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chainside

developed with ❤️ by chainside

btcpy

btcpy is a Python3 SegWit-compliant library which provides tools to handle Bitcoin data structures in a simple fashion. In particular, the main goal of this library is to provide a simple interface to parse and create complex Bitcoin scripts.

N.B.: this library is a work in progress so it is highly discouraged to use it in a production environment. Also, as long as the version is 0.*, API breaking changes should be expected

Table of Contents

Requirements

The strict requirements of this library are:

pip install ecdsa
pip install base58

as an additional requirement, only used for integration testing purposes, this library uses:

pip install python-bitcoinlib==0.7.0

this is used to communicate with the Bitcoin node in order to test transactions validation.

Installation

To install this library and its dependencies one can just run

pip install chainside-btcpy

What it does

The main functionalities provided by this project are the following.

  • Parsing of blocks

  • Parsing, creation and signing of transactions

  • Parsing and creation of scripts. This also includes many nonstandard script types such as:

    • Hashlocked scripts
    • Timelocked scripts, with both absolute and relative times
    • Arbitrarily nested if-else clauses

    all scripts are easily embeddable in P2SH and P2WSH format, also supporting SegWit-over-P2SH formats. This library also offers functions to spend such complex scripts by only providing the necessary data.

What it does not do

This library does not implement the following functionalities:

  • Validation: when blocks, transactions and scripts are parsed, only format errors are reported. No proof-of-work validation, script execution, transaction validation and signature verification is performed. For these consensus-critical functionalities, users of this library should rely on Bitcoin Core or other libraries that perform validation.
  • Communication with the Bitcoin nodes. This is not provided neither on an RPC nor a networking level. For this purpose we highly recommed python-bitcoinlib.

Structure

All important data structures can be found in btcpy.structs, helper modules are located in btcpy.lib. Objects in btcpy.structs are meant as a public interface, while objects located in btcpy.lib are used internally.

Usage examples

Network setup

The first thing to do the first time this package is imported is to set the network on which it has to work. This is achieved by doing:

from btcpy.setup import setup
setup('regtest')

supported network types are:

regtest
testnet
mainnet

The btcpy.setup module also provides the following functions:

is_mainnet() - returns True if 'mainnet' was selected, False otherwise
net_name()   - returns the value that was selected when calling setup()

Parsing and serialization

Transaction, PublicKey, PrivateKey and Block can be extracted from a hex string by doing:

from btcpy.structs.transaction import Transaction
from btcpy.structs.block import Block
from btcpy.structs.crypto import PublicKey, PrivateKey
tx = Transaction.unhexlify(hex_tx)
block = Block.unhexlify(hex_block)
pubk = PublicKey.unhexlify(pubk_hex)
privk = PrivateKey.unhexlify(privk_hex)

PublicKey and PrivateKey can also be extracted from their BIP32 formats using the hd module:

>>> from btcpy.structs.hd import ExtendedPrivateKey, ExtendedPublicKey
>>> priv = ExtendedPrivateKey.decode('tprv8kxXxKwakWDtXvKBjjR5oHDFS7Z21HCVLMVUqEFCSVChUZ26BMDDH1JmaGUTEYGMUyQQBSfTgEK76QBvLephodJid5GTEiGFVGJdEBYptd7')
# priv.key holds a `PrivateKey`
>>> priv.key.hexlify()
'a12618ff6540dcd79bf68fda2faf0589b672e18b99a1ebcc32a40a67acdab608'
>>> pub = ExtendedPublicKey.decode('tpubDHea6jyptsuZRPLydP5gCgsN194xAcPPuf6G7kHVrm16K3Grok2oTVvdkNvPM465uuKAShgba7A2hHYeGGuS9B8AQGABfc6hp7mpcLLJUsk')
# pub.key holds a `PublicKey`
>>> pub.key.hexlify()
'025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260'

All these structures can be converted back to hex by using their hexlify() method.

In the same way, these structures can be serialized and deserialized by using their serialize() and deserialize() methods. These methods respectively return and expect a bytearray type.

