How do I use a X509 certificate with PyCrypto?

Question

I want to encrypt some data in python with PyCrypto.

However I get an error when using key = RSA.importKey(pubkey):

RSA key format is not supported

The key was generated with:

openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout mycert.key -out mycert.pem

The code is:

def encrypt(data):
    pubkey = open('mycert.pem').read()
    key = RSA.importKey(pubkey)
    cipher = PKCS1_OAEP.new(key)
    return cipher.encrypt(data)

Answer 1

PyCrypto does not support X.509 certificates. You must first extract the public key with the command:

openssl x509 -inform pem -in mycert.pem -pubkey -noout > publickey.pem

Then, you can use RSA.importKey on publickey.pem.

---

If you don't want or cannot use openssl, you can take the PEM X.509 certificate and do it in pure Python like this:

from Crypto.Util.asn1 import DerSequence
from Crypto.PublicKey import RSA
from binascii import a2b_base64

# Convert from PEM to DER
pem = open("mycert.pem").read()
lines = pem.replace(" ",'').split()
der = a2b_base64(''.join(lines[1:-1]))

# Extract subjectPublicKeyInfo field from X.509 certificate (see RFC3280)
cert = DerSequence()
cert.decode(der)
tbsCertificate = DerSequence()
tbsCertificate.decode(cert[0])
subjectPublicKeyInfo = tbsCertificate[6]

# Initialize RSA key
rsa_key = RSA.importKey(subjectPublicKeyInfo)