我正在开发一个玩具模块来加密临时文件。这个想法是公开一个类似于tempfile模块中的接口，但使用会话密钥对数据进行透明加密。同样，这只是一个玩具项目，而不是生产代码。
首先，一些技术细节。该项目实施pycrypto并正在使用：
- AES-CTR
- 具有 1 位随机前缀的计数器
基本类的结构是为了模仿文件对象接口。当一个方法如reador被调用时，我们使用类的属性write初始化一个Crypto.Cipher.AES对象。_cipher该属性实现如下：
def _cipher():
doc = doc = "Returns a stateful AES object ready to decrypt at the required \
stream position"
def fget(self):
ctr = Counter.new(nbits=64,
initial_value=self.tell() # alias of self._file.tell
prefix=self._nonce) # set by `Random.new().read(8)
return AES.new(self._key, counter=ctr, mode=self._opmode) # self._opmode = Crypto.Cipher.AES.MODE_CTR
def fset(self, value):
raise AttributeError('Cannot set _cipher')
def fdel(self, value):
raise AttributeError('Cannot delete _cipher')
return locals()
_cipher = property(**_cipher())
这是一个_cipher在调用方法期间如何使用属性进行透明加密的示例write。
def write(self, data):
if not isinstance(data, str):
raise TypeError('Data must be str (or bytestring)')
self._file.write(self._cipher.encrypt(data))
解密时，我们应用相反的交易：
def read(self, size=-1):
return self._cipher.decrypt(self._file.read(size))
这在对 进行单个调用时有效write，但在多个调用write被链接时失败。例如：
ep = EphemeralFile() # the class in question
ep.write('Now is the winter of our discontent')
ep.seek(0)
print ep.read()
>> Now is the winter of our discontent
到目前为止一切都很好......但这就是失败的地方
ep.write(' made glorious summer by this sun of York')
ep.seek(0)
print ep.read()
>> Now is the winter of our discontent"d_"��U�L~ �w���S��h��]"U(��P^��9k
我究竟做错了什么？self._file.tell()属性中的使用不应该_cipher为解密产生适当的计数器位置吗？
请注意，我首先怀疑我可能会偏离一个计数器刻度，因此我尝试将initial_value=self.tell()行修改为initial_value=self.tell() + 1（也尝试使用-1），但无济于事。
为方便起见，这里是完整的类定义。它相当短，可能会产生一些见解。
import tempfile
from Crypto.Cipher import AES
from Crypto import Random
from Crypto.Util import Counter
PRNG = Random.new()
class EphemeralFile(object):
def __init__(self, mode='w+b', bufsize=-1, suffix='', prefix='', dir=None,
key_size=32):
self._key = PRNG.read(key_size)
self._nonce = PRNG.read(8)
self._opmode = AES.MODE_CTR
self._file = tempfile.TemporaryFile(mode=mode, bufsize=bufsize,
suffix=suffix, prefix=prefix, dir=dir)
# alias tempfile methods and parameters
self.close = self._file.close
self.closed = self._file.closed
self.encoding = self._file.encoding
self.errors = self._file.errors
self.fileno = self._file.fileno
self.flush = self._file.flush
self.isatty = self._file.isatty
self.mode = self._file.mode
self.name = self._file.name
self.softspace = self._file.softspace
self.truncate = self._file.truncate
self.seek = self._file.seek
self.tell = self._file.tell
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
self._file.close()
def __iter__(self):
return (line for line in self.readlines())
def _cipher():
doc = "Returns a stateful AES object ready to decrypt at the required \
stream position"
def fget(self):
ctr = Counter.new(nbits=64,
initial_value=self.tell(),
prefix=self._nonce)
return AES.new(self._key, counter=ctr, mode=self._opmode)
def fset(self, value):
raise AttributeError('Cannot set EphemeralFile._cipher')
def fdel(self):
raise AttributeError('Cannot delete EphemeralFile._cipher')
return locals()
_cipher = property(**_cipher())
def write(self, data):
if not isinstance(data, str):
raise TypeError('Data must be str (or bytestring)')
self._file.write(self._cipher.encrypt(data))
def writelines(self, lines):
self.write("\n".join(lines))
def read(self, size=-1):
return self._cipher.decrypt(self._file.read(size))
def readline(self, size=-1):
fptr = self.tell()
bytes = []
got_line = False
while not got_line:
bytes.append(self.read(1))
if not bytes[-1] or ('\n' in bytes[-1]):
bytes[-1] = bytes[-1][0:bytes[-1].find('\n') + 1]
got_line = True
plaintext = ''.join(bytes)
self.seek(fptr + len(plaintext)) # rewind
return plaintext
def readlines(self, size=-1):
return [line for line in self]
def read_ciphertext(self, size=-1):
"""Read ciphertext without decrypting.
size : int (default -1)
Number of bytes to read. Negative values read the entire stream
return : str
Ciphertext
"""
return self._file.read(size)
def next(self):
return self.readline()
在这一点上，我真的不知道问题出在哪里，所以请随时向我提出问题并提出可能的解决方案。
提前谢谢了！