Approve Account

Hello,
After a user has signed up on a web site I am sending an email with a URL to approve its account:
http://www.domain.com/user/approve/232/8b883981b2839b128712937b32
232 is the User Id and 8b883981b2839b128712937b32 would be the key.
I am planning to create a key using some user info like:
String.Concat(User.Username, User.CreatedDate, User.Email);
Then hash that string ...
- Should I use other fields?

- Should I use something MD5 or maybe hash it with the same salt that I used to hash the password?
  Or maybe have a predefined Salt just for create this keys?

Thanks,
Miguel

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