Decrypt using an RSA public key with PyCrypto

11
votes

As far as I understand, I should be able to use RSA to ensure authenticity or privacy, as I wish. In my case, I want to ensure authenticity so I encrypt the data with the private key and allow anyone to decrypt it with the public key. The data is not really secret but I need to guarantee that it was created by the owner of the public (and private) key.

When I try to decrypt using PyCrypto I get No private key error from PyCrypto. The code is this:

def _decrypt_rsa(decrypt_key_file, cipher_text):
    from Crypto.PublicKey import RSA
    from base64 import b64decode

    key = open(decrypt_key_file, "r").read()
    rsakey = RSA.importKey(key)
    raw_cipher_data = b64decode(cipher_text)
    decrypted = rsakey.decrypt(raw_cipher_data)
    return decrypted

I'm calling it with the path to the public key file (in OpenSSH format.) The encrypted data isn't generated by me and it was not done with Python but PHP. In PHP there's a openssl_public_decrypt function that decrypts this data easily.

Is it possible at all to decrypt using the public key with PyCrypto?

2
pythonrsapycrypto
You have it reversed. Public key is used to encrypt and private key is used to decrypt. – jchysk
Public decrypt is the same as encrypt. Your problem can be fixed this way: decrypted = rsakey.encrypt(raw_cipher_data, 0) – 0x2b3bfa0
If the source really has encrypted with the private key, then they've effectively signed the data. Some tools, like Java, let you do this. To decrypt the signature you can use a public key (or the public part of a private key). See: stackoverflow.com/questions/48280670/… – Alastair McCormack

2 Answers

7
votes

That is totally insecure, because you are using raw RSA without padding.

Your application needs a signature, so you should not be dealing with encryptions and decryptions. For instance, PKCS#1 v1.5 is a good protocol, even though the signature is a piece of data that must be appended to what you want to prove the authenticity of.

To verify a PKCS#1 v1.5 signature in Python, you do:

from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA

rsa_key = RSA.importKey(open(verification_key_file, "rb").read())
verifier = PKCS1_v1_5.new(rsa_key)
h = SHA.new(data_to_verify)
if verifier.verify(h, signature_received_with_the_data):
    print "OK"
else:
    print "Invalid"

I would strongly recommend to change the PHP code so that it creates such a signature.

1
votes

Your function is correct. You just need to be giving it the path to your private key in order to decrypt instead of your public key. The public key is for encrypting, the private key is for decrypting.

def _decrypt_rsa(decrypt_key_file, cipher_text):
    '''
    Decrypt RSA encrypted package with private key
    :param decrypt_key_file: Private key
    :param cipher_text: Base64 encoded string to decrypt
    :return: String decrypted
    '''
    from Crypto.PublicKey import RSA
    from base64 import b64decode

    key = open(decrypt_key_file, "r").read()
    rsakey = RSA.importKey(key)
    #optionally could use OAEP
    #from Crypto.Cipher import PKCS1_OAEP
    #rsakey = PKCS1_OAEP.new(rsakey)
    raw_cipher_data = b64decode(cipher_text)
    decrypted = rsakey.decrypt(raw_cipher_data)
    return decrypted