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[liberationtech] [cryptography] New Anonymity Network for Short Messages

Eugen Leitl eugen at leitl.org
Wed Jun 12 07:58:46 PDT 2013


----- Forwarded message from Wasa <wasabee18 at gmail.com> -----

Date: Wed, 12 Jun 2013 15:32:02 +0100
From: Wasa <wasabee18 at gmail.com>
To: cryptography at randombit.net
Subject: Re: [cryptography] [liberationtech] New Anonymity Network for Short Messages
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130510 Thunderbird/17.0.6

On 12/06/13 07:27, Eugen Leitl wrote:
> Additionally to this, CTR allows bit-level maleability of the cleartext:
> a bit flipped in a CTR cipherstream translates into a bit flipped in
> the cleartext.

all encryption modes usually provide confidentiality BUT NOT
integrity. They have been designed to be CPA secure; not CCA secure.
That's why u usually use a MAC along with it... it has nothing to do
with CTR...
The mode that provides both is CGM

> 
> In fact, if there are regions of known cleartext (such as zeroes) the
> adversary can do things like encode the originating IP in the cleartext
> simply by XORing it into the cipherstream.

in CBC if u select the IV incorrectly u also leak info. CBC is only
CPA secure IFF the IVs are unpredictable.

> This property can cause problems if you perform any operations before
> checking the MAC (like evaluating a weak CRC to decide to forward the
> message or not).

This is also irrelevant. it's got nothing to do with CTR or other
modes of encryption; this is all about how u perform authenticated
encryption: u should do encrypt-then-mac rather than something else.

if u want simple primitives to work with; u can have a look at
http://nacl.cr.yp.to/ : implemented by cryptographers.

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----- End forwarded message -----
-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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