Standard Contracts

Standard Contracts

This document introduces standard contracts and libraries in the sCrypt language.

Contracts can call each other. You can create an object of another contract within a contract, requiring the other contract to be unlocked before the current contract can be unlocked.

For example, in a P2PKH contract, you can require the unlocking conditions of P2PKH to simultaneously satisfy the conditions of HashPuzzle and P2PK contracts. The final form on the blockchain is a large UTXO contract that encapsulates several smaller contract codes, all of which must be unlocked to unlock the main contract.

Import

Different contracts or libraries can be encapsulated in different files and then imported using the import statement.

Library

A library is a special type of contract that can only be called by other contracts and cannot contain any public methods. It is mainly used to organize related parameters and static methods.

Some standard contracts provided by the official documentation handle common scenarios and can be used without importing.

https://scryptdoc.readthedocs.io/en/latest/contracts.html#full-list

Contract	Constructor Parameters	Public Function
Utils	None	toLEUnsigned(int n, int l) : bytes fromLEUnsigned(bytes b) : int readVarint(bytes b) : bytes writeVarint(bytes b) : bytes buildOutput(bytes outputScript, int outputSatoshis) : bytes buildPublicKeyHashScript(PubKeyHash pubKeyHash) : bytes buildOpreturnScript(bytes data) : bytes isFirstCall(SigHashPreimage preimage) : bool
Tx	None	checkPreimage(SigHashPreimage preimage) : bool checkPreimageOpt(SigHashPreimage rawTx) : bool checkPreimageOpt_(SigHashPreimage rawTx) : bool checkPreimageSigHashType(SigHashPreimage txPreimage, SigHashType sigHashType) : bool checkPreimageAdvanced(SigHashPreimage rawTx, PrivKey privKey, PubKey pubKey, int inverseK, int r, bytes rBigEndian, SigHashType sigHashType) : bool checkPreimageOCS(SigHashPreimage preimage) : bool checkPreimageOptOCS(SigHashPreimage rawTx) : bool checkPreimageOptOCS_(SigHashPreimage rawTx) : bool checkPreimageSigHashTypeOCS(SigHashPreimage txPreimage, SigHashType sigHashType) : bool checkPreimageAdvancedOCS(SigHashPreimage rawTx, PrivKey privKey, PubKey pubKey, int inverseK, int r, bytes rBigEndian, SigHashType sigHashType) : bool
SigHash	None	nVersion(SigHashPreimage preimage) : bytes hashPrevouts(SigHashPreimage preimage) : bytes hashSequence(SigHashPreimage preimage) : bytes outpoint(SigHashPreimage preimage) : bytes scriptCode(SigHashPreimage preimage) : bytes valueRaw(SigHashPreimage preimage) : bytes value(SigHashPreimage preimage) : int nSequenceRaw(SigHashPreimage preimage) : bytes nSequence(SigHashPreimage preimage) : int hashOutputs(SigHashPreimage preimage) : bytes nLocktimeRaw(SigHashPreimage preimage) : bytes nLocktime(SigHashPreimage preimage) : int sigHashType(SigHashPreimage preimage) : SigHashType
HashedMap<K, V>	bytes data	set(SortedItem\<K> keyWithIdx, V val) : bool canGet(SortedItem\<K> keyWithIdx, V val) : bool delete(SortedItem\<K> keyWithIdx) : bool has(SortedItem\<K> keyWithIdx) : bool clear() : bool size() : int data() : bytes
HashedSet<V>	bytes data	add(SortedItem<\V> entryWithIdx) : bool delete(SortedItem<\V> entryWithIdx) : bool has(SortedItem<\V> entryWithIdx) : bool clear() : bool size() : int data() : bytes

Utils

Utils are some commonly used utility classes and methods. The list of methods is as follows:

Tx

The Tx library contains essential content for reading the current transaction information. This is the core functionality for implementing stateful contracts and constraining subsequent contracts.

The core principle is to use a pseudo-operation OP_PUSH_TX to push the preimage of the current transaction to the contract runtime for verification. The preimage, as part of the unlocking parameters, allows the contract to perceive the characteristics of the transaction unlocking itself and to constrain the transaction based on these characteristics (https://github.com/bitcoin-sv/bitcoin-sv/blob/master/doc/abc/replay-protected-sighash.md).

Note that the transaction preimage requires the SIGHASH_FORKID signature type to be effective. Always verify the signature type SIG_HASH_ALL before verifying the preimage.

The transaction preimage includes the following information:

1. nVersion of the transaction (4-byte little endian)

2. hashPrevouts (32-byte hash)

3. hashSequence (32-byte hash)

4. outpoint (32-byte hash + 4-byte little endian)

5. scriptCode of the input (serialized as scripts inside CTxOuts)

6. value of the output spent by this input (8-byte little endian)

7. nSequence of the input (4-byte little endian)

8. hashOutputs (32-byte hash)

9. nLocktime of the transaction (4-byte little endian)

10. sighash type of the signature (4-byte little endian)

Key preimage checks include:

• checkPreimage(): Original pushTx

• checkPreimageAdvanced(): Advanced version with additional optional parameters

• checkPreimageOCS(): Signature verification with OCS opcodes, verifying only up to the point before checkPreimageOCS, allowing partial preimage usage for signature, reducing script size.

SigHash

The SigHash library is primarily used to access data in the preimage, such as using SigHash.scriptCode to get the scriptCode field and SigHash.value to get the value field.

The transaction preimage includes the following information (https://github.com/bitcoin-sv/bitcoin-sv/blob/master/doc/abc/replay-protected-sighash.md):

1. nVersion of the transaction (4-byte little endian) (version of the SPENDING TX)

2. hashPrevouts (32-byte hash) (hash of all input outpoints of the SPENDING TX)

3. hashSequence (32-byte hash) (hash of all input nSequence of the SPENDING TX)

4. outpoint (32-byte hash + 4-byte little endian) (outpoint corresponding to the current input of the SPENDING TX)

5. scriptCode of the input (serialized as scripts inside CTxOuts) (UTXO script corresponding to the spent TX point)

6. value of the output spent by this input (8-byte little endian) (amount corresponding to the spent TX point)

7. nSequence of the input (4-byte little endian) (sequence corresponding to the spent TX point)

8. hashOutputs (32-byte hash) (hash of all output scripts and amounts of the SPENDING TX, core to restricting the format of subsequent transactions)

9. nLocktime of the transaction (4-byte little endian) (nLockTime of the SPENDING TX)

10. sighash type of the signature (4-byte little endian) (SIGHASH type)

HashedMap

The HashedMap provides map-like functionality, requiring both a key and an index for most functions. Essentially, the key in this map is an aggregated type containing both the key itself and the index used for sorting, referred to as SortedItem<T>.

HashedSet

The HashedSet provides set-like functionality, similar to HashedMap, also requiring the SortedItem aggregate as the key.

Constants

Provides some commonly used business constants.

// number of bytes to denote input sequence
static const int InputSeqLen = 4;
// number of bytes to denote output value
static const int OutputValueLen = 8;
// number of bytes to denote a public key (compressed)
static const int PubKeyLen = 33;
// number of bytes to denote a public key hash
static const int PubKeyHashLen = 20;
// number of bytes to denote a tx id
static const int TxId

Len = 32;
// number of bytes to denote an outpoint
static const int OutpointLen = 36;