Encrypt in java and Decrypt in C# and reverse - bouncycastle

Question

I have read some topic for my title, so I can't use them to fix my problem. So, i open a new topic...
I want to create 2 functions Encrypt and Decrypt in java (for Android app) and C# (my server) using symmetric encryption
Client (java) : Encrypt(ClearText1, Key)=secret --> Server : Decrypt(secret,Key)=ClearText1
and
Server (java) : Encrypt(ClearText2, Key)=secret --> Client: Decrypt(secret,Key)=ClearText2
I have used BouncyCastle Library For java and C#, but the byte in java is -128..127, while the byte in C# is 0..255, have not negative data. So client and Server encryption and decryption is fail.
Please help me (or share me the library for my problem). Thank you!
Here the code in C#:
How to use them in java? Note the data type "byte" in C# and java in not match
(of course, encrypt and decrypt data with 2 following funcions is true)
> public string _secretPhrase = "123abc456";
> public string EncryptData(string plainText)
> {
> DES des = new DESCryptoServiceProvider();
> des.Mode = CipherMode.ECB;
> des.Padding = PaddingMode.PKCS7;
>
> des.Key = Encoding.UTF8.GetBytes(_secretPhrase.Substring(0, 8));
> des.IV = Encoding.UTF8.GetBytes(_secretPhrase.Substring(0, 8));
>
> byte[] bytes = Encoding.UTF8.GetBytes(plainText);
> byte[] resultBytes = des.CreateEncryptor().TransformFinalBlock(bytes, 0, bytes.Length);
>
> return Convert.ToBase64String(resultBytes);
> }
>
> public string DecryptData(string encryptedText)
> {
> DES des = new DESCryptoServiceProvider();
> des.Mode = CipherMode.ECB;
> des.Padding = PaddingMode.PKCS7;
> des.Key = Encoding.UTF8.GetBytes(_secretPhrase.Substring(0, 8));
> des.IV = System.Text.Encoding.UTF8.GetBytes(_secretPhrase.Substring(0, 8));
>
> byte[] bytes = Convert.FromBase64String(encryptedText);
> byte[] resultBytes = des.CreateDecryptor().TransformFinalBlock(bytes, 0, bytes.Length);
>
> return Encoding.UTF8.GetString(resultBytes);
> }

Answer 1

It doesn't look like you are using BouncyCastle. That looks like stock .NET Framework cryptography code. BouncyCastle uses naming conventions like DesEngine.
I recommend that you try using BouncyCastle for both C# and Java. I'm not convinced the byte difference will be a practical problem. The library should account for it correctly.
Also, single DES is now considered very weak. There are single day attacks against it on reasonable hardware, and it's no longer an approved standard. Look at AES instead.
Finally, I'm not sure using the same key and IV are wise. I'm not a cryptography expert, but using two distinct random numbers should be safer.