- Native Scala objects for various protocol types ([transactions](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/transaction/Transaction.scala), [inputs](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/transaction/TransactionInput.scala), [outputs](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/transaction/TransactionOutput.scala), [scripts signatures](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/script/ScriptSignature.scala), [scriptpubkeys](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/script/ScriptPubKey.scala))
- [Serializers and deserializers for bitcoin data structures mentioned above](https://github.com/bitcoin-s/bitcoin-s-core/tree/master/src/main/scala/org/bitcoins/core/serializers)
- [An implementation of Bitcoin's Script programming language](https://github.com/bitcoin-s/bitcoin-s-core/tree/master/src/main/scala/org/bitcoins/core/script)
- Passes all tests found in Bitcoin Core's regression test suite called [script_test.json](https://github.com/bitcoin/bitcoin/blob/master/src/test/data/script_tests.json)
- Passes all tests inside of Bitcoin Core's transaction regression test suite [tx_valid.json](https://github.com/bitcoin/bitcoin/blob/master/src/test/data/tx_valid.json) / [tx_invalid.json](https://github.com/bitcoin/bitcoin/blob/master/src/test/data/tx_invalid.json) /
- Integration with [bitcoin core's optimized secp256k1](https://github.com/bitcoin-core/secp256k1/) library
- Consensus rule set up to date through segregated witness
- A robust set of [generators](https://github.com/bitcoin-s/bitcoin-s-core/tree/master/src/main/scala/org/bitcoins/core/gen), which are used in property based testing
- These are extremely useful for testing bitcoin applications
- Here is an example of a specification for our [ECPrivateKey](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/test/scala/org/bitcoins/core/crypto/ECPrivateKeySpec.scala)
- [Number types](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/number/NumberType.scala#L16) to represent C's unsigned numbers ([`UInt8`](https://github.com/bitcoin-s/bitcoin-s-core/blob/1c7a7b9f46679a753248d9f55246c272bb3d63b9/src/main/scala/org/bitcoins/core/number/NumberType.scala#L91), [`UInt32`](https://github.com/bitcoin-s/bitcoin-s-core/blob/1c7a7b9f46679a753248d9f55246c272bb3d63b9/src/main/scala/org/bitcoins/core/number/NumberType.scala#L102) etc) which are used in the bitcoin protocol
libsecp256k1 needs to be built with the java interface enabled. Use the following commands to build secp256k1 with jni enabled. [Here is the official documentation for doing this in secp256k1](https://github.com/bitcoin-core/secp256k1/blob/master/src/java/org/bitcoin/NativeSecp256k1.java#L35)
This library aims to achieve high level of correctness via property based testing. At the simplest level, you can think of property based testing as specifying a invariant that must always hold true. [Here](https://github.com/bitcoin-s/bitcoin-s-core/blob/89fbf35d78046b7ed21fd93fec05bb57cba023bb/src/test/scala/org/bitcoins/core/protocol/transaction/TransactionSpec.scala#L13-L17) is an example of a property in the bitcoin-s-core test suite
What this property says is that for every transaction we can generate with [`TransactionGenerators.transactions`](https://github.com/bitcoin-s/bitcoin-s-core/blob/1c7a7b9f46679a753248d9f55246c272bb3d63b9/src/main/scala/org/bitcoins/core/gen/TransactionGenerators.scala#L50) we *must* be able to serialize it to hex format, then deserialize it back to a transaction and get the original `tx` back.
A more complex example of property based testing is checking that a [multi signature transaction was signed correctly](https://github.com/bitcoin-s/bitcoin-s-core/blob/89fbf35d78046b7ed21fd93fec05bb57cba023bb/src/test/scala/org/bitcoins/core/crypto/TransactionSignatureCreatorSpec.scala#L33-L40). First we generate a *supposedly* validly signed multisig transaction with [`TransactionGenerators.signedMultiSigTransaction`](https://github.com/bitcoin-s/bitcoin-s-core/blob/1c7a7b9f46679a753248d9f55246c272bb3d63b9/src/main/scala/org/bitcoins/core/gen/TransactionGenerators.scala#L102-L108). These transactions have varying m of n requirements. An interesting corner case if when you have 0 of n signatures. Which means no signature is required. Property based testing is really good at fleshing out these corner cases. We check to see if this transaction is valid by running it through our [`ScriptInterpreter`](https://github.com/bitcoin-s/bitcoin-s-core/blob/89fbf35d78046b7ed21fd93fec05bb57cba023bb/src/main/scala/org/bitcoins/core/script/interpreter/ScriptInterpreter.scala#L29). If we have built our functionality correctly the ScriptInterpreter should always return [`ScriptOk`](https://github.com/bitcoin-s/bitcoin-s-core/blob/89fbf35d78046b7ed21fd93fec05bb57cba023bb/src/main/scala/org/bitcoins/core/script/result/ScriptResult.scala#L15) indicating the script was valid.
