Question

I am using the “BouncyCastle.Crypto.dll” for encrypt/decrypt a string in my app. I am using the
following code from this blog:

1. I have a class BCEngine, exactly the same as the one given in the link mentioned above.

   public class BCEngine
   {
      private readonly Encoding _encoding;
      private readonly IBlockCipher _blockCipher;
      private PaddedBufferedBlockCipher _cipher;
      private IBlockCipherPadding _padding;
   
      public BCEngine(IBlockCipher blockCipher, Encoding encoding)
      {
         _blockCipher = blockCipher;
         _encoding = encoding;
      }
   
      public void SetPadding(IBlockCipherPadding padding)
      {
          if (padding != null)
            _padding = padding;
      }
   
      public string Encrypt(string plain, string key)
      {
          byte[] result = BouncyCastleCrypto(true, _encoding.GetBytes(plain), key);
          return Convert.ToBase64String(result);
      }
   
      public string Decrypt(string cipher, string key)
      {
         byte[] result = BouncyCastleCrypto(false, Convert.FromBase64String(cipher), key);
         return _encoding.GetString(result);
      }
   
      /// <summary>
      ///
      /// </summary>
      /// <param name="forEncrypt"></param>
      /// <param name="input"></param>
      /// <param name="key"></param>
      /// <returns></returns>
      /// <exception cref="CryptoException"></exception>
      private byte[] BouncyCastleCrypto(bool forEncrypt, byte[] input, string key)
      {
          try
          {
              _cipher = _padding == null ? new PaddedBufferedBlockCipher(_blockCipher) : new PaddedBufferedBlockCipher(_blockCipher, _padding);
              byte[] keyByte = _encoding.GetBytes(key);
              _cipher.Init(forEncrypt, new KeyParameter(keyByte));
              return _cipher.DoFinal(input);
          }
          catch (Org.BouncyCastle.Crypto.CryptoException ex)
          {
              throw new CryptoException(ex.Message);
          }
      }
   }
   

I am using an asp.net form in which i have written code as given below:

    public partial class EncryptionForm : System.Web.UI.Page
    {
      Encoding _encoding;
      IBlockCipherPadding _padding;
      string key = "DFGFRT";
       string textToBeEncrypted = "Original text. Please encrypt me.";
       string txtEncryptedText = string.empty;
       string txtDecryptedText = string.empty;

      protected void Page_Load(object sender, EventArgs e)
      {
          _encoding = Encoding.ASCII; 
          Pkcs7Padding pkcs = new Pkcs7Padding();
          _padding = pkcs;   
      }

      protected void btnEncrypt_Click(object sender, EventArgs e)
      {
          txtEncryptedText = AESEncryption(textToBeEncrypted, key, true);
      }

      protected void btnDecrypt_Click(object sender, EventArgs e)
      {
          txtDecryptedText = AESDecryption(txtEncryptedText.Text, key, true);
      }

      public string AESEncryption(string plain, string key, bool fips)
      {
          BCEngine bcEngine = new BCEngine(new AesEngine(), _encoding);
          bcEngine.SetPadding(_padding);
          return bcEngine.Encrypt(plain, key);
      }

      public string AESDecryption(string cipher, string key, bool fips)
      {
          BCEngine bcEngine = new BCEngine(new AesEngine(), _encoding);
          bcEngine.SetPadding(_padding);
          return bcEngine.Decrypt(cipher, key);
      }
    }

Not sure, but due to some reason, I get an exception when I call the btnEncrypt_Click

“Key length not 128/192/256 bits.”

Can anybody please guide? I am a complete newbie to this.
Thanks in Advance.

Answer 1

Your string key = "DFGFRT"; is not 128/192/256 bits.

DFGFRT is 6 characters, which is 6 (or 12?) bytes = 8*12 = 96 bits (at most).

To get a 128 bit key you need a 16 byte string, so I’d go on the safe side and use a 16 character string so it will be a 128 bit key if using single byte characters and 256 if using wide characters.