iOS CryptoSwift AES Encryption to Python Decryption works - but not the inverse

Question

I am using CryptoSwift 1.4.1, iOS 15.2, PyCryptodome 3.12.0, and XCode 13.2.1 to encrypt small string messages that I send to a Raspberry Pi Linux Device over BLE. It works when iOS encrypts the message and sends it to the Raspberry Pi. The Pi can successfully decrypt it. Now I want to do the inverse, encrypt a message on the Pi and have the iOS App read and decrypt it. This, however is not working and the decrypted value is the not the message I encrypted on the Pi.

Working iOS encryption:

func aesEncrypt(stringToEncrypt: String, key: Array<UInt8>, iv: Array<UInt8>) throws -> String {
    let data = stringToEncrypt.data(using: String.Encoding.utf8)
    let encrypted = try AES(key: key, blockMode: CFB(iv: iv), padding: .noPadding).encrypt((data?.bytes)!)
    return encrypted.toHexString()
}

let ivString = "4198816658388141"
let keyString = "9004786896524916"

let key = [UInt8](keyString.utf8)
let iv = [UInt8](ivString.utf8)

let encryptedSsid = try! aesEncrypt(stringToEncrypt: ssid!, key: key, iv: iv)

Working Raspberry Pi decryption in Python:

KEY = b'9004786896524916'
IV = b'4198816658388141'
MODE = AES.MODE_CFB

def decrypt(key, iv, encrypted_text):
    logger.info(f"Encrypted: {encrypted_text}")
    aes = AES.new(key, MODE, iv, segment_size=128)
    encrypted_text_bytes = binascii.a2b_hex(encrypted_text)
    decrypted_text = aes.decrypt(encrypted_text_bytes).decode("utf-8")
    logger.info(f"Decrypted: {decrypted_text}")
    return decrypted_text

I tried to encrypt a message on the Pi with the following code:

KEY = b'9004786896524916'
IV = b'4198816658388141'
MODE = AES.MODE_CFB

def encrypt(key, decrypted_text):
    # Create cipher object and encrypt the data
    logger.info(f"Decrypted: {decrypted_text}")
    cipher = AES.new(key, MODE, segment_size=128)  # Create a AES cipher object with the key using the mode CBC
    #encrypted_text = cipher.encrypt(pad(decrypted_text, AES.block_size))  # Pad the input data and then encrypt
    encrypted_text = cipher.encrypt(decrypted_text)  # Pad the input data and then encrypt
    logger.info(f"Encrypted: {encrypted_text}")
    return encrypted_text
    ...
    encrypt(KEY, returnString('utf-8'))

However, the iOS App fails to decrypt it properly with this method:

func aesDecrypt(stringToDecrypt: String, key: Array<UInt8>, iv: Array<UInt8>) throws -> String {
    let data = stringToDecrypt.data(using: String.Encoding.utf8)
    let decrypted = try AES(key: key, blockMode: CFB(iv: iv), padding: .noPadding).decrypt((data?.bytes)!)
    return decrypted.toHexString()
}
let ivString = "4198816658388141"
let keyString = "9004786896524916"

let key = [UInt8](keyString.utf8)
let iv = [UInt8](ivString.utf8)
var message = try! aesDecrypt(stringToDecrypt: charString, key: key, iv: iv)

How can I get decryption to work properly in the iOS App when a message is encrypted on the Pi?

this SO post may be of use: https://stackoverflow.com/questions/61332076/cross-platform-aes-encryption-between-ios-and-kotlin-java-using-apples-cryptokit/61345165#61345165 — workingdog support Ukraine, Jan 22 '22 at 03:54
Thanks for the suggestion. I tried "encrypt(KEY, IV + returnValue.encode('utf-8'))" but unfortunately it still fails and decrypts to 'nonsense' — PhilBot, Jan 24 '22 at 00:17

Topaco · Accepted Answer · 2022-01-28T10:30:59.000

In the encrypt() method the IV is not considered. As in aesEncrypt(), the IV must be passed and used when creating the AES object.
Furthermore there are bugs in the encoding: The plaintext must be UTF8 encoded and the ciphertext must be hex encoded:

from Crypto.Cipher import AES
import binascii

def encrypt(key, iv, plaintext):
    cipher = AES.new(key, MODE, iv, segment_size=128)  
    plaintext_bytes = plaintext.encode("utf-8")
    ciphertext = cipher.encrypt(plaintext_bytes)
    ciphertext_hex = binascii.b2a_hex(ciphertext)  
    return ciphertext_hex

This function is the counterpart to decrypt(), i.e. encrypt() can be used to generate a ciphertext which can be decrypted with decrypt() (or aesDecrypt()).

In the iOS code there are two bugs, both concerning the encoding: The ciphertext must not be UTF8 encoded, but hex decoded. And the decrypted data must not be hex encoded, but UTF-8 decoded.

A possible fix is:

func aesDecrypt(stringToDecrypt: String, key: Array<UInt8>, iv: Array<UInt8>) throws -> String {
    let data = Array<UInt8>(hex: stringToDecrypt)
    let decrypted = try AES(key: key, blockMode: CFB(iv: iv), padding: .noPadding).decrypt(data)
    return String(bytes: decrypted, encoding: .utf8)!
}

This function is the counterpart to aesEncrypt(), i.e. aesDecrypt() can be used to decrypt a ciphertext generated with aesEncrypt() (or encrypt()).

Regarding security: A static IV is insecure. Instead, the IV should be randomly generated for each encryption. Since the (non-secret IV) is needed for decryption, it is passed along with the ciphertext (typically concatenated).

iOS CryptoSwift AES Encryption to Python Decryption works - but not the inverse

1 Answers1