Components of a Contract Operation
Let's now deep-dive into all the components of a contract operation, which are capable of changing the state of the contract and which are ultimately verified client-side by the legitimate recipient in a deterministic manner.
With the help of the comprehensive diagram above, it's important to point out that any contract operation is composed of some components related to the New State and some components that reference the Old State being updated. The components of the New state are:
The Assignments in which are defined:
The Global State, which can be either mutated or added.
The Valencies which can be defined in State Transition and Genesis.
The Old State is referenced through:
Inputs connected to previous assignments of related State Transitions. Not found in Genesis.
In addition, we also have several operation-specific fields:
FfvorFast-forward versiona 2-byte integer indicating the version of the contract, following the RGB rules of fast-forward versioning. The version of the contract can be updated according to the issuer's choices and needs at certain points in the contract's history, such as concerning re-issuances.TransitionTypeorExtensionTypea 16-bit integer indicating the type of Transition/Extension expressed by the operation encoded in the Contract Schema and representing the manifestation of the Business Logic of the contract. It's not present in Genesis.ContractIdthe 32-byte number that references the processedOpIdof the Genesis of the contract. Naturally, it's present in State Transitions and Extensions, but not in Genesis.SchemaIdis a field found only in Genesis, instead of theContractId. It's a 32-byte hash of the contract Schema used in the contract.Testnetis a boolean variable indicating the use of Bitcoin Testnet or Mainnet. It is only present in Genesis.Altlayers1is a variable that indicates which layer of the Blockchain is used as the Commitment medium for the client-side validated data as an alternative to Bitcoin (e.g. Liquid Sidechain). It is only present in Genesis.Metadatathat allows you to declare temporary variables that are useful for validating complex contracts, but that do not have to be recorded as state properties.
Finally, through a custom hashing methodology, all of the fields in the Contract Operation are summarized into an OpId commitment that is placed in the Transition Bundle.
We will cover each contract component in a dedicated subsection. The complete memory layout of each component of a contract operation is given here.
OpId
Each Contract Operation is identified by a 32-byte hash called OpId, which is, precisely, the ordered SHA-256 hashing of the element contained in the State Transition. Each Contract Operation has its own customized commitment and hashing methodology.
ContractId
As an important additional feature, the ContractId of a smart contract is calculated by using the OpId of its Genesis and applying to it a Reverse byte order operation plus a Base58 encoding.
Contract State
Before addressing each state component, it's essential to clarify through which elements a Contract State is expressed in the RGB protocol. Specifically, in RGB, the State of a contract is fully expressed by:
A single Global State
One or more Owned State(s) that can belong to two different categories:
Private States
Public States
Global States are embedded into Contract Operation as a single component block, while Owned States are defined inside the Assignment construct where they are stored alongside the pertaining Seal Definition.
State update methods and rules
An important feature of RGB, which affects both Global and Owned States, is the way the state is modified. Basically, States may exhibit two different behaviors:
A Mutable behavior, in which each state transition discards the previous state and assigns a new one.
An Accumulating behavior, in which each state transition adds a new state to the previous state.
In all cases where the Contract State is neither Mutated nor Accumulated, the respective components are left empty, meaning that no repetition of data takes place in a Contract Operation.
The choice between mutable or accumulated state is set inside the Business Logic encoded in the Schema of the contract and cannot be changed after the Genesis, except by some State Extension specifically encoded herein.
The table below provides a summary of the rules regarding the permitted modification of Global/Owned States by each Contract Operation:
Genesis | State Extension | State Transition | |
Adds Global State | + | * | + |
Mutates Global State | n/a | * | + |
Adds Owned State | + | * | + |
Mutates Owned State | n/a | No | + |
Adds Valencies | + | + | + |
+ = if allowed by Contract Schema * = if Confirmed by a State Transition
As a final consideration of this section, in the following table we provide a summary of the main scope-related properties that the various kind of state data elements exhibit in the RGB protocol.
Metadata | Global state | Owned state | |
Scope | Defined per contract operation | Defined per contract globally | Defined per single-use-seal (Assignment) |
Who can update | Not updatable | Operation creators | Controlled by right owners (parties able to close single-use-seal) |
Time scope | Defined just for a single operation | State is defined after/as a result of the operation | State is defined before the operation (when the seal definition is embedded in the previous operation) |
Global State
The purpose of Global State can be summarized by the following sentence:"nobody owns, everyone knows" in that it defines certain general features of the contract that must be publicly visible. A Global State is always a public state, and can be written in Genesis by the contract issuer and later modified in state transition or state extensions by a legitimate party defined in the contract schema.
