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.
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Huawei Technologies Co., Ltd.
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From: TLS [mailto:[email protected]] On Behalf Of David Benjamin
Sent: 26 July 2016 04:02
To: [email protected]
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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