>>> from cryptos.keys import gen_key_pair
>>> from cryptos.ecdsa import sign, verify
>>> sk1, pk1 = gen_key_pair()
>>> sk2, pk2 = gen_key_pair()
>>> message = ('pk1 wants to pay pk2 1 BTC').encode('ascii')
>>> sig = sign(sk1, message)
>>> verify(pk1, message, sig)
True
>>> verify(pk2, message, sig)
False

>>> from cryptos.transaction import Tx
>>> from io import BytesIO

>>> # example transaction in Programming Bitcoing Chapter 5
>>> raw = bytes.fromhex('0100000001813f79011acb80925dfe69b3def355fe914bd1d96a3f5f71bf8303c6a989c7d1000000006b483045022100ed81ff192e75a3fd2304004dcadb746fa5e24c5031ccfcf21320b0277457c98f02207a986d955c6e0cb35d446a89d3f56100f4d7f67801c31967743a9c8e10615bed01210349fc4e631e3624a545de3f89f5d8684c7b8138bd94bdd531d2e213bf016b278afeffffff02a135ef01000000001976a914bc3b654dca7e56b04dca18f2566cdaf02e8d9ada88ac99c39800000000001976a9141c4bc762dd5423e332166702cb75f40df79fea1288ac19430600')
>>> tx = Tx.parse(BytesIO(raw))
>>> # we get back a Transaction object with parsed fields
>>> tx

Tx(version=1, tx_ins=[TxIn(prev_tx=b'\xd1\xc7\x89\xa9\xc6\x03\x83\xbfq_?j\xd9\xd1K\x91\xfeU\xf3\xde\xb3i\xfe]\x92\x80\xcb\x1a\x01y?\x81', prev_index=0, script_sig=3045022100ed81ff192e75a3fd2304004dcadb746fa5e24c5031ccfcf21320b0277457c98f02207a986d955c6e0cb35d446a89d3f56100f4d7f67801c31967743a9c8e10615bed01 0349fc4e631e3624a545de3f89f5d8684c7b8138bd94bdd531d2e213bf016b278a, sequence=4294967294, witness=None)], tx_outs=[TxOut(amount=32454049, script_pubkey=OP_DUP OP_HASH160 bc3b654dca7e56b04dca18f2566cdaf02e8d9ada OP_EQUALVERIFY OP_CHECKSIG), TxOut(amount=10011545, script_pubkey=OP_DUP OP_HASH160 1c4bc762dd5423e332166702cb75f40df79fea12 OP_EQUALVERIFY OP_CHECKSIG)], locktime=410393, segwit=False)

>>> tx.validate()
True

from io import BytesIO
from cryptos.block import Block, GENESIS_BLOCK, calculate_new_bits
from cryptos.network import SimpleNode
from cryptos.network import (
    GetHeadersMessage,
    HeadersMessage,
)

# connect to a node and pretty please ask for
# 20 block headers starting with the genesis block

# Start with the genesis block
# https://en.bitcoin.it/wiki/Genesis_block
# class Block:
#     version: int        # 4 bytes little endian
#     prev_block: bytes   # 32 bytes, little endian
#     merkle_root: bytes  # 32 bytes, little endian
#     timestamp: int      # uint32, seconds since 1970-01-01T00:00 UTC
#     bits: bytes         # 4 bytes, current target in compact format
#     nonce: bytes        # 4 bytes, searched over in pow
previous = Block.decode(BytesIO(GENESIS_BLOCK['main']))

# okay now let's crawl the blockchain block headers
node = SimpleNode(
    host='mainnet.programmingbitcoin.com',
    net='main',
)
node.handshake()

blocks = [previous]
for _ in range(20):

    # request next batch of 2,000 headers
    getheaders = GetHeadersMessage(start_block=bytes.fromhex(previous.id()))
    node.send(getheaders)
    headers = node.wait_for(HeadersMessage)

    # extend our chain of block headers
    blocks.extend(headers.blocks)

    previous = headers.blocks[-1]
    print(f"received another batch of blocks, now have {len(blocks)}")

node.close()
# we now have 40,001 blocks total, 80 bytes each in raw, so total of ~3.2MB of data

# now (partially) validate blockchain integrity
for i, block in enumerate(blocks):

    # validate proof of work on this block
    assert block.validate()

    # validate pointer to the previous node matches
    prev = blocks[i - 1]
    expected_prev_block = b'\x00'*32 if i == 0 else bytes.fromhex(prev.id())
    assert block.prev_block == expected_prev_block

    # validate the proof of work target calculation on the block was correct
    if i % 2016 == 0:
        if i == 0:
            # genesis block had hardcoded value for bits
            expected_bits = bytes.fromhex('ffff001d')
        else:
            # recalculate the target at every epoch (2016 blocks), approx 2 week period
            # note that Satoshi had an off-by-one bug in this calculation because we are
            # looking at timestamp difference between first and last block in an epoch,
            # so these are only 2015 blocks apart instead of 2016 blocks apart ¯\_(ツ)_/¯
            prev_epoch = blocks[i - 2016]
            time_diff = prev.timestamp - prev_epoch.timestamp
            expected_bits = calculate_new_bits(prev.bits, time_diff)
    assert block.bits == expected_bits

    if i % 1000 == 0:
        print(f"on block {i+1}/{len(blocks)}")