ethereum_signing_params(matches)?,
+		metrics_params: metrics_params(matches)?,
+		sync_params: HeadersSyncParams {
+			max_future_headers_to_download: MAX_FUTURE_HEADERS_TO_DOWNLOAD,
+			max_headers_in_submitted_status: MAX_SUBMITTED_HEADERS,
+			max_headers_in_single_submit: MAX_SUBMITTED_HEADERS,
+			max_headers_size_in_single_submit: std::usize::MAX,
+			prune_depth: PRUNE_DEPTH,
+			target_tx_mode: TargetTransactionMode::Signed,
+		},
+		eth_contract_address,
+	};
+
+	log::debug!(target: "bridge", "Substrate sync params: {:?}", params);
+
+	Ok(params)
+}
+
+fn ethereum_deploy_contract_params(matches: &clap::ArgMatches) -> Result<EthereumDeployContractParams, String> {
+	let eth_contract_code = parse_hex_argument(matches, "eth-contract-code")?.unwrap_or_else(|| {
+		hex::decode(include_str!("../res/substrate-bridge-bytecode.hex")).expect("code is hardcoded, thus valid; qed")
+	});
+	let sub_initial_authorities_set_id = match matches.value_of("sub-authorities-set-id") {
+		Some(sub_initial_authorities_set_id) => Some(
+			sub_initial_authorities_set_id
+				.parse()
+				.map_err(|e| format!("Failed to parse sub-authorities-set-id: {}", e))?,
+		),
+		None => None,
+	};
+	let sub_initial_authorities_set = parse_hex_argument(matches, "sub-authorities-set")?;
+	let sub_initial_header = parse_hex_argument(matches, "sub-initial-header")?;
+
+	let params = EthereumDeployContractParams {
+		eth_params: ethereum_connection_params(matches)?,
+		eth_sign: ethereum_signing_params(matches)?,
+		sub_params: substrate_connection_params(matches)?,
+		sub_initial_authorities_set_id,
+		sub_initial_authorities_set,
+		sub_initial_header,
+		eth_contract_code,
+	};
+
+	log::debug!(target: "bridge", "Deploy params: {:?}", params);
+
+	Ok(params)
+}
+
+fn ethereum_exchange_submit_params(matches: &clap::ArgMatches) -> Result<EthereumExchangeSubmitParams, String> {
+	let eth_nonce = if let Some(eth_nonce) = matches.value_of("eth-nonce") {
+		Some(
+			relay_ethereum_client::types::U256::from_dec_str(&eth_nonce)
+				.map_err(|e| format!("Failed to parse eth-nonce: {}", e))?,
+		)
+	} else {
+		None
+	};
+
+	let eth_amount = if let Some(eth_amount) = matches.value_of("eth-amount") {
+		eth_amount
+			.parse()
+			.map_err(|e| format!("Failed to parse eth-amount: {}", e))?
+	} else {
+		// This is in Wei, represents 1 ETH
+		1_000_000_000_000_000_000_u64.into()
+	};
+
+	// This is the well-known Substrate account of Ferdie
+	let default_recepient = hex!("1cbd2d43530a44705ad088af313e18f80b53ef16b36177cd4b77b846f2a5f07c");
+
+	let sub_recipient = if let Some(sub_recipient) = matches.value_of("sub-recipient") {
+		hex::decode(&sub_recipient)
+			.map_err(|err| err.to_string())
+			.and_then(|vsub_recipient| {
+				let expected_len = default_recepient.len();
+				if expected_len != vsub_recipient.len() {
+					Err(format!("invalid length. Expected {} bytes", expected_len))
+				} else {
+					let mut sub_recipient = default_recepient;
+					sub_recipient.copy_from_slice(&vsub_recipient[..expected_len]);
+					Ok(sub_recipient)
+				}
+			})
+			.map_err(|e| format!("Failed to parse sub-recipient: {}", e))?
