Browser JS (client side) crypto FUD

Sat Jul 26 08:03:46 PDT 2014

http://matasano.com/articles/javascript-cryptography/

Is surprisingly often passed around as if it is the end-all to the idea of
client side JS crypto.

TL;DR: It's a fantastic load of horse crap, mixed in with some extremely
generalized cryptography issues that most people never thought about before
that do not harm JS crypto at all.

I'm not sure why the guy wrote it. Maybe he's NSA motivated? Maybe he's
worked a lot on secure systems and this just gives him the creeps? Maybe
he's the kind of guy that thinks <dash>JS</dash> dynamic scripted languages
are not a real languages?

Somebody, please, give me something to say against people that claim JS
client side crypto can just never work!

---------------------------------------------------------
Aside from that it's, well, fundamentally moronic to claim that something
is "harmful" when you actually means it does nothing, it's also just
(almost!) never true that no attacks are prevented.

But, let's go with the flow of the article. Rants won't really settle
arguments.

Two example usages are given.

The first is client-side hashing of a password, so that it's never sent in
the clear. This is so legitimate it nearly makes me drop my hat, but, the
author decides to use HMAC-SHA1 instead of SHA2 for reasons that are fully
beyond me. Perhaps just trying to make things less secure?

The second is using AES keys to client side encrypt. The author must've
thought he was being helpful when he imagined the scheme for this. Or maybe
he was drunk. "So you generate an AES key for each note, send it to the
user's browser to store locally, forget the key, and let the user wrap and
unwrap their data.". Somehow trusting the transport layer is all back in
vogue. The only key-generation problem in JS is entropy, which is a problem
everywhere tbh. If you really want to ensure entropy, send a random data
blob and XOR it with whatever client-side best-shot at randomness.
Whatever.

The author bluntheadedly claims "They will both fail to secure users". In
principle I agree, his methods sucked balls. He, however, blames it on JS.
Okay.. Let's go on.

REALLY? WHY?
> For several reasons, including the following:
> 1 Secure delivery of Javascript to browsers is a chicken-egg problem.
> 2 Browser Javascript is hostile to cryptography.
> 3 The "view-source" transparency of Javascript is illusory.

Until those problems are fixed, Javascript isn't a serious crypto research
> environment, and suffers for it.

(points numbered for pointwise addressing)

1 - Yeah. Duh. What do you think of delivering anything client side?
There's the whole SSL infrastructure, if that doesn't cut it for you, well,
welcome to the Internet. (I suggest the next article is about how the
Internet is fundamentally flawed.) I would suggest, however, that once your
delivery pathway is exploited you're fundamentally screwed in every way.
You can't communicate anything, you can't authenticate anyone, you really
can't *do* anything! So let's leave out the "Javascript" part of this
point, and just do whatever we're already doing to alleviate this issue.

2 - This is a conclusion without any basis so far (aside from being..
meaningless to a computer scientist. Hostile?)

3 - Then just look at what data was transferred. Does every crypto
application require checkable source? Is any SSL implementation "considered
harmful" because nobody is able to flawlessly read the code, no compilers
are trusted, etc?

Okay so that chapter meant absolutely nothing. The author goes on to try to
defend his brabble:

"WHAT'S THE "CHICKEN-EGG PROBLEM" WITH DELIVERING JAVASCRIPT CRYPTOGRAPHY?

If you don't trust the network to deliver a password, or, worse, don't
trust the server not to keep user secrets, you can't trust them to deliver
security code. The same attacker who was sniffing passwords or reading
diaries before you introduce crypto is simply hijacking crypto code after
you do."

A fair point against a single thread model. Interestingly the last line
does absolutely not have to hold, sniffing (after the fact) and on-the-fly
rewriting are worlds apart. Take Tempest of Xkeyscore, for example, they
can't do rewrites. They need specialized programs for that. (Conclusion:
nope, nothing to see here)

The next chapter tries to justify the fallacies made earlier on. Equating a
rewrite to a read, ad-homineming the JS crypto "industry" (and failing to
distinguish operational security from actual security), and lastly claiming
that misplaced trust is bad (which is obvious and unrelated).

