Simple Payment and State Verification

It is often useful to allow low resourced clients to participate in a Solana cluster. Be this participation economic or contract execution, verification that a client's activity has been accepted by the network is typically expensive. This proposal lays out a mechanism for such clients to confirm that their actions have been committed to the ledger state with minimal resource expenditure and third-party trust.

A Naive Approach

Validators store the signatures of recently confirmed transactions for a short period of time to ensure that they are not processed more than once. Validators provide a JSON RPC endpoint, which clients can use to query the cluster if a transaction has been recently processed. Validators also provide a PubSub notification, whereby a client registers to be notified when a given signature is observed by the validator. While these two mechanisms allow a client to verify a payment, they are not a proof and rely on completely trusting a validator.

We will describe a way to minimize this trust using Merkle Proofs to anchor the validator's response in the ledger, allowing the client to confirm on their own that a sufficient number of their preferred validators have confirmed a transaction. Requiring multiple validator attestations further reduces trust in the validator, as it increases both the technical and economic difficulty of compromising several other network participants.

Light Clients

A 'light client' is a cluster participant that does not itself run a validator. This light client would provide a level of security greater than trusting a remote validator, without requiring the light client to spend a lot of resources verifying the ledger.

Rather than providing transaction signatures directly to a light client, the validator instead generates a Merkle Proof from the transaction of interest to the root of a Merkle Tree of all transactions in the including block. This Merkle Root is stored in a ledger entry which is voted on by validators, providing it consensus legitimacy. The additional level of security for a light client depends on an initial canonical set of validators the light client considers to be the stakeholders of the cluster. As that set is changed, the client can update its internal set of known validators with receipts. This may become challenging with a large number of delegated stakes.

Validators themselves may want to use light client APIs for performance reasons. For example, during the initial launch of a validator, the validator may use a cluster provided checkpoint of the state and verify it with a receipt.

Receipts

A receipt is a minimal proof that; a transaction has been included in a block, that the block has been voted on by the client's preferred set of validators and that the votes have reached the desired confirmation depth.

Transaction Inclusion Proof

A transaction inclusion proof is a data structure that contains a Merkle Path from a transaction, through an Entry-Merkle to a Block-Merkle, which is included in a Bank-Hash with the required set of validator votes. A chain of PoH Entries containing subsequent validator votes, deriving from the Bank-Hash, is the proof of confirmation.

Transaction Merkle

An Entry-Merkle is a Merkle Root including all transactions in a given entry, sorted by signature. Each transaction in an entry is already merkled here: https://github.com/solana-labs/solana/blob/b6bfed64cb159ee67bb6bdbaefc7f833bbed3563/ledger/src/entry.rs#L205. This means we can show a transaction T was included in an entry E.

A Block-Merkle is the Merkle Root of all the Entry-Merkles sequenced in the block.

Together the two merkle proofs show a transaction T was included in a block with bank hash B.

An Accounts-Hash is the hash of the concatenation of the state hashes of each account modified during the current slot.

Transaction status is necessary for the receipt because the state receipt is constructed for the block. Two transactions over the same state can appear in the block, and therefore, there is no way to infer from just the state whether a transaction that is committed to the ledger has succeeded or failed in modifying the intended state. It may not be necessary to encode the full status code, but a single status bit to indicate the transaction's success.

Currently, the Block-Merkle is not implemented, so to verify E was an entry in the block with bank hash B, we would need to provide all the entry hashes in the block. Ideally this Block-Merkle would be implemented, as the alternative is very inefficient.

Block Headers

In order to verify transaction inclusion proofs, light clients need to be able to infer the topology of the forks in the network

More specifically, the light client will need to track incoming block headers such that given two bank hashes for blocks A and B, they can determine whether A is an ancestor of B (Below section on Optimistic Confirmation Proof explains why!). Contents of header are the fields necessary to compute the bank hash.

A Bank-Hash is the hash of the concatenation of the Block-Merkle and Accounts-Hash described in the Transaction Merkle section above.

