Anchors

Anchors are the client-side validated structures that summarize all the data needed to validate contract commitments, which were described in the previous section. They are composed of the following ordered fields:

• Txid

• MPC Proof

• Extra Transaction Proof ETP

Where:

TxId

Txid is the 32-byte ID of the transaction containing the Opret / Tapret commitment. Note that TxId could theoretically be reconstructed from the off-chain data of state transitions pointing to each witness transaction and thus following the chain of single-use seals. However, to simplify the validation task, they are included in the anchor.

MPC Proof

The MPC Proof of the contract c_i consists of depth, cofactor and Merkle Proof which were described previously.

Extra Transaction Proof - ETP

If an Opret commitment is used, no additional proof is provided in this field, since, as described in the previous section, the verifier inspects the first OP_RETURN output finding the correct mpc::Commitement.

If a Tapret commitment is used, a so called Extra Transaction Proof - ETP must be provided, which consists of:

• Internal Public Key P of the Taproot output used.

• Partner node(s) of the Taproot Script Path Spend which is either:

  • The top left branch (in the example: tHABC) if the Tapret commitment is on the right side of the tree.

  • The left and right nodes of the upper right branch (in the same example they are: tHAB and tHC) if the Tapret commitment is on the left side of the tree.

• The nonce, if used, to optimize the Partner node part of the proof.

In the next section, we will introduce concepts purely concerning the off-chain part of RGB, i.e. contracts, giving an abstract view of the partially replicated finite state machine that gives RGB a much greater expressiveness than can be achieved through Bitcoin Script without sacrificing confidentiality.

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