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

def openssl_public_decrypt(key, data):
"""Decrypt data with RSA public 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_public_decrypt(
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
pubkey = load_pem_public_key(open('public_key.pem').read(), None, backend)
text = openssl_public_encrypt(pubkey, data);

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