+	} else {
+		default_recepient
+	};
+
+	let params = EthereumExchangeSubmitParams {
+		eth_params: ethereum_connection_params(matches)?,
+		eth_sign: ethereum_signing_params(matches)?,
+		eth_nonce,
+		eth_amount,
+		sub_recipient,
+	};
+
+	log::debug!(target: "bridge", "Submit Ethereum exchange tx params: {:?}", params);
+
+	Ok(params)
+}
+
+fn ethereum_exchange_params(matches: &clap::ArgMatches) -> Result<EthereumExchangeParams, String> {
+	let mode = match matches.value_of("eth-tx-hash") {
+		Some(eth_tx_hash) => ethereum_exchange::ExchangeRelayMode::Single(
+			eth_tx_hash
+				.parse()
+				.map_err(|e| format!("Failed to parse eth-tx-hash: {}", e))?,
+		),
+		None => ethereum_exchange::ExchangeRelayMode::Auto(match matches.value_of("eth-start-with-block") {
+			Some(eth_start_with_block) => Some(
+				eth_start_with_block
+					.parse()
+					.map_err(|e| format!("Failed to parse eth-start-with-block: {}", e))?,
+			),
+			None => None,
+		}),
+	};
+
+	let params = EthereumExchangeParams {
+		eth_params: ethereum_connection_params(matches)?,
+		sub_params: substrate_connection_params(matches)?,
+		sub_sign: rialto_signing_params(matches)?,
+		metrics_params: metrics_params(matches)?,
+		instance: instance_params(matches)?,
+		mode,
+	};
+
+	log::debug!(target: "bridge", "Ethereum exchange params: {:?}", params);
+
+	Ok(params)
+}
+
+fn metrics_params(matches: &clap::ArgMatches) -> Result<Option<MetricsParams>, String> {
+	if matches.is_present("no-prometheus") {
+		return Ok(None);
+	}
+
+	let mut metrics_params = MetricsParams::default();
+
+	if let Some(prometheus_host) = matches.value_of("prometheus-host") {
+		metrics_params.host = prometheus_host.into();
+	}
+	if let Some(prometheus_port) = matches.value_of("prometheus-port") {
+		metrics_params.port = prometheus_port
+			.parse()
+			.map_err(|e| format!("Failed to parse prometheus-port: {}", e))?;
+	}
+
+	Ok(Some(metrics_params))
+}
+
+fn instance_params(matches: &clap::ArgMatches) -> Result<Arc<dyn BridgeInstance>, String> {
+	let instance = if let Some(instance) = matches.value_of("sub-pallet-instance") {
+		match instance.to_lowercase().as_str() {
+			"rialto" => Arc::new(RialtoPoA) as Arc<dyn BridgeInstance>,
+			"kovan" => Arc::new(Kovan),
+			_ => return Err("Unsupported bridge pallet instance".to_string()),
+		}
+	} else {
+		unreachable!("CLI config enforces a default instance, can never be None")
+	};
+
+	Ok(instance)
+}
+
+fn parse_hex_argument(matches: &clap::ArgMatches, arg: &str) -> Result<Option<Vec<u8>>, String> {
+	match matches.value_of(arg) {
+		Some(value) => Ok(Some(
+			hex::decode(value).map_err(|e| format!("Failed to parse {}: {}", arg, e))?,
+		)),
+		None => Ok(None),
+	}
+}
+
+#[cfg(test)]
+mod tests {
+
+	// Details: https://github.com/paritytech/parity-bridges-common/issues/118
+	#[test]
+	fn async_std_sleep_works() {
+		let mut local_pool = futures::executor::LocalPool::new();
+		local_pool.run_until(async move {
+			async_std::task::sleep(std::time::Duration::from_secs(1)).await;
+		});
+	}
+}
@@ -0,0 +1,269 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::ethereum_sync_loop::QueuedEthereumHeader;
+use crate::instances::BridgeInstance;
+use crate::rpc_errors::RpcError;
+
+use async_trait::async_trait;
+use bp_eth_poa::AuraHeader as SubstrateEthereumHeader;
+use codec::{Decode, Encode};
+use headers_relay::sync_types::SubmittedHeaders;
+use relay_ethereum_client::types::HeaderId as EthereumHeaderId;
+use relay_rialto_client::{Rialto, SigningParams as RialtoSigningParams};
+use relay_substrate_client::{Client as SubstrateClient, TransactionSignScheme};
+use relay_utils::HeaderId;
+use sp_core::{crypto::Pair, Bytes};
+use std::{collections::VecDeque, sync::Arc};
+
+const ETH_API_IMPORT_REQUIRES_RECEIPTS: &str = "RialtoPoAHeaderApi_is_import_requires_receipts";
+const ETH_API_IS_KNOWN_BLOCK: &str = "RialtoPoAHeaderApi_is_known_block";
+const ETH_API_BEST_BLOCK: &str = "RialtoPoAHeaderApi_best_block";
+const ETH_API_BEST_FINALIZED_BLOCK: &str = "RialtoPoAHeaderApi_finalized_block";
+const EXCH_API_FILTER_TRANSACTION_PROOF: &str = "RialtoCurrencyExchangeApi_filter_transaction_proof";
+
+type RpcResult<T> = std::result::Result<T, RpcError>;
+
+/// A trait which contains methods that work by using multiple low-level RPCs, or more complicated
+/// interactions involving, for example, an Ethereum bridge module.