Keys and addresses

The PublicKey class can handle both compressed and uncompressed public keys. In any case both the compressed and uncompressed version can be extracted. However, the structure will remember how it was initialised, so the hexlify(), hash() and to_address() methods will produce different results depending whether the PublicKey was initialised with a compressed or uncompressed public key. The to_segwit_address() method will always consider the key as compressed (P2WPKH addresses are only allowed with compressed keys). An example of this behaviour follows:

>>> uncomp = PublicKey.unhexlify('04ea4e183e8c751a4cc72abb7088cea79351dbfb7981ceb48f286ccfdade4d42c877d334c1a8b34072400f71b2a900a305ffae8963075fe94ea439b4b57978e9e8')
>>> compr = PublicKey(uncomp.compressed)
>>> uncomp.hexlify()
'04ea4e183e8c751a4cc72abb7088cea79351dbfb7981ceb48f286ccfdade4d42c877d334c1a8b34072400f71b2a900a305ffae8963075fe94ea439b4b57978e9e8'
>>> compr.hexlify()
'02ea4e183e8c751a4cc72abb7088cea79351dbfb7981ceb48f286ccfdade4d42c8'
>>> str(uncomp.to_address())
'mtDD9VFhPaRi6C6McMSnhb7nUZceSh4MnK'
>>> str(uncomp.to_segwit_address())
'tb1qxs0gs9dzukv863jud3wpldtrjh9edeqqqzahcz'  # this actually refers to the compressed version!
>>> str(compr.to_address())
'mkGY1QBotzNCrpJaEsje3BpYJsktksi3gJ'
>>> str(compr.to_segwit_address())
'tb1qxs0gs9dzukv863jud3wpldtrjh9edeqqqzahcz'

Please note that by default the to_address() and to_segwit_address() methods will return an address in the format of the network type specified in setup (regtest in the case of this example) but a flag can be passed to it to return an address for another network:

>>> str(uncomp.to_address(mainnet=True))
'1DhFrSAiaYzTK5cjtnUQsfuTca1wXvXfVY'
>>> str(compr.to_address(mainnet=True))
'15kaiM6q5xvx5hpxXJmGDGcDStABoGTzSX'

The PublicKey derived from a PrivateKey can be obtained by doing:

pubk = PrivateKey.unhexlify(privk_hex).pub()

the pub() method will return by default the compressed public key. The uncompressed version can be obtained by adding the flag compressed=False.

Additionally, one can make sure to use the compressed version of a key by using its compress() method:

>>> compr = uncomp.compress()
>>> str(compr.to_address())
'mkGY1QBotzNCrpJaEsje3BpYJsktksi3gJ'

Addresses can be either created from a PublicKey or from a script. In particular this second use case will be documented in the Script section.

Another low-level way to build an Address or SegWitAddress is by using their constructor and providing the following data:

address = Address(addr_type, hashed_data)
sw_address = SegWitAddress(addr_type, hashed_data, version)

where addr_type can be either 'p2pkh' or 'p2sh' in the case of Address and 'p2wpkh' or 'p2wsh' in the case of SegWitAddress. hashed_data must be a 160 or 256 bits long bytearray.

HD keys

The structs.hd module provides functionalities to handle BIP32 HD keys. Specifically, it provides the following two classes:

  • ExtendedPublicKey
  • ExtendedPrivateKey

These classes both provide the get_child(index, hardened=False) method. If called on an ExtendedPublicKey, hardened must be set to False, otherwise heardened can be either True or False. The ExtendedPublicKey corresponding to an ExtendedPrivateKey can be obtained through the pub() method.

As seen in the example above, ExtendedPublicKey and ExtendedPrivateKey contain the simpler structures PublicKey and PrivateKey, respectively. These structures can be accessed through the key attribute.

