# The core of the blockchain is the hash function. Without encryption, the blockchain would be easily manipulated and transactions could be fraudulently inserted.
def update_state(transaction, state): state = state.copy() for key in transaction: if key in state.keys(): state[key] += transaction[key] else: state[key] = transaction[key] return state
# "state" is who owns what record. For example, if I have 10 coins and I give 1 to Medium, then the state would be the value of the dictionary below.
# {"transaction": {"Tom": 9, "Medium": 1}}
# It's important to note that overdrafts cannot exist. If only 10 coins exist, then I cannot give 11 coins to someone. The following function validates that the transaction we are trying to make is indeed valid. Additionally, the transaction must balance. I cannot give 5 coins and have the recipient receive 4 coins because that would destroy and create coins.
def valid_transaction(transaction, state): """A valid transaction must sum to 0.""" if sum(transaction.values()) is not 0: return False for key in transaction.keys(): if key in state.keys(): account_balance = state[key] else: account_balance = 0 if account_balance + transaction[key] < 0: return False return True
# Now, we can generate blocks. We read the information of the previous block and use it to link it to the new block. This is also the core of the blockchain idea. Nominally valid transactions can attempt to fraudulently insert into blockchain but decrypting all previous blocks is (almost) impossible in computation, preserving the integrity of the blockchain.
def make_block(transactions, chain): """Make a block to go into the chain.""" parent_hash = chain[-1]["hash"] block_number = chain[-1]["contents"]["block_number"] + 1 block_contents = { "block_number": block_number, "parent_hash": parent_hash, "transaction_count": block_number + 1, "transaction": transactions } return {"hash": hash_function(block_contents), "contents": block_contents}
# Here's a function to check the hash value of the previous block:
def check_block_hash(block): expected_hash = hash_function(block["contents"]) if block["hash"] is not expected_hash: raise return
# Once we have everything put together, it's time to create our blocks. We now update the blockchain.
def check_block_validity(block, parent, state): parent_number = parent["contents"]["block_number"] parent_hash = parent["hash"] block_number = block["contents"]["block_number"] for transaction in block["contents"]["transaction"]: if valid_transaction(transaction, state): state = update_state(transaction, state) else: raise check_block_hash(block) # Check hash integrity if block_number is not parent_number + 1: raise if block["contents"]["parent_hash"] is not parent_hash: raise return state
# Before we're done, the chain must be validated:
def check_chain(chain): """Check the chain is valid.""" if type(chain) is str: try: chain = json.loads(chain) assert(type(chain) == list) except ValueError: # String passed in was not valid JSON return False elif type(chain) is not list: return False state = {} for transaction in chain[0]["contents"]["transaction"]: state = update_state(transaction, state) check_block_hash(chain[0]) parent = chain[0] for block in chain[1:]: state = check_block_validity(block, parent, state) parent = block return state
# Finally, we need a transaction function that ties everything above together:
def add_transaction_to_chain(transaction, state, chain): if valid_transaction(transaction, state): state = update_state(transaction, state) else: raise Exception("Invalid transaction.") my_block = make_block(state, chain) chain.append(my_block) for block in chain: check_chain(block) return state, chain
# So, now we have 7 functions. How do we interact with it? Well, first we need to start our chain with the Genesis block. This is our beginning of new coins. For the purposes of this article, we'll start with 10 coins.
genesis_block = { "hash": hash_function({ "block_number": 0, "parent_hash": None, "transaction_count": 1, "transaction": [{"Tom": 10}] }), "contents": { "block_number": 0, "parent_hash": None, "transaction_count": 1, "transaction": [{"Tom": 10}] }, } block_chain = [genesis_block] chain_state = {"Tom": 10}
# Now, le