+#[async_trait]
+pub trait SubstrateHighLevelRpc {
+	/// Returns best Ethereum block that Substrate runtime knows of.
+	async fn best_ethereum_block(&self) -> RpcResult<EthereumHeaderId>;
+	/// Returns best finalized Ethereum block that Substrate runtime knows of.
+	async fn best_ethereum_finalized_block(&self) -> RpcResult<EthereumHeaderId>;
+	/// Returns whether or not transactions receipts are required for Ethereum header submission.
+	async fn ethereum_receipts_required(&self, header: SubstrateEthereumHeader) -> RpcResult<bool>;
+	/// Returns whether or not the given Ethereum header is known to the Substrate runtime.
+	async fn ethereum_header_known(&self, header_id: EthereumHeaderId) -> RpcResult<bool>;
+}
+
+#[async_trait]
+impl SubstrateHighLevelRpc for SubstrateClient<Rialto> {
+	async fn best_ethereum_block(&self) -> RpcResult<EthereumHeaderId> {
+		let call = ETH_API_BEST_BLOCK.to_string();
+		let data = Bytes(Vec::new());
+
+		let encoded_response = self.state_call(call, data, None).await?;
+		let decoded_response: (u64, bp_eth_poa::H256) = Decode::decode(&mut &encoded_response.0[..])?;
+
+		let best_header_id = HeaderId(decoded_response.0, decoded_response.1);
+		Ok(best_header_id)
+	}
+
+	async fn best_ethereum_finalized_block(&self) -> RpcResult<EthereumHeaderId> {
+		let call = ETH_API_BEST_FINALIZED_BLOCK.to_string();
+		let data = Bytes(Vec::new());
+
+		let encoded_response = self.state_call(call, data, None).await?;
+		let decoded_response: (u64, bp_eth_poa::H256) = Decode::decode(&mut &encoded_response.0[..])?;
+
+		let best_header_id = HeaderId(decoded_response.0, decoded_response.1);
+		Ok(best_header_id)
+	}
+
+	async fn ethereum_receipts_required(&self, header: SubstrateEthereumHeader) -> RpcResult<bool> {
+		let call = ETH_API_IMPORT_REQUIRES_RECEIPTS.to_string();
+		let data = Bytes(header.encode());
+
+		let encoded_response = self.state_call(call, data, None).await?;
+		let receipts_required: bool = Decode::decode(&mut &encoded_response.0[..])?;
+
+		Ok(receipts_required)
+	}
+
+	// The Substrate module could prune old headers. So this function could return false even
+	// if header is synced. And we'll mark corresponding Ethereum header as Orphan.
+	//
+	// But when we read the best header from Substrate next time, we will know that
+	// there's a better header. This Orphan will either be marked as synced, or
+	// eventually pruned.
+	async fn ethereum_header_known(&self, header_id: EthereumHeaderId) -> RpcResult<bool> {
+		let call = ETH_API_IS_KNOWN_BLOCK.to_string();
+		let data = Bytes(header_id.1.encode());
+
+		let encoded_response = self.state_call(call, data, None).await?;
+		let is_known_block: bool = Decode::decode(&mut &encoded_response.0[..])?;
+
+		Ok(is_known_block)
+	}
+}
+
+/// A trait for RPC calls which are used to submit Ethereum headers to a Substrate
+/// runtime. These are typically calls which use a combination of other low-level RPC
+/// calls.
+#[async_trait]
+pub trait SubmitEthereumHeaders {
+	/// Submits Ethereum header to Substrate runtime.
+	async fn submit_ethereum_headers(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+		sign_transactions: bool,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError>;
+
+	/// Submits signed Ethereum header to Substrate runtime.
