Encrypt and Decrypt Code for dot net

I have a class called encryptOrDecrypt here I used function EncryptString for encrypt any string and DecryptString for decrypt the encrypt code. both function tale two parameters one in string plaintext and second is passPharse passPharse is code which is used for encrypt we need to pass same passPharse code for decrypt that encrypted text

public class EncryptOrDecrypt
    {

        private const string initVector = "pemgail9uzpgzl88";
        // This constant is used to determine the keysize of the encryption algorithm
        private const int keysize = 256;
        //Encrypt
        public static string EncryptString(string plainText, string passPhrase)
        {
            byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector);
            byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
            PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null);
            byte[] keyBytes = password.GetBytes(keysize / 8);
            RijndaelManaged symmetricKey = new RijndaelManaged();
            symmetricKey.Mode = CipherMode.CBC;
            ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes);
            MemoryStream memoryStream = new MemoryStream();
            CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
            cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
            cryptoStream.FlushFinalBlock();
            byte[] cipherTextBytes = memoryStream.ToArray();
            memoryStream.Close();
            cryptoStream.Close();
            return Convert.ToBase64String(cipherTextBytes);
        }
        //Decrypt
        public static string DecryptString(string cipherText, string passPhrase)
        {
            byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector);
            byte[] cipherTextBytes = Convert.FromBase64String(cipherText);
            PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null);
            byte[] keyBytes = password.GetBytes(keysize / 8);
            RijndaelManaged symmetricKey = new RijndaelManaged();
            symmetricKey.Mode = CipherMode.CBC;
            ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes);
            MemoryStream memoryStream = new MemoryStream(cipherTextBytes);
            CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);
            byte[] plainTextBytes = new byte[cipherTextBytes.Length];
            int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
            memoryStream.Close();
            cryptoStream.Close();
            return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
        }
    }