# A Python library for 'bitcoin cryptography'

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btclib is a Python3 type annotated library intended for teaching, learning, and using bitcoin; the focus is on elliptic curve cryptography and bitcoin's blockchain.

It is rigorously and extensively tested: the test suite covers 100% of the code base and reproduces results from both informal and major reference sources.

Originally developed for the
*Bitcoin and Blockchain Technology*
course at the University of Milano-Bicocca,
btclib is not intended for production environments:
it is often refactored for improved clarity,
without care for backward compatibility; moreover,
some of its algorithms could be broken using side-channel attacks.

The library is not limited to the bitcon elliptic curve secp256k1; anyway, FFI bindings to libsecp256k1 (the optimized C library used by Bitcoin Core) are available for this curve and used by default.

Included features are:

- modulo algebra functions (gcd, inverse, legendre symbol, square root)
- octets / integer / point / var_int / var_bytes helper functions
- elliptic curve class
- fast algebra implemented using Jacobian coordinates
- double scalar multiplication (Straus's algorithm, also known as Shamir's trick)
- multi scalar multiplication (Bos-coster's algorithm)
- point simmetry solution: odd/even, low/high, and quadratic residue
- elliptic curves: SEC 1 v1 and v2, NIST, Brainpool, and low cardinality test curves

- ECDSA signature with (transaction) DER encoding
- ECDSA signature with (message) compact encoding: standard p2pkh and BIP137/Electrum extensions to p2wpkh and p2wpkh-p2sh
- RFC 6979 for deterministic signature schemes
- EC Schnorr signature (according to
BIP340
bitcoin standardization)
- batch validation
- threshold signature (see test-suite)
- MuSig multi-signature (see test-suite)

- Borromean ring signature
- Sign-to-contract commitment
- Diffie-Hellman
- Pedersen committment
- Base58 encoding/decoding
- p2pkh/p2sh addresses and WIFs
- Bech32 encoding/decoding
- p2wpkh/p2wsh native SegWit addresses and their legacy p2sh-wrapped versions
- BIP32 hierarchical deterministic key chains
- SLIP132 key versions (xprv, yprv, zprv, Yprv, Zprv, tprv, uprv, vprv, and Uprv) with corresponding mapping to p2pkh/p2sh, p2wpkh-p2sh, p2wpkh, p2wsh-p2sh, p2wsh and p2tr addresses
- BIP39 wordlists and mnemonic for generating deterministic keys
- Electrum standard for mnemonic
- Script encoding/decoding
- nulldata, p2pk, p2ms, p2pkh, p2sh, p2wpkh, p2wsh and p2tr ScriptPubKeys
- BlockHeader and Block data classes
- OutPoint, TxIn, TxOut, and TX data classes
- legacy, segwit_v0 and taproot transaction hash signatures
- BIP174 partially signed bitcoin transactions (PSBT): PsbtIn, PbstOut, and Psbt data classes

To install (and/or upgrade) btclib:

`python -m pip install --upgrade btclib`

You might want to install btclib into a python virtual environment; e.g. from the root folder:

Shell:

```
python -m venv venv_btclib
source ./venv_btclib/bin/activate
python -m pip install --upgrade btclib
```

Windows CMD or PowerShell:

```
python -m venv venv_btclib
.\venv_btclib\Scripts\activate
python -m pip install --upgrade btclib
```

Windows Git bash shell:

```
python -m venv venv_btclib
cd ./venv_btclib/Scripts
. activate
cd ../..
python -m pip install --upgrade btclib
```

See CONTRIBUTING if you are interested in btclib develoment.

See SECURITY if you have found a security vulnerability.