ExtendedPublicKeys also provide a derive() method which takes as input a string representing a path which either starts with 'm' or with '.'. 'm' indicates an absolute path and can be used only when derive() is called on a master key, '.' represents a relative path and can be used from any starting key. Examples of derivation paths:

  • m/0'/1'/2: absolute path, first two derivations hardened
  • ./0/128/256': relative path, last derivation hardened

Scripts

The main focus of this project is providing a simple way to create complex scripts. Scripts have the following hierarchy

  • BaseScript
    • ScriptSig
    • ScriptPubKey
      • P2pkhscript
      • P2wpkhScript
      • P2shScript
      • P2wshScript
      • P2pkScript
      • NulldataScript
      • MultisigScript
      • IfElseScript
      • TimelockScript
      • RelativeTimelockScript
      • Hashlock256Script
      • Hashlock160Script

Scripts have the following methods:

serialize()              - Returns the script as a bytearray
decompile()              - Returns a string representing the human readable opcodes and pushdata operations
hexlify()                - Returns the script as a hex string
unhexlify(hex_string)    - Creates the script from a hex string
is_standard()            - Returns whether the script complies with standardness rules as of Bitcoin Core commit a90e6d2bffc422ddcdb771c53aac0bceb970a2c4
type                     - A property containing a string which represents the type of the script
get_sigop_count()        - Returns the number of signature operations performed by the script
is_push_only()           - Returns whether the script is only made of push operations
to_address(segwit=False) - (only ScriptPubKey) Returns the script as either a P2SH or a P2WSH address, depending whether
                           the segwit flag is set

Low-level scripting functionalities

This section will introduce low-level creation and template-matching of scripts, for more advanced features please refer to the Transactions section.

This libary allows to create scripts from asm and from hex, as can be seen in the following examples.

Creating a script from asm (i.e. opcodes):

# this returns a bytearray with the compiled script
>>> compiled = Script.compile('OP_DUP OP_HASH160 a33ce8cf2760e2f9ef384bcbbe9a5491759feb14 OP_EQUALVERIFY OP_CHECKSIG')
# the bytearray can be passed to Script() to get a generic script
>>> script = Script(compiled)
# check that everything works as expected
>>> script.decompile()
'OP_DUP OP_HASH160 a33ce8cf2760e2f9ef384bcbbe9a5491759feb14 OP_EQUALVERIFY OP_CHECKSIG'
# beware, this is a generic script, no type recognition has been performed!
>>> script.type
'Script'

Creating a script from hex:

# this returns a bytearray with the compiled script
>>> script = Script.unhexlify('76a914a33ce8cf2760e2f9ef384bcbbe9a5491759feb1488ac')
# check that everything works as expected
>>> script.decompile()
'OP_DUP OP_HASH160 a33ce8cf2760e2f9ef384bcbbe9a5491759feb14 OP_EQUALVERIFY OP_CHECKSIG'
# beware, this is a generic script, no type recognition has been performed!
>>> script.type
'Script'

As we have seen, these are instantiated as generic scripts, if we want to obtain the appropriate script type, the ScriptBuilder class can be used. ScriptBuilder's method identify() will return the appropriate script type by performing template matching on the provided script.

Identifying a P2PKH script:

>>> script = ScriptBuilder.identify('76a914341e8815a2e5987d465c6c5c1fb56395cb96e40088ac')
>>> script.type
'p2pkh'
>>> script.decompile()
'OP_DUP OP_HASH160 341e8815a2e5987d465c6c5c1fb56395cb96e400 OP_EQUALVERIFY OP_CHECKSIG'
>>> script.pubkeyhash
bytearray(b'4\x1e\x88\x15\xa2\xe5\x98}F\\l\\\x1f\xb5c\x95\xcb\x96\xe4\x00')

Identifying a P2SH script

>>> script = ScriptBuilder.identify('a914bb18ed39c2a86f75f7bb5a9b36ba3581d77fd0f087')
>>> script.type
'p2sh'
>>> script.decompile()
'OP_HASH160 bb18ed39c2a86f75f7bb5a9b36ba3581d77fd0f0 OP_EQUAL'
>>> script.scripthash
bytearray(b'\xbb\x18\xed9\xc2\xa8ou\xf7\xbbZ\x9b6\xba5\x81\xd7\x7f\xd0\xf0')

Of course, all the types listed at the beginning of this section can be recognised, see the next section for more complex script types.

