Been advised to use same IV in AES implementation

We've had to extend our website to communicate user credentials to a suppliers website (in the query string) using AES with a 256-bit key, however they are using a static IV when decrypting the information.

I've advised that the IV should not be static and that it 开发者_运维问答is not in our standards to do that, but if they change it their end we would incur the [big] costs so we have agreed to accept this as a security risk and use the same IV (much to my extreme frustration).

What I wanted to know is, how much of a security threat is this? I need to be able to communicate this effectively to management so that they know exactly what they are agreeing to.

*UPDATE:*We are also using the same KEY throughout as well.

Thanks

Using a static IV is always a bad idea, but the exact consequences depend on the Mode of Operation in use. In all of them, the same plaintext will produce the same ciphertext, but there may be additional vulnerabilities: For example, in CFB mode, given a static key, the attacker can extract the cipherstream from a known plaintext, and use it to decrypt all subsequent strings!

Using a static IV is always a bad idea. Using a static key is always a bad idea. I bet that your supplier had compiled the static key into their binaries.

Sadly, I've seen this before. Your supplier has a requirement that they implement encryption and they are attempting to implement the encryption in a manner that's as transparent as possible---or as "checkbox" as possible. That is, they aren't really using encryption to provide security, they are using it to satisfy a checkbox requirement.

My suggestion is that you see if the supplier would be willing to forsake this home-brewed encryption approach and instead run their system over SSL. Then you get the advantage of using a quality standard security protocol with known properties. It's clear from your question that neither your supplier nor you should be attempting to design a security protocol. You should, instead, use one that is free and available on every platform.

As far as I know (and I hope others will correct me if I'm wrong / the user will verify this), you lose a significant amount of security by keeping a static key and IV. The most significant effect you should notice is that when you encrypt a specific plaintext (say usernameA+passwordB), you get the same ciphertext every time.

This is great for pattern analysis by attackers, and seems like a password-equivalent that would give attackers the keys to the kingdom:

• Pattern analysis: The attacker can see that the encrypted user+password combination "gobbbledygook" is used every night just before the CEO leaves work. The attacker can then leverage that information into the future to remotely detect when the CEO leaves.

• Password equivalent: You are passing this username+password in the URL. Why can't someone else pass exactly the same value and get the same results you do? If they can, the encrypted data is a plaintext equivalent for the purposes of gaining access, defeating the purpose of encrypting the data.

What I wanted to know is, how much of a security threat is this? I need to be able to communicate this effectively to management so that they know exactly what they are agreeing to.

A good example of re-using the same nonce is Sony vs. Geohot (on a different algorithm though). You can see the results for sony :) To the point. Using the same IV might have mild or catastrophic issues depending on the encryption mode of AES you use. If you use CTR mode then everything you encrypted is as good as plaintext. In CBC mode your first block of plaintext will be the same for the same encrypted data.