How to load an RSA key from a PEM file and use it in python-crypto

Question

I have not found a way to load an RSA private key from a PEM file to use it in python-crypto (signature).

python-openssl can load a PEM file but the PKey object can't be used to retrieved key information (p, q, ...) to use with Crypto.PublicKey.construct().

Answer 1

is this (close to) what you tried doing?

pk = open( 'public_key.pem', 'rb' ).read()
rsa = M2Crypto.RSA.load_pub_key(pk)

That should work. The issue might be with openssl too, does it work when you just use openssl (not in Python)?

Link to Me Too Crypto

Answer 2

I recommend M2Crypto instead of python-crypto. You will need M2Crypto to parse PEM anyway and its EVP api frees your code from depending on a particular algorithm.

private = """
-----BEGIN RSA PRIVATE KEY-----
MIIBOwIBAAJBANQNY7RD9BarYRsmMazM1hd7a+u3QeMPFZQ7Ic+BmmeWHvvVP4Yj
yu1t6vAut7mKkaDeKbT3yiGVUgAEUaWMXqECAwEAAQJAIHCz8h37N4ScZHThYJgt
oIYHKpZsg/oIyRaKw54GKxZq5f7YivcWoZ8j7IQ65lHVH3gmaqKOvqdAVVt5imKZ
KQIhAPPsr9i3FxU+Mac0pvQKhFVJUzAFfKiG3ulVUdHgAaw/AiEA3ozHKzfZWKxH
gs8v8ZQ/FnfI7DwYYhJC0YsXb6NSvR8CIHymwLo73mTxsogjBQqDcVrwLL3GoAyz
V6jf+/8HvXMbAiEAj1b3FVQEboOQD6WoyJ1mQO9n/xf50HjYhqRitOnp6ZsCIQDS
AvkvYKc6LG8IANmVv93g1dyKZvU/OQkAZepqHZB2MQ==
-----END RSA PRIVATE KEY-----
"""    
message = "python-crypto sucks"

# Grab RSA parameters e, n
from M2Crypto import RSA, BIO
bio = BIO.MemoryBuffer(private)
rsa = RSA.load_key_bio(bio)
n, e = rsa.n, rsa.e

# In Python-crypto:
import Crypto.PublicKey.RSA
pycrypto_key = Crypto.PublicKey.RSA.construct((n, e))

# Use EVP api to sign message
from M2Crypto import EVP
key = EVP.load_key_string(private)
# if you need a different digest than the default 'sha1':
key.reset_context(md='sha256')
key.sign_init()
key.sign_update(message)
signature = key.sign_final()

# Use EVP api to verify signature
public = """
-----BEGIN PUBLIC KEY-----
MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANQNY7RD9BarYRsmMazM1hd7a+u3QeMP
FZQ7Ic+BmmeWHvvVP4Yjyu1t6vAut7mKkaDeKbT3yiGVUgAEUaWMXqECAwEAAQ==
-----END PUBLIC KEY-----
""" 
from M2Crypto import BIO, RSA, EVP
bio = BIO.MemoryBuffer(public)
rsa = RSA.load_pub_key_bio(bio)
pubkey = EVP.PKey()
pubkey.assign_rsa(rsa)
pubkey.reset_context(md="sha256")
pubkey.verify_init()
pubkey.verify_update(message)
assert pubkey.verify_final(signature) == 1

See http://svn.osafoundation.org/m2crypto/trunk/tests/test_rsa.py, but I prefer using the algorithm-independent EVP API http://svn.osafoundation.org/m2crypto/trunk/tests/test_evp.py.

http://stackoverflow.com/questions/544433/how-do-you-verify-an-rsa-sha1-signature-in-python/546476#546476 addresses a similar issue.