+	async fn submit_signed_ethereum_headers(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError>;
+
+	/// Submits unsigned Ethereum header to Substrate runtime.
+	async fn submit_unsigned_ethereum_headers(
+		&self,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError>;
+}
+
+#[async_trait]
+impl SubmitEthereumHeaders for SubstrateClient<Rialto> {
+	async fn submit_ethereum_headers(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+		sign_transactions: bool,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError> {
+		if sign_transactions {
+			self.submit_signed_ethereum_headers(params, instance, headers).await
+		} else {
+			self.submit_unsigned_ethereum_headers(instance, headers).await
+		}
+	}
+
+	async fn submit_signed_ethereum_headers(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError> {
+		let ids = headers.iter().map(|header| header.id()).collect();
+		let submission_result = async {
+			let account_id = params.signer.public().as_array_ref().clone().into();
+			let nonce = self.next_account_index(account_id).await?;
+
+			let call = instance.build_signed_header_call(headers);
+			let transaction = Rialto::sign_transaction(self, &params.signer, nonce, call);
+
+			let _ = self.submit_extrinsic(Bytes(transaction.encode())).await?;
+			Ok(())
+		}
+		.await;
+
+		match submission_result {
+			Ok(_) => SubmittedHeaders {
+				submitted: ids,
+				incomplete: Vec::new(),
+				rejected: Vec::new(),
+				fatal_error: None,
+			},
+			Err(error) => SubmittedHeaders {
+				submitted: Vec::new(),
+				incomplete: Vec::new(),
+				rejected: ids,
+				fatal_error: Some(error),
+			},
+		}
+	}
+
+	async fn submit_unsigned_ethereum_headers(
+		&self,
+		instance: Arc<dyn BridgeInstance>,
+		headers: Vec<QueuedEthereumHeader>,
+	) -> SubmittedHeaders<EthereumHeaderId, RpcError> {
+		let mut ids = headers.iter().map(|header| header.id()).collect::<VecDeque<_>>();
+		let mut submitted_headers = SubmittedHeaders::default();
+
+		for header in headers {
+			let id = ids.pop_front().expect("both collections have same size; qed");
+
+			let call = instance.build_unsigned_header_call(header);
+			let transaction = create_unsigned_submit_transaction(call);
+
+			match self.submit_extrinsic(Bytes(transaction.encode())).await {
+				Ok(_) => submitted_headers.submitted.push(id),
+				Err(error) => {
+					submitted_headers.rejected.push(id);
+					submitted_headers.rejected.extend(ids);
+					submitted_headers.fatal_error = Some(error.into());
+					break;
+				}
+			}
+		}
+
+		submitted_headers
+	}
+}
+
+/// A trait for RPC calls which are used to submit proof of Ethereum exchange transaction to a
+/// Substrate runtime. These are typically calls which use a combination of other low-level RPC
+/// calls.
+#[async_trait]
+pub trait SubmitEthereumExchangeTransactionProof {
+	/// Pre-verify Ethereum exchange transaction proof.
+	async fn verify_exchange_transaction_proof(
+		&self,
+		proof: rialto_runtime::exchange::EthereumTransactionInclusionProof,
+	) -> RpcResult<bool>;
+	/// Submits Ethereum exchange transaction proof to Substrate runtime.
+	async fn submit_exchange_transaction_proof(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		proof: rialto_runtime::exchange::EthereumTransactionInclusionProof,
+	) -> RpcResult<()>;
+}
+
+#[async_trait]
+impl SubmitEthereumExchangeTransactionProof for SubstrateClient<Rialto> {
+	async fn verify_exchange_transaction_proof(
+		&self,
+		proof: rialto_runtime::exchange::EthereumTransactionInclusionProof,
+	) -> RpcResult<bool> {
+		let call = EXCH_API_FILTER_TRANSACTION_PROOF.to_string();
+		let data = Bytes(proof.encode());
+
+		let encoded_response = self.state_call(call, data, None).await?;
+		let is_allowed: bool = Decode::decode(&mut &encoded_response.0[..])?;
+
+		Ok(is_allowed)
+	}
+
+	async fn submit_exchange_transaction_proof(
+		&self,
+		params: RialtoSigningParams,
+		instance: Arc<dyn BridgeInstance>,
+		proof: rialto_runtime::exchange::EthereumTransactionInclusionProof,
+	) -> RpcResult<()> {
+		let account_id = params.signer.public().as_array_ref().clone().into();
+		let nonce = self.next_account_index(account_id).await?;
+
+		let call = instance.build_currency_exchange_call(proof);
+		let transaction = Rialto::sign_transaction(self, &params.signer, nonce, call);
+
+		let _ = self.submit_extrinsic(Bytes(transaction.encode())).await?;
+		Ok(())
+	}
+}
+
+/// Create unsigned Substrate transaction for submitting Ethereum header.
