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

mrex@sap.com (Martin Rex) Fri, 02 August 2019 22:30 UTC

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To: Hubert Kario <hkario@redhat.com>
Date: Sat, 3 Aug 2019 00:30:14 +0200 (CEST)
CC: tls@ietf.org, mrex@sap.com, Martin Thomson <mt@lowentropy.net>
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"
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Hubert Kario <hkario@redhat.com>; wrote:
> On Wednesday, 1 May 2019 01:49:52 CEST Martin Rex wrote:
>> 
>> It is formally provable that from the three protocol versions:
>> 
>>  TLSv1.0, TLSv1.1, TLSv1.2
>> 
>> the weakest one is TLSv1.2, because of the royally stupid downgrade
>> in the strength of digitally signed.
>> 
>> 
>> Disabling TLSv1.0 will only result in lots of interop failures
>> and pain, but no improvement in security.
> 
> We've been over this Martin, the theoretical research shows that for Merkle-
> Damgård functions, combining them doesn't increase their security 
> significantly.
> 
> And the practical research:
> https://eprint.iacr.org/2016/131.pdf
> https://www.iacr.org/archive/asiacrypt2009/59120136/59120136.pdf
> only confirms that.
> 
> So, please, use a bit less inflammatory language when you have no factual 
> arguments behind your assertions.


I recently looked into the practical research paper you referenced,
about what it **REALLY** says.  Maybe you should have read this first,
about the *true* prerequisites of the attack, and what it means *really*
means for the security of digitally_signed using SHA1||MD5 in TLSv1.0+TLSv1.1

From what I can read in that paper, it actually confirms that the security
of digitally_signed using SHA1||MD5 for the TLSv1.0 ServerKeyExchange
handshake message for TLS_ECDHE_RSA_WITH_* is more in the same ballpark
as the security of SHA256, because of the small size of data that gets
signed.



from page 3 of this paper:

   https://eprint.iacr.org/2016/131.pdf

   In this paper, we devise the first second preimage attack on the
   concatenation combiner of Merkle-Damg°ard hash functions which is
   faster than 2^n.  As in related attacks (and in particular, [23])
   we obtain a tradeoff between the complexity of the attack and the
   length of the target message. In particular, our second preimage
   attack is faster than 2^n only for input messages of length at
   least[*2]  2^(2*n/7).  The optimal complexity[*3] of our attack is
   2^(3*n/4), and is obtained for (very) long messages of length 2^(3*n/4).
   Due to these constraints, the practical impact of our second preimage
   attack is limited and its main significance is theoretical.
   Namely, it shows that the concatenation of two Merkle-Damg°ard hash
   functions is not as strong a single ideal hash function.


  [*2]  For example, for n=160 and messge block length 512 bits (as in SHA-1),
        the attack is faster than 2^160 only for messages containing at least
        2^48 blocks, or 2^52 bytes.

  [*3]  The complexity formulas do not take into account (small) constant
        factors, which are generally ignored throughout this paper.


FYI   2^52 bytes == 4,503,599,627,370,496 bytes ==  4 PETA-Bytes

i.e. you need an AWFULLY HUGE amount of "wiggle room" in order to
efficiently construct a multi-collision, and the paper mentions that
if you have just tiny wiggle-room available, you are out-of-luck,
there is no "fast" algorithm for an attack.  More so if you also
have to account for a leading length field in the message.


For comparison size of digitally-signed data in ServerKeyExchange:

      struct {
          select (KeyExchangeAlgorithm) {
              case dh_anon:
                  ServerDHParams params;
              case dhe_dss:
              case dhe_rsa:
                  ServerDHParams params;
                  digitally-signed struct {
                      opaque client_random[32];
                      opaque server_random[32];
                      ServerDHParams params;
                  } signed_params;
              case rsa:
              case dh_dss:
              case dh_rsa:
                  struct {} ;
                 /* message is omitted for rsa, dh_dss, and dh_rsa */
              /* may be extended, e.g., for ECDH -- see [TLSECC] */
          };
      } ServerKeyExchange;

for a DHE-2048 bit keypair, the size of signed data is typically

    32 + 32 + 2 + 256 + 2 + 1 + 2 + 256  = 583 bytes

for ECDHE P-384, the size of signed data is typically

    32 + 32 + 1 + 2 + 1 + 97  = 165 bytes 


So for TLS_ECDHE_RSA_WITH_* cipher suites, you have a *MUCH* stronger
baseline security with TLSv1.0 + TLSv1.1 (SHA1||MD5) than with TLSv1.2
(SHA1-only)


-Martin