使用AES/CBC/NoPadding算法解密字符串

Question

我想AES/CBC/Nopadding在c#Windows Phone 8应用程序中解密加密Sting .我的字符串在IsolatedSorage.我粘贴了垃圾桶HERE.

从这篇文章我使用AesManaged类来解密.但是如何设置填充,NoPadding因为默认情况下填充设置为PKCS7从这里开始.

        string fileName = "titlepage.xhtml";

        if (fileStorage.FileExists(fileName))
        {
            IsolatedStorageFileStream someStream = fileStorage.OpenFile(fileName, System.IO.FileMode.Open, FileAccess.Read);
            using (StreamReader reader = new StreamReader(someStream))
            {
                str1 = reader.ReadToEnd();

                MessageBox.Show(str1);

                try
                {
                    string text = Decrypt(str1, "****************", "****************");

                    MessageBox.Show(text);
                }
                catch (CryptographicException cryptEx)
                {
                    MessageBox.Show(cryptEx.Message, "Encryption Error", MessageBoxButton.OK);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message, "General Error", MessageBoxButton.OK);
                }
            }
        }

    public string Decrypt(string dataToDecrypt, string password, string salt)
    {
        AesManaged aes = null;
        MemoryStream memoryStream = null;

        try
        {
            //Generate a Key based on a Password and HMACSHA1 pseudo-random number generator
            //Salt must be at least 8 bytes long
            //Use an iteration count of at least 1000
            Rfc2898DeriveBytes rfc2898 = new Rfc2898DeriveBytes(password, Encoding.UTF8.GetBytes(salt), 10000);               

            //Create AES algorithm
            aes = new AesManaged();
            //Key derived from byte array with 32 pseudo-random key bytes
            aes.Key = rfc2898.GetBytes(32);
            //IV derived from byte array with 16 pseudo-random key bytes
            aes.IV = rfc2898.GetBytes(16);

            //Create Memory and Crypto Streams
            memoryStream = new MemoryStream();
            CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Write);

            byte[] data = Convert.FromBase64String(dataToDecrypt);
            cryptoStream.Write(data, 0, data.Length);
            cryptoStream.FlushFinalBlock();

            //Return Decrypted String
            byte[] decryptBytes = memoryStream.ToArray();

            //Dispose
            if (cryptoStream != null)
                cryptoStream.Dispose();

            //Retval
            return Encoding.UTF8.GetString(decryptBytes, 0, decryptBytes.Length);
        }
        finally
        {
            if (memoryStream != null)
                memoryStream.Dispose();

            if (aes != null)
                aes.Clear();
        }            
    }

编辑1:

当我在最后一行解密我的加密字符串时

 byte[] data = Convert.FromBase64String(dataToDecrypt);

转移Finally block并获得例外The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters in decrypted string.

这有点混乱,这是在Windows Phone中支持Decrypt的类.

如果我完全错误,请向我建议关于Windows Phone中算法的文章

编辑2:

如下面的答案所示"我将cyperText作为字节获得,它在解密方面很好.但它给出了描述的例外

       [Cryptography_SSD_InvalidDataSize]
    Arguments: 
    Debugging resource strings are unavailable. Often the key and arguments provide 
sufficient information to diagnose the problem

我认为问题是IV [盐键]或设置填充到AesManged.但是我无法在Windows Phone 中将填充属性更改为AesManaged.默认情况下,填充到AesManged是PKCS7.我想换到NoPadding.因为我的cyperText使用AES/CBC/NoPadding算法加密"

Answer 1

如果我理解了这个问题,那么您的数据已经在AES CBC模式下加密,没有填充.但是在要解密数据的手机上,唯一的选择是PKCS#7填充.

好吧,你是幸运的!您可以使用PKCS#7填充来解密密文.您需要做的就是将填充添加到手机上的密文,然后解密.

要在事后添加填充,您将加密一小部分数据并将其附加到密文.然后,您解密修改后的密文,并关闭那一小段数据,并获得原始明文.

