Perfect Forward Secrecy can block the NSA, but almost no one uses it

http://blogs.computerworld.com/encryption/22366/can-nsa-see-through-encrypted-web-pages-maybe-so

by LoganCale • 13 years ago

347 135 13 years ago

blogs.computerworld.com

LoganCale• 13 years ago

Note that despite this headline, the article does correctly state that Google now uses Perfect Forward Secrecy.

y0ghur7_xxx3• 13 years ago

Who cares? The NSA gets it's data directly from google. You need to protect yourself from all hops between you and the one you communicate with, not only from the hops between you and google.

bobwaycott• 13 years ago

I'm assuming you read the article, in which it offers an explanation of how the NSA gets Google data. Are you disputing that very likely explanation for the theory that a Powerpoint slide somehow communicates more technically accurate and viable information?

Obviously, Google could be lying in their most recent denial that disputes such a claim. They are, after all, required to lie if they are directly and materially involved in the program.

On the other hand, were I the business owner of a corporation that held millions of people's information and the NSA approached me for access, I don't think it is at all inconceivable and unlikely that I would attempt to negotiate not having the NSA on my network, in my machines, so I could retain the ability to truthfully tell my users that the company did not allow direct access to company servers.

In the article, Mr. Horowitz suggests that, given other leaks and information we've had in the prior years regarding NSA tapping the internet, the slurping of company data is happening at the on/off ramps to the networks, where data is coming in/out. This is far more likely than Google shipping off deltas in cron batches. Thus, Google et al. can appear on the PRISM Powerpoint, which just says their data was added to the system. That can mean anything, you know. The Powerpoint presentation is hardly a source of empirical data presenting the technical architecture of the program.

This article is attempting to help educate and achieve protection at all hops, not just the hops between you and Google. Perfect Forward Secrecy is just for that purpose--it removes the ability to find your encrypted data crackable via single master key decryption by using temporary keys.

So, while Google can only do so much in resisting NSLs, perhaps there is a little bit worth appreciating here. Google can't stop the NSA from slurping up traffic. But they can prevent that data from being massively decrypted via a single key.

Strilanc• 13 years ago

I agree with the article. Perfect forward secrecy (given master private key, still can't figure out derived session keys) is a wonderful property.

However, it's a bit over-reaching to say it can "block the NSA". It won't stop them from backdoor-ing your hardware/software (keyloggers, compromised random numbers, etc). It won't stop them from storing the encrypted communication until (for example) quantum computers make it possible to decrypt it (assuming RSA).

cantankerous• 13 years ago

..Or stop the company you're talking to from shipping off information to a 3rd party after it's been received.

senorprogrammer• 13 years ago

Yes, in the same way that an excellent front-door lock doesn't stop someone from smashing your front window.

For the purposes of encrypting traffic to prevent eavesdropping, one of the recipients sending the unencrypted data to a third-party is not an attack vector of consideration. In your scenario the only viable measure is to send no data at all (aka "don't play").

tptacek11• 13 years ago

In the forward secrecy modes of TLS, RSA isn't used to encrypt anything.

Groxx• 13 years ago

Since we're assuming quantum computers, isn't Diffie-Hellman vulnerable as well? A bit of wikipediaing suggests DH is equivalent to a "discrete logarithm problem", which it also states has an "efficient" (polynomial time) quantum algorithm.

chongli• 13 years ago

backdoor-ing your hardware/software

Yeah, gotta build your own computer and operating system from scratch!

(assuming RSA)

A great reason to look at elliptic curve crypto.

redblacktree• 13 years ago

I'll guess you've seen this[1], but if you want perfect security, you really do have to build from scratch. Of course, I doubt that any single human is really capable of building a truly-secure system.

I guess we have to live with a situation where some faith is required if we want to use computers.

[1] http://cm.bell-labs.com/who/ken/trust.html

lambda• 13 years ago

Sure, it won't "block the NSA" from discovering anything about you. But it raises the stakes.

It is much easier for the NSA to backdoor/crack/make deals with a few telecoms, collect all of the encrypted data, and then decrypt it at their leisure (via stealing RSA keys, getting them via legal channels, cracking RSA, taking advantage of compromised random numbers, or the like), than it is for them to backdoor your hardware (much larger surface of possible discovery), install keyloggers (need physical access, much more expensive to do so only makes sense to do in a targeted fashion) etc.