The next chapter claims SSL is safe, and "real" crypto unlike JS crypto.
Then firmly cements his baseless ridicule by claiming that if you use
non-JS crypto to make JS crypto work, then obviously there's no point.

The next chapter "WHAT'S HARD ABOUT DEPLOYING JAVASCRIPT CRYPTO CODE OVER
SSL/TLS?" claims all the page has to be SSL/TLS and that makes it hard.
It's not hard and you should already be doing it to have /*any*/ security.
Not to mention it's not true, only that interpreted as page contents has to
be SSL'ed (eg, images don't need to be transported over SSL).

So, point 1 has no merit against JS whatsoever. There's also a lot of
FUD-like text that denies reality. Especially the assumption that SSL and
desktop programs are somehow more secure.

So point 2.

(letterized for pointwise addressing)
"HOW ARE BROWSERS HOSTILE TO CRYPTOGRAPHY?
In a dispriting variety of ways, among them:

a - The prevalence of content-controlled code.
b - The malleability of the Javascript runtime.
c - The lack of systems programming primitives needed to implement crypto.
d - The crushing weight of the installed base of users.

Each of these issues creates security gaps that are fatal to secure crypto.
Attackers will exploit them to defeat systems that should otherwise be
secure. There may be no way to address them without fixing browsers."

a, c, d are, at first sight, all rubbish. b is a very genuine point
however. With prototyping and the like it can be VERY hard to see what's
going on. It's an often mentioned thing about JS that it's too powerful in
some ways, and it can be true. The same goes for C and memory control.

Next chapter confirms that a is rubbish.

Chapter after that explains some basic Comp Sci about when you can trust
something (and discredits something that can help in a lot of cases, any if
you do it correctly (which is too hard))

Chapter after that rehashes the idea that you can't trust the environment
unless you trust the whole environment, which is also the same everywhere.
(I also refer to trusting the compiler)

Next chapter is titled "WELL THEN, COULDN'T I WRITE A SIMPLE BROWSER
EXTENSION THAT WOULD ALLOW JAVASCRIPT TO VERIFY ITSELF?". And guess what,
the author agrees. You can indeed do this. If you're just doing it for
yourself or a single kind of crypto you could also make a plugin for that.
Which is what the WhatWG is doing with the HTML5 crypto extension. Then
claims crypto is to PGP as programming languages are to Lisp, which is
rubbish.

The author then goes on to actually talk about random generators. Which are
not always required, but who cares, right? Then Secure erase, which is only
important if you expect the client device to be exploited. Then ?timing
attacks? which is even more specific and can be alleviated easily enough.

Then tries to generalize his claim to remove JS specifically from the
equation, removing is last viable (but not definitive) arguments.

Some hating on key management, which is justified but again bullocks wrt
the main argument. (not to mention it's a problem everywhere, and it can be
solved like everywhere)

Some hate on people running old browsers, which has actually been solved by
background-auto-updating by now. (huzzah for all the added insecurity there)

Then something about graceful degrading. Which is fair except for him not
sufficiently providing any reason JS crypto never works. (and not
relevant). He apparently meant this with d. Depends greatly on the
deployment situation, but in general it's FUD.

"AND WHEN YOU SAID "VIEW-SOURCE TRANSPARENCY WAS ILLUSORY"?

We meant that you can't just look at a Javascript file and know that it's
secure, even in the vanishingly unlikely event that you were a skilled
cryptographer, because of all the reasons we just cited."

Yeah. Welcome to programming. There's absolutely no truth to this claim
btw. Vagely referring to a large body of rubbish is not an argument.

The rest does not even try to take a direct shot anymore. Something about
how users that use 100 programs are more likely to find an insecure one
than people that use only 2 or 3.

He's the kind of guy that claims cracking and rewriting SSL connections is
easy, whereas using AES for a secure "cryptosystem" is hard. I don't know
what's up with this guy.
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