Invariant Properties

I’ve previously mentioned providing an encryption key via a JNDI value provided by an appserver, or better yet splitting the key between that JNDI value and a file outside of the webapp directory. A recent question at work reminded me that this isn’t the only solution. Another approach is a simple key server – a webservice that provides the encryption key on demand.

This sounds wildly insecure but it’s fairly easy to lock down since the use case is so limited:

1. Run it on a private network (in a data center you own) or a VPC (in AWS). This restricts access to systems in the same data center since there should be a physical router blocking access from the internet at large.

2. Consider using IP address white-listing. You know the IP addresses of your servers, or at least their IP address range. Configure the webservice to only accept connections from these IP addresses. This can’t hurt but a knowledgeable attacker can perform ARP poisoning to hijack a white-listed address.

3. Use mutual authentication with digital certificates. You can create these certificates in-house. This forces an attacker to know something non-trivial in addition to being physically located near the key server.

4. Use SSL to prevent eavesdropping.

5. Use a keystore with a password provided by JNDI if you are returning private keys. It’s harder to protect secret keys but you could use a JNDI-provided key to wrap them. This would limit exposure if the appserver value is leaked due to poor backup security or compromised sysadmins.

I don’t think we gain anything by requiring a nonce since the code to handle it will also require secure initialization.

The biggest benefits to a key server are scalability and the ability to provide more than one key. It also prevents key disclosure if an attacker is able to look at arbitrary files on your system or, worse, obtain a backup copy of the system. The biggest drawback is that it’s one more system to maintain.

In production an AWS ‘micro’ instance should be more than adequate to support many client servers. The cost of a ‘micro’ instance is negligible. In a data center you could put it on a minimal virtual server.

For development and testing you can use a keyserver under the same appserver as your application.

I hope it goes without saying that this is not adequate for systems that require the highest security. However if you’re looking for something beyond a key provided by JNDI then this should be one of the options on the table.