The whole point of this brouhaha is that the NSA appears to be hoovering up massive amounts of data, which it keeps and which even fairly junior employees at government contractors have access to and could abuse. I don't think that anyone (or most people, anyhow), object to specific, targeted wiretaps of targets that you have appropriate warrants for. It's the incredibly broad based collection of all records, without targeting, for later data mining, coupled with the first-amendment destroying demands of secrecy about discussion these practices, that have people up in arms.

The NSA has admitted that they retain encrypted data for longer than unencrypted, as they cannot as easily discard it as irrelevant. If you use a protocol with perfect forward secrecy, it substantially undermines the value of that collection.

Edit to add:

It won't stop them from storing the encrypted communication until (for example) quantum computers make it possible to decrypt it (assuming RSA).

Actually, it will stop them from using quantum computers to crack RSA and thus be able to read past communications. That's the who point of perfect forward secrecy; even if you can crack the RSA layer (or obtain the private RSA keys), which is used for bootstrapping the authenticated encrypted channel, perfect forward secrecy generates the symmetric key in a way in which it is never actually communicated over that channel.

Perfect forward secrecy protocols, such as Diffie-Hellman key exchange, work by both sides coming up with two values, one of which is secret and one of which is passed over the channel, such that both of them can compute the same secret value by combining the private and public values. Even if the "public" values are intercepted, for instance by cracking RSA, you cannot determine the secret values, because you do not have the secret portions of the keys that each side generated.

The reason to use RSA in addition to key exchange is that Diffie-Hellman key exchange is not authenticated; someone could MITM you, doing both sides of the Diffie-Hellman protocol, and intercept your communication that way. RSA allows for signing of certificates which can be used to bootstrap the authentication process.

So, if RSA gets cracked in the future, then the NSA could at that point MITM all of your future connections. But they could not decrypt your prior conversations, which had been encrypted using a private key generated via a key-exchange protocol that provides perfect forward secrecy.

edit 2:

Hmm. After further investigation, it looks like quantum computers would be able able to break the common "perfect forward secrecy" algorithms as well. So yes, in the case of RSA being broken due to quantum computing taking off, it's likely that DH and other discrete log based perfect forward secrecy algorithms will be broken as well.

Sigh. Why can't we have any NP-complete problems that work cryptographically? While even that isn't a guarantee of security against quantum algorithms (after all, it may still be the case that P == NP), there's a fairly strong belief that P!=NP and that NP-complete is a disjoint category than quantum polynomial algorithms, and so would give us a better security margin against quantum attacks.

ReidZB• 13 years ago

Cryptography needs problems that are hard on average, not in the worst case. This doesn't fit well with P/NP/NP-complete, which are all about the worst-case difficulty of a problem.

For example, see the Merkle-Hellman knapsack cryptosystem [1], which is based on the subset sum problem --- a NP-complete problem! Nevertheless, it is broken. As you can see, using a NP-complete problem as a computational basis for a cryptosystem is not a silver bullet.

[1] https://en.wikipedia.org/wiki/Merkle-Hellman

rocky1138• 13 years ago

It would have been a fantastic effect and win for computerworld.com if their site supported what this article talked about.

paulsutter• 13 years ago

Chrome should change the lock icon to something weaker for sessions that don't use ephemeral keys.

This may be the most important article on HN related to the NSA leaks. Fact is, most of us haven't paid enough attention to the details of https.

EDIT: "something weaker" - didn't mention color. People dont need to understand "forward secrecy", the browser just needs to raise the bar for what's considered secure. The goal is to change server-side behavior, not consumer behavior.

jlgreco4• 13 years ago

This is a great idea, but I wonder how far you can go with it. Could you also change the lock icon when larger more secure keys are used? Could you perhaps have the lock icon change color between a continuum of red to green (or better, black to white, or something accessible to colorblind people) depending on how the site is set up?

dllthomas2• 13 years ago

Probably it should be a rolling standard, based on current best-practices.

Skalman• 13 years ago

Assuming you're sarcastic, but don't you think it's right to punish sites that don't use the best encryption strategies?

I'd support three stages: 1) no encryption, 2) some/partial encryption, 3) "perfect" encryption.

For the partial encryption, the browser would provide an explanation of exactly how it could be compromised. For the case where the encrypted version does not have perfect forward secrecy, it could explain that if the server is compromised, this session can also be decrypted.

I don't think browsers should cover the banks' asses here.

