如何在Python中解密OpenSSL AES加密文件？

Question

OpenSSL提供了一个流行的(但不安全的--见下文！)用于AES加密的命令行界面：

openssl aes-256-cbc -salt -in filename -out filename.enc

Python以PyCrypto包的形式支持AES，但它只提供工具。如何使用Python/PyCrypto对使用OpenSSL加密的文件进行解密？

通知

这个问题过去也涉及到Python中使用相同方案的加密。后来，我删除了这一部分，以阻止任何人使用它。不要用这种方式加密更多的数据，因为按照今天的标准，它是不安全的。您应该只使用解密，除了向后兼容之外没有其他原因，即当您没有其他选择的时候。想要加密吗？如果可能的话，使用NaCl/libsodium。

Answer 1

考虑到Python的流行，一开始我对这个问题没有完整的答案感到失望。我花了相当多的时间阅读了这块板上的不同答案，以及其他资源，才把它弄对了。我想我可能会分享结果以供将来参考或审查；我绝不是密码学专家！但是，下面的代码似乎可以无缝地工作：

from hashlib import md5
from Crypto.Cipher import AES
from Crypto import Random

def derive_key_and_iv(password, salt, key_length, iv_length):
    d = d_i = ''
    while len(d) < key_length + iv_length:
        d_i = md5(d_i + password + salt).digest()
        d += d_i
    return d[:key_length], d[key_length:key_length+iv_length]

def decrypt(in_file, out_file, password, key_length=32):
    bs = AES.block_size
    salt = in_file.read(bs)[len('Salted__'):]
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    next_chunk = ''
    finished = False
    while not finished:
        chunk, next_chunk = next_chunk, cipher.decrypt(in_file.read(1024 * bs))
        if len(next_chunk) == 0:
            padding_length = ord(chunk[-1])
            chunk = chunk[:-padding_length]
            finished = True
        out_file.write(chunk)

用法：

with open(in_filename, 'rb') as in_file, open(out_filename, 'wb') as out_file:
    decrypt(in_file, out_file, password)

如果您看到有机会改进这一点或将其扩展为更灵活(例如，让它在没有盐的情况下工作，或提供Python 3兼容性)，请随时这样做。

通知

这个答案过去也涉及到Python中使用相同方案的加密。后来，我删除了这一部分，以阻止任何人使用它。不要用这种方式加密更多的数据，因为按照今天的标准，它是不安全的。您应该只使用解密，除了向后兼容之外没有其他原因，即当您没有其他选择的时候。想要加密吗？如果可能的话，使用NaCl/libsodium。

Answer 2

我正在重新发布您的代码，并进行了一些更正(我不想使您的版本变得模糊)。虽然您的代码可以正常工作，但它不会检测到有关填充的一些错误。特别是，如果提供的解密密钥不正确，填充逻辑可能会做一些奇怪的事情。如果您同意我的更改，您可以更新您的解决方案。

from hashlib import md5
from Crypto.Cipher import AES
from Crypto import Random

def derive_key_and_iv(password, salt, key_length, iv_length):
    d = d_i = ''
    while len(d) < key_length + iv_length:
        d_i = md5(d_i + password + salt).digest()
        d += d_i
    return d[:key_length], d[key_length:key_length+iv_length]

# This encryption mode is no longer secure by today's standards.
# See note in original question above.
def obsolete_encrypt(in_file, out_file, password, key_length=32):
    bs = AES.block_size
    salt = Random.new().read(bs - len('Salted__'))
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    out_file.write('Salted__' + salt)
    finished = False
    while not finished:
        chunk = in_file.read(1024 * bs)
        if len(chunk) == 0 or len(chunk) % bs != 0:
            padding_length = bs - (len(chunk) % bs)
            chunk += padding_length * chr(padding_length)
            finished = True
        out_file.write(cipher.encrypt(chunk))

def decrypt(in_file, out_file, password, key_length=32):
    bs = AES.block_size
    salt = in_file.read(bs)[len('Salted__'):]
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    next_chunk = ''
    finished = False
    while not finished:
        chunk, next_chunk = next_chunk, cipher.decrypt(in_file.read(1024 * bs))
        if len(next_chunk) == 0:
            padding_length = ord(chunk[-1])
            if padding_length < 1 or padding_length > bs:
               raise ValueError("bad decrypt pad (%d)" % padding_length)
            # all the pad-bytes must be the same
            if chunk[-padding_length:] != (padding_length * chr(padding_length)):
               # this is similar to the bad decrypt:evp_enc.c from openssl program
               raise ValueError("bad decrypt")
            chunk = chunk[:-padding_length]
            finished = True
        out_file.write(chunk)

Answer 3

下面的代码应该与Python3兼容，并与代码中记录的小更改兼容。我也想用os.urandom代替Crypto.Random。'Salted__‘被替换为salt_header，如果需要，可以定制或保留为空。

from os import urandom
from hashlib import md5

from Crypto.Cipher import AES

def derive_key_and_iv(password, salt, key_length, iv_length):
    d = d_i = b''  # changed '' to b''
    while len(d) < key_length + iv_length:
        # changed password to str.encode(password)
        d_i = md5(d_i + str.encode(password) + salt).digest()
        d += d_i
    return d[:key_length], d[key_length:key_length+iv_length]

def encrypt(in_file, out_file, password, salt_header='', key_length=32):
    # added salt_header=''
    bs = AES.block_size
    # replaced Crypt.Random with os.urandom
    salt = urandom(bs - len(salt_header))
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    # changed 'Salted__' to str.encode(salt_header)
    out_file.write(str.encode(salt_header) + salt)
    finished = False
    while not finished:
        chunk = in_file.read(1024 * bs) 
        if len(chunk) == 0 or len(chunk) % bs != 0:
            padding_length = (bs - len(chunk) % bs) or bs
            # changed right side to str.encode(...)
            chunk += str.encode(
                padding_length * chr(padding_length))
            finished = True
        out_file.write(cipher.encrypt(chunk))

def decrypt(in_file, out_file, password, salt_header='', key_length=32):
    # added salt_header=''
    bs = AES.block_size
    # changed 'Salted__' to salt_header
    salt = in_file.read(bs)[len(salt_header):]
    key, iv = derive_key_and_iv(password, salt, key_length, bs)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    next_chunk = ''
    finished = False
    while not finished:
        chunk, next_chunk = next_chunk, cipher.decrypt(
            in_file.read(1024 * bs))
        if len(next_chunk) == 0:
            padding_length = chunk[-1]  # removed ord(...) as unnecessary
            chunk = chunk[:-padding_length]
            finished = True 
        out_file.write(bytes(x for x in chunk))  # changed chunk to bytes(...)