I think your security model has a big problem.

Yes, you could use DES-3 to encrypt a user/name password, or some other public key encryption scheme, but the user would have to enter the private key. Which means they have to in effect enter a password. Since they have to enter something:

Why not just let Oracle authenticate each user, like it would normally do? Just ask let Oracle ask the user for the credentials or you ask them and pass them to Oracle -

why do you have to have an intervening layer?