You can not decrypt the password because it is not stored encrypted in the database dictionary!

The gibberish values that can be seen in DBA_USERS.PASSWORD column are not encripted passwords, they are hash values of passwords + corresponding usernames. So they are obteined by implementing some hashing alghorytm, not some encription alghorytm.

There is a huge difference between encryption and hashing. Encrypton is a reversible proces, while hashing is not. What that means? With encryption, you can allways get the original value from the encrypted value, provided that you have propper encryption key and an alghorytm. But with hashing process it is different. You can (generally speaking) never get the original value from the hashed value, even if you know the hashing algorythm. It is mathematicaly one-way process.

Oracle stores only hashed values of the passwords, so noone (including everyone in Oracle Corp.) can ever "guess" what the real password is, even if he got access to thos hashed passwords. The only way to get real password from its hash value is by use of brute force.