Heartbleed and Perfect Forward Secrecy

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#heartbleed #PFS #infosec

Last week was a crazy week for information security. That's probably also the understatement of the year. With the public exposure of Heartbleed, everyone was talking about what to do and how to do it to help customers and the Internet, in general, deal with the ramifications of such a pervasive vulnerability.

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The most significant impact on organizations was related to what amounts to the invalidation of the private keys used to ensure secure communications. Researchers found that not only did exploitation of the vulnerability result in the sharing of passwords or sensitive data, but the keys to the organization's kingdom.

That meant, of course, that anyone who'd managed to get them could decrypt any communication they'd snatched over the past couple of years while the vulnerable versions of OpenSSL were in use.

Organizations must not not only patch hundreds (or thousands) of servers, but they must also go through the process of obtaining new keys. That's not going to be simple - or cheap.

That's all because of the way PKI (Public key infrastructure) works. Your private key. And like the One Ring, Gandalf's advice to Frodo applies to organizations: keep it secret; keep it safe.

What Heartbleed did was to make that impossible. There's really no way to know for sure how many private keys were exposed, because the nature of the vulnerability was such that exploitation left no trail, no evidence, no nothing. No one knows just what was exposed, only what might have been exposed. And that is going to drive people to assume that keys were compromised because playing with a potentially compromised key is ... as insane as Gollum after years of playing with a compromised Ring.

There's no debating this is the right course of action and this post is not about that anyway, not really. Post-mortem blogs and discussions are generally around how to prevent similar consequences in the future, and this is definitely that kind of post.

Now, it turns out that in the last year or so (and conspiracy theorists will love this) support for PFS (Perfect Forward Secrecy) has been introduced by a whole lot of folks. Both Microsoft and Twitter introduced support for the protocol late last year, and many others have followed suit. PFS was driven by a desire for providers to protect consumer privacy from government snooping, but it turns out that PFS would have done that as well in the case of Heartbleed being exploited.

Even though PFS relies on a single private key, just as current encryption mechanisms, what PFS (and even FS) do with that key means that even if the key is compromised, it's not going to open up the world to the attacker.

PFS uses the private key to generate what are called ephemeral keys; that is, they're keys based on the original but unique to either the conversation or a few, selected messages within a conversation, depending on the frequency with which ephemeral keys are generated.That means you can't use the private key to decrypt communication that's been secured using an ephemeral key. They're only related, not the same, and cryptography is pretty unforgiving when it comes to even a single bit difference in the data.

 

In cryptography, forward secrecy (also known as perfect forward secrecy or PFS^[1]) is a property of key-agreement protocols ensuring that a session key derived from a set of long-term keys will not be compromised if one of the long-term keys is compromised in the future. The key used to protect transmission of data must not be used to derive any additional keys, and if the key used to protect transmission of data was derived from some other keying material, that material must not be used to derive any more keys. Thus, compromise of a single key will permit access only to data protected by a single key.

-- Wikipedia, Forward secrecy

This is the scenario for which PFS was meant to shine: the primary key is compromised, yet if enabled, no conversations (or transactions or anything else) can be decrypted with that key. Similarly, if the key currently being used to encrypt communications is compromised, it can only impact the current communication - no one else.

PFS has only recently begun being supported, more recently than Heartbleed has been in existence. But now that we know it does exist, and the very real threat of vulnerabilities that compromise consumer privacy and organizational confidentiality, we should take a look at PFS and how it might benefit us to put it in place - before we find out about the next bleeding organ.