Edit (since I can't reply deeper): I thought you were being sarcastic since I consider a gradient to be a really horrible idea. I'd want something where the browser tells me something. I don't want to wonder whether the new browser version changed the gradients slightly or a site improved its encryption strategy. Clear symbols and language is a must.

roc3• 13 years ago

People are not going to pay attention to a color-coded internet-weather-report sort of icon.

But maybe if you switched the iconography/expectation: use a 'person' icon, instead of the lock, for vanilla SSL. It would indicate you have verified the server's identity -- which is not nothing -- but you can reserve the lock icon for forward-secure servers.

tptacek11• 13 years ago

People barely pay attention to the lock icon. Why would color-coding the lock make a difference?

rhizome• 13 years ago

Because they can learn.

ankeshk• 13 years ago

Make it blink or jiggle a bit. Problem of winning attention solved.

malandrew• 13 years ago

Part of the problem is that the lock icon is just grey. The lock icon should be pulsing red, solid yellow or green. This would work the same way Google's anti-black-hat-SEO advisories work. Force people into security by calling attention to bad practices and reducing the prominence in search of any site that violates this.

Clicking on the pulsing red icon, would pop up a window that tells people exactly what they should be careful about.

Clicking on the certificate, should present a much clearer dialog for the layman.

IvyMike5• 13 years ago

I don't think the goal would be getting everyone to pay attention, because if it is, you're absolutely correct.

But if you look at it as a way to spread awareness to web admins, to be confronted with the question "how come we don't have the bestest type of icon", it's not so bad.

A related example: I have recently been wondering how widespread DNSSEC is.

I installed the DNSSEC validator chrome plugin, and now (almost) every website on the planet shows me a sad "no DNSSEC" icon. I can't help but feel that part of the low deployment of DNSSEC is because almost nobody, including web admins, gets any feedback about DNSSEC. (I am aware that there are other costs and risks to deployment.)

Spooky232• 13 years ago

The average customer does not, but security professionals do.

I know that in many government scenarios, (think local governments) for example, policy guidance drives purchasing and other behavior. So if compliance with some security standard requires a "green lock" that lock matters.

That will drive behavior -- banks, insurance companies, and other businesses will be disqualified for all sorts of RFPs.

schtev• 13 years ago

Or just throw away the lock entirely, given that Google is in bed with the NSA.

mpyne• 13 years ago

Google is the only major player even employing perfect forward secrecy; hardly a case of a company trying to get extra-cozy with the NSA.

jiggy2011• 13 years ago

How do you explain that to people though?

It's simple enough to say "if you don't see the green padlock, don't enter your credit card number" but then what if the person sees the weaker symbol?

You could say "go ahead" in which case they will go ahead all of the time or you could say "only go ahead under these additional circumstances" , what would they be?

inopinatus2• 13 years ago

How about a big red nose next to the padlock?

andrewflnr• 13 years ago

I vote for having a rope tied in a knot as the icon: still sort of secure, just less so.

rdl4• 13 years ago

Maybe, but in the long run the positive change would be by changing defaults everywhere, and then eventually throwing warnings for non-ephemeral SSL in the browser.

lazyjones2• 13 years ago

This article seems to start out with wrong assumptions and therefore ends up presenting dangerously wrong conclusions:

"Suppose, for example, the NSA was recording all HTTPS encrypted traffic to/from joeswebsite.com in January. Then, in February, they learned the private key for joeswebsite.com. Almost always, that lets them decrypt everything from January, February, March and beyond."

We should instead suppose that the NSA has had the private keys all the time (since it started recording encrypted communication), so every piece of communication that has been recorded can be decrypted even with Perfect Forward Secrecy. I.e. let's assume they got Google's private key immediately when it was last changed, then all communication with Google's servers is decryptable (correct me if I'm wrong).

The NSA doesn't have to brute-force private keys, they just use legal/strong-arm techniques to obtain them, so it doesn't take a lot of computing power and time.

Scaevolus• 13 years ago

No. The point of Perfect Forward Secrecy is that, even if you know one side's private key and have a log of all communications, you cannot determine what _session-key_ was agreed upon, and thus can't decrypt the rest of the session.

