Badly implementing encryption: Part VI–malleable encryption
In the previous post, we managed to get to a fairly complete state, the full code is less than 50 lines of code, but has enough functionality to be able to make use of it.
Don’t actually do that. This code is horribly broken, and the adage of “don’t implement your own encryption” holds very strongly here.
Let’s consider a fairly typical encryption setup. You log in with me, I generate an encrypted cookie and hand it back to you. In all future interactions, you give me back the cookie. I can decrypt that and make decisions based on that.
For example, let’s assume that we want to send the user the following:
We compute the user name and role, pass it to the genCookie() function and have an encrypted string to work with. In this case, here is the cookie in question:
To go the other way, we decrypt the cookie using the nonce and the key, then parse the JSON into the cookie struct, like so:
With this in place, we can start making use of this foundation. Here is some user level code:
At this point, this is awesome. We know that the cookie we got was encrypted with my key (which the user doesn’t have, obviously). So I handed an encrypted blob to the user, got it back and now I can make decisions based on this.
Or can I? A proper crypto system is defined as one where everything is known, aside from the secret key, and it maintains all its properties. The plain text of the cookie is:
But I don’t have that. However… can I play with this? Remember the last post, we saw that XOR on the encrypted text can give us a lot of insight. What about in this case? Here is what I know:
- The role value starts at position 9 and lasts 5 characters.
- The value it currently has is “users”, we would like it to be “admin”.
Since we got the encrypted text from the server, we can return something else at a later point. Can we take advantage of that? Well, XOR is cumulative, right? So we can do this:
Let’s see what we’ll get when we run this on this cookie:
And the output would be:
Now, if I send this to the server, it will properly decrypt this into:
And we are off to the races with full administrator privileges.
This is not a theoretical issue, it has been exploited in the past, to a devastating effect.
The question is, how do we prevent that? The key issue is that encryption (for stream ciphers) is basically just XORing with a secret key stream. Even for block ciphers, the encrypted data is malleable. We can’t assume that it will not decrypt properly, or that the decryption of the modified encrypted text wouldn’t result in valid plain text.
Luckily, cryptography also has an answer, we can create a signature of the data (with the secret key) and then use that to verify that the data hasn’t been tampered with. In fact, we are already using HMAC, which is meant for this exact purpose. This is a pretty big topic, so I’ll discuss that in the next post.