How to change padding option on an existing encrypt/decrypt code?

Question

I have the following code, and it was working for almost two years. But now we have started to see random issue with the Padding. When I say random, I mean same thing works one day but doesn't work the other day. And someday it decides to work randomly.

Now, if I add the padding to none like mentioned in answers above, I could mess up all the previously encrypted files. I'm thinking to create different approach using GOTO statement in catch block in this method same way as I did when I changed the encryption key. Or is there any better approach to change padding to None?

/// <summary>
/// 
/// </summary>
[Serializable]
public static class EncryptDecrypt
{
    private static string EncryptionKey_old = "MAKV2SPBNI99212";
    private static string EncryptionKey = "Yi9BpGG1cXR01gBwGPZRTOznoJHpkGBOzisBg5jl3iRu48yhcFGdZu76fDpa5FUu";

    /// <summary>
    /// 
    /// </summary>
    /// <param name="clearText"></param>
    /// <returns></returns>
    public static string Encrypt(string clearText)
    {
        byte[] whitebs = Encoding.Unicode.GetBytes(clearText);

        using (Aes encryptor = Aes.Create())
        {

            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });

            encryptor.Key = pdb.GetBytes(32);

            encryptor.IV = pdb.GetBytes(16);
            encryptor.Mode = CipherMode.ECB;
            encryptor.Padding = PaddingMode.PKCS7;

            using (MemoryStream ms = new MemoryStream())
            {

                using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                {

                    cs.Write(whitebs, 0, whitebs.Length);
                    cs.FlushFinalBlock();
                    cs.Close();
                }
                clearText = Convert.ToBase64String(ms.ToArray());
            }
        }
        return clearText.EndsWith("==") ? clearText.Remove(clearText.Length - 2) : clearText;
    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="cipherText"></param>
    /// <returns></returns>
    public static string Decrypt(string cipherText)
    {
        int attempts = 0;
        string exception = string.Empty;


        StartHere:
        cipherText = cipherText.Replace(" ", "+");

        byte[] cipherBytes;
        try { cipherBytes = Convert.FromBase64String(cipherText); }
        catch { cipherBytes = Convert.FromBase64String(cipherText + "=="); }

        using (Aes encryptor = Aes.Create())
        {

            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });

            encryptor.Key = pdb.GetBytes(32);

            encryptor.IV = pdb.GetBytes(16);
            encryptor.Mode = CipherMode.ECB;
            encryptor.Padding = PaddingMode.PKCS7;
            try
            {
                using (MemoryStream ms = new MemoryStream())
                {

                    using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                    {

                        cs.Write(cipherBytes, 0, cipherBytes.Length);
                        cs.FlushFinalBlock();
                        cs.Close();
                    }

                    cipherText = Encoding.Unicode.GetString(ms.ToArray());

                }
            }
            catch
            {
                if (attempts == 2) throw;
                EncryptionKey = EncryptionKey_old;
                attempts++;
                goto StartHere;
            }
        }
        return cipherText;
    }
    '

Changing this now is not a good idea also I don't know how I would go about doing that because there are thousands of files we encrypted with this code.

Answer 1

It looks like a padding error is being used to determine is decryption is successful, that is wrong and will not work! You need another method to determine successful decryption, in this case if the correct key is used.

See PKCS#7 padding.

If decryption with the wrong key the result will be essentially random data and there is a 1/256 probability that the last byte will be 0x01 which is correct padding and there will be no padding error reported. To a lesser extent other valid padding will randomly occur.

Another method is necessary to determine if correct decryption is obtained, generally a MAC is used to authenticate the encryption. But it can be something in the data that is known, this is generally called a crib and the success rate is determined by the uniqueness of it.

Re: padding: Unless the length of the data to be encrypted is always an exact multiple of the block size (16-bytes for AES) padding is required and PKCS#7 padding is the correct padding