Re: [saag] AD review of draft-iab-crypto-alg-agility-06

Eliot Lear <> Thu, 03 September 2015 21:02 UTC

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To: Russ Housley <>
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From: Eliot Lear <>
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Date: Thu, 3 Sep 2015 23:01:59 +0200
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Subject: Re: [saag] AD review of draft-iab-crypto-alg-agility-06
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Much better.


On 9/3/15 10:30 PM, Russ Housley wrote:
> Eliot:
> We are talking about section 2.6...
>>>   Without clear mechanisms for algorithm and suite transition,
>>>   preserving interoperability becomes a difficult social problem.  For
>>>   example, consider web browsers.  Dropping support for an algorithm
>>>   suite can break connectivity to some web sites, and the browser
>>>   vendor will lose users by doing so.  This situation creates
>>>   incentives to support algorithm suites that would otherwise be
>>>   deprecated in order to preserve interoperability.
>> Honestly this paragraph is confusing.  It's opaque because it's not
>> clear whether you're aiming at a strawman of where TLS doesn't support
>> agility or the case of long lived root or intermediate certificates.  If
>> it's the former, can you find a more current example?  And the last
>> sentence is just flat out ambiguous, although in an amusing sort of way
>> (who deprecates in order to preserve interoperability?).
> Kathleen also had a comment on this part of the document in her IESG ballot.
> I'm trying to address both comments with this proposal:
> 2.6.  Preserving Interoperability
>    Cryptographic algorithm deprecation is very difficult.  People do not
>    like to introduce interoperability problems, even to preserve
>    security.  As a result, flawed algorithms are supported for far too
>    long.  The impact of legacy software and long support tails on
>    security can be reduced by making it easy to transition from old
>    algorithms and suites to new ones.  Social pressure is often needed
>    to cause the transition to happen.
>    Implementers have been reluctant to remove deprecated algorithms or
>    suites from server software, and server administrators have been
>    reluctant to diable them over concerns that some party will no longer
>    have the ability to connect to their server.  Implementers and
>    administrators want to improve security by using the best supported
>    algorithms, but their actions are tempered by the desire to preserve
>    connectivity.  Recently, some browser vendors have started to provide
>    visual warnings when a deprecated algorithm or suite is used.  These
>    visual warnings provide a new incentive to transition away from
>    deprecated algorithms and suites.
>    Transition in Internet infrastructure is particularly difficult.  The
>    digital signature on the certificate for an intermediate
>    certification authority (CA) [RFC5280] is often expected to last
>    decades, which hinders the transition away from a weak signature
>    algorithm or short key length.  Once a long-lived certificate is
>    issued with a particular signature algorithm, that algorithm will be
>    used by many relying parties, and none of them can stop supporting it
>    without invalidating all of the subordinate certificates.  In a
>    hierarchical system, many subordinate certificates could be impacted
>    by the decision to drop support for a weak signature algorithm or an
>    associated hash function.
>    Institutions, being large or dominant users within a large user base,
>    can assist by coordinating the demise of an algorithm suite, making
>    the transition easier for their own users as well as others.
> Russ