Pycryptodome official example unclear

Question

I have an issue with https://www.pycryptodome.org/en/latest/src/examples.html#encrypt-data-with-rsa

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

file_out = open("encrypted_data.bin", "wb")

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

# Encrypt the session key with the public RSA key
cipher_rsa = PKCS1_OAEP.new(recipient_key)
file_out.write(cipher_rsa.encrypt(session_key))

# Encrypt the data with the 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 (cipher.nonce, tag, ciphertext) ]

It says I should use cipher.nonce, but cipher is undefined in the example. Am I supposed to use

cipher = AES.new(key, AES.MODE_EAX)
cipher = AES.new(key, AES.MODE_EAX, nonce)

or something else? I bet on the cipher = AES.new(key, AES.MODE_EAX, nonce), but I want to be sure it is cryptographically safe. In that case, key needed for cipher should be recipient_key, correct?

Answer 1

This code works, but I am not sure about it's security, so if anyone could confirm this, it would be highly appreciated.

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

data = "abc".encode("utf-8")

#print (open("pubkey.der").read())
recipient_key = RSA.import_key(open("pubkey.der").read())
session_key = get_random_bytes(16)
cipher_aes = AES.new(session_key, AES.MODE_EAX)

# Encrypt the session key with the public RSA key
cipher_rsa = PKCS1_OAEP.new(recipient_key)

# Encrypt the data with the AES session key
ciphertext, tag = cipher_aes.encrypt_and_digest(data)
enc_session_key = (cipher_rsa.encrypt(session_key))
encoded = [x for x in (cipher_aes.nonce, tag, ciphertext, enc_session_key) ]

#------------------------------------------------------

(cipher_aes_nonce, tag, ciphertext, enc_session_key) = encoded

private_key = RSA.import_key(open("privkey.der").read())

#print (enc_session_key, cipher.nonce, tag, ciphertext)

# Decrypt the session key with the public RSA key
cipher_rsa = PKCS1_OAEP.new(private_key)
session_key = cipher_rsa.decrypt(enc_session_key)

# Decrypt the data with the AES session key
cipher_aes = AES.new(session_key, AES.MODE_EAX, cipher_aes_nonce)
data = cipher_aes.decrypt_and_verify(ciphertext, tag)

print (data.decode("utf-8"))