Python Blockchain

This project is intended as a learning tool to understand the fundamentals of blockchain. It is a proof-of-work blockchain client loosely based on Bitcoin, with a number of major simplifications to make it easier to read (see Limitations).

Prerequisites

• Python 3
• Docker (assuming you don't just have a Redis instance lying around)

Running

First, you'll need to spin up a redis instance.

docker pull redis
docker run -p 6379:6379 --name my-redis -d redis

Next, you'll need to update settings.py to point to a valid, passwordless RSA public/private key pair.

...
PRIVATE_KEY_FILE = "C:\\Users\\MYUSERNAME\\.ssh\\id_rsa"
PUBLIC_KEY_FILE = "C:\\Users\\MYUSERNAME\\.ssh\\id_rsa.pub"

Install python dependencies.

pip install --user -r dependencies.txt

Now you can spin up a node.

python .\app.py

To spin up another node, clone the repo to another location (so that you have a fresh settings.py file), point settings.py to a different public/private key pair, and run that app.py.

Once your node has mined a block or two, you can spend your block reward by sending a transaction.

python .\send.py 1 RecepientPublicKey

Finally, when you are ready to close everything out, kill the nodes (they may take a minute to exit gracefully) and shutdown the Redis container.

docker stop my-redis
docker container prune -f

Limitations

Compared with most other blockchains, this one currently has the following limitations (either for readability, or because I haven't built it yet):

• There is no concept of contract scripting (e.g. Bitcoin Script, Ethereum Solidity). Transactions simply send a quantity to a given public key.
• The mining block reward is hardcoded, and does not decrease as the chain lengthens.
• The difficulty is hardcoded, and does not scale based how fast the network mines blocks.
• Transactions do not support multi-input/multi-output. This means each transaction must send the entire balance of the input transaction. Since the mining reward is hardcoded to 1, this means every transaction must send exactly 1.
• There is no concept of a transaction fee.
• RSA public/private keys are used instead of a base58 of a RIPEMD160 of a SHA256 of a ECDSA key.
• Redis pub/sub is used to broadcast messages to other nodes in place of a true p2p network.
• It is written in python.

Further reading

Here are some resources I have found useful learning about blockchains:

• A really good blockchain introduction. My favorite is the part where the author says "you won't build your own blockchain".
• Why most cryptocurrencies use ECDSA keys
• P2P networks are really complicated
• Some other interesting blockchain implementations are Etherium and Hyperledger Fabric