Contract Operations
Last updated
Last updated
The approach followed in this paragraph is the same as that developed in the using our beloved cryptographic characters Alice and Bob.
This time the explanation contains an important difference: Bob is not simply validating the client-side validated data that Alice shows him. He is actually asking Alice to add some additional data that will give Bob some degree of ownership over the contract expressed as a hidden reference to one of his Bitcoin . Let's see how the process works in practice for a (one of the fundamental ):
Alice has a of client-side validated data, which references a Bitcoin UTXO owned by her. This means that in her client-side validated data there is a that points to one of her UTXOs.
Bob, in turn, also has unspent UTXOs. These UTXOs are completely unrelated to Alice's, which means that there is no direct spending event that creates a link between them. If Bob doesn't possess any UTXO it is also possible to spend towards him the witness transaction output containing the commitment to the client-side data and implicitly associate the ownership of the contract to him.
Validates every RGB state transition in the Consignment, including the one creating some New State assigned to his own UTXO.
A transition bundle is essentially the collection of all state transitions in a given witness transaction that operate on a certain contract. In the simplest case, such as the one shown above between Alice and Bob, a Transition Bundle consists of a single state transition.
BundleId = SHA-256(SHA-256(bundle_tag) || SHA-256(bundle_tag) || InputMap)
Where:
bundle_tag = urn:lnp-bp:rgb:bundle#2024-02-03
The InputMap associates N inputs of the witness transaction (by their vin) to the set of K_N RGB transitions that involve state that used to be linked to the spent outpoints and it gets serialized in the following way:
Where:
N is the total number of inputs of the witness transaction that refer to some OpIds.
OpId_i(input_j) is the Operation Identifier of the i-th State Transition that spends some state associated with the j-th input of the witness transaction.
Note: In general, there is a many-to-many relationship between witness transaction inputs and the transitions. This for instance allows to:
use allocations from two different UTXOs as inputs to the same state transition, e.g.:
Alice owns 10 assets on utxo_1 plus 5 assets on utxo_2 and wants to send 12 assets to Bob
The input map will look like: {utxo_1: {opid_1}, utxo_2: {opid_1}}
use different transitions to move more than one allocation on a given UTXO
Alice owns both assets and an inflation right on utxo_1 and wants to spend it without burning anything
The input map will look like: {utxo_1: {opid_1, opid_2}}
To prevent double spends, client-side validation thus needs to check that every allocation only appears once in state transition inputs.
The following figure shows the three components of contract operations along with their position in the RGB contract DAG, linked to their respective anchors in the Bitcoin Blockchain. State Transitions are represented by the red blocks.
It is important to note that the main difference between ordinary State Transitions and Genesis lies in the lack of the closing part of the seal. Hence Genesis, in order to appear into the blockchain history, requires a State Transition that closes one of the seals defined by it.
To give an example, in the case of a contract defining the creation of a token, the following data are likely to be inscribed in Genesis:
The number of tokens issued and their owner(s) (the owner(s) of the UTXO referred to in the seal definition's genesis).
The maximum number of tokens to be issued in total.
The possibility of inflation and the designed party that possesses this right.
As a natural implication, Genesis does not refer to any previous state transition, nor does it close any previously defined seal. As mentioned above, to be effectively validated in the history of the chain, a Genesis must be referenced by a first state transition (e.g. an auto-spend to the issuer or a first round of distribution), which finalizes the "first ownership" of the contract through a commitment to the Bitcoin Blockchain.
Bob, through some informational data, encoded in an , instructs Alice to create a New state that follows the rules of the contract and which embeds a new seal definition pointing to one of his UTXOs in a concealed form (more on that ). In this way, Alice assigns Bob some ownership of the new state: for example, ownership of a certain amount of tokens.
After that, Alice, using some wallet tool, prepares a transaction that spends the UTXO that was indicated by the previous seal definition (the very same one that granted her ownership of some elements of the contracts). In this transaction, which is a , Alice embeds in one output a commitment to the new state data that uses or rules depending on the chosen method. As explained earlier, Opret or Tapret commitments are derived from an tree which may collect more than one contract's state transition.
