回答问题

我正在尝试编写一个 Python 模块来加密我们现有的 .NET 类可以解密的文本。据我所知,我的代码行向上但没有解密(我在 C# 端收到“无效填充长度”错误)。我的 pkcs7 代码看起来不错,但研究表明无效密钥可能会导致同样的问题。

这两种设置有什么不同? Python:

derived_key = PBKDF2(crm_key, salt, 256 / 8, iterations)
iv = PBKDF2(crm_key, salt, 128 / 8, iterations)

encoder = pkcs7.PKCS7Encoder()

cipher = AES.new(derived_key, AES.MODE_CBC, iv)
decoded = cipher.decrypt(encoded_secret)

#encode - just stepped so i could debug. 
padded_secret = encoder.encode(secret)              # 1
encodedtext = cipher.encrypt(padded_secret)         # 2
based_secret = base64.b64encode(encodedtext)        # 3

我认为 based_secret 可以传递给 C# 并在那里解码。但它失败了。相同的加密 c# 代码是:

var rfc = new Rfc2898DeriveBytes(key, saltBytes);


        // create provider & encryptor
        using (var cryptoProvider = new AesManaged())
        {
            // Set cryptoProvider parameters
            cryptoProvider.BlockSize = cryptoProvider.LegalBlockSizes[0].MaxSize;
            cryptoProvider.KeySize = cryptoProvider.LegalKeySizes[0].MaxSize;

            cryptoProvider.Key = rfc.GetBytes(cryptoProvider.KeySize / 8);
            cryptoProvider.IV = rfc.GetBytes(cryptoProvider.BlockSize / 8);

            using (var encryptor = cryptoProvider.CreateEncryptor())
            {
                // Create a MemoryStream.
                using (var memoryStream = new MemoryStream())
                {
                    // Create a CryptoStream using the MemoryStream and the encryptor.
                    using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                    {
                        // Convert the passed string to a byte array.
                        var valueBytes = Encoding.UTF8.GetBytes(plainValue);

                        // Write the byte array to the crypto stream and flush it.
                        cryptoStream.Write(valueBytes, 0, valueBytes.Length);
                        cryptoStream.FlushFinalBlock();

                        // Get an array of bytes from the
                        // MemoryStream that holds the
                        // encrypted data.
                        var encryptBytes = memoryStream.ToArray();

                        // Close the streams.
                        cryptoStream.Close();
                        memoryStream.Close();

                        // Return the encrypted buffer.
                        return Convert.ToBase64String(encryptBytes);
                    }
                }
            }

我正在使用的 Python pkcs7 实现是:https://gist.github.com/chrix2/4171336

Answers

首先,我验证了 Rfc2898 和 PBKDF2 是一回事。然后,如上所述,问题似乎是 .net 主义。我在[msdn](http://msdn.microsoft.com/en-us/library/system.security.cryptography.rfc2898derivebytes.getbytes(v=vs.110).aspx 上找到)

Rfc2898DeriveBytes 内部的 GetBytes 实现在每次调用时都会发生变化,即。它拥有状态。 (见页面一半左右的备注)

Python 中的示例(伪输出):

derived_key = PBKDF2(key, salt, 32, 1000)
iv = PBKDF2(key, salt, 16, 1000)
print(base64.b64encode(derived_key))
print(base64.b64encode(iv))
$123456789101112134==
$12345678==

.NET 中的相同(ish)代码(再次,伪输出):

var rfc = new Rfc2898DeriveBytes(key, saltBytes);
    using (var cryptoProvider = new AesManaged())
    {
        // Set cryptoProvider parameters
        cryptoProvider.BlockSize = cryptoProvider.LegalBlockSizes[0].MaxSize;
        cryptoProvider.KeySize = cryptoProvider.LegalKeySizes[0].MaxSize;

        cryptoProvider.Key = rfc.GetBytes(cryptoProvider.KeySize / 8);
        cryptoProvider.IV = rfc.GetBytes(cryptoProvider.BlockSize / 8);
    }
Console.Writeline(Convert.ToBase64(cryptoProvider.Key));
Console.Writeline(Convert.ToBase64(cryptoProvider.IV));

$123456789101112134==
$600200300==

对 rfc.GetBytes 的后续调用总是产生不同的结果。 MSDN 表示,它会在呼叫中增加密钥大小。因此,如果您调用 GetBytes(20) 两次,则与调用 GetBytes(20+20) 或 GetBytes(40) 相同。从理论上讲,这应该只是增加密钥的大小,而不是完全改变它。

有一些解决方案可以解决此问题,可能是在第一次调用时生成更长的密钥,然后将其分割成派生密钥和 IV,或者随机生成 IV,将其附加到编码消息并剥离它在解密之前。

对 python 输出进行切片会产生与 .NET 相同的结果。它看起来像这样:

derived_key = PBKDF2(key, salt, 32, 1000)
iv = PBKDF2(key, salt, 32 + 16, 1000) # We need 16, but we're compensating for .NETs 'already called' awesomeness on the GetBytes method
split_key = iv[32:]

print(base64.b64encode(derived_key))
print(base64.b64encode(iv))
print(base64.b64encode(split_key))

$ 123456789101112134==   # matches our derived key
$ 12345678== # doesn't match
$ 600200300== # matches. this is the base 64 encoded version of the tailing 16 bytes.

享受,