When you really do need to store a password in a database what are best practices

I have a web service using HMAC to verify signed requests. In this scenario there is a key (password) assigned to each caller of my service. The user of course gets that key, but I also need to store the key in my database so that my service can verify the signature of each request coming in.

So this is a case where I really do need to store passwords in the database in a form that I can retrieve and use them. I can't use the best practice of only storing a salted hash of the password in the database.

I could encrypt the keys but then I need to store the encryption key somewhere. This is a fairly common scenario f开发者_如何转开发or secured RESTful web services, so the likes of Amazon (AWS) and Microsoft (Azure) have to deal with this problem.

What are best practices in this situation?

---

The only time password should be stored in a database is if the password is needed to connect to some other system. If one merely needs to check the credentials supplied by some other entity, one should store a hash of the password.

Even if it's necessary to use a password to connect with another party, if some other credentials are needed for you to perform such access (e.g. someone logs into your system, and then you log into another system on their behalf) it's desirable when practical to store the external password encrypted with a hash of the supplied password (but NOT the same hash as the one stored in the database!). If there are multiple credentials which could be used to log into your server, store a separate copy of the encryption key for the remote password, encrypted using each acceptable credential.

---

If you really need the password (in order to connect to another system, for example), I would recommend putting in somewhere far away from the other information. Perhaps another database, an encrypted file on the file system, etc. This is so if someone gets your main database, they are not guaranteed to also get the passwords in question.

It may be obvious but you want the location of the passwords to be encrypted with another key (to make it less likely that someone who somehow gained access to the primary data source will also gain access to the password data store).

---

It sounds like the best practice for your scenario would be to use public key cryptography.