python-secp256k1

WARNING: In python you do not control memory. Regardless of how secrets are passed to the underlying lib, it still was an object in python before. It will linger in the heap for some time even after going out of scope. It is also impossible to mlock() secrets, your private keys may end up on disk in swap. Use with caution!!!

Python FFI bindings for libsecp256k1 (an experimental and optimized C library for EC operations on curve secp256k1) using ctypes. Alternative implementation that uses cffi instead of ctypes is secp256k1-py. CFFI is heavier, needs compiler for API mode (parses C headers) while ctypes does not need dependencies at all.

Rationale and goal

This library aims to provide a standard way to wrap libsecp256k1 using ctypes.

Implementation Details

• Scratch spaces are not implemented.
• methods from secp256k1_preallocated.h are not implemented
• This library creates default contexts (sign/verify) at the initialization phase, randomizes them and uses them the whole time, you do not need to worry about contexts. In case you need to randomize more often (to protect against side-channel leakage) use pysecp256k1.context_randomize.
• way to provide own hash functions is not implemented - default hash functions are used
• Default illegal callback function (that is added to default contexts) logs to stderr.
• Method names are the same as in libsecp256k1 but without secp256k1_ prefix (i.e. secp256k1_ec_pubkey_serialize -> ec_pubkey_serialize)
• Modules are structured same as in secp256k1 include/ directory but without secp256k1_ prefix.

secp256k1 modules	pysecp256k1 modules	importing
secp256k1.h	pysecp256k1.__init__.py	from pysecp256k1 import *
secp256k1_ecdh.h	pysecp256k1.ecdh.py	from pysecp256k1.ecdh import *
secp256k1_extrakeys.h	pysecp256k1.extrakeys.py	from pysecp256k1.extrakeys import *
secp256k1_recovery.h	pysecp256k1.recovery.py	from pysecp256k1.recovery import *
secp256k1_schnorrsig.h	pysecp256k1.schnorrsig.py	from pysecp256k1.schnorrsig import *

Validation and data types

This library tries to supplement libsecp256k1 with valid data ONLY, therefore heavy input type validation is in place. Validation is implemented via enforce_type((can be found in pysecp256k1.low_level.util)) which check for correct type (based on type hints) and correct length if possible.

Internal (opaque) secp256k1 data structures are represented as ctypes.c_char_Array to get bytes from c_char_Array use .raw (see examples).

pysecp256k1 class	type
Secp256k1Pubkey	c_char_Array_64
Secp256k1ECDSASignature	c_char_Array_64
Secp256k1XonlyPubkey	c_char_Array_64
Secp256k1Keypair	c_char_Array_96
Secp256k1ECDSARecoverableSignature	c_char_Array_65
Secp256k1Context	c_void_p

Apart from ctypes.c_char_Array and ctypes.c_void_p this library uses a limited number of standard python types.

python type	usage
bool	result of signature verification functions ecdsa_verify and schnorrsig_verify
int	recovery id, pubkey parity, result of ec_pubkey_cmp and xonly_pubkey_cmp
bytes	tags, tweaks, messages, message hashes, serialized pubkeys, serialized signatures, seckeys
List[Secp256k1Pubkey]	list of initialized pubkeys for ec_pubkey_combine, and ec_pubkey_sort
Tuple[Secp256k1XonlyPubkey, int]	initialized xonly public key and its parity
Tuple[bytes, int]	serialized recoverable signature and its recovery id
Optional[bytes]	optional random data for schnorrsig_sign{32,_custom}

Installation and dependencies

Only dependency of pysecp256k1 is python3.6+ and libsecp256k1 itself. To use full feature set build secp256k1 this way:

git clone https://github.com/bitcoin-core/secp256k1.git
cd secp256k1/
git checkout 0cdc758a56360bf58a851fe91085a327ec97685a  # v0.6.0
./autogen.sh
./configure --enable-module-recovery
make
make check
sudo make install

if one builds secp256k1 without schnorrsig for example and then tries to import from it from pysecp256k1.schnorrsig import schnorrsig_sign32 RuntimeError is raised hinting that libsecp256k1 is built without shnorrsig support. Same applies for all optional modules.