PFS means that even with private keys, you can't read sessions passively. Of course, it can't help with a MITM or taps on unencrypted data.

methehack3• 13 years ago

I guess this is exactly why this is over my head. Maybe you can help me even though it's a little unfair to pick on you :) If one can decrypt the conversation that negotiates the session key and thereby obtain the session key, couldn't one also read the conversation? This must be the whole trick to the ephemeral part...

peter487• 13 years ago

Yes you are wrong :D If NSA is not actively man-in-the-middling you, Perfect forward secrecy still works.

ECDHE has been in Openssl since version 1.0.0 so its out there just not used.

Btw. Is there any addon for Firefox that shows the “encrypted communication” details like google does?

sliverstorm2• 13 years ago

They don't have to take an active role though if they just sniff all the traffic, right?

lazyjones2• 13 years ago

Yes you are wrong :D If NSA is not actively man-in-the-middling you, Perfect forward secrecy still works.

OK, I guess I was reading too much into the (not very enlightening) definition of PFS on Wikipedia and too little of the actual implementation based on Diffie-Hellman, which has the desired properties.

The question that arises is: how feasible is a MITM attack on this phase of session initiation? Can it be kept undetected?

tlb• 13 years ago

FWIW, news.ycombinator.com provides perfect forward secrecy with ECDHE_RSA. Thanks, Nick!

adventured• 13 years ago

Since this is a forum site (versus a site that might get sensitive data), I hadn't bothered to check to see if it was always on HTTPS, until this post. Glad to see that it's always on.

e12e5• 13 years ago

Of course, there is no way for us to know whether or not yc archives a copy of the session keys... (Not that I think yc does that, but we're still talking a shared secret -- you need to trust both parties, if you want to trust forward secrecy...).

jgrahamc2• 13 years ago

I really hate the name "Perfect Forward Secrecy". There's no guarantee in ECDHE that the connections cannot be decrypted at some future time. All that's being implied is that the key changes per connection.

Sure, it's 'better' than RSA with a long lived private key, but there could be advances that would breach ECDH and make none of this 'perfect'. Such as: http://www.wired.com/politics/security/commentary/securityma... If the NIST supplied curves used by the TLS standard had a backdoor none of this would be perfect.

tptacek11• 13 years ago

What you hate is the word "perfect". "Forward secrecy", though, has a technical meaning: a compromise in the long-lived key material for the cryptosystem doesn't enable an adversary to go back and decrypt earlier saved sessions.

The link you've provided doesn't implicate Diffie Hellman; it's in an obscure, virtually unused NIST standard that uses elliptic curves to generate random numbers.

There are other things not to like about the NIST curves, but the methodology used to generate them is documented (I suppose you could generate your own using the same method) and is based on the literature on ECC from the late '90s.

jgrahamc2• 13 years ago

Yes, it's the word 'perfect' that really bothers me. I think it gives the general public the wrong impression.

flatfilefan• 13 years ago

Suppose the data captured is not a dump of a database but rather a https log. Doesn't it imply that NSA has to essentially rebuild the server functionality to make sense of the data?

mtgx• 13 years ago

If Google is offering perfect forward secrecy for e-mails, why don't they have the same for Hangouts (OTR with perfect forward secrecy)?

Sprint• 13 years ago

How important is the difference between ECDHE_RSA and DHE_RSA security-wise?

edit: I asked because Opera showed a site's free StartCom certificate as "TLS v1.0 256 bit AES (1024 bit DHE_RSA/SHA)". I checked with Chromium and there it is shown as ECDHE_RSA though. This is confusing...

JshWright• 13 years ago

They both provide PFS, but ECDHE_RSA is _much_ faster.

Do you have TLS 1.1/1.2 enabled in Opera?

lucian19002• 13 years ago

I don't see how this really helps, though. The NSA can still force a CA to generate certs for any domain they wish to intercept and MITM everyone they care about. They are likely to have the resources for that.

gojomo3• 13 years ago

But that requires the greater cost/intrusiveness of an MITM, and could potentially be noticed as an unexpected change in the certificate. See for example the SSL Observatory project:

https://www.eff.org/observatory

e12e5• 13 years ago

In the case where providers/servers are under NSA jurisdiction (or control, in the case of them hacking servers) -- they could also keep a copy of session keys.

But that would force them to intercept at many more points than simply various edge routers (which is what they may or may not be doing now, having (or not having) equipment at ISPs/TelCos).