In the code:

https://github.com/solana-labs/solana/blob/b6bfed64cb159ee67bb6bdbaefc7f833bbed3563/runtime/src/bank.rs#L3468-L3473

        let mut hash = hashv(&[
            // bank hash of the parent block
            self.parent_hash.as_ref(),
            // hash of all the modifed accounts
            accounts_delta_hash.hash.as_ref(),
            // Number of signatures processed in this block
            &signature_count_buf,
            // Last PoH hash in this block
            self.latest_blockhash().as_ref(),
        ]);

A good place to implement this logic along existing streaming logic in the validator's replay logic: https://github.com/solana-labs/solana/blob/b6bfed64cb159ee67bb6bdbaefc7f833bbed3563/core/src/replay_stage.rs#L1092-L1096

Optimistic Confirmation Proof

Currently optimistic confirmation is detected via a listener that monitors gossip and the replay pipeline for votes: https://github.com/solana-labs/solana/blob/b6bfed64cb159ee67bb6bdbaefc7f833bbed3563/core/src/cluster_info_vote_listener.rs#L604-L614.

Each vote is a signed transaction that includes the bank hash of the block the validator voted for, i.e. the B from the Transaction Merkle section above. Once a certain threshold T of the network has voted on a block, the block is considered optimistically confirmed. The votes made by this group of T validators is needed to show the block with bank hash B was optimistically confirmed.

However other than some metadata, the signed votes themselves are not currently stored anywhere, so they can't be retrieved on demand. These votes probably need to be persisted in Rocksdb database, indexed by a key (Slot, Hash, Pubkey) which represents the slot of the vote, bank hash of the vote, and vote account pubkey responsible for the vote.

Together, the transaction merkle and optimistic confirmation proofs can be provided over RPC to subscribers by extending the existing signature subscription logic. Clients who subscribe to the "Confirmed" confirmation level are already notified when optimistic confirmation is detected, a flag can be provided to signal the two proofs above should also be returned.

It is important to note that optimistically confirming B also implies that all ancestor blocks of B are also optimistically confirmed, and also that not all blocks will be optimistically confirmed.

B -> B'

So in the example above if a block B' is optimistically confirmed, then so is B. Thus if a transaction was in block B, the transaction merkle in the proof will be for block B, but the votes presented in the proof will be for block B'. This is why the headers in the Block headers section above are important, the client will need to verify that B is indeed an ancestor of B'.

Proof of Stake Distribution

Once presented with the transaction merkle and optimistic confirmation proofs above, a client can verify a transaction T was optimistically confirmed in a block with bank hash B. The last missing piece is how to verify that the votes in the optimistic proofs above actually constitute the valid T percentage of the stake necessary to uphold the safety guarantees of "optimistic confirmation".

One way to approach this might be for every epoch, when the stake set changes, to write all the stakes to a system account, and then have validators subscribe to that system account. Full nodes can then provide a merkle proving that the system account state was updated in some block B, and then show that the block B was optimistically confirmed/rooted.

Account State Verification

An account's state (balance or other data) can be verified by submitting a transaction with a TBD Instruction to the cluster. The client can then use a Transaction Inclusion Proof to verify whether the cluster agrees that the account has reached the expected state.

Validator Votes

Leaders should coalesce the validator votes by stake weight into a single entry. This will reduce the number of entries necessary to create a receipt.

Chain of Entries

A receipt has a PoH link from the payment or state Merkle Path root to a list of consecutive validation votes.

It contains the following:

• Transaction -> Entry-Merkle -> Block-Merkle -> Bank-Hash

And a vector of PoH entries:

• Validator vote entries
• Ticks
• Light entries

/// This Entry definition skips over the transactions and only contains the
/// hash of the transactions used to modify PoH.
LightEntry {
    /// The number of hashes since the previous Entry ID.
    pub num_hashes: u64,
    /// The SHA-256 hash `num_hashes` after the previous Entry ID.
    hash: Hash,
    /// The Merkle Root of the transactions encoded into the Entry.
    entry_hash: Hash,
}

The light entries are reconstructed from Entries and simply show the entry Merkle Root that was mixed in to the PoH hash, instead of the full transaction set.

Clients do not need the starting vote state. The fork selection algorithm is defined such that only votes that appear after the transaction provide finality for the transaction, and finality is independent of the starting state.

Verification

A light client that is aware of the supermajority set validators can verify a receipt by following the Merkle Path to the PoH chain. The Block-Merkle is the Merkle Root and will appear in votes included in an Entry. The light client can simulate fork selection for the consecutive votes and verify that the receipt is confirmed at the desired lockout threshold.

Synthetic State

Synthetic state should be computed into the Bank-Hash along with the bank generated state.

For example:

• Epoch validator accounts and their stakes and weights.
• Computed fee rates

These values should have an entry in the Bank-Hash. They should live under known accounts, and therefore have an index into the hash concatenation.