加密文件大小的计算与真实大小不匹配

Question

我需要计算使用Rijndael加密的文件的大小。

根据本网站上其他的答案，并在谷歌，以下是计算加密数据长度的正确方法：

EL = UL + (BS - (UL Mod BS) Mod BS) 

Where: 
EL = Encrypted Length 
UL = Unencrypted Length 
BS = Block Size 

在我的情况下，未加密的文件长度为5101972个字节，和我正在使用128位加密密钥，给我一个块大小的16字节。 因此，等式为：

EL = 5101972 + (16 - (5101972 Mod 16) Mod 16) 
EL = 5101972 + (16 - 4 Mod 16) 
EL = 5101972 + (12 Mod 16) 
EL = 5101972 + 12 
EL = 5101984 

布施5101984个字节一个加密文件长度。

但是，我的文件在加密后的大小在5,242,896 重量为140,912字节的大小差异！

现在..我明显在做错了，但我不知道它是什么。 下面是我的加密和解密测试代码，以及用于计算加密大小的方法：

private static void Enc(string decryptedFileName, string encryptedFileName) 
{ 
    PasswordDeriveBytes passwordDB = new PasswordDeriveBytes("ThisIsMyPassword", Encoding.ASCII.GetBytes("thisIsMysalt!"), "MD5", 2); 
    byte[] passwordBytes = passwordDB.GetBytes(128/8); 

    using (FileStream fsOutput = File.OpenWrite(encryptedFileName)) 
    { 
     using(FileStream fsInput = File.OpenRead(decryptedFileName)) 
     { 
      byte[] IVBytes = Encoding.ASCII.GetBytes("123456789"); 

      fsOutput.Write(BitConverter.GetBytes(fsInput.Length), 0, 8); 
      fsOutput.Write(IVBytes, 0, 16); 

      RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.Zeros}; 
      ICryptoTransform encryptor = symmetricKey.CreateEncryptor(passwordBytes, IVBytes); 

      using (CryptoStream cryptoStream = new CryptoStream(fsOutput, encryptor, CryptoStreamMode.Write)) 
      { 
       for (long i = 0; i < fsInput.Length; i += chunkSize) 
       { 
        byte[] chunkData = new byte[chunkSize]; 
        int bytesRead = 0; 
        while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0) 
        { 
         cryptoStream.Write(chunkData, 0, chunkSize); 
        } 
       } 
      } 
     } 
    }    
} 

private static void Dec(string encryptedFileName, string decryptedFileName) 
{ 
    PasswordDeriveBytes passwordDB = new PasswordDeriveBytes("ThisIsMyPassword", Encoding.ASCII.GetBytes("thisIsMysalt!"), "MD5", 2); 
    byte[] passwordBytes = passwordDB.GetBytes(128/8); 

    using (FileStream fsInput = File.OpenRead(encryptedFileName)) 
    { 
     using (FileStream fsOutput = File.OpenWrite(decryptedFileName)) 
     { 
      byte[] buffer = new byte[8]; 
      fsInput.Read(buffer, 0, 8); 

      long fileLength = BitConverter.ToInt64(buffer, 0); 

      byte[] IVBytes = new byte[16]; 
      fsInput.Read(IVBytes, 0, 16); 


      RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.Zeros}; 
      ICryptoTransform decryptor = symmetricKey.CreateDecryptor(passwordBytes, IVBytes); 

      using (CryptoStream cryptoStream = new CryptoStream(fsOutput, decryptor, CryptoStreamMode.Write)) 
      { 
       for (long i = 0; i < fsInput.Length; i += chunkSize) 
       { 
        byte[] chunkData = new byte[chunkSize]; 
        int bytesRead = 0; 
        while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0) 
        { 
         cryptoStream.Write(chunkData, 0, bytesRead); 
        } 
       } 
       fsOutput.SetLength(fileLength); 
      }      
     } 
    } 
} 

private static void CalcEncSize(string decryptedFileName) 
{ 
    FileInfo fi = new FileInfo(decryptedFileName); 
    if (fi.Exists) 
    {     
     long blockSize = 128/8; 
     long fileLength = fi.Length; 
     long encryptedSize = fileLength + ((blockSize - (fileLength % blockSize)) % blockSize); 
     encryptedSize += 24; //16 bytes for the IV, and 8 more for the filelength, both stored at the start of the file. 

     Console.WriteLine("Estimated Encryption Size: " + encryptedSize.ToString());   
    } 
} 

注：在开始计算，我不包括在使用额外的24个字节我自己的加密文件开始存储原始文件长度和IV ...我知道这一点，但不希望复杂的方程超过必要的。

Answer 1

在实际加密过程之前，您不应写入输出文件。当关闭时，CryptoStream将处理所有必要的填充。此外for循环不是必需的，因为inner while循环将读取整个文件。你也应该只写和从文件中读取的一样多。尝试这些更改。

private static void Enc(string decryptedFileName, string encryptedFileName) 
{ 
    PasswordDeriveBytes passwordDB = new PasswordDeriveBytes("ThisIsMyPassword", Encoding.ASCII.GetBytes("thisIsMysalt!"), "MD5", 2); 
    byte[] passwordBytes = passwordDB.GetBytes(128/8); 

    using (FileStream fsOutput = File.OpenWrite(encryptedFileName)) 
    { 
     using(FileStream fsInput = File.OpenRead(decryptedFileName)) 
     { 
      byte[] IVBytes = Encoding.ASCII.GetBytes("123456789"); 

      RijndaelManaged symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC,Padding=PaddingMode.Zeros}; 
      ICryptoTransform encryptor = symmetricKey.CreateEncryptor(passwordBytes, IVBytes); 

      using (CryptoStream cryptoStream = new CryptoStream(fsOutput, encryptor, CryptoStreamMode.Write)) 
      { 
       byte[] chunkData = new byte[chunkSize]; 
       int bytesRead = 0; 
       while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) > 0) 
       { 
        cryptoStream.Write(chunkData, 0, bytesRead); //KEY FIX 
       } 
      } 
     } 
    }    
} 

[编辑]

哦，我已经上了很多的信息，错过了。我误解了你的尺寸想法140,912是大小，而不是差异。现在我明白了，我可以做出更明智的回应。根据您的代码，大小的差异应该与您的块大小相当。由于chunkSize可能有点大，因此您的代码通常会写入更多的数据，而不是实际在输入文件中的数据（如Greg和caf指出的那样）。我标出的行是之间的差异。