If one needs to have older version of libsecp256k1 installed in standard path and recent one for this library, do NOT do last step from above (sudo make install) and instead export absolute path to desired .so file in environment variable.

export PYSECP_SO=/home/johndoe/secp256k1/.libs/libsecp256k1.so.0.0.0

Install python-secp256k1 python package from pypi:

python3 -m pip install -U pip wheel
python3 -m pip install python-secp256k1

Examples

import os
from pysecp256k1 import *


seckey = tagged_sha256(b"seckey", os.urandom(32))
print("seckey:", seckey.hex())
ec_seckey_verify(seckey)
pubkey = ec_pubkey_create(seckey)
print("Compressed pubkey:", ec_pubkey_serialize(pubkey).hex())
msg = b"message to be signed"
print("msg:", msg.decode())
msg_hash = tagged_sha256(b"message", msg)
print("msg hash:", msg_hash.hex())
sig = ecdsa_sign(seckey, msg_hash)
print("DER signature:", ecdsa_signature_serialize_der(sig).hex())
print("compact signature:", ecdsa_signature_serialize_compact(sig).hex())
print("Correct signature for pubkey and msg hash:", ecdsa_verify(sig, pubkey, msg_hash))

Schnorrsig

import os
from pysecp256k1 import tagged_sha256
from pysecp256k1.low_level.constants import *
from pysecp256k1.extrakeys import *
from pysecp256k1.schnorrsig import *


seckey = tagged_sha256(b"seckey", os.urandom(32))
print("seckey:", seckey.hex())
keypair = keypair_create(seckey)
xonly_pubkey, pk_parity = keypair_xonly_pub(keypair)
print("xonly pubkey:", xonly_pubkey_serialize(xonly_pubkey).hex())
msg = b"message to be signed"
print("msg:", msg.decode())
msg_hash = tagged_sha256(b"message", msg)
print("msg hash:", msg_hash.hex())
rand_32 = os.urandom(32)
sig = schnorrsig_sign32(keypair, msg_hash, aux_rand32=rand_32)
print("schnorr signature:", sig.hex())
print("Correct signature for xonly pubkey and msg hash:", schnorrsig_verify(sig, msg_hash, xonly_pubkey))
# you can also sign variable length messages
extraparams = SchnorrsigExtraparams(
    SCHNORRSIG_EXTRAPARAMS_MAGIC,
    None,  # custom nonce function goes here
    ctypes.cast(ctypes.create_string_buffer(rand_32), ctypes.c_void_p),
)
sig0 = schnorrsig_sign_custom(keypair, msg, extraparams)
print("schnorr signature:", sig0.hex())
print("Correct signature for xonly pubkey and msg hash:", schnorrsig_verify(sig0, msg, xonly_pubkey))

Recovery

import os
from pysecp256k1 import tagged_sha256
from pysecp256k1 import ec_pubkey_create
from pysecp256k1.recovery import *


msg = b"message to be signed"
print("msg:", msg.decode())
msg_hash = tagged_sha256(b"message", msg)
print("msg hash:", msg_hash.hex())
seckey = tagged_sha256(b"seckey", os.urandom(32))
pubkey = ec_pubkey_create(seckey)
rec_sig = ecdsa_sign_recoverable(seckey, msg_hash)
compact_rec_sig_ser, recid = ecdsa_recoverable_signature_serialize_compact(rec_sig)
print("compact signature:", compact_rec_sig_ser.hex(), "recovery id:", recid)
rec_sig_parsed = ecdsa_recoverable_signature_parse_compact(compact_rec_sig_ser, recid)
assert rec_sig_parsed.raw, rec_sig.raw
rec_pubkey = ecdsa_recover(rec_sig, msg_hash)
print("recovered pubkey is the same as original:", pubkey.raw == rec_pubkey.raw)                                            

