[ANN] PyCoin (PYC) - a from-scratch PoW coin in pure Python, open source, no pre

PyCoin (PYC)

Hi everyone,

I wrote a small proof-of-work cryptocurrency from scratch in Python. It is not a Bitcoin fork, the code is about a dozen small files, and every node keeps and verifies its own copy of the chain (no central server). It is a hobby/educational project: it works, and I'd like people to run nodes, break things and tell me what is wrong with the networking and consensus code.

Source code: https://github.com/litenonce/pycoin
Info and downloads: https://renderon.net

Specs

Algorithm: double SHA-256, same idea as Bitcoin
Signatures: ECDSA on secp256k1, with per-address nonces against replay
Addresses: SHA-256 + RIPEMD-160, base58 with checksum
Block reward: 10 PYC per block (plus fees), no halving
Difficulty: starts at 1 bit, +1 bit every 50 blocks (based on height, not on time)
Supply: no cap, it grows by 10 PYC per block
Premine: none. ICO: none. Dev fee: none. The genesis block is empty and every coin comes from mining
Network: home-made JSON-over-TCP protocol, default port 9333
Language: Python 3, the only dependency is cryptography

How the network works

Nodes handshake, exchange peer lists, and gossip new blocks and transactions
A node that sees a peer with more cumulative work finds the last common block (binary search), downloads the missing blocks, re-validates everything from scratch and switches if the branch is valid and heavier
Transactions from orphaned blocks go back to the mempool
Everything received from the network is treated as untrusted: signatures, balances, nonces, PoW, difficulty, coinbase amount, timestamps and malformed fields are all checked
A peer that sends an invalid chain is banned for the session

Wallet / miner

Tkinter wallet with a built-in miner, a block explorer tab and a Transaction History tab where each transaction is shown as Pending (in the mempool) or Done (in a block). Optional 6-word recovery phrase (PBKDF2, locks after 3 wrong attempts).

Running a node

Code:

git clone https://github.com/litenonce/pycoin
cd pycoin/server
pip install -r requirements.txt
python node_server.py --peer IP:PORT

Open TCP 9333 if you want other people to be able to connect to you. A node behind NAT still works (it connects outward, syncs and mines). Seed node: PUT YOUR NODE IP:PORT HERE

Downloads (Windows x64)

Pre-built executables, for people who don't want to install Python. The wallet + miner (client.exe) and the node (node.exe) are on the download page: https://renderon.net

Code:

SHA-256 client.exe: ec4a7866ef39c93e8f2193cd2534d5ad3972cae710bd888b2f8d83f9064883f8
SHA-256 node.exe:   2cdbe409d832dd88dbb82a03ccaaca5f842614ec644e0c4460196b61c9b08563

Check the hash after downloading (certutil -hashfile client.exe SHA256 on Windows). The executables are built with PyInstaller from the source in the repo, and they are not code-signed, so Windows SmartScreen will probably warn you. If you don't trust an exe from a stranger (you shouldn't, blindly), run it from source instead: it's a few short files.

Known limitations (please read)

The recovery phrase travels in clear inside its transaction until it is mined, and on a P2P network it is gossiped to every node. Someone relaying it could swap in their own key. Fixing it properly needs a commit-reveal scheme, which I have not done
wallet.dat (the private key) is not encrypted
No encryption between nodes, no Sybil protection, only basic size limits against DoS
It has only been tested with a few nodes on my own machine (gossip, pending to done, fork reorganization, restart from disk). I don't know how it behaves on a real network
Difficulty depends on height only, so with a lot of hash power the chain will run faster than any target time

What this is not

PYC has no market value and I am not promising any. It is not an investment, it is not listed anywhere, and I'm not asking for money. If you want to mine it, run a node or read the code, great.

What I'd like feedback on

The fork choice / reorganization logic
The sync protocol (is there an obvious way to make a node waste bandwidth or stall another one?)
A sane replacement for the height-based difficulty
A safer recovery scheme

Questions and criticism welcome.