XCM: Remote account converter (paritytech#6662)

* Draft remote account converter

* Tests and doc comments

* Adapt naming, tests fix

* Try with prefix in account

* Revert "Try with prefix in account"

This reverts commit 4617597.

* Adapt code and test to review suggestions

* adapt to take into account parent in conversion

* fix: docs

* fix: try doc fix v2

xcm/xcm-builder/src/location_conversion.rs

@@ -22,6 +22,111 @@ use sp_std::{borrow::Borrow, marker::PhantomData};
 use xcm::latest::prelude::*;
 use xcm_executor::traits::Convert;
 
+/// Prefix for generating alias account for accounts coming  
+/// from chains that use 32 byte long representations.
+pub const FOREIGN_CHAIN_PREFIX_PARA_32: [u8; 37] = *b"ForeignChainAliasAccountPrefix_Para32";
+
+/// Prefix for generating alias account for accounts coming  
+/// from chains that use 20 byte long representations.
+pub const FOREIGN_CHAIN_PREFIX_PARA_20: [u8; 37] = *b"ForeignChainAliasAccountPrefix_Para20";
+
+/// Prefix for generating alias account for accounts coming  
+/// from the relay chain using 32 byte long representations.
+pub const FOREIGN_CHAIN_PREFIX_RELAY: [u8; 36] = *b"ForeignChainAliasAccountPrefix_Relay";
+
+/// This converter will for a given `AccountId32`/`AccountKey20`
+/// always generate the same "remote" account for a specific
+/// sending chain.
+/// I.e. the user gets the same remote account
+/// on every consuming para-chain and relay chain.
+///
+/// Can be used as a converter in `SovereignSignedViaLocation`
+///
+/// ## Example
+/// Assuming the following network layout.
+///
+/// ```notrust
+///              R
+///           /    \
+///          /      \   
+///        P1       P2
+///        / \       / \
+///       /   \     /   \
+///     P1.1 P1.2  P2.1  P2.2
+/// ```
+/// Then a given account A will have the same alias accounts in the
+/// same plane. So, it is important which chain account A acts from.
+/// E.g.
+/// * From P1.2 A will act as
+///    * hash(ParaPrefix, A, 1, 1) on P1.2
+///    * hash(ParaPrefix, A, 1, 0) on P1
+/// * From P1 A will act as
+///    * hash(RelayPrefix, A, 1) on P1.2 & P1.1
+///    * hash(ParaPrefix, A, 1, 1) on P2
+///    * hash(ParaPrefix, A, 1, 0) on R
+///
+/// Note that the alias accounts have overlaps but never on the same
+/// chain when the sender comes from different chains.
+pub struct ForeignChainAliasAccount<AccountId>(PhantomData<AccountId>);
+impl<AccountId: From<[u8; 32]> + Clone> Convert<MultiLocation, AccountId>
+	for ForeignChainAliasAccount<AccountId>
+{
+	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
+		let entropy = match location.borrow() {
+			// Used on the relay chain for sending paras that use 32 byte accounts
+			MultiLocation {
+				parents: 0,
+				interior: X2(Parachain(para_id), AccountId32 { id, .. }),
+			} => ForeignChainAliasAccount::<AccountId>::from_para_32(para_id, id, 0),
+
+			// Used on the relay chain for sending paras that use 20 byte accounts
+			MultiLocation {
+				parents: 0,
+				interior: X2(Parachain(para_id), AccountKey20 { key, .. }),
+			} => ForeignChainAliasAccount::<AccountId>::from_para_20(para_id, key, 0),
+
+			// Used on para-chain for sending paras that use 32 byte accounts
+			MultiLocation {
+				parents: 1,
+				interior: X2(Parachain(para_id), AccountId32 { id, .. }),
+			} => ForeignChainAliasAccount::<AccountId>::from_para_32(para_id, id, 1),
+
+			// Used on para-chain for sending paras that use 20 byte accounts