ECDH

import os
from pysecp256k1 import tagged_sha256
from pysecp256k1 import ec_pubkey_create
from pysecp256k1.ecdh import ecdh


bob_seckey = tagged_sha256(b"seckey", os.urandom(32))
bob_pubkey = ec_pubkey_create(bob_seckey)
alice_seckey = tagged_sha256(b"seckey", os.urandom(32))
alice_pubkey = ec_pubkey_create(alice_seckey)
shared_secret_bob = ecdh(bob_seckey, alice_pubkey)
shared_secret_alice = ecdh(alice_seckey, bob_pubkey)
print("bob and alice shared secret equals:", shared_secret_bob == shared_secret_alice)

Tweaking

import os
from pysecp256k1 import (
    ec_pubkey_create, ec_seckey_tweak_add, ec_seckey_negate, ec_seckey_verify,
    tagged_sha256
)
from pysecp256k1.extrakeys import (
    keypair_create, keypair_sec, keypair_xonly_pub, xonly_pubkey_from_pubkey,
    xonly_pubkey_serialize, xonly_pubkey_tweak_add_check, xonly_pubkey_parse,
    xonly_pubkey_tweak_add, keypair_xonly_tweak_add
)


seckey = tagged_sha256(b"seckey", os.urandom(32))
raw_pubkey = ec_pubkey_create(seckey)
keypair = keypair_create(seckey)
xonly_pub, parity = xonly_pubkey_from_pubkey(raw_pubkey)
xonly_pub1, parity1 = keypair_xonly_pub(keypair)
assert xonly_pub.raw == xonly_pub1.raw
assert parity == parity1
ser_xonly_pub = xonly_pubkey_serialize(xonly_pub)
assert xonly_pubkey_parse(ser_xonly_pub).raw == xonly_pub.raw

valid_tweak = tagged_sha256(b"tweak", seckey)  # this is random
assert ec_seckey_verify(valid_tweak) is None
# tweak keypair
tweaked_keypair = keypair_xonly_tweak_add(keypair, valid_tweak)
# below returns standard pubkey (not xonly)
tweaked_pubkey = xonly_pubkey_tweak_add(xonly_pub, valid_tweak)
tweaked_xonly_pub, parity2 = xonly_pubkey_from_pubkey(tweaked_pubkey)
tweaked_xonly_pub1, parity3 = keypair_xonly_pub(tweaked_keypair)
assert tweaked_xonly_pub.raw == tweaked_xonly_pub1.raw
assert parity2 == parity3
ser_tweaked_xonly_pub = xonly_pubkey_serialize(tweaked_xonly_pub)
assert xonly_pubkey_tweak_add_check(
    ser_tweaked_xonly_pub, parity2, xonly_pub, valid_tweak
) is True
# https://github.com/bitcoin-core/secp256k1/issues/1021
if parity == 0:
    tweaked_seckey = ec_seckey_tweak_add(seckey, valid_tweak)
else:
    tweaked_seckey = ec_seckey_tweak_add(
        ec_seckey_negate(seckey), valid_tweak
    )
assert tweaked_seckey == keypair_sec(tweaked_keypair)

Negations

import os
from pysecp256k1 import ec_pubkey_create, ec_pubkey_negate, ec_seckey_negate, tagged_sha256


seckey = tagged_sha256(b"seckey", os.urandom(32))
pubkey = ec_pubkey_create(seckey)
# double negation - result is the same seckey
assert seckey == ec_seckey_negate(ec_seckey_negate(seckey))
# double negation - result is the same pubkey
assert pubkey.raw == ec_pubkey_negate(ec_pubkey_negate(pubkey)).raw

Testing

cd python-secp256k1
python3 -m unittest -vvv

or with tox against multiple python interpreters

cd python-secp256k1
tox