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$$ \newcommand \LogicSig {\mathrm{LSig}} \newcommand \LogicSigMaxSize {\LogicSig_{\max}} \newcommand \LogicSigMaxCost {\LogicSig_{c,\max}} $$

Execution Environment for Logic Signatures

Logic Signatures execute as part of testing a proposed transaction to see if it is valid and authorized to be committed into a block. If an authorized program executes and finishes with a single non-zero uint64 value on the Stack, then that program has validated the transaction it is attached to.

The program has access to data from the transaction it is attached to (txn opcode), any transactions in a transaction group it is part of (gtxn opcode), and a few global values like consensus parameters (global opcode).

Some arguments may be attached to a transaction being validated by a program. Arguments are an array of byte strings. A common pattern would be to have the key to unlock some contract as an argument.

Be aware that Logic Signature arguments are recorded on the blockchain and publicly visible when the transaction is submitted to the network, even before the transaction has been included in a block. These arguments are not part of the transaction ID nor of the transaction group hash (grp). They also cannot be read from other programs in the group of transactions.

Bytecode Size

The size of a Logic Signature is defined as the length of its bytecode plus the length of all its arguments. The sum of the sizes of all Logic Signatures in a group MUST NOT exceed \( \LogicSigMaxSize \) bytes times the number of transactions in the group1.

Opcode Budget

Each opcode has an associated cost, usually \( 1 \), but a few slow operations have higher costs.

Before Version 4, the program’s cost was estimated as the static sum of all the opcode costs in the program (whether they were actually executed or not).

Beginning with Version 4, the program’s cost is tracked dynamically while being evaluated. If the program exceeds its opcode budget, it fails.

The total program cost of all Logic Signatures in a group MUST NOT exceed \( \LogicSigMaxCost \) times the number of transactions in the group1.

⚙️ IMPLEMENTATION

Opcode budget tracker reference implementation.

Modes

A program can either authorize some delegated action on a normal signature-based or multisignature-based account or be wholly in charge of a contract account.

Delegated Signature Mode

If the account has signed the program (by providing a valid Ed25519 signature or valid multisignature for the authorizer address on the string Program concatenated with the program bytecode) then, if the program returns True, the transaction is authorized as if the account had signed it. This allows an account to hand out a signed program so that other users can carry out delegated actions that are approved by the program. Note that Logic Signature arguments are not signed.

Contract Account Mode

If the SHA-512/256 hash of the program (prefixed by Program) is equal to the authorizer address of the transaction sender, then this is a contract account wholly controlled by the program. No other signature is necessary or possible. The only way to execute a transaction against the contract account is for the program to approve it.

  1. See the Ledger parameters section. ↩ ↩2