Global Variables
Global time lock variables
Bitcoin Cash has support for different kinds of time locks, which can be used to specify time-based conditions inside Bitcoin Cash contracts. These time locks can be separated by three main attributes: location, targeting and metric. The location can be the transaction level or the contract level, but contract level locks also require you to use a corresponding transaction level lock. The targeting can be relative (e.g. at least 4 hours have passed) or absolute (e.g. the block number is at least 600,000). The metric can be blocks or seconds.
It can be difficult to fully grasp the intricacies of time locks, so if you're starting out it is recommended to start off with the simplest version: absolute block-based time locks. If you do want to dive into the more advanced uses of time locks, James Prestwich wrote the best article explaining time locks in Bitcoin that also fully applies to Bitcoin Cash.
tx.time
tx.time is used to create absolute time locks. The value of tx.time can either represent the block number of the spending transaction or its timestamp. When comparing it with values below 500,000,000, it is treated as a blocknumber, while higher values are treated as a timestamp.
tx.time corresponds to the nLocktime field of the current transaction using the OP_CHECKLOCKTIMEVERIFY opcode. Because of this tx.time can only be used in the following way:
require(tx.time >= <expression>);
To access the value of the nLocktime field for assignment, use the tx.locktime introspection variable.
Because of the way time locks work, a corresponding time lock needs to be added to the transaction. The CashScript SDK automatically sets this transaction level time lock to the most recent block number, as this is the most common use case. If you need to use a different block number or timestamp, this should be passed into the CashScript SDK using the withTime() function. If the default matches your use case, no additional actions are required.
tx.age
tx.age is used to create relative time locks. The value of tx.age can either represent a number of blocks, or a number of chunks, which are 512 seconds. The corresponding transaction level time lock determines which of the two options is used.
tx.age corresponds to the nSequence field of the current UTXO using the OP_CHECKSEQUENCEVERIFY opcode. Because of this tx.age can only be used in the following way:
require(tx.age >= <expression>);
To access the value of the nSequence field for assignment, use the tx.inputs[i].sequenceNumber introspection variable.
Because of the way time locks work, a corresponding time lock needs to be added to the transaction. This can be done in the CashScript SDK using the withAge() function. However, the value passed into this function will always be treated as a number of blocks, so it is currently not supported to use tx.age as a number of second chunks.
Introspection variables
Introspection functionality is used to create covenant contracts. Covenants are a technique used to put constraints on spending the money inside a smart contract. The main use case of this is limiting the addresses where money can be sent and the amount sent. To explore the possible uses of covenants inside smart contracts, read the CashScript Covenants Guide.
this.activeInputIndex
int this.activeInputIndex
During the validation of a BCH transaction, every transaction input is evaluated in order, and the contract's code is evaluated in the context of the different inputs. this.activeInputIndex represents the index of the input that is currently being evaluated. This can be used in conjunction with the properties under tx.inputs.
this.activeBytecode
bytes this.activeBytecode
During the validation of a BCH transaction, every transaction input is evaluated in order, and the contract's code is evaluated in the context of the different inputs. this.activeBytecode represents the contract bytecode of the input that is currently being evaluated.
tx.version
int tx.version
Represents the version of the current transaction. Different transaction versions can have differences in functionality. Currently only version 1 and 2 exist, where only version 2 has support for BIP68.
tx.locktime
int tx.locktime
Represents the nLocktime field of the transaction. This is similar to the tx.time global variable but tx.time can only be used in require statements, not for variable declaration.
The use case for tx.locktime is to read the nLocktime value and add to the local state. Example usage for this is demonstrated in the Sablier example.
tx.inputs
Represents the list of inputs of the evaluated transaction. This is an array, and cannot be used on itself. You need to access an input with a specific index and specify the properties you want to access.
tx.inputs.length
int tx.inputs.length
Represents the number of inputs in the transaction.
tx.inputs[i].value
int tx.inputs[i].value
Represents the value of a specific input (in satoshis).
tx.inputs[i].lockingBytecode
bytes tx.inputs[i].lockingBytecode
Represents the locking bytecode (scriptPubKey) of a specific input.
tx.inputs[i].unlockingBytecode
bytes tx.inputs[i].unlockingBytecode
Represents the unlocking bytecode (scriptSig) of a specific input.
tx.inputs[i].outpointTransactionHash
bytes32 tx.inputs[i].outpointTransactionHash
Represents the outpoint transaction hash where a specific input was initially locked.
tx.inputs[i].outpointIndex
int tx.inputs[i].outpointIndex
Represents the outpoint index where a specific input was initially locked.
tx.inputs[i].sequenceNumber
int tx.inputs[i].sequenceNumber
Represents the nSequence number of a specific input.
tx.inputs[i].tokenCategory
bytes tx.inputs[i].tokenCategory
Represents the tokenCategory of a specific input. Returns 0 when that specific input contains no tokens. When the input contains an NFT with a capability, the 32-byte tokenCategory is concatenated together with 0x01 for a mutable NFT and 0x02 for a minting NFT.