Transactions

Creating transactions

Transactions can be created by using the following classes:

  • TxIn, takes as input the following parameters:
    • txid, the txid of the transaction being spent
    • txout, the output number of the output being spent
    • script_sig, a scriptSig
    • sequence, the sequence number of the TxIn
  • Sequence, the constructor takes a sequence number, but it offers a couple of helper static methods for creation:
    • create(), which takes seq, lower 16 bits of sequence number, blocks, whether the seq param expresses blocks or a timestamp, and disable which sets the disable bit. For further info on how this all works, please refer to BIP68 specification.
    • max(), this automatically creates a Sequence object with the maximum sequence number (i.e. 0xffffffff).
  • ScriptSig, this can be initialised with a bytearray representing the script, but offers the following static methods:
    • empty(), this creates an empty ScriptSig, useful when initialising a transaction which has not been signed yet
  • StackData, this class represents data that scripts push on the stack, it offers methods to convert between the push operations and the actual data pushed.
  • Witness, this represents a SegWit witness, it is constructed with an array of StackData.
  • TxOut, takes as input the following parameters: value the value spent, in satoshis, n, the output number, script_pubkey, an object of type ScriptPubKey where the coins are being sent.
  • ScriptPubKey and derived classes, they take as input a bytearray representing the script but can also be created through the ScriptBuilder.identify() method or in the way displayed later in this section.
  • Locktime, takes as input a number representing the transaction's locktime field.
  • Transaction, takes as inputs: a version number, a list of TxIns, a list of TxOuts, a Locktime.
  • SegWitTransaction, has the same interface as Transaction

All the aforementioned classes are Immutable, this means that, after construction, their attributes can't be mutated. This helps caching values returned by their methods. The classes Transaction, SegWitTransaction and TxIn have mutable versions, unsurprisingly called MutableTransaction, MutableSegWitTransaction and MutbleTxIn, respectively. These mutable versions are mainly used to create unsigned transactions which then are mutated to add signatures to them. We will see how to use these in the rest of this section.

Example of a transaction creation:

>>> from btcpy.structs.transaction import Transaction, TxIn, Sequence, TxOut, Locktime
>>> script_sig = Script.unhexlify('48304502210083e6e7507e838a190f0443441c0b62d2df94673887f4482e27e89ff415a90392022050575339c649b85c04bb410a00b62325c1b82c537135fa62fb34fae2c9a30b0b01210384478d41e71dc6c3f9edde0f928a47d1b724c05984ebfb4e7d0422e80abe95ff')
>>> script_pubkey = ScriptBuilder.identify('76a914905f77004d081f20dd421ba5288766d56724c3b288ac')
>>> tx = Transaction(version=1,
...                  ins=[TxIn(txid='1a5a4f9a0d34cfca187db4fe6a3316f46264984c4b4c9fdb582123815afd508f',
...                            txout=0,
...                            script_sig=script_sig,
...                            sequence=Sequence.max())],
...                  outs=[TxOut(value=193000000,
...                              n=0,
...                              script_pubkey=script_pubkey)],
...                  locktime=Locktime(0))
>>> tx.txid
'14e6afbae7d2b1825b7ee711cbcad77d519767b70f5a1e70e5ba7f0bfc902e81'

Example creation of a SegWit transaction:

>>> from btcpy.structs.transaction import SegWitTransaction, Witness
>>> from btcpy.structs.script import StackData, empty_script
>>> witness_sig = StackData.from_bytes(unhexlify('304402200d0fbf48270e690be17cb0c47ee6ce2df3b671c2e4b196065e09c6df649b807c022056d8f10da83b2856458152c7f09e53a3495f3fbdd2e20638586a52ddff4f495b01'))
>>> witness_pubkey = StackData.from_bytes(unhexlify('02a079cb0269c933b1ee041a933092c9c439dd1b3a4eebd32ae391cf815002d378'))
>>> witness = Witness([witness_sig, witness_pubkey])
>>> script_pubkey = ScriptBuilder.identify('a914b2eb061810dac0614ac3e06d1bc55077b32b3b2687')
>>> tx = SegWitTransaction(version=1,
...                        ins=[TxIn(txid='1a5a4f9a0d34cfca187db4fe6a3316f46264984c4b4c9fdb582123815afd508f',
...                                  txout=0,
...                                  script_sig=empty_script,
...                                  sequence=Sequence.max(),
...                                  witness=witness],
...                        outs=[TxOut(value=193000000,
...                                    n=0,
...                                    script_pubkey=script_pubkey)],
...                        locktime=Locktime(0))
>>> tx.txid
'14dd31532ca06d62121fd13d35a2c9090246291960e73bf2bb3615abcb1bedab'