+fn create_unsigned_submit_transaction(call: rialto_runtime::Call) -> rialto_runtime::UncheckedExtrinsic {
+	rialto_runtime::UncheckedExtrinsic::new_unsigned(call)
+}
@@ -0,0 +1,82 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use relay_ethereum_client::Error as EthereumNodeError;
+use relay_substrate_client::Error as SubstrateNodeError;
+use relay_utils::MaybeConnectionError;
+
+/// Contains common errors that can occur when
+/// interacting with a Substrate or Ethereum node
+/// through RPC.
+#[derive(Debug)]
+pub enum RpcError {
+	/// The arguments to the RPC method failed to serialize.
+	Serialization(serde_json::Error),
+	/// An error occured when interacting with an Ethereum node.
+	Ethereum(EthereumNodeError),
+	/// An error occured when interacting with a Substrate node.
+	Substrate(SubstrateNodeError),
+}
+
+impl From<RpcError> for String {
+	fn from(err: RpcError) -> Self {
+		match err {
+			RpcError::Serialization(e) => e.to_string(),
+			RpcError::Ethereum(e) => e.to_string(),
+			RpcError::Substrate(e) => e.to_string(),
+		}
+	}
+}
+
+impl From<serde_json::Error> for RpcError {
+	fn from(err: serde_json::Error) -> Self {
+		Self::Serialization(err)
+	}
+}
+
+impl From<EthereumNodeError> for RpcError {
+	fn from(err: EthereumNodeError) -> Self {
+		Self::Ethereum(err)
+	}
+}
+
+impl From<SubstrateNodeError> for RpcError {
+	fn from(err: SubstrateNodeError) -> Self {
+		Self::Substrate(err)
+	}
+}
+
+impl From<ethabi::Error> for RpcError {
+	fn from(err: ethabi::Error) -> Self {
+		Self::Ethereum(EthereumNodeError::ResponseParseFailed(format!("{}", err)))
+	}
+}
+
+impl MaybeConnectionError for RpcError {
+	fn is_connection_error(&self) -> bool {
+		match self {
+			RpcError::Ethereum(ref error) => error.is_connection_error(),
+			RpcError::Substrate(ref error) => error.is_connection_error(),
+			_ => false,
+		}
+	}
+}
+
+impl From<codec::Error> for RpcError {
+	fn from(err: codec::Error) -> Self {
+		Self::Substrate(SubstrateNodeError::ResponseParseFailed(err))
+	}
+}
@@ -0,0 +1,195 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Rialto-Substrate -> Ethereum PoA synchronization.
+
+use crate::ethereum_client::EthereumHighLevelRpc;
+use crate::rpc_errors::RpcError;
+
+use async_trait::async_trait;
+use codec::Encode;
+use headers_relay::{
+	sync::HeadersSyncParams,
+	sync_loop::TargetClient,
+	sync_types::{HeadersSyncPipeline, QueuedHeader, SourceHeader, SubmittedHeaders},
+};
+use relay_ethereum_client::{
+	types::Address, Client as EthereumClient, ConnectionParams as EthereumConnectionParams,
+	SigningParams as EthereumSigningParams,
+};
+use relay_rialto_client::{HeaderId as RialtoHeaderId, Rialto, SyncHeader as RialtoSyncHeader};
+use relay_substrate_client::{
+	headers_source::HeadersSource, Chain as SubstrateChain, Client as SubstrateClient,
+	ConnectionParams as SubstrateConnectionParams,
+};
+use relay_utils::{metrics::MetricsParams, relay_loop::Client as RelayClient};
+use sp_runtime::Justification;
+
+use std::fmt::Debug;
+use std::{collections::HashSet, time::Duration};
+
+pub mod consts {
+	use super::*;
+
+	/// Interval at which we check new Ethereum blocks.
