Bitcoin Single-use Seals

This section briefly describes the possible Single-use Seal structures that can be implemented using bitcoin as a publication medium and outlines the set of choices taken by the RGB protocol in particular.

There are 2 main ways in which a Single-use Seal can be defined in Bitcoin transactions:

• Public keys or addresses - the seal is defined by selecting an address or public key that has not yet been used (i.e. it has not been used by any locking script, so it is not locking any bitcoin).

• Bitcoin transaction outputs – the seal is defined by the selection of a specific UTxO available to some wallet.

The defined methods can be used in a combination of closing methods that differ according to how a spending transaction:

1. uses the seal definition: use of the address in the locking script or spending of the UTXO;

2. hosts the message on which the seal is closed according to a commitment scheme (i.e. in which part of the transaction the message is committed and stored).

The following table shows the 4 possible combinations of defining and closing a seal:

Seal type	Seal Definition	Seal Closing	Additional Requirements	Main application	Commitment schemes
PkO	Public key value	Transaction output	P2(W)PKH	none yet	keytweak, tapret, opret
TxO2	Transaction output	Transaction output	Requires Deterministic Bitcoin Commitments	RGB	keytweak, tapret, opret
PkI	Public key value	Transaction input	Taproot-only - Not working with legacy wallets	Bitcoin-based identities	sigtweak, witweak
TxOI	Transaction output	Transaction input	Taproot-only - Not working with legacy wallets	none yet	sigtweak, witweak

RGB protocol uses the TxO2 scheme in which both the Seal Definition and the Seal Closing use transaction outputs.

As shown in the table above, several commitment schemes can be used for each seal type. Each method differs in the location used by related transactions to host the commitment and, in particular, whether the message is committed to a location belonging to the input or output of the transaction:

• Transaction Input:

  • Sigtweak - the commitment is placed within the 32-byte random $r$ component that forms the ECDSA signature pair $<r,s>$ of an input. It makes use of Sign-to-contract (S2C).

  • Witweak - commitment is placed within the segregated witness data of the transaction.

• Transaction Output (scriptPubKey):

  • Keytweak - It uses the Pay-to-contract construction by which the public key of the output of the output is "tweaked" (i.e. modified) to contain a deterministic reference to the message.

  • Opret - used in RGB, the committed message is placed in an unspendable output after the opcode OP_RETURN.

  • Tapret (Taptweak) - This scheme, used in RGB, represents a form of tweak in which the commitment is an OP_RETURN leaf in the Script path of a taproot output which then modifies the value of the PubKey.

The different seal closing methods in Bitcoin transaction.

After reading this overview, it should now be easier to dive into details of RGB Single-use Seals construction.