Before transmitting the transaction thus prepared, Alice passes to Bob a data packet called which contains the organized stash of client-side data already in possession of Alice in addition to the new state. Bob, at this point, using RGB consensus rules:
Verifies that every RGB state transition is committed to in a valid , ensuring legitimacy and uniqueness of the whole history from Genesis to the new state.
After checking the correctness of the Consignment, Bob may optionally give Alice a "go-ahead" signal (e.g., by GPG signing the ). Alice, even without Bob's clearance, can now broadcast this last witness transaction, containing the New State. Once confirmed, such a witness transaction represents the conclusion of the from Alice to Bob.
For the detailed process of a contract transfer with the RGB stack, see the .
It's useful to see the full details of a DAG of two RGB contract operations - ( + a ) - both from the RGB client-side components, which will be covered in the next few paragraphs, and from the connection points to the Bitcoin Blockchain which embeds the seal definition and the witness transaction.
Just to give an introduction to the context of the above diagram, let us introduce terminology that will be discussed in greater technical detail:
The construct, which is pointed at Alice (in this example by the Genesis), and later used by Alice and pointed at Bob, is responsible for two things:
The which points to a specific UTXO (to Alice's first, by the Genesis created by a contract issuer, and later to Bob's by Alice herself).
The association of the Seal Definition to specific sets of data called which, depending on the properties of the contract, can be chosen from several types. To give a simple context example, the amount of tokens transferred is a common kind of Owned State.
, on the other hand, reflects general and public properties of a contract that are not meant to be owned, such as the total issued supply for a inflatable fungible asset.
As mentioned , a State Transition represents the main form within (in addition to ). State Transitions refer to one or more previously defined state(s) - in Genesis or another State Transition - and update them to a New State.
As an interesting scalability feature of RGB, multiple State Transitions can be aggregated in a Transition Bundle, so that each bundling operation fits one and only one contract leaf in the tree:
A collects all transitions that refer to a given contract.
The Transition Bundle is hashed to produce its , which is included in a leaf of the Tree at a position that is determined by its contract ID.
When all bundles are included in the tree, the empty leaves are filled with random data and its merkle root is computed. The MPC commitment, composed by the merkle root and parameters used in the tree construction, is finally included into a Tapret or Opret output thanks to , so that the bitcoin transaction unequivocally commits to a set of RGB state transitions.
The represents the connection point between the Bitcoin Blockchain and the RGB client-side validation structure.
In the following paragraphs, we will delve into all the elements and processes involved in the State Transition operation. All topics discussed from now on belong to RGB Consensus, which is encoded in the .
However, RGB natively supports batching operations, so that one or more payers can send assets to one or more payees and multiple transition types and state types are involved (e.g. atomically sending tokens and inflation rights). In these cases, all the operations involving a certain contract would belong to the same position in the MPC tree, so they need to be deterministically bundled in order to fit in a single MPC leaf. The corresponding then needs to close all the seals that are spent by each state transition.
From a more technical angle, the BundleId to be inserted in the leaf of the is from a tagged hash of the strict serialization of the InputMap field of the bundle in the following way:
State transitions, just covered in the previous sections, allow the transfer of ownership of certain state properties from one party to another. However, state transitions are only one of the allowed in RGB. The other one, which allows to generate new state at the issuance of a contract, is called Genesis.
Another obvious, but crucial, aspect to keep in mind is that the Active State(s) are the last state(s) at the leaves of the of contract operations that reference themselves in the order committed into the Bitcoin Blockchain, starting from the Genesis. All other states associated with spent UTXOs are no longer valid but are critical to the validation process.
Genesis represents the initial data block of each RGB contract. It is constructed by a and any state transitions must be connected to it through the DAG of the contract operations. In Genesis, according to the rules defined in the , are defined several properties affecting deeply the contract states, both of and form.