+	pub const ETHEREUM_TICK_INTERVAL: Duration = Duration::from_secs(5);
+	/// Max Ethereum headers we want to have in all 'before-submitted' states.
+	pub const MAX_FUTURE_HEADERS_TO_DOWNLOAD: usize = 8;
+	/// Max Ethereum headers count we want to have in 'submitted' state.
+	pub const MAX_SUBMITTED_HEADERS: usize = 4;
+	/// Max depth of in-memory headers in all states. Past this depth they will be forgotten (pruned).
+	pub const PRUNE_DEPTH: u32 = 256;
+}
+
+/// Substrate synchronization parameters.
+#[derive(Debug)]
+pub struct SubstrateSyncParams {
+	/// Substrate connection params.
+	pub sub_params: SubstrateConnectionParams,
+	/// Ethereum connection params.
+	pub eth_params: EthereumConnectionParams,
+	/// Ethereum signing params.
+	pub eth_sign: EthereumSigningParams,
+	/// Ethereum bridge contract address.
+	pub eth_contract_address: Address,
+	/// Synchronization parameters.
+	pub sync_params: HeadersSyncParams,
+	/// Metrics parameters.
+	pub metrics_params: Option<MetricsParams>,
+}
+
+/// Substrate synchronization pipeline.
+#[derive(Clone, Copy, Debug)]
+#[cfg_attr(test, derive(PartialEq))]
+pub struct SubstrateHeadersSyncPipeline;
+
+impl HeadersSyncPipeline for SubstrateHeadersSyncPipeline {
+	const SOURCE_NAME: &'static str = "Substrate";
+	const TARGET_NAME: &'static str = "Ethereum";
+
+	type Hash = rialto_runtime::Hash;
+	type Number = rialto_runtime::BlockNumber;
+	type Header = RialtoSyncHeader;
+	type Extra = ();
+	type Completion = Justification;
+
+	fn estimate_size(source: &QueuedHeader<Self>) -> usize {
+		source.header().encode().len()
+	}
+}
+
+/// Queued substrate header ID.
+pub type QueuedRialtoHeader = QueuedHeader<SubstrateHeadersSyncPipeline>;
+
+/// Rialto node as headers source.
+type SubstrateHeadersSource = HeadersSource<Rialto, SubstrateHeadersSyncPipeline>;
+
+/// Ethereum client as Substrate headers target.
+#[derive(Clone)]
+struct EthereumHeadersTarget {
+	/// Ethereum node client.
+	client: EthereumClient,
+	/// Bridge contract address.
+	contract: Address,
+	/// Ethereum signing params.
+	sign_params: EthereumSigningParams,
+}
+
+impl EthereumHeadersTarget {
+	fn new(client: EthereumClient, contract: Address, sign_params: EthereumSigningParams) -> Self {
+		Self {
+			client,
+			contract,
+			sign_params,
+		}
+	}
+}
+
+#[async_trait]
+impl RelayClient for EthereumHeadersTarget {
+	type Error = RpcError;
+
+	async fn reconnect(&mut self) -> Result<(), RpcError> {
+		self.client.reconnect();
+		Ok(())
+	}
+}
+
+#[async_trait]
+impl TargetClient<SubstrateHeadersSyncPipeline> for EthereumHeadersTarget {
+	async fn best_header_id(&self) -> Result<RialtoHeaderId, RpcError> {
+		// we can't continue to relay headers if Ethereum node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
+		self.client.best_substrate_block(self.contract).await
+	}
+
+	async fn is_known_header(&self, id: RialtoHeaderId) -> Result<(RialtoHeaderId, bool), RpcError> {
+		self.client.substrate_header_known(self.contract, id).await
+	}
+
+	async fn submit_headers(&self, headers: Vec<QueuedRialtoHeader>) -> SubmittedHeaders<RialtoHeaderId, RpcError> {
+		self.client
+			.submit_substrate_headers(self.sign_params.clone(), self.contract, headers)
+			.await
+	}
+
+	async fn incomplete_headers_ids(&self) -> Result<HashSet<RialtoHeaderId>, RpcError> {
+		self.client.incomplete_substrate_headers(self.contract).await
+	}
+
+	async fn complete_header(&self, id: RialtoHeaderId, completion: Justification) -> Result<RialtoHeaderId, RpcError> {
+		self.client
+			.complete_substrate_header(self.sign_params.clone(), self.contract, id, completion)
+			.await
+	}
+
+	async fn requires_extra(&self, header: QueuedRialtoHeader) -> Result<(RialtoHeaderId, bool), RpcError> {
+		Ok((header.header().id(), false))
+	}
+}
+
+/// Run Substrate headers synchronization.