Of course, nobody would like to create transactions in such a cumbersome way. In fact, this library provides the appropriate tools to create complex scriptPubKeys in an easy fashion and to automatically fill in scriptSigs and witnesses of a spending transaction based on the minimum needed parameters. In the following sections we will show some examples of these features.

The supported scripts can be created by using their constructor and passing them the needed parameters. All the constructors of these classes can take an input of type Script. In this case they try to match it to their template and raise a WrongScriptTypeException if the script does not match the desired template. Otherwise, they take the following parameters:

Class Description Parameters
P2pkhscript, P2wpkhScript A P2PKH/P2WPKH script Either a PublickKey, a bytearray representing a public key hash or an Address
P2shScript A P2SH script Either a ScriptPubKey representing the redeemScript, a bytearray representing the redeemScript's hash or an Address
P2wshScript A P2WSH script Either a ScriptPubKey representing the witnessScript, a bytearray representing the witnessScript's hash or an Address
P2pkScript A P2PK script A PublicKey
NulldataScript An OP_RETURN script A StackData representing the data to store in the transaction
MultisigScript A multisig script, where m out of n keys are needed to spend m, the number of signatures needed to spend this output, an arbitrary number of PublicKeys, n the number of public keys provided
IfElseScript A script consisting of an OP_IF, a script, an OP_ELSE, another script and an OP_ENDIF Two ScriptPubKey scripts, the first to be executed in the if branch, the second to be executed in the else branch
TimelockScript A script consisting of <pushdata> OP_CHECKLOCKTIMEVERIFY OP_DROP and a subsequent script which can be spent only after the absolute time expressed by the <pushdata> is expired A Locktime, expressing the absolute time/number of blocks after which the subsequent script can be spent, and the locked ScriptPubKey
RelativeTimelockScript A script consisting of <pushdata> OP_CHECKSEQUENCEVERIFY OP_DROP and a subsequent script which can be spent only after the relative time time expressed by the <pushdata> is expired A Sequence, expressing the relative time/ number of blocks after which the subsequent script can be spent, and the locked ScriptPubKey
Hashlock256Script A script consisting of OP_HASH256 <pushdata> OP_EQUALVERIFY and a subsequent script which can be spent only after providing the preimage of <pushdata> for the double SHA256 hash function Either a bytearray or StackData representing the hashed value that locks the subsequent script, plus the locked ScriptPubKey
Hashlock160Script A script consisting of OP_HASH160 <pushdata> OP_EQUALVERIFY and a subsequent script which can be spent only after providing the preimage of <pushdata> for the RIPEMPD160 of the SHA256 hash function Either a bytearray or StackData representing the hashed value that locks the subsequent script, plus the locked ScriptPubKey

Please note that in the following sections we will frequently use the same keypair for ease of documenting, of course this is a very bad practice in a production environment and should be avoided at all costs.

Spending a transaction

This library offers Solvers to spend a previous transaction's output. Solvers expect as input all the data needed to create the appropriate scriptSig and witness. To create a Solver, the Sighash class is needed. This class represents a SIGHASH and its constructor takes two parameters:

  • sighash, either of the literal strings 'ONE', 'ALL' or 'NONE'
  • anyonecanpay, a flag defaulting to False.

The following solvers take one sighash as last parameter, defaulting to Sighash('ALL'):

  • P2pkhSolver
  • P2wpkhV0Solver
  • P2pkSolver

The MultisigSolver class takes many sighashes as additional last parameters, all defaulting to Sighash('ALL'). All other classes do not accept sighashes.

