Exploit Notes

AES (Advanced Encryption Standard)

Last modified: 2023-07-28

Cryptography

AES is a specification for the encryption of electronic data.

Decrypt

GPG

gpg --decrypt example.gpg

OpenSSL

# AES-256-CBC
openssl aes-256-cbc -d -in encrypted_text -out decrypted.txt

# AES-256-CBC (PBKDF2)
openssl aes-256-cbc -pbkdf2 -iter 10000 -d -in encrypted_text -out decrypted.txt

Encrypt

GPG

gpg --symmetric --cipher-algo CIPHER message.txt

OpenSSL

# AES-256-CBC
openssl aes-256-cbc -e -in message.txt -out encrypted_message

# AES-256-CBC (PBKDF2)
openssl aes-256-cbc -pbkdf2 -iter 10000 -e -in message.txt -out encrypted_message

Implement AES in Python

Reference: https://medium.com/quick-code/aes-implementation-in-python-a82f582f51c2

We need pycryptodome for implementing AES easily.

pip install pycryptodome

Below is a Python script for AES encryption and decryption.

import hashlib
from Crypto import Random
from Crypto.Cipher import AES
from base64 import b64encode, b64decode

class AESCipher(object):
	def __init__(self, key):
		# AES block size is 128 bit
		self.block_size = AES.block_size
		# Generate unique 256 bit key
		self.key = hashlib.sha256(key.encode()).digest()

	def encrypt(self, plaintext):
		plaintext = self.__pad(plaintext)
		# IV (Initialization Vector) is the same size as the block size
		iv = Random.new().read(self.block_size)
		cipher = AES.new(self.key, AES.MODE_CBC, iv)
		ciphertext = cipher.encrypt(plaintext.encode())
		return b64encode(iv + ciphertext).decode('utf-8')

	def decrypt(self, ciphertext):
		ciphertext = b64decode(ciphertext)
		iv = ciphertext[:self.block_size]
		cipher = AES.new(self.key, AES.MODE_CBC, iv)
		plaintext = cipher.decrypt(ciphertext[self.block_size:]).decode('utf-8')
		return self.__unpad(plaintext)

	# Pad a plaintext for it to be a multiple of 128 bits.
	def __pad(self, plaintext):
		num_of_bytes_to_pad = self.block_size - len(plaintext) % self.block_size
		ascii_str = chr(num_of_bytes_pad)
		pad_str = num_of_bytes_to_pad * ascii_str
		pad_plaintext = plaintext + pad_str
		return pad_plaintext

	@staticmethod
	def __unpad(plaintext):
		last_char = plaintext[len(plaintext) -1:]
		bytes_to_remove = ord(last_char)
		return plaintext[:-bytes_to_remove]

References