Essentially, if you use gmail or outlook.com -- you'll just have to trust that no one has forced (or covertly) installed backdoored crypto libraries. I do think it is very likely security agencies (both foreign and domestic) have agents/assets working at large companies like Google and Microsoft -- I don't think it is very likely that they have been able to covertly subvert their infrastructure. But it certainly is possible.

lucian19002• 13 years ago

They don't need to do that much. If they control a CA and a few ISPs (especially networks to the outside of the US, since apparently us in the rest of the world are fair game), they can MITM anyone reliably. The only defence is checking fingerprints, but few will bother.

codereflection• 13 years ago

Gmail uses Perfect Forward Secrecy, so what? If the NSA really does have direct access to Google's servers, then PFS will not provide you any extra protection from them. Sure, PFS will make it harder for others to snoop. But the current context for the general population is PRISM and the NSA.

I'm not saying that PFS (ugh, what a name, perfect, really?) isn't valuable, I'm just pointing out that no one should think this is going to make it any harder for the NSA to read/access your gmail account.

AJ007• 13 years ago

The NSA is getting all of the attention, but this applies to all of the world's intelligence services.

Even if you absolutely believe and support what the NSA is doing, you probably have good reason to want any number of governments, foreign competitors, and individuals from knowing what you are doing or (at the least) your financial status.

y0ghur7_xxx3• 13 years ago

you probably have good reason to want any number of governments,
foreign competitors, and individuals from knowing what you are
doing or (at the least) your financial status.

...and you should therefore ignore PFS in https, and use something that encrypts your data so that only the recipient can read it, and not someone in the middle like google, microsoft or facebook.

mafribe• 13 years ago

Sure, but if Googles stores all session keys, as is likely (key phrase: meta-data), then the session data store is a rich and likely target for all the world's intelligence services, and one that is not invulnerable. After all, Google is a big organisation with a large turnover of staff.

gojomo3• 13 years ago

Two notes:

(1) While Google uses forward-security on their HTTPS connections, I've not yet seen evidence either way as to whether the SMTP-TLS connections (relaying email to other domains) use forward-security. (The report at checktls.com mentions only the cipher "RC4-SHA", not the key-exchange mechanism.)

(2) If one side of the connection chooses to retain its session keys, or chooses session keys in a poor/predictable manner, or leaks information about session keys via a side-channel (either by mistake or intent), then the forward-security could be destroyed, and in a very subtle/undetectable way.

(This could be a cheap and sly way to grant visibility to a third-party: adopt forward-security outwardly, but ensure your session keys only look random to people who don't know the bug/secret-seed-shared-with-the-third-party.)

e12e5• 13 years ago

Thanks for asking this -- apparently there is no really easy tool for this (I was thinking openssl s_client could do this automagically -- apparently not) -- however, see:

http://superuser.com/questions/109213/is-there-a-tool-that-c...

And now I realized I hadn't tuned the SSL ciphers on one of my servers to not accept (among other thing NULL ciphers... at least if that bash-script works -- I'll have to double check).

jervisfm• 13 years ago

You should check out https://ssllabs.com. They accept a URL and do SSL checks to determine if your server is not configured in the most secure manner possible.

It is already interesting to run the check against online banking sites to see first hand how seriously they take security in practice. I was surprised to see that some banks score pretty poorly in this regard.

e12e5• 13 years ago

I'm not sure I'm brave enough to force forward secrecy in my Exim config just yet -- I can't find an option to log handshakes (unless it is logged as part of the message logs -- which can be kept) -- but I suppose refused messages would show up in the reject log.

Still, I'm not sure if I'm even ready to force SSL at all... for incomming SMTP. Sounds like a good way to break your email infrastructure (and reduce spam ;-).

Essentially mail transport is pretty much unencrypted -- I see SSL/TLS having potential to help fight spam by forcing some form of authentication (via DNS sec, CAs etc) -- but not really a useful tool for securing email from snooping -- for that I would advocate S/MIME and/or GnuPG (Gnu Privacy Guard).

X-Istence• 13 years ago

Every so often I get a mail server from Google that uses DHE outgoing, but most of the time it is RC4-SHA at 128 bits/128 bits.

There are plenty of other mail servers sending me mail though that are sending it over DHE-RSA-AES256-SHA.

I may go tighten up the ciphers I accept. Maybe if only given the option for PFS ciphers Google's mail servers will use them.

Amadou• 13 years ago

How much SSL traffic is offloaded to dedicated hardware on the server side nowadays? How many different companies are there in that space? How many of them have secret agreements with the NSA (or their respective state intelligence agencies) to retain session keys and dump them out when they receive the right kind of query?