```scala
property("generate valid signatures for a multisignature transaction") =
- Simple [`TransactionBuilder`](https://github.com/MetacoSA/NBitcoin/blob/56bfd1cbca535424b160d8a40688a79de4800630/NBitcoin/TransactionBuilder.cs#L172) similar nbitcoin
- Hardware wallet support (trezor, ledger etc)
- Lightning network scriptpubkey types (some work done [here](https://github.com/Christewart/bitcoin-s-core/commits/lightning_contracts))
Every bitcoin protocol data structure (and some other data structures) extends [`NetworkElement`](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/protocol/NetworkElement.scala). NetworkElement provides easier methods to convert the data structure to hex or a byte representation. When paired with our [`Factory`](https://github.com/bitcoin-s/bitcoin-s-core/blob/master/src/main/scala/org/bitcoins/core/util/Factory.scala) we can easily serialize and deserialize data structures. Most data structures have companion objects that extends `Factory` to be able to easily create protocol data structures. An example of this is the [`ScriptPubKey`](https://github.com/bitcoin-s/bitcoin-s-core/blob/1c7a7b9f46679a753248d9f55246c272bb3d63b9/src/main/scala/org/bitcoins/core/protocol/script/ScriptPubKey.scala#L462) companion object. You can use this companion object to create a SPK from hex or a byte array.
scala> val hexTx = "0100000001ccf318f0cbac588a680bbad075aebdda1f211c94ba28125b0f627f9248310db3000000006b4830450221008337ce3ce0c6ac0ab72509f8$9c1d52701817a2362d6357457b63e3bdedc0c0602202908963b9cf1a095ab3b34b95ce2bc0d67fb0f19be1cc5f7b3de0b3a325629bf01210241d746ca08da0a668735c3e01c1$a02045f2f399c5937079b6434b5a31dfe353ffffffff0210335d05000000001976a914b1d7591b69e9def0feb13254bace942923c7922d88ac48030000000000001976a9145e$90c865c2f6f7a9710a474154ab1423abb5b9288ac00000000"
This gives us an example of a bitcoin transaction that is encoded in hex format that is deserialized to a native Scala object called a [`Transaction`](https://github.com/bitcoin-s/bitcoin-s-core/blob/6358eb83067909771f989d615b422759222d060a/src/main/scala/org/bitcoins/core/protocol/transaction/Transaction.scala#L14-L42). You could also serialize the transaction to bytes using `tx.bytes` instead of `tx.hex`. These methods are available on every data structure that extends NetworkElement, like [`ECPrivateKey`](https://github.com/bitcoin-s/bitcoin-s-core/blob/6358eb83067909771f989d615b422759222d060a/src/main/scala/org/bitcoins/core/crypto/ECKey.scala#L23-L67), [`ScriptPubKey`](https://github.com/bitcoin-s/bitcoin-s-core/blob/6358eb83067909771f989d615b422759222d060a/src/main/scala/org/bitcoins/core/protocol/script/ScriptPubKey.scala#L23), [`ScriptWitness`](https://github.com/bitcoin-s/bitcoin-s-core/blob/6358eb83067909771f989d615b422759222d060a/src/main/scala/org/bitcoins/core/protocol/script/ScriptWitness.scala#L13), and [`Block`](https://github.com/bitcoin-s/bitcoin-s-core/blob/6358eb83067909771f989d615b422759222d060a/src/main/scala/org/bitcoins/core/protocol/blockchain/Block.scala#L17).
Transactions are run through the interpreter to check the validity of the Transaction. These are packaged up into an object called ScriptProgram, which contains the following:
scala> val spendingTx = Transaction("0100000001ccf318f0cbac588a680bbad075aebdda1f211c94ba28125b0f627f9248310db3000000006b4830450221008337ce3ce0c6ac0ab72509f889c1d52701817a2362d6357457b63e3bdedc0c0602202908963b9cf1a095ab3b34b95ce2bc0d67fb0f19be1cc5f7b3de0b3a325629bf01210241d746ca08da0a668735c3e01c1fa02045f2f399c5937079b6434b5a31dfe353ffffffff0210335d05000000001976a914b1d7591b69e9def0feb13254bace942923c7922d88ac48030000000000001976a9145e690c865c2f6f7a9710a474154ab1423abb5b9288ac00000000")