+			MultiLocation {
+				parents: 1,
+				interior: X2(Parachain(para_id), AccountKey20 { key, .. }),
+			} => ForeignChainAliasAccount::<AccountId>::from_para_20(para_id, key, 1),
+
+			// Used on para-chain for sending from the relay chain
+			MultiLocation { parents: 1, interior: X1(AccountId32 { id, .. }) } =>
+				ForeignChainAliasAccount::<AccountId>::from_relay_32(id, 1),
+
+			// No other conversions provided
+			_ => return Err(()),
+		};
+
+		Ok(entropy.into())
+	}
+
+	fn reverse_ref(_: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
+		Err(())
+	}
+}
+
+impl<AccountId> ForeignChainAliasAccount<AccountId> {
+	fn from_para_32(para_id: &u32, id: &[u8; 32], parents: u8) -> [u8; 32] {
+		(FOREIGN_CHAIN_PREFIX_PARA_32, para_id, id, parents).using_encoded(blake2_256)
+	}
+
+	fn from_para_20(para_id: &u32, id: &[u8; 20], parents: u8) -> [u8; 32] {
+		(FOREIGN_CHAIN_PREFIX_PARA_20, para_id, id, parents).using_encoded(blake2_256)
+	}
+
+	fn from_relay_32(id: &[u8; 32], parents: u8) -> [u8; 32] {
+		(FOREIGN_CHAIN_PREFIX_RELAY, id, parents).using_encoded(blake2_256)
+	}
+}
+
 pub struct Account32Hash<Network, AccountId>(PhantomData<(Network, AccountId)>);
 impl<Network: Get<Option<NetworkId>>, AccountId: From<[u8; 32]> + Into<[u8; 32]> + Clone>
 	Convert<MultiLocation, AccountId> for Account32Hash<Network, AccountId>
@@ -225,4 +330,215 @@ mod tests {
 		let inverted = UniversalLocation::get().invert_target(&input);
 		assert_eq!(inverted, Err(()));
 	}
+
+	#[test]
+	fn remote_account_convert_on_para_sending_para_32() {
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(Parachain(1), AccountId32 { network: None, id: [0u8; 32] }),
+		};
+		let rem_1 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				181, 186, 132, 152, 52, 210, 226, 199, 8, 235, 213, 242, 94, 70, 250, 170, 19, 163,
+				196, 102, 245, 14, 172, 184, 2, 148, 108, 87, 230, 163, 204, 32
+			],
+			rem_1
+		);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(
+				Parachain(1),
+				AccountId32 { network: Some(NetworkId::Polkadot), id: [0u8; 32] },
+			),
+		};
+
+		assert_eq!(ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap(), rem_1);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(Parachain(2), AccountId32 { network: None, id: [0u8; 32] }),
+		};
+		let rem_2 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				183, 188, 66, 169, 82, 250, 45, 30, 142, 119, 184, 55, 177, 64, 53, 114, 12, 147,
+				128, 10, 60, 45, 41, 193, 87, 18, 86, 49, 127, 233, 243, 143
+			],
+			rem_2
+		);
+
+		assert_ne!(rem_1, rem_2);
+	}
+
+	#[test]
+	fn remote_account_convert_on_para_sending_para_20() {
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(Parachain(1), AccountKey20 { network: None, key: [0u8; 20] }),
+		};
+		let rem_1 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				210, 60, 37, 255, 116, 38, 221, 26, 85, 82, 252, 125, 220, 19, 41, 91, 185, 69,
+				102, 83, 120, 63, 15, 212, 74, 141, 82, 203, 187, 212, 77, 120
+			],
+			rem_1
+		);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(
+				Parachain(1),
+				AccountKey20 { network: Some(NetworkId::Polkadot), key: [0u8; 20] },
+			),
+		};
+
+		assert_eq!(ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap(), rem_1);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X2(Parachain(2), AccountKey20 { network: None, key: [0u8; 20] }),
+		};
+		let rem_2 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				197, 16, 31, 199, 234, 80, 166, 55, 178, 135, 95, 48, 19, 128, 9, 167, 51, 99, 215,
