easyutxo

EasyUTXO

A simple yet extendable, programmable and powerful UTXO ledger model with advanced concepts

EasyUTXO at this stage is PoC, however it has intention to become a fully-functional UTXO ledger used in DLT projects.

EasyUTXO uses EasyFL as the language for its functional constraints. Here is a dexription of EasyFL.

Introduction

The EasyUTXO is an implementation of the UTXO concept well-known in distributed ledger technologies, such as Bitcoin or IOTA (Stardust release).

The EasyUTXO model of the UTXO ledger intends to go beyond the Stardust ledger model, which is hardcoded and language-dependent to certain extent. The EasyUTXO introduces programmability of the ledger transition function (transaction model), while same time preserving robustness and determinism of the UTXO ledger model by adding formal verifiability and minimizing the global trust assumptions. The reference code EasyUTXO model is written in Go, however the transaction model (UTXO types/behavior) are completely platform independent.

The above is achieved by:

• building very generic and minimalistic transaction model (the minimization of assumptions)
• programming almost all the state transition logic as the finite functional constraints, expressed in the EasyFL functional constraint language.

The whole model of the state transition is finite and non-Turing complete, because EasyFL language does not allow loops and recursions.

This makes EasyFL program (expression) and its specification is the same thing. With this trait of the EasyUTXO makes it possible automatic prove of the ledger properties, e.g. if it can achieve desired (or undesired) states. For example the answer to the questions Is it possible to spend funds without knowing private key? or Is the output with specific constraint unlockable? can be automated. This is not possible with Turing-complete models without adding formal specifications to the program code./

This way essentially all the Stardust UTXO types are expressed in provable EasyFL constraints: all kind of transfers, chain constraints, aliases, NFTs, time lock, expiry conditions etc (by the time of writing, native tokens and foundries remains in future plans).

This also makes extension and programmability of the UTXO transactions by embedding new constraints right into the transaction (inline), or extending library of constrains with new constraints, available globally.

The functional nature of the EasyFL makes it easy to extend the computation model of the ledger even further, for example with Turing-complete computations of WebAssembly, EVM, Move or any other.

The above makes EasyUTXO a powerful yet safe architectural abstraction layer for any further developments of the UTXO ledgers.

Transaction model

Output

UTXO ledger consists of UTXOs (Unspent TransaXion Outputs). Each output is an array of constraints: (C1, C1m ..., CN). Each constraint Ci is an EasyFL expression (formula) in its bytecode form.

For the output to be valid, all constraints of the output are evaluated. If all formulas does not panic and return true, the output is valid. If constraint evaluates to false or panics, the whole output is invalidated.

Each constraint is evaluated by supplying to the EasyFL engine:

• global context of the transaction being validated
• local place (path) in the transaction where being evaluated constraint is located. This constraints the data in the local context. For example, constraint addressED25519(0x12345...) by knowing its location, reaches out the corresponding unlock block or signature to check if the signature is valid

Any data which appears in the output comes in the form of constraint, i.e. is runnable. Examples of constraints:

• amount(1000) check validity of the amount
• timestamp(123456) checks validity of the timestamp in the output
• chainLock(0xaaaaaaaaaaaaaa) check if output can be consumed, i.e. if the chain 0xaaaaaaaaaaaaa.. is transited in the same transaction

Transaction

Ledger is updated in atomic units, called transaction. Each transaction consist of:

• consumed outputs, up to 255
• produced outputs, up to 255
• transaction level data, such as transaction timestamp, input commitment, user signature, provided constraint scripts etc

We distinguish full validation context of the transaction and transaction it self.

Validation context includes all consumed inputs in its entirety. It is necessary for validation of the transaction. The transaction itself only contains inputIDs instead of consumed outputs. The transaction is the data transferred over the wire between nodes. To validate it, the node rebuild validation context on its own copy of the ledger state.

Validation of the transaction

The node runs the following steps fo each transaction received by the wire:

• rebuilds validation context
• it evaluates all constraints on each consumed output
• it evaluates all constraints on each produced output
• checks integrity of global constraints, which cannot be expressed in EasyFL (unbounded):
  • balances of amount(N) constraints must be equal on consumed and produced side
  • each outputs must have one of predefined locks (addresses)
  • checks validity of the input commitment

The above loop is the only global assumption of the ledger. It is completely agnostic of what constraint are provided. This way UTXO behavior is encoded into the constraints on outputs.

Examples

Example of user-readable transaction printout:

  Transaction. ID: 32xdf79cf3a04c4030d733553db328611413929da07c0be6ee5ac6a3385f0d45c42, size: 390
  Timestamp: 4x636b89b1 (1667991985)
  Input commitment: 32x8152eb3defc209036763c044e2b509953708536458a39e07434dc397bc800bb8
  Inputs (consumed outputs): 
    #0: [1]32xf0a31a9cf03579b51e5365b5738a8c7980c7f2e8bebcdc62766a6e31d1bffb84 (97 bytes)
       0: amount(u64/2000) (11 bytes)
       1: timestamp(u32/1667991981) (7 bytes)
       2: addressED25519(0xb400219a67085d3a22c1fee43844ab06995e1e1d2384b3f98ceddba6e2b75273) (35 bytes)
       3: chain(0x0000000000000000000000000000000000000000000000000000000000000000ffffff) (38 bytes)
       Unlock data: [0x,0x,1xff,3x000300]
    #1: [1]32xc3b6521c9415f8be70d15de2827d66b46e9167b6ecec9ed4e4b60e64774363e2 (58 bytes)
       0: amount(u64/1000) (11 bytes)
       1: timestamp(u32/1667991984) (7 bytes)
       2: chainLock(0xb3051a25fef3ee5163c469646600323e654b871e8aec34330ce85e81ec42fc04) (35 bytes)
       Unlock data: [0x,0x,2x0003]
  Outputs (produced): 
    #0 (97 bytes) :
       0: amount(u64/2500) (11 bytes)
       1: timestamp(u32/1667991985) (7 bytes)
       2: addressED25519(0xb400219a67085d3a22c1fee43844ab06995e1e1d2384b3f98ceddba6e2b75273) (35 bytes)
       3: chain(0xb3051a25fef3ee5163c469646600323e654b871e8aec34330ce85e81ec42fc04000300) (38 bytes)
    #1 (58 bytes) :
       0: amount(u64/500) (11 bytes)
       1: timestamp(u32/1667991985) (7 bytes)
       2: addressED25519(0xb400219a67085d3a22c1fee43844ab06995e1e1d2384b3f98ceddba6e2b75273) (35 bytes)

Example of output constraint in EasyFL (timelock in this case):

// enforces output can be unlocked only after specified time
// $0 is Unix seconds of the time lock
func timelock: or(
    and(
        selfIsProducedOutput,
        equal(len8($0), 4),             // must be 4-bytes long
        lessThan(txTimestampBytes, $0)  // time lock must be after the transaction (not very necessary)
    ),
    and(
        selfIsConsumedOutput,
        lessThan($0, txTimestampBytes)  // is unlocked if tx timestamp is strongly after the time lock 
    )
)