Changes to PoX and Clarity

PoX Contract Changes

PoX Failure

In the event that PoX does not activate, the chain halts. If there are no Stackers, then block production cannot happen.

Changes to PoX

To support tenure changes, Nakamoto uses a new PoX contract, .pox-4. The .pox-4 contract would be altered over the current PoX contract (.pox-3) to meet the following requirements:

  • Stackers register a WSTS signing key when they call stack-stx or a delegate provides the signing key with delegate-stack-stx.

In addition, a .stackers boot contracts will be created which carries out the following tasks:

  • The .stackers contract will expose each reward cycle's full reward set, including the signing keys for each stacker, via a public function. Internally, the Stacks node will call a private function to load the next reward set into the .stackers data space after it identifies the PoX anchor block. This is required for some future Stacks features that have been discussed.

  • The .stackers contract will expose two private functions to update the signing set for the following reward cycle.

    • update-signer-set: will update the reward set for the upcoming reward cycle. It will take in a list of at maximum size 4096 of signer pubkeys.

    • update-current-signer: will update the the specified signer with their corresponding key-ids.

  • The .stackers contract will expose a function to vote on the aggregate public key used by the Stackers to sign new blocks in the current tenure.

    • vote-for-aggregated-public-key takes the key of the signer calling the contract, the reward cycle number, the round number, and the list of all tapleaves.

New Stacks-on-Chain Rules

The stack-stx Stacks-on-Chain transaction will be extended to include the Stackerโ€™s signing key, as follows:

        0      2  3              19           20           53
         magic  op  uSTX to lock    num-cycles  signing key

The new field, signing key, will contain the compressed secp256k1 ECDSA public key for the stacker.

In addition, the following two new Stacks-on-Chain transactions are added:


In Nakamoto, it will now be possible for a stacking delegate to lock up their delegated STX via a Bitcoin transaction. It shall contain an OP_RETURN with the following payload:

        0      2  3              19           20             21        25
         magic  op  uSTX to lock    num-cycles  has-pox-addr?  index

Where op = +, uSTX to lock is the number of microSTX to lock up, num-cycles is the number of reward cycles to lock for. The field has-pox-addr? can be 0x00 or 0x01. If it is 0x01, then index is treated as a 4-byte big-endian integer, and is used as an index into the transaction outputs to identify the PoX address the delegate must use. The value 0 refers to the first output after the OP_RETURN. The outputโ€™s scriptPubKey is then decoded and converted to the PoX address. If index points to a nonexistent transaction output or if scriptPubKey cannot be decoded into a PoX address, then this transaction is treated as invalid and has no effect. If has-pox-addr? is 0x00 instead, then index is not decoded and the delegate may choose the PoX address.


In Nakamoto, it will now be possible for a stacking delegate to commit delegated STX behind a PoX address. It shall contain an OP_RETURN with the following payload:

        0      2  3       7           11
         magic  op  index  reward cycle

Where op = *, and as with delegate-stack-stx above, index is a big-endian 4-byte integer which points to a transaction output whose scriptPubKey must decode to a supported PoX address. The reward cycle field is a 4 byte big-endian integer which identifies the reward cycle for which to aggregate the stacked STX.

Changes to Clarity

Nakamoto produces Stacks blocks occur at a much greater frequency than in the past. Many contracts rely on the block-height primitive to approximate a time assuming that a block takes, on average, 10 minutes. To release faster blocks while preserving the functionality of existing contracts that make this block frequency assumption, Nakamoto will use a new version of Clarity, version 3, which includes the following changes.

  1. A new primitive stacks-block-height that indicates the block height under Nakamoto

  2. A new Clarity global variable tenure-height will be introduced, which evaluates to the number of tenures that have passed. When the Nakamoto block-processing starts, this will be equal to the chain length.

  3. The clarity global variable block-height will continue to be supported in the existing Clarity 2 contracts by returning the same value as tenure-height, but usage of block-height in a Clarity 3 contract will trigger an analysis error.

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