Decode text encoded with RSA

Question

I have a RSA public/private key pair and passphrase. I am trying to decode text encrypted using using above key(s). The encoded text is always 512 chars long alpha-num string.

I have tried using the code provided at SOF question Decrypt using an RSA public key with PyCrypto

First I used my private key which was encoded with AES-256-CBC from a PEM file. This is start of privkey.pem which made me think its AES-256 encrypted

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-256-CBC
<rest of the data>
-----END RSA PRIVATE KEY-----

but I received following error message.

ValueError: PEM encryption format not supported.

So I asked the source for a private key without AES encryption which they gave me. Now using this key the decrypted works and the decrypted text looks like below (I am only showing some of the text)

b'\x93\n(\x92\x02\x9aF*?\x18"\x19\x12Gn\xc2\<rest of the text>'

This is not my plain text. What am I doing wrong? can someone help me to decode this text.

EDIT 1:

Based on Maarten's answer below, I have tried the following code but I am still getting errors.

Here is my code for decryption

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

encrypted_text = "39085fc25e<HIDDEN>2fcce845760391ff"

key = RSA.importKey(open("\\path_to_key\\private.der", encoding="utf8").read())
cipher = PKCS1_OAEP.new(key)

message = cipher.decrypt(ast.literal_eval(str(uid)))

and I get error:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x82 in position 1: invalid start byte

Note that I had to convert my private key from PEM to DER using the code below becasue using PEM file I was getting SyntaxError: unexpected EOF while parsing

openssl rsa -in private_key.pem -out private_key.der -outform DER

becasue

I've voted this off topic because besides the idea of the padding, there is simply not enough information to go on. Try with a test key and post the required information here. — Maarten Bodewes, Nov 11 '16 at 12:49
I am not sure what extra information do u need. My keys were created using ```openssl``` and they are already in use so I can't change them by recreating them. I have been given a public_key.txt, private_key.pem (which is AES-256 encrypted) and the passphrase. How do I use python to decrypt text using these keys? — Khurram Majeed, Nov 11 '16 at 13:25
I have encrypted text using .net c# code and want to decrypt code using python but where I can get RSA.importKey ?? — Neo, Oct 18 '17 at 10:30

Khurram Majeed · Accepted Answer · 2016-11-15T14:16:05.837

Here is the solution that I have found.

First of all I am using pycryptodome librray instead of pycrypto.

Below are my encode and decode functions.

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

def encode_rsa(message, key_path):    
    key = RSA.importKey(open(key_path).read())
    cipher = PKCS1_OAEP.new(key)
    ciphertext = cipher.encrypt(message)
    return ciphertext

def decode_rsa(ciphertext, key_path):
    key = RSA.importKey(open(key_path).read())
    cipher = PKCS1_OAEP.new(key)
    # before decrypt convert the hex string to byte_array 
    message = cipher.decrypt(bytearray.fromhex(ciphertext))
    return message

Using above two functions I was able to encode/deode the data correctly.

Decode text encoded with RSA

1 Answers1