#!/usr/bin/python3

'''
Decrypts a message using RSA private key (possibly generated with rsa_key_gen.py)
	message may have been generated using rsa_encrypt.py

Author: Tim Pierson, Dartmouth CS55, Winter 2021
				Based Wenliang Du https://github.com/kevin-w-du/BookCode/tree/master/Public_Key_Cryptography
Usage: 	python3 rsa_decrypt.py <private key file name> <password> <ciphertext file name>
			where password is the password used during key generation (from rsa_key_gen.py)
		python3 rsa_decrypt.py privateKey.pem cs55 ciphertext.bin
				
'''

from Crypto.Cipher import PKCS1_OAEP
from Crypto.PublicKey import RSA
import sys
import base64

if __name__ == '__main__':
	#usage check: make sure we got a filename to read
	if len(sys.argv) != 4:
		print("Usage: python3 rsa_decrypt.py <private key file name> <password> <ciphertext file name")
		print("Did you provide these?")
		exit()

	print("Reading ciphertext from",sys.argv[3])
	ciphertext = open(sys.argv[3], 'rb').read()

	print("Reading private key from",sys.argv[1])
	private_key_pem = open(sys.argv[1],'rb').read()
	private_key = RSA.importKey(private_key_pem, passphrase=sys.argv[2])
	cipher = PKCS1_OAEP.new(private_key)

	print("Decrypting ciphertext")
	message = cipher.decrypt(base64.b64decode(ciphertext))

	print("Message:",message)