ValueError: Ciphertext with incorrect length using Python

Question

Can some please help me I tried googling this error but could not understand why it is being raised. Can you point out the problem in my code I am fairly new to encryption this is my first time trying to use it.

session_key = cipher_rsa.decrypt(enc_session_key) , ValueError("Ciphertext with incorrect length."), ValueError: Ciphertext with incorrect length.

Encryption code

from Crypto.PublicKey import RSA
from Crypto import Random
from Crypto.Cipher import AES, PKCS1_OAEP
from Crypto.Random import get_random_bytes
random_generator = Random.new().read
print (random_generator,"HI")
key = RSA.generate(1024, random_generator)
print(key) 
code = 'totalyundetectable' #******************important ****************
encrypted_key = key.exportKey(format='PEM', passphrase=code, pkcs=8,
                          protection="scryptAndAES128-CBC")
with open('C:/Users/Arnav/Documents/Project/my_private_key.bin', 'wb') as f:
    f.write(encrypted_key)
with open('C:/Users/Arnav/Documents/Project/my_rsa_public.pem', 'wb') as f:
    f.write(key.publickey().exportKey())
with open('C:/Users/Arnav/Documents/Project/encrypted_data.bin', 'wb') as out_file:
recipient_key = RSA.import_key(
    open('C:/Users/Arnav/Documents/Project/my_rsa_public.pem').read())
session_key = get_random_bytes(16)
cipher_rsa = PKCS1_OAEP.new(recipient_key)
out_file.write(cipher_rsa.encrypt(session_key))

cipher_aes = AES.new(session_key, AES.MODE_EAX)
data = b'blah blah bl'
ciphertext, tag = cipher_aes.encrypt_and_digest(data)

out_file.write(cipher_aes.nonce)
out_file.write(tag)
out_file.write(ciphertext)
code = 'totalyundetectable'

Decryption code

    with open('C:/Users/Arnav/Documents/Project/encrypted_data.bin', 'rb') as fobj:
    private_key = RSA.import_key(
        open('C:/Users/Arnav/Documents/Project/my_private_key.bin','rb').read(),
        passphrase=code)

    enc_session_key, nonce, tag, ciphertext = [fobj.read(x)
                                               for x in (private_key.size_in_bytes(),
                                                         16, 16, -1)]

    cipher_rsa = PKCS1_OAEP.new(private_key)
    session_key = cipher_rsa.decrypt(enc_session_key)

    cipher_aes = AES.new(session_key, AES.MODE_EAX, nonce)
    data = cipher_aes.decrypt_and_verify(ciphertext, tag)

print(data)

Tried that but I cant figure out how because its value is coming directly from the library function @LukePark — ARNAV CHAUHAN, Nov 12 '17 at 08:47

invzbl3 · Answer 1 · 2019-11-11T09:38:31.763

In your case it looks like wrong white spaces/tabs. Because if I write like:

from Crypto.PublicKey import RSA
from Crypto import Random
from Crypto.Cipher import AES, PKCS1_OAEP
from Crypto.Random import get_random_bytes

random_generator = Random.new().read
print (random_generator,"HI")
key = RSA.generate(1024, random_generator)
print(key)

code = 'totalyundetectable' #******************important ****************
encrypted_key = key.exportKey(format='PEM', passphrase=code, pkcs=8,
                          protection="scryptAndAES128-CBC")

with open('my_private_key.bin', 'wb') as f:
    f.write(encrypted_key)
with open('my_rsa_public.pem', 'wb') as f:
    f.write(key.publickey().exportKey())
with open('encrypted_data.bin', 'wb') as out_file:
    recipient_key = RSA.import_key(
    open('my_rsa_public.pem').read())
    session_key = get_random_bytes(16)
    cipher_rsa = PKCS1_OAEP.new(recipient_key)
    out_file.write(cipher_rsa.encrypt(session_key))

    cipher_aes = AES.new(session_key, AES.MODE_EAX)
    data = b'blah blah bl'
    ciphertext, tag = cipher_aes.encrypt_and_digest(data)

    out_file.write(cipher_aes.nonce)
    out_file.write(tag)
    out_file.write(ciphertext)
code = 'totalyundetectable'

with open('encrypted_data.bin', 'rb') as fobj:
    private_key = RSA.import_key(
        open('my_private_key.bin','rb').read(),
        passphrase=code)

    enc_session_key, nonce, tag, ciphertext = [fobj.read(x)
                                               for x in (private_key.size_in_bytes(),
                                                         16, 16, -1)]

    cipher_rsa = PKCS1_OAEP.new(private_key)
    session_key = cipher_rsa.decrypt(enc_session_key)

    cipher_aes = AES.new(session_key, AES.MODE_EAX, nonce)
    data = cipher_aes.decrypt_and_verify(ciphertext, tag)

print(data)

I'm getting as result:

<bound method _UrandomRNG.read of <Crypto.Random._UrandomRNG object at 0x000001696F4B8700>> HI
Private RSA key at 0x1696FB5CE20
b'blah blah bl'

Also, be careful, if you're using the construction in code without with statement, you need to close file at the end.

E.g. in this case will be the error ValueError: Ciphertext with incorrect length:

#...

out_file = open("encrypted_data.bin", "wb")
recipient_key = RSA.import_key(
    open('my_rsa_public.pem').read())
session_key = get_random_bytes(16)
cipher_rsa = PKCS1_OAEP.new(recipient_key)
out_file.write(cipher_rsa.encrypt(session_key))

cipher_aes = AES.new(session_key, AES.MODE_EAX)
data = b'blah blah bl'
ciphertext, tag = cipher_aes.encrypt_and_digest(data)

out_file.write(cipher_aes.nonce)
out_file.write(tag)
out_file.write(ciphertext)

#...

To solve it you need to write like:

#...

out_file = open("encrypted_data.bin", "wb")
recipient_key = RSA.import_key(
    open('my_rsa_public.pem').read())
session_key = get_random_bytes(16)
cipher_rsa = PKCS1_OAEP.new(recipient_key)
out_file.write(cipher_rsa.encrypt(session_key))

cipher_aes = AES.new(session_key, AES.MODE_EAX)
data = b'blah blah bl'
ciphertext, tag = cipher_aes.encrypt_and_digest(data)

out_file.write(cipher_aes.nonce)
out_file.write(tag)
out_file.write(ciphertext)

out_file.close() # important to add at the end

#...

Alternatively, you can do:

#...

with open('encrypted_data.bin', 'wb') as out_file:
    recipient_key = RSA.import_key(
    open('my_rsa_public.pem').read())
    session_key = get_random_bytes(16)
    cipher_rsa = PKCS1_OAEP.new(recipient_key)
    out_file.write(cipher_rsa.encrypt(session_key))

    cipher_aes = AES.new(session_key, AES.MODE_EAX)
    data = b'blah blah bl'
    ciphertext, tag = cipher_aes.encrypt_and_digest(data)

    out_file.write(cipher_aes.nonce)
    out_file.write(tag)
    out_file.write(ciphertext)

#...

ValueError: Ciphertext with incorrect length using Python

1 Answers1