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# SPDX-License-Identifier: GPL-3.0+
# Copyright (C) 2020 nlscc
""" FUS authentication functions (decrypting nonce, calculating auth token) """
import base64
from Cryptodome.Cipher import AES
# Constant key input values.
KEY_1 = "hqzdurufm2c8mf6bsjezu1qgveouv7c7"
KEY_2 = "w13r4cvf4hctaujv"
# PKCS#7 padding functions.
pkcs_unpad = lambda d: d[:-d[-1]]
pkcs_pad = lambda d: d + bytes([16 - (len(d) % 16)]) * (16 - (len(d) % 16))
def aes_encrypt(inp: bytes, key: bytes) -> bytes:
""" Perform an AES-CBC encryption. Encrypts /inp/ with key /key/. """
enc_iv = key[:16] # IV is first 16 bytes of key
cipher = AES.new(key, AES.MODE_CBC, enc_iv)
return cipher.encrypt(pkcs_pad(inp))
def aes_decrypt(inp: bytes, key: bytes) -> bytes:
""" Perform an AES-CBC decryption. Decrypts /inp/ with key /key/. """
enc_iv = key[:16]
cipher = AES.new(key, AES.MODE_CBC, enc_iv)
return pkcs_unpad(cipher.decrypt(inp))
def derive_key(nonce: str) -> bytes:
""" Calculate the AES key from the FUS input nonce. """
key = ""
# First 16 bytes are offsets into KEY_1
for i in range(16):
key += KEY_1[ord(nonce[i]) % 16]
# Last 16 bytes are static
key += KEY_2
return key.encode()
def getauth(nonce: str) -> str:
""" Calculate the response token from a given nonce. """
nkey = derive_key(nonce)
auth_data = aes_encrypt(nonce.encode(), nkey)
return base64.b64encode(auth_data).decode()
def decryptnonce(inp: str) -> str:
""" Decrypt the nonce returned by the server. """
inp_data = base64.b64decode(inp)
nonce = aes_decrypt(inp_data, KEY_1.encode()).decode()
return nonce
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