Commitment Schemes within Bitcoin and RGB
In this chapter we will explore the application of Client-side Validation and Single-use Seal to Bitcoin Blockchain, introducing the main architectural features behind RGB protocol related to the commitment layer (layer 1).
As mentioned in the previous chapter, these cryptographic operations can be applied in general to different blockchains and also to different publication media. However, the outstanding properties of Bitcoin consensus algorithm, particularly those related to decentralization, censorship resistance, and permissionlessness, make it the ideal technology stack for the development of advanced programmability features, such as those required by digital bearer rights and smart contracts.
From the previous section, we recall that the creation of Single-use Seals is subject to two basic operations: Seal Definition and Seal Closing. We will now explore how these two operations are implemented using Bitcoin Transactions as a publication medium for the RGB protocol. A a more generic overview on single-use seals constructions on the bitcoin blockchain can be found here.
Single-use Seals in RGB
RGB seals are represented by bitcoin unspent transaction outputs (UTXO), so that:
A seal is defined by some state that points to a UTXO that identifies its owner
A seal is closed around a message when a transaction spends the UTXO and commits to a message
These two operations are then chained together by including a seal definition in the message around which the previous seal is closed, so that the witness transaction:
spends the UTXO on which a seal was defined, thus closing the seal
has an output that contains a commitment to a new seal definition
RGB Client-side Validation
In the next paragraphs, we will focus on client-side validation combined with the above construction of a single-use seal, showing them step by step trough the usual cryptographic characters: Alice, who deals with a seal operation, and Bob as an observer.
First of all, Alice has a UTXO that is linked to some client-side validated data known only by her.
Alice informs Bob that the spending of these UTXO represents a sign that some seal was closed.
Once Alice spends her UTXO, only Bob knows that this event has some additional meaning and consequences, even though everyone (i.e. the Bitcoin Blockchain audience) can see it.
In fact, the UTXO spent by Alice through the witness transaction contains a commitment to a change in the client-side validated data. By passing the original data to Bob, she is able to prove him that it is properly referenced by the commitment that Alice included in the witness transaction. Bob can independently verify that the new seal definition is unequivocally bound to the seal closing, so that no other players can be convinced to accept a different one as valid, effectively preventing double spend.
The key point of using the single-use seal in combination with client-side validation is the uniqueness of the spending event and the data committed (i.e., the message) in it, which cannot be changed in the future. The whole operation can be summarized in the following terms.
The next important step is to illustrate precisely how the two commitment schemes adopted in RGB protocol, Opret and Tapret, work and which features they must meet, particularly concerning commitment determinism.
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