Re: [TLS] Keeping TLS extension points working

David Benjamin <davidben@chromium.org> Tue, 02 August 2016 13:59 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Tue, 02 Aug 2016 13:59:39 +0000
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To: Steven Valdez <svaldez@google.com>, Raja ashok <raja.ashok@huawei.com>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Keeping TLS extension points working
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To expand on that a little, since it seems comments (a) and (b) are really
the same one:

The purpose of having an explicitly reserved list (b) is precisely so we do
not have to do a second handshake (a). The purpose here is to ensure we
exercise the little-used codepaths, not introduce new ones. This is
intended to be an extremely minimal mechanism. Clients add a tiny bit of
code to their ClientHello and no server code changes at all. (Note that
every MUST in the document is just reiterating what TLS already requires.)

David

On Tue, Aug 2, 2016 at 9:47 AM Steven Valdez <svaldez@google.com> wrote:

> a) It seems like if an implementation has updated to be able to handle a
> specific GREASE alert, it should be able to handle not sending an invalid
> cipher suite. In general, its probably cleaner for the connection to
> fatally shutdown and then restart if the server is behaving that poorly.
> Servers that are sending back non-existent ciphers are also potentially
> broken in other ways, and I don't know whether a client should trust that
> it can reset any handshake state correctly if it were to try doing a
> "warning" alert.
>
> b) The reasoning behind having an explicit list is so that implementations
> don't send a value that ends up being defined as some other valid value.
> Otherwise its possible that some implementations will update to include
> GREASE values, but they might not update immediately upon new values being
> assigned by IANA, which means that there will be periods of times that some
> clients might send "fake" values that collide with real values, confusing
> the peer implementation into believing they actually support something that
> they don't and resulting in more intolerance issues between outdated GREASE
> clients and newly updated servers, with this intolerance being firmly the
> GREASE clients fault. The hardcoded list gets around this by making sure
> GREASE never overlaps with an actual value, though at the trade-off that
> badly designed implementations could choose to just hard-code ignore the
> GREASE codepoints.
>
> On Tue, Aug 2, 2016 at 2:59 AM Raja ashok <raja.ashok@huawei.com> wrote:
>
>> Hi Benjamin,
>>
>>
>>
>> I have gone through the GREASE mechanism which you proposed in your new
>> draft. It’s really a nice idea for finding a buggy server before it thrives.
>>
>>
>>
>> I am having few doubts on this, which are listed below.
>>
>> a)      What should be the behaviour of client incase if a buggy server
>> responded for a GREASE value ?
>>
>> -          Consider a client sends a GREASE cipher value at first place
>> and followed by valid cipher suites, in its client hello.
>>
>> -          If a buggy server selects that cipher then it will response
>> server hello with that GREASE cipher value. At this case if client sends
>> FATAL alert then both side TLS and TCP needs to be closed and client needs
>> to recreate a new TCP connection, and then restart TLS handshake without
>> GREASE cipher value.
>>
>> -          Instead of this we can make client to send warning alert
>> (with new TLS alert code GREASE_CIPHER_VALUE_SELECTED(111)) and restart TLS
>> handshake by sending client hello again.
>>
>> -          If a server receives this new warning, then it should be
>> ready to receive new client hello to restart handshake.
>>
>>
>>
>> SERVER
>>                                 CLIENT
>>
>> CH (GREASE Cipher value & Valid Cipher value)          ------>
>>
>>
>> <-------                  SH (GREASE cipher value)
>>
>> Fatal alert                    -------->
>>
>> TCP (SYN)                    -------->
>>
>>
>>                                                 <--------
>> TCP(SYN ACK)
>>
>> TCP (ACK)                    -------->
>>
>> CH (Valid cipher
>> value)                                                          ------->
>>
>>                         Scenario 1 : Sending FATAL alert for server
>> selecting GREASE value
>>
>>
>>
>> SERVER
>>                                 CLIENT
>>
>> CH (GREASE Cipher value & Valid Cipher value)          ------->
