If I control both sides of a connection, is there any reason to support alternate cipher suites?

Question

If I have a system where I have 100% control over the client operating system and the server operating system, is there any use case for enabling more than one cipher suite (or any of the options that something like openssl will let you configure)?

Given the fact that I want to take advantage of processor extensions in AES and want to avoid that recent terrapin attack against chachapoly and cbc, is there a use case for falling back to another suite if I know that the one I choose is going to work?

Answer 1

If you control both ends of the tunnel, and there are no issues with intermediary server support, then it definitely makes sense to limit the ciphers. Additional ciphers just increases the attack space, and just increases the opportunity for various downgrade/degradation attacks. The only risks that I can think of is that if you are hardcoding the cipher suite they won't automatically be upgraded when patches and changes occur, so it is a good idea to make sure that there is a robust process to review the cipher suites and updating them as necessary.

Answer 2

If you have complete control over both sides, you can limit the number of cipher suites.

A use case for supporting multiple suites in such an environment would be an upgrade or change in cipher suite. You can enable the new suite on both sides and remove the old suite without the need of doing it all at the same time. This would leave you with a (short) period of having multiple cipher suites, but makes the upgrade/changes more flexible.

Answer 3

The ability to switch between cryptographic primitives is called cryptographic agility. The idea is that when a certain algorithm is broken, it is easy to switch to another algorithm.

However, in practice this switching support often contains bugs. We are more confident about our cryptographic primitives. So cryptographic agility is not uniformly viewed as a good property anymore.

More info:

• ImperialViolet - Cryptographic Agility
• Automatic Cipher Suite Ordering in crypto/tls