Python Decryption using private key

Question

I have an encrypted string. The Encryption is done using java code. I decrypt the encrypted string using following java code

InputStream fileInputStream = getClass().getResourceAsStream(
                    "/private.txt");
            byte[] bytes = IOUtils.toByteArray(fileInputStream);



private String decrypt(String inputString, byte[] keyBytes) {
        String resultStr = null;
        PrivateKey privateKey = null;
        try {
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(keyBytes);
            privateKey = keyFactory.generatePrivate(privateKeySpec);
        } catch (Exception e) {
            System.out.println("Exception privateKey:::::::::::::::::  "
                    + e.getMessage());
            e.printStackTrace();
        }
        byte[] decodedBytes = null;
        try {
            Cipher c = Cipher.getInstance("RSA/ECB/NoPadding");
            c.init(Cipher.DECRYPT_MODE, privateKey);
            decodedBytes = c.doFinal(Base64.decodeBase64(inputString));

        } catch (Exception e) {
            System.out
                    .println("Exception while using the cypher:::::::::::::::::  "
                            + e.getMessage());
            e.printStackTrace();
        }
        if (decodedBytes != null) {
            resultStr = new String(decodedBytes);
            resultStr = resultStr.split("MNSadm")[0];
            // System.out.println("resultStr:::" + resultStr + ":::::");
            // resultStr = resultStr.replace(salt, "");
        }
        return resultStr;

    }

Now I have to use Python to decrypt the encrypted string. I have the private key. When I use Cryptography package using following code

key = load_pem_private_key(keydata, password=None, backend=default_backend())

It throws ValueError: Could not unserialize key data.

Can anyone help what I am missing here?

Never use textbook RSA. Using no padding or a bad padding is very insecure. Nowadays, you should use OAEP instead of the default PKCS#1 v1.5 padding. So you should probably use Cipher.getInstance("RSA/ECB/OAEPWithSHA-256AndMGF1Padding"); — Artjom B.
– Artjom B., Commented Aug 24, 2016 at 19:33

David Ongaro · Accepted Answer · 2019-06-20 04:06:31Z

6

I figured out the solution:

from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from base64 import b64decode

rsa_key = RSA.importKey(open('private.txt', "rb").read())
cipher = PKCS1_v1_5.new(rsa_key)
raw_cipher_data = b64decode(<your cipher data>)
phn = cipher.decrypt(raw_cipher_data, <some default>)

This is the most basic form of code. What I learned is first you have to get the RSA_key(private key). For me RSA.importKey took care of everything. Really simple.

edited Jun 20, 2019 at 4:06

David Ongaro

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answered Aug 25, 2016 at 16:39

Dutta

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3 Comments

Charles L. Over a year ago

Should it be from Crypto.Cipher import PKCS1_v1_5 instead?

Sunny Chaudhari Over a year ago

what is <some default> ? Please can you elaborate

java-addict301 Over a year ago

@SunnyChaudhari this is probably the default data to decrypt if raw_cipher_data is None

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