Durable Transaction Nonces

Problem

To prevent replay, Solana transactions contain a nonce field populated with a "recent" blockhash value. A transaction containing a blockhash that is too old (~2min as of this writing) is rejected by the network as invalid. Unfortunately certain use cases, such as custodial services, require more time to produce a signature for the transaction. A mechanism is needed to enable these potentially offline network participants.

Requirements

1. The transaction's signature needs to cover the nonce value
2. The nonce must not be reusable, even in the case of signing key disclosure

A Contract-based Solution

Here we describe a contract-based solution to the problem, whereby a client can "stash" a nonce value for future use in a transaction's recent_blockhash field. This approach is akin to the Compare and Swap atomic instruction, implemented by some CPU ISAs.

When making use of a durable nonce, the client must first query its value from account data. A transaction is now constructed in the normal way, but with the following additional requirements:

1. The durable nonce value is used in the recent_blockhash field
2. An AdvanceNonceAccount instruction is the first issued in the transaction

Contract Mechanics

TODO: svgbob this into a flowchart

Start
Create Account
  state = Uninitialized
NonceInstruction
  if state == Uninitialized
    if account.balance < rent_exempt
      error InsufficientFunds
    state = Initialized
  elif state != Initialized
    error BadState
  if sysvar.recent_blockhashes.is_empty()
    error EmptyRecentBlockhashes
  if !sysvar.recent_blockhashes.contains(stored_nonce)
    error NotReady
  stored_hash = sysvar.recent_blockhashes[0]
  success
WithdrawInstruction(to, lamports)
  if state == Uninitialized
    if !signers.contains(owner)
      error MissingRequiredSignatures
  elif state == Initialized
    if !sysvar.recent_blockhashes.contains(stored_nonce)
      error NotReady
    if lamports != account.balance && lamports + rent_exempt > account.balance
      error InsufficientFunds
  account.balance -= lamports
  to.balance += lamports
  success

A client wishing to use this feature starts by creating a nonce account under the system program. This account will be in the Uninitialized state with no stored hash, and thus unusable.

To initialize a newly created account, an InitializeNonceAccount instruction must be issued. This instruction takes one parameter, the Pubkey of the account's authority. Nonce accounts must be rent-exempt to meet the data-persistence requirements of the feature, and as such, require that sufficient lamports be deposited before they can be initialized. Upon successful initialization, the cluster's most recent blockhash is stored along with specified nonce authority Pubkey.

The AdvanceNonceAccount instruction is used to manage the account's stored nonce value. It stores the cluster's most recent blockhash in the account's state data, failing if that matches the value already stored there. This check prevents replaying transactions within the same block.

Due to nonce accounts' rent-exempt requirement, a custom withdraw instruction is used to move funds out of the account. The WithdrawNonceAccount instruction takes a single argument, lamports to withdraw, and enforces rent-exemption by preventing the account's balance from falling below the rent-exempt minimum. An exception to this check is if the final balance would be zero lamports, which makes the account eligible for deletion. This account closure detail has an additional requirement that the stored nonce value must not match the cluster's most recent blockhash, as per AdvanceNonceAccount.

The account's nonce authority can be changed using the AuthorizeNonceAccount instruction. It takes one parameter, the Pubkey of the new authority. Executing this instruction grants full control over the account and its balance to the new authority.

AdvanceNonceAccount, WithdrawNonceAccount and AuthorizeNonceAccount all require the current nonce authority for the account to sign the transaction.

Runtime Support

The contract alone is not sufficient for implementing this feature. To enforce an extant recent_blockhash on the transaction and prevent fee theft via failed transaction replay, runtime modifications are necessary.

Any transaction failing the usual check_hash_age validation will be tested for a Durable Transaction Nonce. This is signaled by including a AdvanceNonceAccount instruction as the first instruction in the transaction.

If the runtime determines that a Durable Transaction Nonce is in use, it will take the following additional actions to validate the transaction:

1. The NonceAccount specified in the Nonce instruction is loaded.
2. The NonceState is deserialized from the NonceAccount's data field and confirmed to be in the Initialized state.
3. The nonce value stored in the NonceAccount is tested to match against the one specified in the transaction's recent_blockhash field.

If all three of the above checks succeed, the transaction is allowed to continue validation.

Since transactions that fail with an InstructionError are charged a fee and changes to their state rolled back, there is an opportunity for fee theft if an AdvanceNonceAccount instruction is reverted. A malicious validator could replay the failed transaction until the stored nonce is successfully advanced. Runtime changes prevent this behavior. When a durable nonce transaction fails with an InstructionError aside from the AdvanceNonceAccount instruction, the nonce account is rolled back to its pre-execution state as usual. Then the runtime advances its nonce value and the advanced nonce account stored as if it succeeded.