From builtin ASP.NET membership to custom membership password problem

We have a production site using ASP.NET built in membership. Now we wan't to extend the tables with some custom properties so we built a custom membership provider using ADO.Net entities.

Everything is working fine, we can create new users and the login controls are also working. We imported every existing user from the aspnet_Membership table into our custom table, and copied the password field into our custom password field in the custom table.

Unfortunately, these existing user cannot login into the system. We found out while debugging the system that the password is not properly decrypted (we're using passwordFormat="Encrypted").

For example: We have an user with password testing123. When we decrypt the password from the database, it returns 睝렱ꀱ꘲䧥꾖饱testing123

Does anybody have an idea what we're doing wrong here?

EDIT: I find this answer where is described you should remove the salt (8 bytes) from your password. In that case that is the solution for me, but then I have a problem with new use开发者_如何学运维rs passwords which are working OK by default.. Is there no other possibility?