Additionally, the following solvers are available and they take the following inputs:

Class Inputs Solves
P2pkhSolver a PrivateKey P2pkhScript
P2wpkhV0Solver a PrivateKey P2wpkhV0Script
P2pkSolver a PrivateKey P2pkhScript
P2shSolver a ScriptPubKey, representing the redeemScript and a Solver which solves the redeemScript P2shScript
P2wshV0Solver a ScriptPubKey, representing the witnessScript and a Solver which solves the inner witnessScript P2wshV0Script
MultisigSolver an arbitrary number of PrivateKeys MultisigScript
IfElseSolver an object of type Branch. This is an enum and its values are Branch.IF and Branch.ELSE, these are used to specify whether we are spending the if or else branch of the script. The second parameter is a Solver for the script inside the desired branch. IfElseScript
TimelockSolver a Solver of the inner timelocked script TimelockedScript, RelativeTimelockScript
HashlockSolver the preimage needed to spend the script, as a bytearray, and a Solver for the hashlocked script Hashlock256Script, Hashlock160Script

To spend a previous transaction, the MutableTransaction class provides the spend() method. The spend() method expects the following inputs:

  • txouts, an array of TxOuts being spent by the transaction's inputs, in the correct order.
  • solvers, an array of Solvers, one per input, in the correct order

for example:

>>> to_spend = Transaction.unhexlify('...')
>>> unsigned = MutableTransction(version=1,
...                              ins=[TxIn(txid=to_spend.txid,
...                                        txout=0,
...                                        script_sig=ScriptSig.empty(),
...                                        sequence=Sequence.max())]
...                              outs=[TxOut(value=100000,
...                                          n=0,
...                                          script_pubkey=P2pkhScript(pubk))],
...                              locktime=Locktime(0))
>>> solver = P2pkhSolver(privk)
>>> signed = unsigned.spend([to_spend.outs[0]], [solver])

In particular, the spend() method automatically recognises whether we are spending a SegWit transaction, hence returning either a Transaction or a SegWitTransaction.

Now, let's see how more complex scripts can be created and spent. In the following examples, in solvers, we will always use the default SIGHASH_ALL, to change this, as described above, one can use the last parameter of the solvers that accept SIGHASHes.

P2PKH

This is how a P2PKH script can be created:

# create public key
>>> pubk = PublicKey.unhexlify('025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260')
# create P2PKH script
>>> p2pkh_script = P2pkhScript(pubk)
>>> p2pkh_script.hexlify()
'76a914905f77004d081f20dd421ba5288766d56724c3b288ac'
>>> str(p2pkh_script)
'OP_DUP OP_HASH160 905f77004d081f20dd421ba5288766d56724c3b2 OP_EQUALVERIFY OP_CHECKSIG'

and this is an example of a P2PKH solver:

>>> privk = PrivateKey.unhexlify('a12618ff6540dcd79bf68fda2faf0589b672e18b99a1ebcc32a40a67acdab608')
>>> p2pkh_solver = P2pkhSolver(privk)

now let's assume we have an unsigned mutable transaction, we will use this solver to fill in the transaction's scriptSig:

>>> unsigned_tx = MutableTransaction(...)
>>> previous_txout = TxOut(value=1000, n=0, script_pubkey=p2pkh_script)
>>> signed_tx = unsigned_tx.spend([previous_txout], [p2pkh_solver])

P2SH

Creating a P2SH script that embeds a P2PKH script:

>>> p2sh_script = P2shScript(P2pkhScript(pubk))
>>> p2sh_script.hexlify()
'a914cd1ab43e7c01a08886fd0e699988d2f44c9c57cc87'
>>> str(p2sh_script)
'OP_HASH160 cd1ab43e7c01a08886fd0e699988d2f44c9c57cc OP_EQUAL'

A solver to spend it would be:

>>> privk = PrivateKey.unhexlify('a12618ff6540dcd79bf68fda2faf0589b672e18b99a1ebcc32a40a67acdab608')
>>> solver = P2shSolver(P2pkhScript(pubk),  # the redeemScript
                        P2pkhSolver(privk)) # the redeemScript's solver

P2WSH

Creating a P2WSH script that embeds a P2PKH script:

