Metadata for offline signers

Bastian Köcher
Core Dev @ Polkadot

Status Quo

• Ledger:
  • Hard coded each runtime
  • Relied on transaction_version to be bumped when TX format changed
• Polkadot Vault
  • Required the entire metadata to be transferred to the device (~500KB+)
  • Metadata was signed by some central entity 🤮

Why Metadata?

• Contains all the information for constructing a transaction
  • Type information
  • Calls
  • Signed extensions

Problems with Metadata

• Metadata is too big
  • Does not fit onto a ledger device
  • Takes too long to transfer
• Contains also information on all the storage types, docs etc

Chunk the Metadata

• Each type is their own chunk
• Each call is their own chunk, referencing the type chunks
• Reduces the size to some kilobytes per transaction

Merkleize the Metadata

• Each chunk is a leaf node
• Put all together into one tree
• Calculate the root hash of this tree => Metadata hash

How does it work?

• Online wallet generates a proof of all type/call nodes
• Proof is send/streamed alongside the transaction to the signer
• Signer uses type/call nodes in the proof to decode and show the transaction

How does it work?

• Signer includes the metadata hash in the transaction and signs
• Runtime checks on inclusion if the metadata hash matches the current on chain hash
  • If not, it rejects the transaction

Integration

In your runtime:

pub type SignedExtra = (
	frame_system::CheckNonZeroSender<Runtime>,
	...
	frame_metadata_hash_extension::CheckMetadataHash<Runtime>,
);

Integration

In your build.rs:

fn main() {
	substrate_wasm_builder::WasmBuilder::init_with_defaults()
		.enable_metadata_hash("UNIT", 12)
		.build();
}

Questions?

• More info
  • RFC78
  • Integration guide