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.
- The transaction's signature needs to cover the nonce value
- The nonce must not be reusable, even in the case of signing key disclosure
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
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:
- The durable nonce value is used in the
AdvanceNonceAccountinstruction is the first issued in the transaction
TODO: svgbob this into a flowchart
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
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.
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
The account's nonce authority
can be changed using the
AuthorizeNonceAccount instruction. It takes one parameter,
Pubkey of the new authority. Executing this instruction grants full
control over the account and its balance to the new authority.
AuthorizeNonceAccountall require the current nonce authority for the account to sign the transaction.
The contract alone is not sufficient for implementing this feature. To enforce
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
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:
NonceAccountspecified in the
Nonceinstruction is loaded.
NonceStateis deserialized from the
NonceAccount's data field and confirmed to be in the
- The nonce value stored in the
NonceAccountis tested to match against the one specified in the transaction's
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.