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[liberationtech] DecryptoCat

Jacob Appelbaum jacob at appelbaum.net
Tue Jul 9 07:29:35 PDT 2013


Patrick Mylund Nielsen:
> On Tue, Jul 9, 2013 at 9:22 AM, Eugen Leitl <eugen at leitl.org> wrote:
> 
>> On Tue, Jul 09, 2013 at 09:12:21AM -0400, Patrick Mylund Nielsen wrote:
>>> If it's so easy, go ahead and produce a more secure alternative that
>> people
>>
>> You mean something like http://dee.su/ ?
>>
>> And http://dee.su/cables ?
>>
>>
> No, I mean an alternative to Cryptocat (i.e. an OTR client with multiparty
> communication) that is more secure, and as easy to use.
> 

While Cryptocat has OTR - the multi-party communication is not the OTR
protocol.

Cables is as easy to use as email. Generally it is used with an email
client.

If you boot liberte - there is little to no configuration beyond
establishing communication and verifying that you've done so correctly.
Once that is done, you do not need to do it again - a key defense
against active attackers. As I understand things this critical step
(verification and persistence, or merely verification in a usable
manner) cannot be done in CryptoCat at the moment. Active attackers will
win against everyone without verification. The last bug ensured that
*passive* attackers won against everyone on the main server and they
would also win against everyone not using forward secret TLS modes. As I
understand, we do not have numbers on how many users are using the less
secure TLS modes.

Please read this page:

  https://www.ssllabs.com/ssltest/analyze.html?d=crypto.cat

On three computers near me, I see it using non-forward secret modes
today - SSL_RSA_WITH_RC4_128_SHA - this isn't good news.

This also means that if CryptoCat's security may be reduced to SSL, it
is now possible to reduce that to plaintext by forcing disclosure of the
current website's key. This may happen legally or it may happen through
exploitation. I'm not sure why CryptoCat doesn't just exclusively offer
everything with forward secret modes, and encourage everyone else to
upgrade their browser when they use a less secure mode? I suggested this
to Nadim on another mailing list, I'm not sure if he is working on this
already? Perhaps so? I hope so...

In any case, "more secure than CryptoCat" is not a high bar during the
time of this bug. Any CA could have subverted the very little security
provided the web browser trust model. Also the security provided by
non-forward secret TLS connections is a really serious problem.

If you mean "as easy to use" as a plugin in a browser and that it can be
as secure as just chatting over HTTPS protected servers without any
other security, I think that the requirement is not proportional.

Usability is absolutely critical - but we're not looking to build usable
software without any security - if we were, we'd all be using Facetime,
Skype, GChat and so on, without any complaints.

All the best,
Jacob



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