Bitcoin Scripting Language (BSL) provides a high level language to write bitcoin L2 contracts then test these contracts with a simulated blockchain. The contracts and the blockchain are described in BSL and executed with the BSL execution environment. BSL supports contracts that require out of band communication between participants and the simulator supports these communications too.
BSL is best described with examples.
This example shows a coinbase transaction and how it is spent into a contract that can spent by two parties.
# Define keys
key alice, bob, carol
# Create coinbase with p2pkh contract, confirm it at given height and grab a reference to them
alice_coins = p2pkh alice amount 50 confirmation height 100
# Send alice coins to a contract allowing either bob or carol to spend further
spend alice_coins, signedby alice, receiver p2pkh bob amount 50 or p2pkh carol amount 50, confirmation height 200
This example shows a coinbase transaction and how it is spent into an HTLC and how the HTLC is spent as well.
# Define keys
key alice, bob, carol
# Create coinbase with p2pkh contract and confirm it
alice_coins = p2pkh alice, amount 50, confirmation height 100
# Define an HTLC contract with "secret" as the preimage
htlc_contract = htlc from alice, to bob, timelock 1000, preimage "secret", amount 10
# Spend alice coins to an htlc
bob_coins = spend alice_coins, signedby alice, receiver htlc_contract, confirmation height 200
# Spend bob's htlc contract to carol by revealing preimage
carol_coins = p2pkh carol, amount 10
spend htlc_contract, signedby bob, revealing "secret", receiver carol_coins, confirmation height 300
Next we show how BSL can be used to specify a contract for a one way channel.
# Define keys
key alice, bob
# Create coinbase with p2pkh contract and confirm it
bob_coins = p2pkh bob, amount 50, confirmation-height 100
# Multisig contract, that is the channel
channel = p2sh multisig 2 of 2, alice bob, amount 50
# Bond transaction
bond = spend bob_coins, signedby bob, receiver channel, unconfirmed
# bob p2pkh for refund
bob_return = p2pkh bob, amount 50
# Alice signs bond transaction after verifying bond spends to channel
refund = spend channel, signedby alice, receiver bob_return, after 1000, unconfirmed
# Bob sends bond to alice, who signs and broadcasts it
spend bond, signedby alice, receiver channel, confirmation height 200
# Make micropayments over the channel
channel_v1 = p2sh multisig 2 of 2 alice bob amounts alice 10 bob 40
pay_alice_v1 = spend channel signedby bob receiver channel_v1 unconfirmed
channel_v2 = p2sh multisig 2 of 2 alice bob amounts alice 20 bob 30
pay_alice_v2 = spend channel signedby bob receiver channel_v2 unconfirmed
# alice closes by spending v2
spend pay_alice_v2 signedby alice receiver channel_v2 confirmation height 300
There is a fair bit happening in the above example scripts, we use
high level constructs like p2pkh, htlc, sign, spend and
confirmation height to write out a series of statements that
describe the workings of a contract. The BSL execution engine then
runs these statements in the context of a simulated blockchain.
We can also use assertions to check the validity of our scripts. With the script in the above example, we can assert if a UTXO has been spent and verify they were spent to a specific output.
assert alice_coins are spent to htlc_contract
Note, unlike miniscript, the goal of BSL is not to generate production ready, optimised bitcoin Scripts. For generating bitcoin Scripts in production contracts, miniscript is the best choice at the moment.
We next show how BSL can be used to compose contracts.
In the HTLC example above we used a construct called htlc. We
implied that BSL provides the implementation of BSL as part of the
execution engine. In reality, the htlc construct is implemented
using the inbuilt operations provided by BSL.
define htlc from $sender to $receiver timelock $time preimage $secret amount $amount
receiver_spends = p2pkh $receiver and reveal sha256 $secret amount $amount
sender_spends = p2pkh $sender and after $time amount $amount
receiver_spends or sender_spends
define is used to compose new contracts from core constructs
provided by BSL. With define we can then describe contracts like DLC
and LN channel contracts. These can then be used to describe other
higher level contracts.
[TODO - List and explain core constructs supported by BSL]
To run tests raco test . for now. No package support is provided yet.
[TODO - Briefly describe related work and explain how BSL's goals are different. ]