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This code is used to encrypt random input using RSA. I understand this line will write the output into a new file but can't grasp the theory of using x and list ([]) in the code.

cipher_aes = AES.new(session_key, AES.MODE_EAX)
ciphertext, tag = cipher_aes.encrypt_and_digest(data)
[ file_out.write(x) for x in (enc_session_key, cipher_aes.nonce, tag, ciphertext) ]
print("Message encrypted \n")
file_out.close()

The full code is as follows:

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

key = RSA.generate(2048)
private_key = key.export_key()
file_out = open("private.pem", "wb")
file_out.write(private_key)
file_out.close()

public_key = key.publickey().export_key()
file_out = open("receiver.pem", "wb")
file_out.write(public_key)
file_out.close()

print("Keys Generated \n")

data = input("Insert message to encrypt: ").encode("utf-8")
file_out = open("encrypted_data.bin", "wb")

recipient_key = RSA.import_key(open("receiver.pem").read())
session_key = get_random_bytes(16)

# Encrypt session key (random) guna public key
cipher_rsa = PKCS1_OAEP.new(recipient_key)
enc_session_key = cipher_rsa.encrypt(session_key)

# Encrypt data guna AES session key
cipher_aes = AES.new(session_key, AES.MODE_EAX)
ciphertext, tag = cipher_aes.encrypt_and_digest(data)
[ file_out.write(x) for x in (enc_session_key, cipher_aes.nonce, tag, ciphertext) ]
print("Message encrypted \n")
file_out.close()

Any help is appreciated!

Ruch
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    It's a [list comprehension](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions). Rather an odd usage for this case, I believe. – Fred Larson Feb 02 '21 at 18:00
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    There's no reason to use a list comprehension there. It's being used as a shortcut for a `for` loop. – Barmar Feb 02 '21 at 18:01

1 Answers1

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The list is unnecessary. They're using a list comprehension, but not saving the list anywhere. So it's equivalent to

for x in (enc_session_key, cipher_aes.nonce, tag, ciphertext):
    file_out.write(x)

There isn't even a need for the loop, you can simply concatenate all the variables:

file_out.write(enc_session_key + cipher_aes.nonce + tag + ciphertext)
Barmar
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