# FROM: [url]https://github.com/klokantech/flask-fastspring/blob/master/flask_fastspring.py[/url]

from cryptography.hazmat.backends.openssl.backend import backend
from cryptography.hazmat.primitives.serialization import load_pem_private_key

def openssl_private_encrypt(key, data):
    """Encrypt data with RSA private key.
    This is a rewrite of the function from PHP, using cryptography
    FFI bindings to the OpenSSL library. Private key encryption is
    non-standard operation and Python packages either don't offer
    it at all, or it's incompatible with the PHP version.
    The backend argument MUST be the OpenSSL cryptography backend.
    """
    length = backend._lib.EVP_PKEY_size(key._evp_pkey)
    buffer = backend._ffi.new('unsigned char[]', length)
    result = backend._lib.RSA_private_encrypt(
        len(data), data, buffer,
        backend._lib.EVP_PKEY_get1_RSA(key._evp_pkey),
        backend._lib.RSA_PKCS1_PADDING)
    backend.openssl_assert(result == length)
    return backend._ffi.buffer(buffer)[:]

#usage
privkey = load_pem_private_key(open('private_key.pem').read(), None, backend)
cipher = openssl_private_encrypt(privkey, '123456');