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akira_decrypt_file.py
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akira_decrypt_file.py
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# MIT License
#
# Copyright (c) 2023-2024 Andrey Zhdanov (rivitna)
# https://github.com/rivitna
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files
# (the "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to permit
# persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be included
# in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
# DEALINGS IN THE SOFTWARE.
import sys
import io
import os
import shutil
import struct
from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_v1_5
import chacha
RANSOM_EXT = '.akira'
# RSA
RSA_KEY_SIZE = 512
# ChaCha20/8
CHACHA_KEY_SIZE = 32
CHACHA_NONCE_SIZE = 8
CHACHA_ROUNDS = 8
METADATA_SIZE = RSA_KEY_SIZE + 22
ENC_BLOCK_SIZE = 0x10000
SENTINEL_SIZE = 16
def rsa_decrypt(enc_data: bytes, priv_key_data: bytes) -> bytes:
"""RSA decrypt data"""
key = RSA.import_key(priv_key_data)
sentinel = os.urandom(SENTINEL_SIZE)
cipher = PKCS1_v1_5.new(key)
data = cipher.decrypt(enc_data[::-1], sentinel)
if data == sentinel:
return None
return data
def decrypt_file(filename: str, priv_key_data: bytes) -> bool:
"""Decrypt file"""
with io.open(filename, 'rb+') as f:
# Read metadata
try:
f.seek(-METADATA_SIZE, 2)
except OSError:
return False
metadata = f.read(METADATA_SIZE)
# Encryption mode (0 - full, 1 - part, 2 - spot)
enc_mode = metadata[RSA_KEY_SIZE + 12]
if not (0 <= enc_mode <= 2):
return False
# Encryption percent (0..100)
enc_percent = metadata[RSA_KEY_SIZE + 13]
if not (0 <= enc_percent <= 100):
return False
file_size, = struct.unpack_from('<Q', metadata, RSA_KEY_SIZE + 14)
if enc_mode == 0:
# full
# (.avdx .vhd .pvm .bin .avhd .vsv .vmx .vmsn .vmsd .vmrs .vmem
# .vmcx .vhdx .vmdk .nvram .iso .raw .qcow2 .vdi .subvol)
num_chunks = 1
chunk_size = file_size
chunk_step = 0
elif enc_mode == 1:
# part (file size <= 2000000)
num_chunks = 1
chunk_size = (file_size * enc_percent) // 100
chunk_step = 0
else:
# spot (file size > 2000000)
enc_size = (file_size * enc_percent) // 100
n = 3 if (enc_percent < 50) else 5
chunk_size = enc_size // n
num_chunks = 2 if (enc_percent < 50) else 4
chunk_step = (file_size - chunk_size * num_chunks) // n
# Decrypt ChaCha20 key and nonce
enc_key_data = metadata[:RSA_KEY_SIZE]
key_data = rsa_decrypt(enc_key_data, priv_key_data)
if key_data is None:
return False
key = key_data[:CHACHA_KEY_SIZE]
nonce = key_data[CHACHA_KEY_SIZE :
CHACHA_KEY_SIZE + CHACHA_NONCE_SIZE]
# Remove metadata
f.truncate(file_size)
# Decrypt file data
chacha_blocks_per_chunk = (chunk_size + (64 - 1)) // 64
pos = 0
for i in range(num_chunks):
# Decrypt chunk
cipher = chacha.ChaCha(key, nonce, i * chacha_blocks_per_chunk,
CHACHA_ROUNDS)
p = pos
size = chunk_size
while size != 0:
block_size = min(size, ENC_BLOCK_SIZE)
f.seek(p)
enc_data = f.read(block_size)
if enc_data == b'':
break
dec_data = cipher.decrypt(enc_data)
f.seek(p)
f.write(dec_data)
size -= block_size
p += block_size
else:
pos += chunk_step
continue
break
return True
#
# Main
#
if len(sys.argv) != 2:
print('Usage:', os.path.basename(sys.argv[0]), 'filename')
sys.exit(0)
filename = sys.argv[1]
with io.open('./rsa_privkey.pem', 'rb') as f:
priv_key_data = f.read()
# Copy file
new_filename = filename
if new_filename.endswith(RANSOM_EXT):
new_filename = new_filename[:-len(RANSOM_EXT)]
else:
new_filename += '.dec'
shutil.copy(filename, new_filename)
# Decrypt file
if not decrypt_file(new_filename, priv_key_data):
print('Error: Failed to decrypt file')
sys.exit(1)