CryptoKey

Python (>= 3.7) library for asymmetric cryptography with algorithms such as RSA and ECC.

Various backends implement wrappers around other crypto libraries (such as https://cryptography.io/) and makes them available using a unified API. The actual cryptographic operations are carried out by those backend libraries.

No hard dependencies on any non-python libraries such as OpenSSL exist.

CryptoKey is meant to be used by other libraries that need to carry out cryptographic operations. That could e.g. be an ACME client, TrustedTimeStamp service or an SSH client.

Users can implement their own backends to utilise their favourite HSM or smartcard, cloud HSMs or their own ECC implementation (don't!), etc.

CryptoKey can thus be seen as a python alternative to PKCS#11.

There are high-level interfaces such as key.sign(msg) which just do the right thing, and low-level interfaces such as rsakey.sign_int to calculate s = m ** d % n which, if used incorrectly, opens up security holes.

Implementations for padding schemes such as PSS are given. They can be used for low-level plumbing like extracting the salt from a PSS signature or creating a PSS signature with a specific salt.

One stated goal is to provide interfaces for unsafe operations too. If you want to shoot yourself in the foot, here's the tool to do it!

Examples

Sign a message

from asyncio import run
from cryptography.hazmat.primitives import serialization
from cryptokey.backend.cryptography import backend
from cryptokey.backend.cryptography.rsa import RsaPrivateKey

# Load a private key using normal cryptography.io operations.
with open('private.key', 'rb') as fp:
   cryptography_key = serialization.load_pem_private_key(
      fp.read(),
      password=None,
      backend=backend,
   )

# Create wrapper
key = RsaPrivateKey(cryptography_key)

# Sign a message. By default, PSS and SHA2_256 are used. The
# signature object also contains the parameters that were used.
sig = run(key.sign(b'Hello, World!'))

# Write signature to a file.
with open('hello.sig', 'wb') as fp:
   fp.write(sig.value)

Verifying the signature using openssl

echo -n 'Hello, World!' | openssl sha256 -binary | openssl pkeyutl \
     -verify -inkey private.key -sigfile hello.sig                 \
     -pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pss

Solving homework

from asyncio import run
from cryptokey.backend.textbook.rsa import TextbookRsaPrivateKey

key = TextbookRsaPrivateKey(public_exponent=7, primes=(17, 31))
print(f'Private exponent: {key.private_exponent}')
print(f'Signature for M=2: {run(key.sign_int(2)).int_value}')

Security

This library is supposed to be just as (in)secure as the used backend. If in doubt, use the cryptography backend, which builds upon OpenSSL.

The textbook backend is deliberately insecure and should not be used for real applications.