As an important feature, the Global State is usually made available by contract issuers or contract participants and distributed through both centralized and decentralized public networks (e.g. websites, IPFS, Nostr, Torrent, etc.) in form of a contract consignment. It's important to note that the availability of the Global State is incentivized only by economic means of using and sharing the contract to the wider public: the parties involved are committed to bear the cost of the necessary storage solution that enables the accessibility of this kind of contract data.
Each component of a Global State consists of a 2-field structure that includes:
A
GlobalTypewhich embeds a deterministic reference to the global propriety expressed in the Schema.The actual Data expressing the property.
For example, a Global State of newly issued token written in Genesis, dependent on the Non inflatable Asset Schema and Contract Interface RGB 20, contains generally, as common GlobalTypes:
The
ticker.The full name of the token:
name.The precision of decimal digits:
precision.The maximum supply of the token:
issuedSupply.The date of issuance:
created.A text with some Legal disclaimer:
data.
Assignments
Assignments are the core constructs responsible for the Seal Definition operation and related Owned State to which that Seal Definition is bound. They are the central part that enables the rightful transfer of a digital property, described in the Owned State, to a New Owner identified by the possession of a specific Bitcoin UTXO. An Assignment can be compared to a Bitcoin Transaction Output, but possibly embedding more expressiveness and potential.
Each Assignment consists of the following components:
The
AssignmentTypewhich is the identifier of the digital property that is stored in the Assignment (e.g. theassetOwnerdeclaration used to declare the transfer function in token transfers).The
Seal Definitionwhich is a sub-construct containing the reference to the UTXO.The
Owned Statewhich specifies how the properties associated with theAssignmentTypeare modified.
Revealed / Concealed form
As a unique feature of RGB, both the Seal Definition and the Owned State can be expressed in Revealed or Concealed form. This is particularly useful for maintaining - selectively - high privacy and scalability both in the construction of state transitions and in subsequent validation by the various parties that may be involved in the contract. The constructs in Revealed form can be used to validate the same data entered in the previous State Transitions with their hash digest representing the Concealed form of the construct. In the diagram below, all four Reveal/Conceal form combinations are shown:
Since the concealment methodology of each construct may vary, we will discuss the respective forms for each construct when necessary. As a final remark in this paragraph, according to the RGB consensus rules the OpId of the State Transition is always calculated from the concealed data.
Seal Definition
The first main component of the Assignment construct is the Seal Definition which, in its Revealed form, is itself a structure consisting of four fields: txptr vout blinding method.
txptris a more complex object than a simple hash of a Bitcoin Transaction. In particular, it can have two distinct kinds:Graph sealwhich can have, itself, two separate forms:A simpler case where, as in in
Genesis seal, thetxidpoints to the UTXO chosen as a seal.A
WitnessTxform which should be interpreted as a "self-referenced" definition. The use of this construct means that the transaction used as the seal definition coincides with the witness transaction that includes the present Assignment. Since the finaltxidof the transaction depends on all the state transition data, includingtxptr, it would be impossible to compute it because of the implied circular reference. In practice theWitnessTxis a null field that has become necessary to handle several situations where no external UTXO is available: a notable example is the generation and update of Lightning Network's commitment transactions or when the recipient doesn't have any available UTXO.
voutis the transaction output number of the Transaction Id entered intxptr, and it is only present iftxptris aGraph seal. Thetxptrfield together withvoutfield constitute the standard outpoint representation of Bitcoin transactions.blindingis a random number of 8 bytes, which allows the seal data to be effectively hidden once they have been hashed, improving resistance to brute-force attacks.methodis a 1-byte field indicating the seal closing method, which will be used in the related witness transaction. It's either Tapret or Opret.
The concealed form of the Seal Definition is simply the SHA-256 tagged hash of the concatenation of the four fields:
SHA-256(SHA-256(seal_tag) || SHA-256(seal_tag) || txptr || vout || blinding || method)
Where:
seal_tag = urn:lnp-bp:seals:secret#2024-02-03
Owned States
This second Assignment component is responsible for defining and storing the data assigned by the Seal Definition. Before proceeding with the features of Owned States, a brief digression about Conceal/Reveal feature of this construct is necessary. Unlike the Global State, Owned States come in two forms:
Public Owned States: in which related data must always be kept and transferred in a revealed form by their owner recursively. For example, they may apply to some image files that must be bound to ownership, but are always publicly shown. This form can be described by the phrase: "someone owns, everybody knows".
Private Owned States: in which related data is kept hidden and revealed only if it is part of the history for validation purposes. For example, the number of tokens transferred in a token contract is generally kept private. This form can be summarized by the sentence: "someone owns, nobody knows".
