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POPadding in AES CBC
 

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    <p>I am trying to test CBC with Random IV using (128-bit AES) in C#.</p> <p>In my question to solve, I have 12-byte input message. The condition is that if PlainText is less than block-size (16-bytes) the padding to be used starts with 0x01 and then upto 6 0x00. </p> <p><strong>Example:</strong></p> <pre><code>in ASCII PT = Pay Bob 100% in hex PT = 50 61 79 20 42 6f 62 20 31 30 30 24 PT with Padding = 50 61 79 20 42 6f 62 20 31 30 30 24 01 00 00 00 </code></pre> <p>I don't seem to be able to find this <strong>PaddingMode</strong> in <strong>RijndaelManaged</strong>.</p> <p>Can any one suggest me how to do the following? </p> <ul> <li>Variable Length padding </li> </ul> <p><strong>EDIT:</strong></p> <pre><code>public class CBC { public CBC() { } private static readonly byte[] SALT = new byte[] {0x26, 0xdc, 0xff, 0x00, 0xad, 0xed, 0x7a, 0xee, 0xc5, 0xfe, 0x07, 0xaf, 0x4d, 0x08, 0x22, 0x3c}; public static byte[] EncryptCBC(string plainText, string passPhrase, PaddingMode paddingMode ) { byte[] result; using (RijndaelManaged cryptoProvider = new RijndaelManaged()) { Rfc2898DeriveBytes derivedKey = new Rfc2898DeriveBytes(passPhrase, SALT); cryptoProvider.Mode = CipherMode.CBC; cryptoProvider.GenerateIV(); // generate random IV cryptoProvider.Padding = paddingMode; cryptoProvider.Key = derivedKey.GetBytes(cryptoProvider.KeySize / 8); using (MemoryStream msEncrypt = new MemoryStream()) { using (ICryptoTransform encryptor = cryptoProvider.CreateEncryptor(cryptoProvider.Key, cryptoProvider.IV)) { using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write)) { using (StreamWriter swEncrypt = new StreamWriter(csEncrypt)) { swEncrypt.Write(plainText); } } } // concatenate iv to ciphertext result = cryptoProvider.IV.Concat(msEncrypt.ToArray()).ToArray(); } cryptoProvider.Clear(); } return result; } public static string DecryptCBC(byte[] cipherTextBytes, string passPhrase, PaddingMode paddingMode) { string result = null; using (RijndaelManaged cryptoProvider = new RijndaelManaged()) { Rfc2898DeriveBytes derivedKey = new Rfc2898DeriveBytes(passPhrase, SALT); cryptoProvider.Mode = CipherMode.CBC; // take the iv off the beginning of the ciphertext message cryptoProvider.IV = cipherTextBytes.Take(cryptoProvider.BlockSize / 8).ToArray(); cryptoProvider.Padding = paddingMode;//PaddingMode.ANSIX923; cryptoProvider.Key = derivedKey.GetBytes(cryptoProvider.KeySize / 8); using (MemoryStream msEncrypt = new MemoryStream(cipherTextBytes.Skip(cryptoProvider.BlockSize / 8).ToArray())) // skip the IV bytes { using (ICryptoTransform encryptor = cryptoProvider.CreateDecryptor(cryptoProvider.Key, cryptoProvider.IV)) { using (CryptoStream cryptoStream = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Read)) { byte[] plainTextBytes = new byte[cipherTextBytes.Length - cryptoProvider.BlockSize / 8]; int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length); result = Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount); cryptoStream.Close(); } } } cryptoProvider.Clear(); } return result; } } </code></pre> <p><strong>My Padding function</strong></p> <pre><code> private byte[] PaddPlainTextBytes(byte[] plainTextBytes ) { byte[] padding = utils.HexToBytes("01000000"); MemoryStream s = new MemoryStream(); s.Write(plainTextBytes, 0, plainTextBytes.Length); s.Write(padding, 0, padding.Length); byte[] paddedPt = s.ToArray(); return paddedPt; } </code></pre> <p><strong>Method to test my CBC</strong></p> <pre><code> private void btnTestCBC_Click(object sender, EventArgs e) { string plainText = "Pay Bob 100%"; string passPhrase = "Thisismypassphrase"; ShowMessage(@"Plain Text = " + plainText); byte[] pBytes = PaddPlainTextBytes(Encoding.ASCII.GetBytes(plainText)); string message = Encoding.ASCII.GetString(pBytes); byte[] encryptedBytes = CBC.EncryptCBC(plainText: message, passPhrase: passPhrase, paddingMode: PaddingMode.None); ShowMessage("Encrypted String = " + Encoding.ASCII.GetString(encryptedBytes)); ShowMessage("Encrypted HEX = " + utils.BytesToHex(encryptedBytes)); string decryptedString = CBC.DecryptCBC(encryptedBytes, passPhrase, PaddingMode.None); ShowMessage("Deccrypted String = " + decryptedString); } </code></pre>
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    1. USKhurram Majeed
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    1. COYou also could use a mode of operation that does not require padding, like CTR. Not built-in, alas.
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    2. COYou seem to assume padding is always a fixed length, and not an alignment function.
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      1. USleppie
 

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