+				147, 94, 171, 28, 157, 29, 107, 240, 22, 10, 104, 99, 186
+			],
+			rem_2
+		);
+
+		assert_ne!(rem_1, rem_2);
+	}
+
+	#[test]
+	fn remote_account_convert_on_para_sending_relay() {
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X1(AccountId32 { network: None, id: [0u8; 32] }),
+		};
+		let rem_1 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				227, 12, 152, 241, 220, 53, 26, 27, 1, 167, 167, 214, 61, 161, 255, 96, 56, 16,
+				221, 59, 47, 45, 40, 193, 88, 92, 4, 167, 164, 27, 112, 99
+			],
+			rem_1
+		);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X1(AccountId32 { network: Some(NetworkId::Polkadot), id: [0u8; 32] }),
+		};
+
+		assert_eq!(ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap(), rem_1);
+
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X1(AccountId32 { network: None, id: [1u8; 32] }),
+		};
+		let rem_2 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				143, 195, 87, 73, 129, 2, 163, 211, 239, 51, 55, 235, 82, 173, 162, 206, 158, 237,
+				166, 73, 254, 62, 131, 6, 170, 241, 209, 116, 105, 69, 29, 226
+			],
+			rem_2
+		);
+
+		assert_ne!(rem_1, rem_2);
+	}
+
+	#[test]
+	fn remote_account_convert_on_relay_sending_para_20() {
+		let mul = MultiLocation {
+			parents: 0,
+			interior: X2(Parachain(1), AccountKey20 { network: None, key: [0u8; 20] }),
+		};
+		let rem_1 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				25, 251, 15, 92, 148, 141, 236, 238, 50, 108, 133, 56, 118, 11, 250, 122, 81, 160,
+				104, 160, 97, 200, 210, 49, 208, 142, 64, 144, 24, 110, 246, 101
+			],
+			rem_1
+		);
+
+		let mul = MultiLocation {
+			parents: 0,
+			interior: X2(Parachain(2), AccountKey20 { network: None, key: [0u8; 20] }),
+		};
+		let rem_2 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				88, 157, 224, 235, 76, 88, 201, 143, 206, 227, 14, 192, 177, 245, 75, 62, 41, 10,
+				107, 182, 61, 57, 239, 112, 43, 151, 58, 111, 150, 153, 234, 189
+			],
+			rem_2
+		);
+
+		assert_ne!(rem_1, rem_2);
+	}
+
+	#[test]
+	fn remote_account_convert_on_relay_sending_para_32() {
+		let mul = MultiLocation {
+			parents: 0,
+			interior: X2(Parachain(1), AccountId32 { network: None, id: [0u8; 32] }),
+		};
+		let rem_1 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				45, 120, 232, 0, 226, 49, 106, 48, 65, 181, 184, 147, 224, 235, 198, 152, 183, 156,
+				67, 57, 67, 67, 187, 104, 171, 23, 140, 21, 183, 152, 63, 20
+			],
+			rem_1
+		);
+
+		let mul = MultiLocation {
+			parents: 0,
+			interior: X2(
+				Parachain(1),
+				AccountId32 { network: Some(NetworkId::Polkadot), id: [0u8; 32] },
+			),
+		};
+
+		assert_eq!(ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap(), rem_1);
+
+		let mul = MultiLocation {
+			parents: 0,
+			interior: X2(Parachain(2), AccountId32 { network: None, id: [0u8; 32] }),
+		};
+		let rem_2 = ForeignChainAliasAccount::<[u8; 32]>::convert(mul).unwrap();
+
+		assert_eq!(
+			[
+				97, 119, 110, 66, 239, 113, 96, 234, 127, 92, 66, 204, 53, 129, 33, 119, 213, 192,
+				171, 100, 139, 51, 39, 62, 196, 163, 16, 213, 160, 44, 100, 228
+			],
+			rem_2
+		);
+
+		assert_ne!(rem_1, rem_2);
+	}
+
+	#[test]
+	fn remote_account_fails_with_bad_multilocation() {
+		let mul = MultiLocation {
+			parents: 1,
+			interior: X1(AccountKey20 { network: None, key: [0u8; 20] }),
+		};
+		assert!(ForeignChainAliasAccount::<[u8; 32]>::convert(mul).is_err());
+	}
 }