Is .NET DESCryptoServiceProvider secure in this case? I have the following piece of .NET code (see below). I know that DES is not quite secure, I saw that MSDN does not recommend using DES, only for compatibility with legacy programs. I also saw that some people say that IV has to be regenerated every time new message is encrypted.

Q: How secure is the code? Assuming attacker has several hundred messages and corresponding ciphers (and can get more), how easy to break this and find key/IV?. How could it be done? But most importantly, how it could be made secure? Will change to AES help?
I'm a beginner and any help or references are very appreciated.

internal static class Encryptor
{      
  private static readonly byte[] Key = { *** };
  private static readonly byte[] Iv = { *** };

  internal static String Encrypt(string source)
  {
    var des = new DESCryptoServiceProvider();
    var enc = des.CreateEncryptor(Key, Iv);
    var b = Encoding.ASCII.GetBytes(source);
    var encId = enc.TransformFinalBlock(b, 0, b.Length);
    return Convert.ToBase64String(encId);
  }

  internal static string Decrypt(string encrypted)
  {
    var des = new DESCryptoServiceProvider();
    var dec = des.CreateDecryptor(Key, Iv);
    var b = Convert.FromBase64String(encrypted);
    var decId = dec.TransformFinalBlock(b, 0, b.Length);
    return Encoding.ASCII.GetString(decId);
  }
}
The key space for DES is far too small (56 bits). Therefore, any use of DES is not secure. It doesn't matter what mode you use. If the attacker has one plaintext, ciphertext pair, they can brute force the key space and recover the key in a feasible amount of time (24 hours using the cloud).


  But most importantly, how it could be made secure? Will change to AES help?


Moving to AES will definitely help. What will help the most is first, defining the desired protection model (i.e., what does "secure" mean in your context?). Do you only care about confidentiality? Do you also care about integrity? How about authenticity? Deciding this will help you decide which mode to use. In particular, do you need a MAC or authenticated encryption mode?

Once you have this figured out, choose the cipher and mode to match your needs, not just security needs, but also performance needs.