+pub fn run(params: SubstrateSyncParams) -> Result<(), RpcError> {
+	let SubstrateSyncParams {
+		sub_params,
+		eth_params,
+		eth_sign,
+		eth_contract_address,
+		sync_params,
+		metrics_params,
+	} = params;
+
+	let eth_client = EthereumClient::new(eth_params);
+	let sub_client = async_std::task::block_on(async { SubstrateClient::<Rialto>::new(sub_params).await })?;
+
+	let target = EthereumHeadersTarget::new(eth_client, eth_contract_address, eth_sign);
+	let source = SubstrateHeadersSource::new(sub_client);
+
+	headers_relay::sync_loop::run(
+		source,
+		Rialto::AVERAGE_BLOCK_INTERVAL,
+		target,
+		consts::ETHEREUM_TICK_INTERVAL,
+		(),
+		sync_params,
+		metrics_params,
+		futures::future::pending(),
+	);
+
+	Ok(())
+}
@@ -0,0 +1,76 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Converting between Ethereum headers and bridge module types.
+
+use bp_eth_poa::{
+	AuraHeader as SubstrateEthereumHeader, LogEntry as SubstrateEthereumLogEntry, Receipt as SubstrateEthereumReceipt,
+	TransactionOutcome as SubstrateEthereumTransactionOutcome,
+};
+use relay_ethereum_client::types::{
+	Header as EthereumHeader, Receipt as EthereumReceipt, HEADER_ID_PROOF as ETHEREUM_HEADER_ID_PROOF,
+};
+
+/// Convert Ethereum header into Ethereum header for Substrate.
+pub fn into_substrate_ethereum_header(header: &EthereumHeader) -> SubstrateEthereumHeader {
+	SubstrateEthereumHeader {
+		parent_hash: header.parent_hash,
+		timestamp: header.timestamp.as_u64(),
+		number: header.number.expect(ETHEREUM_HEADER_ID_PROOF).as_u64(),
+		author: header.author,
+		transactions_root: header.transactions_root,
+		uncles_hash: header.uncles_hash,
+		extra_data: header.extra_data.0.clone(),
+		state_root: header.state_root,
+		receipts_root: header.receipts_root,
+		log_bloom: header.logs_bloom.unwrap_or_default().data().into(),
+		gas_used: header.gas_used,
+		gas_limit: header.gas_limit,
+		difficulty: header.difficulty,
+		seal: header.seal_fields.iter().map(|s| s.0.clone()).collect(),
+	}
+}
+
+/// Convert Ethereum transactions receipts into Ethereum transactions receipts for Substrate.
+pub fn into_substrate_ethereum_receipts(
+	receipts: &Option<Vec<EthereumReceipt>>,
+) -> Option<Vec<SubstrateEthereumReceipt>> {
+	receipts
+		.as_ref()
+		.map(|receipts| receipts.iter().map(into_substrate_ethereum_receipt).collect())
+}
+
+/// Convert Ethereum transactions receipt into Ethereum transactions receipt for Substrate.
+pub fn into_substrate_ethereum_receipt(receipt: &EthereumReceipt) -> SubstrateEthereumReceipt {
+	SubstrateEthereumReceipt {
+		gas_used: receipt.cumulative_gas_used,
+		log_bloom: receipt.logs_bloom.data().into(),
+		logs: receipt
+			.logs
+			.iter()
+			.map(|log_entry| SubstrateEthereumLogEntry {
+				address: log_entry.address,
+				topics: log_entry.topics.clone(),
+				data: log_entry.data.0.clone(),
+			})
+			.collect(),
+		outcome: match (receipt.status, receipt.root) {
+			(Some(status), None) => SubstrateEthereumTransactionOutcome::StatusCode(status.as_u64() as u8),
+			(None, Some(root)) => SubstrateEthereumTransactionOutcome::StateRoot(root),
+			_ => SubstrateEthereumTransactionOutcome::Unknown,
+		},
+	}
+}