>>> p2wsh_script = P2wshV0Script(P2pkhScript(pubk))
>>> p2wsh_script.hexlify()
'002058f04cd072784e9dede6821772a195cef65424f2e4957e14232e642bbbdf1aec'
>>> str(p2wsh_script)
'OP_0 58f04cd072784e9dede6821772a195cef65424f2e4957e14232e642bbbdf1aec'

Solving it:

>>> solver = P2wshV0Solver(P2pkhScript(pubk),  # witness script
...                        P2pkhSolver(privk)) # witness script's solver

P2WSH-over-P2SH

Let's now create a P2SH scriptPubKey that embeds a P2WSH that, in turn, embeds a P2PKH:

>>> p2wsh_over_p2sh = P2shScript(P2wshV0Script(P2pkhScript(pubk)))
>>> p2wsh_over_p2sh.hexlify()
'a914efbd1b969b0e15e7a3dc9b1128e4cf493974e62187'
>>> str(p2wsh_over_p2sh)
'OP_HASH160 efbd1b969b0e15e7a3dc9b1128e4cf493974e621 OP_EQUAL'
>>> solver = P2shSolver(
...              P2wshV0Script(P2pkhScript(pubk)),   # redeemScript
...              P2wshV0Solver(            # redeemScript solver
...                  P2pkhScript(pubk),    # witnessScript
...                  P2pkhSolver(privk)    # witnessScript solver
...              )
...           )

P2PK

>>> p2pk_script = P2pkScript(pubk)
>>> p2pk_script.hexlify()
'21025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260ac'
>>> str(p2pk_script)
'025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260 OP_CHECKSIG'
>>> solver = P2pkSolver(privk)

Multisig

>>> privk2 = PrivateKey.unhexlify('710b464f020b676fd9ec3af28d014dec9c8582e6a9059731a3e14aa762527ae4')
>>> pubk2 = privk2.pub()
>>> multisig_script = MultisigScript(1, pubk, pubk2, 2)  # a 1-of-2 multisig
>>> multisig_script.hexlify()
'5121025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe882602102a5f22a78db5c38eaa18f73390e82e000bd52ab84edbcb3ad9b4124460acaf5ee52ae'
>>> str(multisig_script)
'OP_1 025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260 02a5f22a78db5c38eaa18f73390e82e000bd52ab84edbcb3ad9b4124460acaf5ee OP_2 OP_CHECKMULTISIG'
>>> multisig_solver = MultisigSolver(privk)  # this could potentially be passed a list of SIGHASHES in the end to use them when signing

As one will usually embed this in a P2SH format, this could be done as follows:

>>> p2sh_multisig = P2shScript(multisig_script)
>>> solver = P2shSolver(multisig_script, multisig_solver)

Timelocks, Hashlocks, IfElse

Now we are going to create a very complex output. This output can be spent in two ways:

  1. at any time, with two out of two signatures
  2. 5 blocks after it has entered a block, with only one signature. This script is hence composed of two possible execution flows: an if branch and an else branch. Inside the first branch, a 2-of-2 multisig script can be found. Inside the second branch there is a timelocked script. Such a script has a time (a relative time in this case, expressed as a Sequence number) and an inner script, which is the one that can be executed after the relative time has expired. We can create such a script in the following way:
>>> timelocked_multisig = IfElseScript(
...     # if branch
...     MultisigScript(  # a multisig script, as above
...         2,
...         pubk,
...         pubk2,
...         2
...     ),
...     # else branch
...     RelativeTimelockScript(  # timelocked script
...         Sequence(5),  # expiration, 5 blocks
...         P2pkhScript(  # locked script
...             pubk
...         )
...     )
... )

Let's see this script a bit more in depth:

