If you don’t follow NSA news obsessively, you might have missed yesterday’s massive Snowden document dump
from Der Spiegel. The documents provide a great deal of insight into how the NSA breaks our cryptographic systems. I was very lightly involved in looking at some of this material, so I’m glad to see that it’s been published.
Unfortunately with so much material, it can be a bit hard to separate the signal from the noise. In this post I’m going to try to do that a little bit — point out the bits that I think are interesting, the parts that are old news, and the things we should keep an eye on.
Those who read this blog will know that I’ve been wondering for a long time how NSA works its way around our encryption. This isn’t an academic matter, since it affects just about everyone who uses technology today.
What we’ve learned since 2013 is that NSA and its partners hoover up vast amounts of Internet traffic from fiber links around the world. Most of this data is plaintext and therefore easy to intercept. But at least some of it is encrypted — typically protected by protocols such as SSL/TLS or IPsec.
Conventional wisdom pre-Snowden told us that the increasing use of encryption ought to have shut the agencies out of this data trove. Yet the documents we’ve seen so far indicate just the opposite. Instead, the NSA and GCHQ have somehow been harvesting massive amounts of SSL/TLS and IPSEC traffic, and appear to be making inroads into other technologies such as Tor as well.
How are they doing this? To repeat an old observation, there are basically three ways to crack an encrypted connection:
- Go after the mathematics. This is expensive and unlikely to work well against modern encryption algorithms (with a few exceptions). The leaked documents give very little evidence of such mathematical breaks — though a bit more on this below.
- Go after the implementation. The new documents confirm a previously-reported and aggressive effort to undermine commercial cryptographic implementations. They also provide context for how important this type of sabotage is to the NSA.
- Steal the keys. Of course, the easiest way to attack any cryptosystem is simply to steal the keys. Yesterday we received a bit more evidence that this is happening.
I can’t possibly spend time on everything that’s covered by these documents — you should go read them yourself
— so below I’m just going to focus on the highlights.
Not so Good Will Hunting
First, the disappointing part. The NSA may be the largest employer of cryptologic mathematicians in the United States, but — if the new story
is any indication — those guys really aren’t pulling their weight.
In fact, the only significant piece of cryptanalytic news in the entire stack comes is a 2008 undergraduate research project looking at AES. Sadly, this is about as unexciting as it sounds — in fact it appears to be nothing more than a summer project by a visiting student. More interesting is the context it gives around the NSA’s efforts to break block ciphers such as AES, including the NSA’s view of the difficulty of such cryptanalysis, and confirmation that NSA has some ‘in-house techniques’.
Additionally, the documents include significant evidence that NSA has difficulty decrypting certain types of traffic, including Truecrypt, PGP/GPG, Tor and ZRTP from implementations such as RedPhone. Since these protocols share many of the same underlying cryptographic algorithms — RSA, Diffie-Hellman, ECDH and AES — some are presenting this as evidence that those primitives are cryptographically strong.
As with the AES note above, this ‘good news’ should also be taken with a grain of salt. With a small number of exceptions, it seems increasingly obvious that the Snowden documents are geared towards NSA’s analysts and operations staff. In fact, many of the systems actually seem aimed at protecting knowledge of NSA’s cryptanalytic capabilities from NSA’s own operational staff (and other Five Eyes partners). As an analyst, it’s quite possible you’ll never learn why a given intercept was successfully decrypted.
To put this a bit more succinctly: the lack of cryptanalytic red meat in these documents may not truly be representative of the NSA’s capabilities. It may simply be an artifact of Edward Snowden’s clearances at the time he left the NSA.
One of the most surprising aspects of the Snowden documents — to those of us in the security research community anyway — is the NSA’s relative ineptitude when it comes to de-anonymizing users of the Tor anonymous communications network.
The reason for our surprise is twofold. First, Tor was never really designed to stand up against a global passive adversary — that is, an attacker who taps a huge number of communications links. If there’s one thing we’ve learned from the Snowden leaks, the NSA (plus GCHQ) is the very definition of the term. In theory at least, Tor should be a relatively easy target for the agency.