The actual owner of the SSL offload hardware would have a hell of time even detecting that their keys were being handed out that way - picking out a once-a-day SSL connection that the offload hardware talks to directly versus one that it passes through in the clear to the systems behind it is going to be exceptionally difficult.

thrownaway24243• 13 years ago

I think the amount of SSL hardware in the field is closer to zero. Nobody with any clue uses that stuff because it's power hungry, the APIs are awful, the implementations can't be verified, and any such asic will look slow in a year whereas cloud server lifetime is five years. AESNI killed those guys a long time ago. The only users of them are embedded platforms without the host CPU power to do crypto.

Spooky232• 13 years ago

More than you think. ADC/load balances like F5 have hardware security modules that offload SSL.

Folks who host PKI often offload key generation. This might e as complex as some sort of appliance, or as simple as a smartcard.

inopinatus2• 13 years ago

Quick log check. For the last month at least, all SMTP-TLS that my servers received from Google was using ECDHE-RSA-RC4-SHA. Same for Yahoo mail.

Hotmail, by contrast, never uses it, nor is STARTTLS offered by their MXes.

IvyMike5• 13 years ago

I've had a question about PFS for a while now: How 'expensive' is it to implement?

First up, let me state for the record that I don't have personal experience with running SSL on a server. But back in the day, generating a public/private key pair for PGP was a semi-expensive operation. Definitely not the kind of thing I could imagine running on a per session basis.

Since PFS requires per-session public/private key pairs, how "costly" is it?

(I understand that the goog uses ECDHE, so I guess I'm asking for a general feel for how quickly and efficient this compares to RSA key generation.)

agilebyte• 13 years ago

The article mentions 23% reduction in speed for ECDHE-RSA vs RSA. Another example states between 15% and 27% reduction.

IvyMike5• 13 years ago

Thanks! I knew I should have read the article more carefully.

(The same section of the article also references http://vincent.bernat.im/en/blog/2011-ssl-perfect-forward-se... which answers some of my follow on questions.)

diminoten• 13 years ago

The line has always been, "You can't stop a determined hacker with any single solution."

The NSA may be a group of determined hackers.

One technology isn't going to suddenly cure the whole problem. It takes a solid security policy set at and followed by the top levels of a company, and constant vigilance to respond to threats. You're not going to stop anyone from getting in, but you can stop them from getting anything of value, and you can kick them out quickly and quietly.

Nursie• 13 years ago

Does it need to be DHE or will DH do just as well? What exactly is the difference here?

It still uses a key per connection at the least, and it's still not on the NIST/FIPS 140-2 standards because you can't decode it after the fact so it breaks auditing (all AFAICT, and I have looked at this stuff a lot).

That said, we're best using AES_GCM, 2048-bit RSA and ECDHE as a matter of course, because at the very least it requires a valid, active MITM to break and can't just be logged and scanned later.

casca• 13 years ago

It's odd that there seems to be no definitive instructions for nginx or apache for enabling PFS. Given that it's clearly not obvious how to set this up, how many here are inadvertently running non-PFS without knowing it?

baudehlo• 13 years ago

Funny, I think in part it's because few people care. Maybe I didn't have the right keywords, but I just wrote an article about this for nginx/stud, and it got 2 upvotes on HN.

Here's the HN link: https://news.ycombinator.com/item?id=5935988

joepub• 13 years ago

What about more use of one-time or ephemeral email addresses? The info we have so far seems to suggest NSA likes to track email addresses. Compare tracking an email address that was only used to send or receive one or a few messages with one that has a long history of use.

lucasjans• 13 years ago

It appears this very site is using Perfect Forward Security. http://cloud.lucasjans.com/image/081m1V3c3O1z

mattme• 13 years ago

PGP complete lacks perfect forward secrecy. Suppose an eavesdropper is recording your email, even though they can't understand it. If your private key is ever compromised (say by torture or subpoena), then all your previous messages can be immediately decrypted.

You could say that Bob losing control of his private key was the problem. But with today’s easily-compromised personal computers, this is an all-too-likely occurence. We would really prefer to be able to handle such failures gracefully, and not simply give away the farm.

http://www.cypherpunks.ca/otr/otr-wpes.pdf

Perfect Forward Secrecy can block the NSA, but almost no one uses it

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