The tokenCategory is returned in the original unreversed order. Explorers and wallets change the byte order by default but for CashScript you need to be careful to use the unreversed order.
tx.inputs[i].nftCommitment
bytes tx.inputs[i].nftCommitment
Represents the NFT commitment data of a specific input.
tx.inputs[i].tokenAmount
int tx.inputs[i].tokenAmount
Represents the amount of fungible tokens of a specific input.
tx.outputs
Represents the list of outputs of the evaluated transaction. This is an array, and cannot be used on itself. You need to access an output with a specific index and specify the properties you want to access.
tx.outputs.length
int tx.outputs.length
Represents the number of outputs in the transaction.
tx.outputs[i].value
int tx.outputs[i].value
Represents the value of a specific output (in satoshis).
tx.outputs[i].lockingBytecode
bytes tx.outputs[i].lockingBytecode
Represents the locking bytecode (scriptPubKey) of a specific output.
tx.output[i].tokenCategory
bytes tx.output[i].tokenCategory
Represents the tokenCategory of a specific output. Returns 0 when that specific output contains no tokens. When the output contains an NFT with a capability, the 32-byte tokenCategory is concatenated together with 0x01 for a mutable NFT and 0x02 for a minting NFT.
The tokenCategory is returned in the original unreversed order. Explorers and wallets change the byte order by default but for CashScript you need to be careful to use the unreversed order.
tx.output[i].nftCommitment
bytes tx.output[i].nftCommitment
Represents the NFT commitment data of a specific output.
tx.output[i].tokenAmount
int tx.output[i].tokenAmount
Represents the amount of fungible tokens of a specific output.
Constructing locking bytecode
One of the main use cases of covenants is enforcing transaction outputs (where money is sent). To assist with enforcing these outputs, there is a number of LockingBytecode objects that can be instantiated. These locking bytecodes can then be compared to the locking bytecodes of transaction outputs.
Example
bytes25 lockingBytecode = new LockingBytecodeP2PKH(pkh);
int value = 10000;
require(tx.outputs[0].lockingBytecode == lockingBytecode);
require(tx.outputs[0].value == value);
LockingBytecodeP2PKH
new LockingBytecodeP2PKH(bytes20 pkh): bytes25
Creates new P2PKH locking bytecode for the public key hash pkh.
LockingBytecodeP2SH20
new LockingBytecodeP2SH20(bytes20 scriptHash): bytes23
Creates new P2SH20 locking bytecode for the script hash, where scriptHash is the hash160() of your script.
Because the old P2SH20 addresses use hashes which are only 20-bytes in length, they are vulnerable to hash collision in situations where an attacker can introduce arbitrary data to a contract. To solve these security issues P2SH32 has been introduced. It is recommended to always use LockingBytecodeP2SH32 from now on.
LockingBytecodeP2SH32
new LockingBytecodeP2SH32(bytes32 scriptHash): bytes35
Creates new P2SH32 locking bytecode for the script hash, where scriptHash is the hash256() of your script.
LockingBytecodeNullData
new LockingBytecodeNullData(bytes[] chunks): bytes
Creates new OP_RETURN locking bytecode with chunks as its OP_RETURN data.
Globally available units
An integer literal can take a suffix of either monetary or temporary units to add semantic value to these integers and to simplify arithmetic. When these units are used, the underlying integer is automatically multiplied by the value of the unit.
BCH Units
The units sats, finney, bits and bitcoin can be used to denominate Bitcoin Cash amounts.
Example
require(1 sats == 1);
require(1 finney == 10);
require(1 bits == 100);
require(1 bitcoin == 1e8);
Time Units
The units seconds, minutes, hours, days and weeks can be used to denote time.
Be careful when using these units in precise calendar calculations though, because not every year equals 365 days and not even every minute has 60 seconds because of leap seconds.
Example
require(1 seconds == 1);
require(1 minutes == 60 seconds);
require(1 hours == 60 minutes);
require(1 days == 24 hours);
require(1 weeks == 7 days);
