Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"

mrex@sap.com (Martin Rex) Wed, 08 May 2019 00:32 UTC

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To: Hubert Kario <hkario@redhat.com>
Date: Wed, 8 May 2019 02:31:57 +0200 (CEST)
CC: mrex@sap.com, tls@ietf.org, Martin Thomson <mt@lowentropy.net>
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From: mrex@sap.com (Martin Rex)
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/F4SVamEyDexrVR88k28UMeQSMe4>
Subject: Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"
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Hubert Kario <hkario@redhat.com> wrote:
>> 
>> Thanks to Peter Gutmann for the summary:
>> 
>>     https://mailarchive.ietf.org/arch/msg/tls/g0MDCdZcHsvZefv4V8fssXMeEHs
>> 
>> which you may have missed.
> 
> yes, Joux paper also shows that attacking MD5||SHA1 is harder than attacking  
> SHA1 alone
> 
> but that doesn't matter, what matters is _how much harder it is_ and Joux 
> paper says that it's less than a work factor of two, something also knows
> as a "rounding error" for cryptographic attacks

collision attacks and real-time 2nd preimage attacks on randomly keyed
hashes are substantially different things.

simple math seems hard.


TLSv1.0 + TLSv1.1 both use   (rsa, MD5||SHA1)

TLSv1.2 (rfc5246) permitted (rsa, MD5) and allows (rsa,SHA1)

if we assumed that there *existed* (it currently doesn't, mind you)

a successful preimage attack on MD5  with effort  2^20
a successful preimage attack on SHA1 with effort  2^56

then if Joux would apply not just to multicollisons, but also 2nd preimage,

then the efforts would be:

  TLSv1.2 (rsa,MD5)  2^20
  TLSv1.2 (rsa,SHA1) 2^56

  TLSv1.0 (rsa, MD5||SHA1) >= 2^57 (slightly more than the stronger of the two)


Comparing  TLSv1.0 (rsa,MD5||SHA1) 2^57  with TLSv1.2 (rsa,MD5) 2^20

A factor 2^37 is significantly more than "marginally stronger".


If you are aware of successfull 2nd preimage attacks on
either MD5 or SHA1, please provide references.

-Martin