>>
>>
>> <-------                  SH (GREASE cipher value)
>>
>> Warning alert             -------->
>>
>> CH (Valid cipher
>> value)                                                          ------->
>>
>>                         Scenario 2 : Sending WARNING alert for server
>> selecting GREASE value
>>
>>
>>
>> -          I hope sending warning msg and restarting TLS handshake will
>> be efficient.
>>
>> -          TLS Server must notify the application, whenever it receives
>> a GREASE warning alert.
>>
>>
>>
>> b)      Why only few values are specified as GREASE value ? Basically
>> all value which are not specified by IANA should be considered as GREASE
>> value right ?
>>
>> -          Basically client should maintain the list of values (cipher
>> suite, extensions) specified by IANA. The range of values.
>>
>> -          For example IANA specified cipher suite values are
>> {{0x0000,0x005C}, {0x0060,0x006D}, {0x0084, 0x00C5}, {0x00FF, 0x00FF} …..
>> }. This should be maintained in client.
>>
>> -          We should make the client to choose a random value which are
>> not in this supported value list. That cipher value should be considered as
>> GREASE value and need to keep in first place of its cipher list.
>>
>> -          If its selected by a buggy server, then client should behave
>> as mentioned in above scenario 2.
>>
>> -          Even in future if IANA provides new values to new cipher,
>> then this list also should be updated. Consider in this case server is
>> supporting that new cipher and client is not aware about it, so client can
>> propose that value as GREASE value, then still connection will work with
>> the 2nd handshake.
>>
>> -          I am not understanding why we are planning to maintain a few
>> set of GREASE values {0x0A0A, 0x1A1A …. }. If I am missing something,
>> please clarify me.
>>
>>
>>
>> Regards,
>>
>> R Ashok
>>
>>
>> ------------------------------
>>
>> Raja Ashok VK
>> 华为技术有限公司 Huawei Technologies Co., Ltd.
>> [image: Company_logo]
>>
>> Phone:
>> Fax:
>> Mobile:
>> Email:
>> Huawei Technologies Co., Ltd.
>> Bangalore, India
>>
>> http://www.huawei.com
>> ------------------------------
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>> *From:* TLS [mailto:tls-bounces@ietf.org] *On Behalf Of *David Benjamin
>> *Sent:* 26 July 2016 04:02
>> *To:* tls@ietf.org
>> *Subject:* [TLS] Keeping TLS extension points working
>>
>>
>>
>> Hi folks,
>>
>>
>>
>> I'm not sure how this process usually works, but I would like to reserve
>> a bunch of values in the TLS registries to as part of an idea to keep our
>> extension points working. Here's an I-D:
>>
>> https://tools.ietf.org/html/draft-davidben-tls-grease-00
>>
>>
>>
>> (The name GREASE is in honor of AGL's rusted vs. well-oiled joints
>> analogy from https://www.imperialviolet.org/2016/05/16/agility.html )
>>
>>
>>
>> One problem we repeatedly run into is servers failing to implement TLS's
>> various extension points correctly. The most obvious being version
>> intolerance. When we deployed X25519 in Chrome, we discovered an intolerant
>> implementation. (Thankfully it was rare enough to not warrant a fallback or
>> revert!) It appears that signature algorithms maybe also be gathering rust.
>> Ciphers and extensions seem to have held up, but I would like to ensure
>> they stay that way.
>>
>>
>>
>> The root problem here is these broken servers interoperate fine with
>> clients at the time they are deployed. It is only after new values get
>> defined do we notice, by which time it is too late.
>>
>>
>>
>> I would like to fix this by reserving a few values in our registries so
>> that clients may advertise random ones and regularly exercise these
>> codepaths in servers. If enough of the client base does this, we can turn a
>> large class of tomorrow's interop failures into today's interop failures.
>> This is important because an bug will not thrive in the ecosystem if it
>> does not work against the current deployment.
>>
>>
>>
>> If you were in Berlin, you may recognize this idea from the version
>> negotiation debate. Alas that all happened in the wrong order as I hadn't
>> written this up yet. This idea can't be applied to versioning without
>> giving up on ClientHello.version, but we can start with the rest of the
>> protocol.
>>
>>
>>
>> David
>>
>>
>>
>> PS: This is actually my first I-D, so apologies if I've messed it up
>> somewhere!
>>
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