Overview
PyCrypto is a pure-Python library that provides the foundational cryptographic primitives needed for Bitcoin and blockchain development. It is split into two distinct layers: a cryptographic core implementing elliptic curve operations on the secp256k1 curve, and a transaction parsing layer that deserializes raw Bitcoin transaction hex into structured data.
The library requires no compiled extensions — all finite field arithmetic, point operations, and ECDSA signing are implemented directly in Python. This makes it an excellent educational resource for understanding the mathematics behind Bitcoin, as well as a reusable library for building blockchain tools.
Key Features
- Finite field arithmetic:
FieldElementclass with modular addition, subtraction, multiplication, division (via Fermat's Little Theorem), exponentiation, and square root over prime fields - Elliptic curve operations: Point addition, scalar multiplication (double-and-add), and curve membership validation on secp256k1
- ECDSA signing and verification: Deterministic k generation via RFC 6979 (HMAC-based), signature creation, and verification against the Bitcoin generator point
- Key generation and encoding:
PrivateKeyclass with WIF (Wallet Import Format) encoding, compressed and uncompressed public key derivation - Bitcoin address generation: Base58Check encoding for legacy addresses, Bech32 encoding for SegWit addresses
- Transaction parsing: Full deserialization of legacy and SegWit transactions — version, inputs (prevout, script, sequence, witness), outputs (value, script, address extraction), locktime
- Testnet API fetcher:
TxFetcherandRemoteApiCallerclasses that fetch raw transactions from the Blockstream Testnet API
Architecture
A two-layer architecture separating pure cryptography from Bitcoin wire-format parsing:
| Layer | Role |
|---|---|
| ECC Core | FieldElement → Point/S256Point → PrivateKey → Signature |
| Transaction | Transaction, Input, Output, Script, Address, TxFetcher |
┌──────────────────────────────────┐
│ ECC Core (ecc/) │
│ FieldElement ── S256FieldElement │
│ Point ──────── S256Point │
│ PrivateKey ──── ECDSA sign() │
│ Signature ───── DER encode/decode│
│ utils ──────── Base58, hash160 │
└──────────────┬───────────────────┘
│ used by
┌──────────────▼───────────────────┐
│ Transaction Layer (src/) │
│ Transaction ─── parse hex │
│ Input ───────── prevout, script │
│ Output ──────── value, address │
│ TxFetcher ───── Blockstream API │
└──────────────────────────────────┘
Tech Stack
| Layer | Technology |
|---|---|
| Language | Python 3 (pure, no compiled extensions) |
| Crypto curve | secp256k1 (Bitcoin's curve, prime P = 2²⁵⁶ - 2³² - 977) |
| Hashing | SHA-256, RIPEMD-160 (stdlib + low-level ops) |
| HTTP client | requests (for Blockstream API) |
| Config | python-dotenv (API keys) |
| Testing | unittest (stdlib) |
| License | MIT |