这是你如何做到的:

1. 在手机上取一个密文.即使没有填充,这也是16个字节的倍数.没有其他可能性 - AES密文始终是16字节的倍数.

2. 将密文的最后16个字节放在一边,并将其设置为AES ENCRYPT的IV.(加密,而不是解密.)使用您将要用于稍后解密的相同密钥.

3. 现在加密小于16字节的内容,例如字符'$'.手机将为此添加PKCS#7填充.

4. 将生成的16字节密文附加到步骤1中的原始密文,现在您有一个正确的PKCS#7填充密文,其中包括原始明文和添加的"$".

5. 使用原来的IV,和相同的密钥,现在DECRYPT这个组合的密文.您现在可以删除将出现在明文末尾的"$"(或者您在步骤3中添加的任何内容).

当使用原始密文的最后16个字节对小位进行加密时,实际上是在真正的AES CBC模式下扩展密文,而您恰好使用PKCS#7填充进行加密,因此您现在可以解密整个密文并取消一点点.你将拥有没有填充的原始明文.

我认为在代码中展示会很有趣:

var rfc2898 = new Rfc2898DeriveBytes("password", new byte[8]);

using (var aes = new AesManaged())
{
    aes.Key = rfc2898.GetBytes(32);
    aes.IV = rfc2898.GetBytes(16);

    var originalIV = aes.IV; // keep a copy

    // Prepare sample plaintext that has no padding
    aes.Padding = PaddingMode.None;
    var plaintext = Encoding.UTF8.GetBytes("this plaintext has 32 characters");
    byte[] ciphertext;
    using (var encryptor = aes.CreateEncryptor())
    {
        ciphertext = encryptor.TransformFinalBlock(plaintext, 0, plaintext.Length);
        Console.WriteLine("ciphertext: " + BitConverter.ToString(ciphertext));
    }

    // From this point on we do everything with PKCS#7 padding
    aes.Padding = PaddingMode.PKCS7;

    // This won't decrypt -- wrong padding
    try
    {
        using (var decryptor = aes.CreateDecryptor())
        {
            var oops = decryptor.TransformFinalBlock(ciphertext, 0, ciphertext.Length);
        }
    }
    catch (Exception e)
    {
        Console.WriteLine("caught: " + e.Message);
    }

    // Last block of ciphertext is used as IV to encrypt a little bit more
    var lastBlock = new byte[16];
    var modifiedCiphertext = new byte[ciphertext.Length + 16];

    Array.Copy(ciphertext, ciphertext.Length - 16, lastBlock, 0, 16);
    aes.IV = lastBlock;

    using (var encryptor = aes.CreateEncryptor())
    {
        var dummy = Encoding.UTF8.GetBytes("$");
        var padded = encryptor.TransformFinalBlock(dummy, 0, dummy.Length);

        // Set modifiedCiphertext = ciphertext + padded
        Array.Copy(ciphertext, modifiedCiphertext, ciphertext.Length);
        Array.Copy(padded, 0, modifiedCiphertext, ciphertext.Length, padded.Length);
        Console.WriteLine("modified ciphertext: " + BitConverter.ToString(modifiedCiphertext));
    }

    // Put back the original IV, and now we can decrypt...
    aes.IV = originalIV;

    using (var decryptor = aes.CreateDecryptor())
    {
        var recovered = decryptor.TransformFinalBlock(modifiedCiphertext, 0, modifiedCiphertext.Length);
        var str = Encoding.UTF8.GetString(recovered);
        Console.WriteLine(str);

        // Now you can remove the '$' from the end
    }
}

Answer 2

您链接到的字符串不是Base -64。它看起来好像是原始加密字节，被解释为字符。要么在加密端工作以输出原始字节的 Base-64 字符串编码，要么在解密端工作以将密文读取为原始字节，而不是文本，并且忘记删除 Base-64。

一般来说，在加密方面工作会更好，因为传递 Base-64 文本比传递原始字节更不容易出错。