>>> timelocked_multisig.type
'if{ multisig }else{ [relativetimelock] p2pkh }'
>>> str(timelocked_multisig)
'OP_IF OP_2 025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260 02a5f22a78db5c38eaa18f73390e82e000bd52ab84edbcb3ad9b4124460acaf5ee OP_2 OP_CHECKMULTISIG OP_ELSE OP_5 OP_CHECKSEQUENCEVERIFY OP_DROP OP_DUP OP_HASH160 905f77004d081f20dd421ba5288766d56724c3b2 OP_EQUALVERIFY OP_CHECKSIG OP_ENDIF'
>>> timelocked_multisig.if_script
MultisigScript(2, 025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260, 02a5f22a78db5c38eaa18f73390e82e000bd52ab84edbcb3ad9b4124460acaf5ee, 2)
>>> str(timelocked_multisig.if_script)
'OP_2 025f628d7a11ace2a6379119a778240cb70d6e720750416bb36f824514fbe88260 02a5f22a78db5c38eaa18f73390e82e000bd52ab84edbcb3ad9b4124460acaf5ee OP_2 OP_CHECKMULTISIG'
>>> timelocked_multisig.else_script
RelativeTimelockScript(5, OP_DUP OP_HASH160 905f77004d081f20dd421ba5288766d56724c3b2 OP_EQUALVERIFY OP_CHECKSIG)
>>> str(timelocked_multisig.else_script)
'OP_5 OP_CHECKSEQUENCEVERIFY OP_DROP OP_DUP OP_HASH160 905f77004d081f20dd421ba5288766d56724c3b2 OP_EQUALVERIFY OP_CHECKSIG'
>>> timelocked_multisig.else_script.locked_script
P2pkh(905f77004d081f20dd421ba5288766d56724c3b2)
>>> timelocked_multisig.else_script.locked_script.decompile()
'OP_DUP OP_HASH160 905f77004d081f20dd421ba5288766d56724c3b2 OP_EQUALVERIFY OP_CHECKSIG'

Let's write the solvers for this script:

>>> solver_if = IfElseSolver(Branch.IF,                      # branch selection
...                          MultisigSolver(privk, privk2))  # inner solver
>>> solver_else = IfElseSolver(Branch.ELSE,
...                            TimelockSolver(P2pkhSolver(privk)))

Low-level signing

If one wants to sign a transaction by hand, instead of using solvers, one of the following procedures can be used:

  • Manually writing the scriptSig (this can be seen in the Creating transactions section)
  • Creating the scriptSig by computing and signing the digest of the transaction

Let's see an example of this last case:

>>> unsigned = MutableTransaction(...)
>>> digest = unsigned.get_digest(2,   # the input to be signed
                                 prev_script,  # the previous script to spend (this is the redeem/witness script in case of P2SH/P2WSH ouputs)
                                 sighash=Sighash('NONE', anyonecanpay=True))  # sighash: 0x02 | 0x80
>>> privk.sign(digest)

In case one wants to sign a SegWit digest for the transaction, the following can be done:

>>> unsigned = SegWitTransaction(...)
>>> digest = unsigned.get_segwit_digest(2,   # the input to be signed
                                        prev_script,  # the previous script to spend (this is the redeem/witness script in case of P2SH/P2WSH ouputs)
                                        prev_amount,  # the amount of the output being spent
                                        sighash=Sighash('NONE', anyonecanpay=True))  # sighash: 0x02 | 0x80
>>> privk.sign(digest)

Contributing and running tests

This library has two testing tools that can be found in the tests/ folder:

  • unit.py, this runs basic unit testing
  • integration.py, this runs tests of signed transactions, to do this, transactions are signed and sent to a Bitcoin Core node through the sendrawtransaction command.

To make sure these tests are using the code in the current repository and not a stale copy installed in a virtualenv or system wide, please make sure to run the following commands from the root of the repo:

python3 -m unittest tests/unit.py
python3 -m unittest tests/integration.py

Contributors are invited to run these tests before submitting PRs. Also, contributions to improve and expand these tests are highly welcome.

TODO

Since this library is still a work in progress, the following roadmap lists the improvements to be done:

  • Expanding the test suites
  • Improving and expanding this documentation
  • Adding docstrings where missing (many places)
  • Handling OP_CODESEPARATORs in the signing process
  • Adding caching to segwit digest computation to avoid quadratic hashing
  • Add further transaction creation helpers
  • Add RPC calls to Bitcoin Core nodes
  • Add networking with Bitcoin Core nodes
  • Add methods to generate private keys from entropy