Re: [TLS] padding bug
Ben Laurie <benl@google.com> Tue, 24 September 2013 16:04 UTC
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Date: Tue, 24 Sep 2013 17:03:55 +0100
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From: Ben Laurie <benl@google.com>
To: Bodo Moeller <bmoeller@acm.org>
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Subject: Re: [TLS] padding bug
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On 24 September 2013 15:28, Bodo Moeller <bmoeller@acm.org> wrote: > >> > I take your question that you actually read RFC2104 and the >> > Preneel-van-Oorschot paper and you disagree with their points? > > >> >> c) In any case, the whole point of the truncation defence is to make >> >> it harder to go from _observing_ a colliding MAC to the ability to >> construct more collisions. But it mostly does this by making it easier >> to observe MAC collisions. > > > I think this reasoning is not quite right because that attack requires > *internal* collisions (and the internal state wouldn't be truncated). That's what makes it harder :-) > Of > course, it's not an attack on HMAC. > > >> >> d) If you _really_ want to defend against this attack, the right >> answer is to design hash functions with bigger internal state. > > > Exactly. Given a particular l-bit MAC with an l-bit internal state, > truncating its outputs (such as to l/2 bits) isn't demonstrably an > improvement. Rather, if we want an l-bit MAC, it makes sense to have an > internal state larger than l bits. > > Nikos, you pointed to RFC 2104. The following is a literal quote from that > RFC: "The results in this area are not absolute as for the overall security > advantages of truncation." In particular, encrypt-then-MAC with untruncated > MACs isn't a broken design. (Assuming hypothetically there's a key recovery > attack on the MAC alone, the encryption step means that the attacker can't > pick MAC inputs after all.) > > Bodo >
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Nikos Mavrogiannopoulos
- Re: [TLS] padding bug Blumenthal, Uri - 0558 - MITLL
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Blumenthal, Uri - 0558 - MITLL
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Eric Rescorla
- Re: [TLS] padding bug Eric Rescorla
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Adam Langley
- Re: [TLS] padding bug Hugo Krawczyk
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Nikos Mavrogiannopoulos
- Re: [TLS] padding bug Ben Laurie
- Re: [TLS] padding bug Nikos Mavrogiannopoulos
- Re: [TLS] padding bug Ben Laurie
- Re: [TLS] padding bug Nikos Mavrogiannopoulos
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Blumenthal, Uri - 0558 - MITLL
- Re: [TLS] padding bug Paterson, Kenny
- Re: [TLS] padding bug Bodo Moeller
- Re: [TLS] padding bug Michael D'Errico
- Re: [TLS] padding bug Ben Laurie
- Re: [TLS] padding bug Christian Kahlo
- Re: [TLS] padding bug Hovav Shacham
- Re: [TLS] padding bug Blumenthal, Uri - 0558 - MITLL
- Re: [TLS] padding bug Martin Rex
- Re: [TLS] padding bug Andrei Popov
- Re: [TLS] padding bug Ben Laurie
- Re: [TLS] padding bug Kelly John Rose
- Re: [TLS] padding bug Lewis, Nick
- Re: [TLS] padding bug Alfredo Pironti
- Re: [TLS] padding bug Peter Gutmann
- Re: [TLS] padding bug Peter Gutmann