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POCalculation of encrypted file size does not match true size
 

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  1. POCalculation of encrypted file size does not match true size
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    <p>I have the need to calculate the size of a file I am encrypting using Rijndael.</p> <p>According to other answers on this site, and on google, the following is the correct way of calculating encrypted data length:</p> <pre><code>EL = UL + (BS - (UL Mod BS) Mod BS) Where: EL = Encrypted Length UL = Unencrypted Length BS = Block Size </code></pre> <p>In my instance, the <strong>unencrypted file length is 5,101,972 bytes</strong>, and I am using a 128bit encryption key, giving me a <strong>block size of 16 bytes</strong>. Therefore, the equation is:</p> <pre><code>EL = 5101972 + (16 - (5101972 Mod 16) Mod 16) EL = 5101972 + (16 - 4 Mod 16) EL = 5101972 + (12 Mod 16) EL = 5101972 + 12 EL = 5101984 </code></pre> <p>Giving an <strong>encrypted file length of 5,101,984 bytes</strong>.</p> <p>However, the size of my file after encryption weighs in at 5,242,896 A massive difference in sizes of 140,912 bytes!</p> <p>Now.. I'm obviously doing SOMETHING wrong, but I can't work out what it is. Below is my encryption and decryption test code, as well as the method used to calculate the encrypted size:</p> <pre><code>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("1234567890123456"); 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 &lt; fsInput.Length; i += chunkSize) { byte[] chunkData = new byte[chunkSize]; int bytesRead = 0; while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) &gt; 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 &lt; fsInput.Length; i += chunkSize) { byte[] chunkData = new byte[chunkSize]; int bytesRead = 0; while ((bytesRead = fsInput.Read(chunkData, 0, chunkSize)) &gt; 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()); } } </code></pre> <p>Note: In the calculations at the start, I am NOT including the extra 24 bytes that are used at the start of the encrypted file by myself to store the original file length, and the IV... I know this, but didn't want to complicate the equation more than necessary.</p>
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    1. COWhat is the value of chunkSize?
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    2. COchunk size is currently 16 bytes. However, this has no influence on the overall encrypted size as far as I can tell (using increments of 16, from 16 to 10240)
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    3. COIs that what you used for the large file? I'd imagine you used something larger
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