pycrypto - Ciphertext with incorrect length

Question

I've generated a public and private key with pycrypto, and I save them to a file using export key:

from Crypto.PublicKey import RSA
bits=2048
new_key = RSA.generate(bits, e=65537)

prv = open('keymac.pem','w')
prv.write(new_key.exportKey('PEM'))
prv.close()
pub = open('pubmac.pem', 'w')
pub.write(new_key.publickey().exportKey('PEM'))
pub.close()

I use the public key to encrypt a file (following http://insiderattack.blogspot.com/2014/07/encrypted-file-transfer-utility-in.html#comment-form)

When I read the file to decrypt it, I get "Ciphertext with incorrect length."

I added a try-except block around the decryption code on Deepal Jayasekara example:

try:
    encryptedonetimekey = filetodecrypt.read(512)
    privatekey = open("keymac.pem", 'r').read()
    rsaofprivatekey = RSA.importKey(privatekey)
    pkcs1ofprivatekey = PKCS1_OAEP.new(rsaofprivatekey)
    aesonetimekey = pkcs1ofprivatekey.decrypt(encryptedonetimekey)

   except Exception as decrypprivkeyerr:
       print "Decryption of the one time key using the private key        failed!!"
       print "Key error == %s" %decrypprivkeyerr
     raise Exception("Decryption using Private key failed error = %s" %decrypprivkeyerr)

Am I missing something? Should I save the private key differently? Am I not reading the private key correctly?

Answer 1

The error message, "Ciphertext with incorrect length", has told us all. That means, cipher text exceeded the limit length which can be calculated by (length of key, 1024.2048..)/8. to solve this problem, you can separate the cipher text and decrypt them within a loop, then assemble all the decrypted byte string. My code in Python 3.6 for reference:

# 1024/8
default_length = 128
encrypt_str = str(data["content"])
sign_str = str(data["sign"])
try:
    rsa_private_key = RSA.importKey(private_key)
    encrypt_byte = base64.b64decode(encrypt_str.encode())
    length = len(encrypt_byte)
    cipher = PKCS115_Cipher(rsa_private_key)
    if length < default_length:
        decrypt_byte = cipher.decrypt(encrypt_byte, 'failure')
    else:
        offset = 0
        res = []
        while length - offset > 0:
            if length - offset > default_length:
                res.append(cipher.decrypt(encrypt_byte[offset: offset + 
                    default_length], 'failure'))
            else:
                res.append(cipher.decrypt(encrypt_byte[offset:], 'failure'))
            offset += default_length
        decrypt_byte = b''.join(res)
    decrypted = decrypt_byte.decode()

Answer 2

This doesnt answer your question directly but it may give you some clues to the problem. Im using two functions for encrypting content to a file rather than encrypting a file directly. One for encrypting (in my case username and password) to a file then another to decrypt that data to use as needed.

Note the need for the padding

Creat Encrypted Content In File:

from Crypto.Cipher import AES
import base64
import os
import argparse

parser = argparse.ArgumentParser(description='Arguments used to generate new credentials file, Use: -u for username, -p for password')
parser.add_argument('-u', help='Specify username', required=True)
parser.add_argument('-p', help='Specify password', required=True)
parser.add_argument('-b', help='Specify debug', required=False, action='store_true')
args = vars(parser.parse_args())


def encrypt(username, password):
    #Encrypt Credentials To '.creds' file, including 'secret' for username and password
    dir_path = os.path.dirname(os.path.realpath(__file__))
    # the block size for the cipher object; must be 16 per FIPS-197
    BLOCK_SIZE = 16

    # the character used for padding--with a block cipher such as AES, the value
    # you encrypt must be a multiple of BLOCK_SIZE in length.  This character is
    # used to ensure that your value is always a multiple of BLOCK_SIZE
    PADDING = '{'

    # one-liner to sufficiently pad the text to be encrypted
    pad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * PADDING

    # generate a random secret key
    user_secret = os.urandom(BLOCK_SIZE)
    pass_secret = os.urandom(BLOCK_SIZE)

    # one-liners to encrypt/encode and decrypt/decode a string
    # encrypt with AES, encode with base64

    EncodeAES = lambda c, s: base64.b64encode(c.encrypt(pad(s)))
    # create a cipher object using the random secret
    user_cipher = AES.new(user_secret)
    pass_cipher = AES.new(pass_secret)

    # encode a string
    user_encoded = EncodeAES(user_cipher, username)
    pass_encoded = EncodeAES(pass_cipher, password)
    try:
        with open('.creds', 'w') as filename:
            filename.write(user_encoded + '\n')
            filename.write(user_secret + '\n')
            filename.write(pass_encoded + '\n')
            filename.write(pass_secret + '\n')
            filename.close()
            print '\nFile Written To: ', dir_path + '/.creds'
    except Exception, e:
        print e

    if args['b']:
        print((user_encoded, user_secret), (pass_encoded, pass_secret))

username = args['u']
password = args['p']

encrypt(username, password)

Decrypt The Data

def decrypt(dir_path, filename):
    #Read '.creds' file and return unencrypted credentials (user_decoded, pass_decoded)

    lines = [line.rstrip('\n') for line in open(dir_path + filename)]

    user_encoded = lines[0]
    user_secret = lines[1]
    pass_encoded = lines[2]
    pass_secret = lines[3]

    # the character used for padding--with a block cipher such as AES, the value
    # you encrypt must be a multiple of BLOCK_SIZE in length.  This character is
    # used to ensure that your value is always a multiple of BLOCK_SIZE
    PADDING = '{'

    DecodeAES = lambda c, e: c.decrypt(base64.b64decode(e)).rstrip(PADDING)

    # create a cipher object using the random secret
    user_cipher = AES.new(user_secret)
    pass_cipher = AES.new(pass_secret)

    # decode the encoded string
    user_decoded = DecodeAES(user_cipher, user_encoded)
    pass_decoded = DecodeAES(pass_cipher, pass_encoded)

    return (user_decoded, pass_decoded)