From: "Jonathan M. Bresler" <jmb@FreeBSD.ORG>
Date: Fri, 15 Dec 1995 10:13:47 +0800
To: Sten Drescher <dreschs@austnsc.tandem.com>
Subject: Re: kocher's timing attack
In-Reply-To: <55loofy5qn.fsf@galil.austnsc.tandem.com>
Message-ID: <Pine.BSF.3.91.951214200818.22735C-100000@Aspen.Woc.Atinc.COM>
MIME-Version: 1.0
Content-Type: text/plain


On 14 Dec 1995, Sten Drescher wrote:

> On Firewalls, "Jonathan M. Bresler" <jmb@FreeBSD.ORG> said:
> 
> JMB> regarding kocher's timing attack paper:
> 
> JMB> RSA attack.  only known ciphertext is needed.  dont know how many
> JMB> known ciphertexts are required (related to key size surely).  the
> JMB> paper's example is digital signature, rephrase that to Alice signs
> JMB> Bob's public key certifying that (you know the story).  After
> JMB> several large key signing parties hundreds of known ciphertexts
> JMB> could have been generated using Alice's key--each one a public key
> JMB> of someone else.  over several years it piles up.  the known
> JMB> ciphertexts can be tested/analyzed to yield Alice's secret key.
> JMB> ouch.  ;/
> 
> 	Are you sure about this?  It would seem that the same principle
> would then apply to signed messages as well, and I find it a bit hard to
> believe that signing messages would make ones key pair vulnerable.

	no, i am not sure.  but after reading the paper carefully that is 
what i conclude.  on page 4 start of the 4th paragraph "The Chinese 
Remainder Theorem RSA attack can also be adapted to use only known 
ciphertext, and thus can be used to attack RSA digital signatures."

	the key here is "known ciphertext": you have both the message and 
its encrypted version.  When Alice signs Bob's public key, with her 
private key of course, she is encrypting Bob's public key.  this allows 
Charlie to use Alice's public key to decrypt the signature, recovering a 
message that is identical to Bob's public key.  that's the proof that 
Alice was the signer.

	no, i am not sure.   anyone see holes in this?


Jonathan M. Bresler        FreeBSD Postmaster         jmb@FreeBSD.ORG
play go. ride bike. hack FreeBSD.--ah the good life 
i am moving to a new job.                 PLEASE USE: jmb@FreeBSD.ORG