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)

First response in a google search: http://stackoverflow.com/questions/10569189/how-to-read-a-rsa-public-key-in-pem-pkcs1-format-in-python — tMC, Oct 16 '12 at 13:46
@tMC doesn't work with me, I use certificate, not a public key file. — eshizhan, Oct 16 '12 at 14:32

SquareRootOfTwentyThree · Accepted Answer · 2012-10-17T05:40:50.007

38

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)

edited Oct 17 '12 at 05:40

answered Oct 16 '12 at 19:14

SquareRootOfTwentyThree

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As a note, PEM->DER conversion could be done easier using builtin [`ssl.PEM_cert_to_DER_cert()`](http://docs.python.org/2/library/ssl.html#ssl.PEM_cert_to_DER_cert). – Michał Górny Aug 04 '13 at 21:16
Can you explain how to crypt a string after this step? – Pierluigi B Web Developer Oct 21 '14 at 14:51
is it still the case in 2016 ? – allan.simon Mar 01 '16 at 06:39
In 2016, one can use [pycryptodome](https://www.pycryptodome.org), which can read in X.509 certificates directly with `importKey`. – SquareRootOfTwentyThree Mar 01 '16 at 12:02
This is required for anyone verifying google oauth tokens on their own because their published public keys are x509 certs, https://www.googleapis.com/oauth2/v1/certs – casey Mar 30 '18 at 06:52

score 2 · Answer 2 · edited Apr 21 '22 at 12:30

Here's a good example: https://www.dlitz.net/software/pycrypto/api/2.6/Crypto.Cipher.PKCS1_OAEP-module.html

from Crypto.Cipher import PKCS1_OAEP
from Crypto.PublicKey import RSA

# sender side
message = 'To be encrypted'
key = RSA.importKey(open('pubkey.der').read())
cipher = PKCS1_OAEP.new(key)
ciphertext = cipher.encrypt(message)

# receiver side
key = RSA.importKey(open('privkey.der').read())
cipher = PKCS1_OAEP.new(key)
message = cipher.decrypt(ciphertext)

How do I use a X509 certificate with PyCrypto?

2 Answers2

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