Decryption using Rijndael in C#

Your problem is a subtle error in your Encrypt method. You are losing data from your returned ciphertext by messing with the the hexOutput string. Instead of:

//store result as a hex value string hexOutput = BitConverter.ToString(cipherTextBytes).Replace("-", ""); hexOutput = hexOutput.Substring(0, plaintext.Length * 2); //finially return encrypted string return hexOutput; 

You should just return the output:

return BitConverter.ToString(cipherTextBytes).Replace("-", ""); 

You will also need to change the padding mode in your Decrypt method to None. Though this will now correctly decrypt it will also include the manual padding characters that you add in your encrypt method. As you don't know your plain text you have no GOOD way of removing them. You could always add a method to remove all bytes in your array that dont match your padding value of zero:

int endMarker = decryptedData.Length; do { endMarker--; } while (decryptedData[endMarker] == 0); Array.Resize(ref decryptedData, endMarker + 1); 

However this isn't really a good idea as you're possibly discarding otherwise valid data. A better solution would be to update your encrypt and decrypt methods to let the cipher handle the padding. Putting it all together we get (showing only what i've changed):

private static RijndaelManaged BuildRigndaelCommon(out byte[] rgbIV, out byte[] key) { rgbIV = new byte[] { 0x0, 0x1, 0x2, 0x3, 0x5, 0x6, 0x7, 0x8, 0xA, 0xB, 0xC, 0xD, 0xF, 0x10, 0x11, 0x12 }; key = new byte[] { 0x0, 0x1, 0x2, 0x3, 0x5, 0x6, 0x7, 0x8, 0xA, 0xB, 0xC, 0xD, 0xF, 0x10, 0x11, 0x12 }; //Specify the algorithms key & IV RijndaelManaged rijndael = new RijndaelManaged{BlockSize = 128, IV = rgbIV, KeySize = 128, Key = key, Padding = PaddingMode.PKCS7 }; return rijndael; } public static string Encrypt(string plaintext) { byte[] rgbIV; byte[] key; RijndaelManaged rijndael = BuildRigndaelCommon(out rgbIV, out key); //convert plaintext into a byte array byte[] plaintextBytes = Encoding.UTF8.GetBytes(plaintext); byte[] cipherTextBytes = null; //create uninitialized Rijndael encryption obj using (RijndaelManaged symmetricKey = new RijndaelManaged()) { //Call SymmetricAlgorithm.CreateEncryptor to create the Encryptor obj var transform = rijndael.CreateEncryptor(); //Chaining mode symmetricKey.Mode = CipherMode.CFB; //create encryptor from the key and the IV value ICryptoTransform encryptor = symmetricKey.CreateEncryptor(key, rgbIV); //define memory stream to hold encrypted data using (MemoryStream ms = new MemoryStream()) using (CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write)) { //encrypt contents of cryptostream cs.Write(plaintextBytes, 0, plaintextBytes.Length); cs.Flush(); cs.FlushFinalBlock(); //convert encrypted data from a memory stream into a byte array ms.Position = 0; cipherTextBytes = ms.ToArray(); ms.Close(); cs.Close(); } } //store result as a hex value return BitConverter.ToString(cipherTextBytes).Replace("-", ""); } public static string Decrypt(string disguisedtext) { byte[] disguishedtextBytes = FromHexString(disguisedtext); byte[] rgbIV; byte[] key; BuildRigndaelCommon(out rgbIV, out key); string visiabletext = ""; //create uninitialized Rijndael encryption obj using (var symmetricKey = new RijndaelManaged()) { //Call SymmetricAlgorithm.CreateEncryptor to create the Encryptor obj symmetricKey.Mode = CipherMode.CFB; symmetricKey.BlockSize = 128; //create encryptor from the key and the IV value // ICryptoTransform encryptor = symmetricKey.CreateEncryptor(key, rgbIV); ICryptoTransform decryptor = symmetricKey.CreateDecryptor(key, rgbIV); //define memory stream to hold encrypted data using (MemoryStream ms = new MemoryStream(disguishedtextBytes)) { //define cryptographic stream - contains the transformation to be used and the mode using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read)) { byte[] decryptedData = new byte[disguishedtextBytes.Length]; int stringSize = cs.Read(decryptedData, 0, disguishedtextBytes.Length); cs.Close(); //Trim the excess empty elements from the array and convert back to a string byte[] trimmedData = new byte[stringSize]; Array.Copy(decryptedData, trimmedData, stringSize); visiabletext = Encoding.UTF8.GetString(trimmedData); } } } return visiabletext; } 

Hope this helps point you on your way. As an aside I maintain a set of encryption utilities on Snipt that may be of use to you, particularly the SymmetricEncrypt and SymmetricDecrypt methods.

------ EDIT ------

As noted in the comment below, we are not allowed to alter the Encrypt method. I do like a good challenge! With appropriate byte mangling applied, here's a decrypt that honours the return coming form the Encrypt method:

public static string Decrypt(string disguisedtext) { byte[] disguishedtextBytes = FromHexString(disguisedtext); var originalLength = disguishedtextBytes.Length; int BlockSize; BlockSize = 16 * (1 + (originalLength / 16)); Array.Resize(ref disguishedtextBytes, BlockSize); // fill the remaining space with 0 for (int i = originalLength; i < BlockSize; i++) { disguishedtextBytes[i] = 0; } byte[] rgbIV; byte[] key; BuildRigndaelCommon(out rgbIV, out key); string visiabletext = ""; //create uninitialized Rijndael encryption obj using (var symmetricKey = new RijndaelManaged()) { //Call SymmetricAlgorithm.CreateEncryptor to create the Encryptor obj symmetricKey.Mode = CipherMode.CFB; symmetricKey.BlockSize = 128; symmetricKey.Padding = PaddingMode.None; // ICryptoTransform encryptor = symmetricKey.CreateEncryptor(key, rgbIV); ICryptoTransform decryptor = symmetricKey.CreateDecryptor(key, rgbIV); //define memory stream to hold encrypted data using (MemoryStream ms = new MemoryStream(disguishedtextBytes)) using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read)) { byte[] decryptedData = new byte[disguishedtextBytes.Length]; int stringSize = cs.Read(decryptedData, 0, disguishedtextBytes.Length); cs.Close(); //Trim the excess empty elements from the array and convert back to a string byte[] trimmedData = new byte[stringSize]; Array.Copy(decryptedData, trimmedData, originalLength); Array.Resize(ref trimmedData, originalLength); visiabletext = Encoding.UTF8.GetString(trimmedData); } } return visiabletext; }