EncryptString.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace myClasses
{
    public static class EncryptString
    {
        // This constant string is used as a "salt" value for the PasswordDeriveBytes function calls.
        // This size of the IV (in bytes) must = (keysize / 8).  Default keysize is 256, so the IV must be
        // 32 bytes long.  Using a 16 character string here gives us 32 bytes when converted to a byte array.
        private static readonly byte[] initVectorBytes = Encoding.ASCII.GetBytes("tu89geji340t89u2");

        // This constant is used to determine the keysize of the encryption algorithm.
        private const int keysize = 256;

        public static string Encrypt(string plainText, string passPhrase)
        {
            try { 
            if (plainText != "" & plainText != null)
            {
                byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
                using (PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null))
                {
                    byte[] keyBytes = password.GetBytes(keysize / 8);
                    using (RijndaelManaged symmetricKey = new RijndaelManaged())
                    {
                        symmetricKey.Mode = CipherMode.CBC;
                        using (ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes))
                        {
                            using (MemoryStream memoryStream = new MemoryStream())
                            {
                                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                                {
                                    cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
                                    cryptoStream.FlushFinalBlock();
                                    byte[] cipherTextBytes = memoryStream.ToArray();
                                    return Convert.ToBase64String(cipherTextBytes);
                                }
                            }
                        }
                    }
                }
            }
            }
            catch { }
            return null;
        }

        public static string Decrypt(string cipherText, string passPhrase)
        {
            try
            {
                if (cipherText != "" & cipherText != null)
                {
                    byte[] cipherTextBytes = Convert.FromBase64String(cipherText);
                    using (PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null))
                    {
                        byte[] keyBytes = password.GetBytes(keysize / 8);
                        using (RijndaelManaged symmetricKey = new RijndaelManaged())
                        {
                            symmetricKey.Mode = CipherMode.CBC;
                            using (ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes))
                            {
                                using (MemoryStream memoryStream = new MemoryStream(cipherTextBytes))
                                {
                                    using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                                    {
                                        byte[] plainTextBytes = new byte[cipherTextBytes.Length];
                                        int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
                                        return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
                                    }
                                }
                            }
                        }
                    }
                }
            }
            catch { }
            return null;
        }

        public static string[] EncryptLines(string plainText, string passPhrase) {
            try { 
            if (plainText != "" & plainText != null)
            {
                List<string> list = new List<string>();
                plainText = plainText.Replace(";", "[dc]").Replace("\r\n", ";");
                string[] array = plainText.Split(';');

                string encryptText;
                foreach (string item in array) {
                    encryptText = Encrypt(item.Replace("[dc]", ";"), passPhrase);
                    if (encryptText != null & encryptText != "") list.Add(encryptText);
                }
                return list.ToArray();
            }
            }
            catch { }
            return null;
        }

        public static string[] DecryptLines(string cipherText, string passPhrase)
        {
            try { 
            if (cipherText != "" & cipherText != null)
            {
                List<string> list = new List<string>();
                cipherText = cipherText.Replace(";", "[dc]").Replace("\r\n", ";");
                string[] array = cipherText.Split(';');

                string decryptText;
                foreach (string item in array) {
                    decryptText = Decrypt(item.Replace("[dc]", ";"), passPhrase);
                    if (decryptText != null & decryptText != "") list.Add(decryptText);
                }
                return list.ToArray();
            }
            }
            catch { }
            return null;
        }

        public static string[] EncryptLines(string[] linesArray, string passPhrase)
        {
            if (linesArray.Length != 0)
            {
                List<string> list = new List<string>();

                string encryptText = "";
                foreach (string item in linesArray)
                {
                    try {
                        encryptText = Encrypt(item, passPhrase);
                    }
                    catch { }
                    if (!String.IsNullOrEmpty(encryptText)) list.Add(encryptText);
                }
                return list.ToArray();
            }
            return null;
        }

        public static string[] DecryptLines(string[] linesArray, string passPhrase)
        {
            if (linesArray.Length != 0)
            {
                List<string> list = new List<string>();

                string decryptText = "";
                foreach (string item in linesArray)
                {
                    try {
                        decryptText = Decrypt(item, passPhrase);
                    }
                    catch { }
                    if (!String.IsNullOrEmpty(decryptText)) list.Add(decryptText);
                }
                return list.ToArray();
            }
            return null;
        }


    }
}

Question: why use tu89geji340t89u2as your IV/salt? Any particular reason you chose that particular value?