In RGB, an Owned State can only be defined with one of the four StateTypes: Declarative, Fungible, Structured, Attachments, each of which has its concealed and Revealed form:
Declarativeis a StateType with no data, representing some form of governance rights that can be performed by a contract party. For example, it can be used for voting rights. Concealed and Revealed forms of it coincide.Fungibleis the StateType that allows for the transfer of fungible units such as those in a token contract. In the Revealed form it consists of two fields: anamountand ablindingfactor, while in concealed form it is transformed into a 1-field structure containing a Pedersen commitment which commits to theamountand to theblindingfactor of the revealed form. In a future update, it would be possible to implement Zero-KnowledgeIts cryptographic proofs such asBulletproofthat will be able to prove that within the same State Transition the sum ofInputsthat refer to a fungible state equals the sum of fungibleOwned Stateswithout revealing the actual amounts.Structuredis a State Type that can accommodate ordered and limited data collections of arbitrary content, which can be used as input for complex contract validation schemes. Its maximum storage size is limited to a maximum of 64 KiB. The Revealed form is simply the blob of data serialized into bytes, while the concealed form is the SHA-256 tagged hash of that data blob:SHA-256(SHA-256(tag_data) || SHA-256(tag_data) || blob), wheretag_data = urn:lnp-bp:rgb:state-data#2024-02-12.Attachmentstype is used to attach an arbitrary file with a defined purpose, such as a media file, audio file, text, binary, etc. The actual file is kept separated by the Owned State construct itself, as, in revealed form, the Attachment structure contains three fields: the SHA-256file_hash, the MIMEmedia typeand asaltfactor that provides additional privacy. In concealed form, this StateType is the SHA-256 tagged hash of the three fields just described:SHA-256(SHA-256(tag_attachment) || SHA-256(tag_attachment) || file_hash || media_type || salt), wheretag_attachment = urn:rgb:state-attach#2024-02-12.
The diagram below shows a summary of the four state types and their Concealed and Revealed forms:
In addition, a summary of the technical characteristics of each StateType is provided in the table below:
| Item | Declarative | Fungible | Structured | Attachments |
|---|---|---|---|---|
Data | None | 64-bit signed/unsigned integer | Any strict data type | Any file |
Type info | None | Signed/unsigned | Strict Types | MIME type |
Confidentiality | Not Required | Pedersen commitment | Hashing with blinding | Hashed file id |
Size limits | N/A | 256 Byte | Up to 64 kByte | Up to ~500 GByte |
Inputs
Similar to Bitcoin Transactions, Inputs represent the "other half" of the Assignment construct. They have the basic role of referencing Assignments from a previous State Transition or Genesis. Inputs are not present in Genesis and State Extension Operation and consist of the following fields:
PrevOpIdcontaining the identifier of the previous Assignment operation being referenced.AssignmentTypecontaining the identifier of the contract property being modified by the referenced Assignment.Indexis the index number of the Assignment being referenced within the Assignment list of thePrevOpId. TheIndexis calculated implicitly from the lexicographic sorting of the hashes of the Concealed Seal of the referenced Assignments.
The validation procedure of RGB, in addition to checking the correct closure of the Seal, is also responsible for checking the consistency between the inputs and outputs, particularly for the Fungible StateType. In this case, the validation procedure, embedded in the AluVM script part of the Schema, checks that the amount of tokens of each Input of a specific AssignmentType matches the number of tokens of the Assignments with the same AssignmentType.
As a natural property, Genesis has no Inputs as well as all State Transitions that don't change some Owned States of any kind. For example, a State Transition that changes only the Global State has no Inputs.
Metadata
The metadata construct is a particular field that contains all the information that is not useful to be stored as part of the contract state history. It has a maximum size of 64 KiB and can be used, for example, to host temporary data from a complex contract validation procedure by the AluVm engine.
Valencies
Valencies are a unique-in-its-kind construct that can be present in all three forms of Contract Operation. In essence, they are a set of digital rights that can be redeemed in State Extensions and "put into effect" by a subsequent State Transition. In RGB, Valencies are encoded simply by enumerating each ValencyType, which is a list of 16-bit fields that define the particular right encoded by the Valency. As GlobalType and AssignmentType, the appropriate meaning and semantics are encoded and defined in the Contract Schema and decoded into human form by a related Interface.
Redeems
Redeems are similar to State Transition's Inputs for Valencies. They are included only in State Extensions and are responsible for "activating" the digital right embedded in the Valency itself. An example of Redeem might be the execution of a coinswap or a distributed issuance. Redeems consist of two fields:
The 32-byte
PrevOpIdfield that refers to the hash of the Operation in which the Valency to be redeemed is defined.The 16-bit
ValencyTypefield that is retrieved from the previous operation in which the Valency is defined. Each ValencyType can be redeemed only once within the same State Extension.
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