The real surprise, though, is that despite this huge signals intelligence advantage, the NSA has barely even tested their ability to de-anonymize users. In fact, this leak provides the first concrete evidence that NSA is experimenting with traffic confirmation attacks to find the source of Tor connections. Even more surprising, their techniques are relatively naive, even when compared to what’s going on in the ‘research’ community.
This doesn’t mean you should view Tor as secure against the NSA. It seems very obvious that the agency has identified Tor as a high-profile target, and we know they have the resources to make much more headway against the network. The real surprise is that they haven’t tried harder. Maybe they’re trying now.
SSL/TLS and IPSEC
A few months ago I wrote a long post speculating about how the NSA breaks SSL/TLS. Because it’s increasingly clear that the NSA does break these protocols, and at relatively large scale.
The new documents don’t tell us much we didn’t already know, but they do confirm the basic outlines of the attack. The first portion requires endpoints around the world that are capable of performing the raw decryption of SSL/TLS sessions provided they know the session keys. The second is a separate infrastructure located on US soil that can recover those session keys when needed.
All of the real magic happens within the key recovery infrastructure. These documents provide the first evidence that a major attack strategy for NSA/GCHQ involves key databases containing the private keys for major sites. For the RSA key exchange ciphersuites of TLS, a single private key is sufficient to recover vast amounts of session traffic — in real time or even after the fact.
The interesting question is how the NSA gets those private keys. The easiest answer may be the least technical. A different Snowden leak shows gives some reason to believe that the NSA may have relationships with employees at specific named U.S. entities, and may even operate personnel “under cover”. This would certainly be one way to build a key database.
But even without the James Bond aspect of this, there’s every reason to believe that NSA has other means to exfiltrate RSA keys from operators. During the period in question, we know of at least one vulnerability (Heartbleed) that could have been used to extract private keys from software TLS implementations. There are still other, unreported vulnerabilities that could be used today.
Pretty much everything I said about SSL/TLS also applies to VPN protocols, with the additional detail that many VPNs use broken protocols and relatively poorly-secured pre-shared secrets that can in some cases be brute-forced. The NSA seems positively gleeful about this.
Open Source packages: Redphone, Truecrypt, PGP and OTR
The documents provide at least circumstantial evidence that some open source encryption technologies may thwart NSA surveillance. These include Truecrypt, ZRTP implementations such as RedPhone, PGP implementations, and Off the Record messaging. These packages have a few commonalities:
- They’re all open source, and relatively well studied by researchers.
- They’re not used at terribly wide scale (as compared to e.g., SSL or VPNs)
- They all work on an end-to-end basis and don’t involve service providers, software distributers, or other infrastructure that could be corrupted or attacked.
What’s at least as interesting is which packages are not included on this list. Major corporate encryption protocols such as iMessage make no appearance in these documents, despite the fact that they ostensibly provide end-to-end encryption. This may be nothing. But given all we know about NSA’s access to providers, this is definitely worrying.
A note on the ethics of the leak
Before I finish, it’s worth addressing one major issue with this reporting: are we, as citizens, entitled to this information? Would we be safer keeping it all under wraps? And is this all ‘activist nonsense‘?
This story, more than some others, skates close to a line. I think it’s worth talking about why this information is important.
To sum up a complicated issue, we live in a world where targeted surveillance is probably necessary and inevitable. The evidence so far indicates that NSA is very good at this kind of work, despite some notable failures in actually executing on the intelligence it produces.
Unfortunately, the documents released so far also show that a great deal of NSA/GCHQ surveillance is not targeted at all. Vast amounts of data are scooped up indiscriminately, in the hope that some of it will someday prove useful. Worse, the NSA has decided that bulk surveillance justifies its efforts to undermine many of the security technologies that protect our own information systems. The President’s own hand-picked review council has strongly recommended this practice be stopped, but their advice has — to all appearances — been disregarded. These are matters that are worthy of debate, but this debate hasn’t happened.
Unfortunate if we can’t enact changes to fix these problems, technology is probably about all that’s left. Over the next few years encryption technologies are going to be widely deployed, not only by individuals but also by corporations desperately trying to reassure overseas customers who doubt the integrity of US technology.
In that world, it’s important to know what works and doesn’t work. Insofar as this story tells us that, it makes us all better off.