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Re: [TLS] Requesting working group adoption of draft-stebila-tls-hybrid-design

Douglas Stebila <douglas@stebila.ca> Thu, 13 February 2020 11:48 UTC

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From: Douglas Stebila <douglas@stebila.ca>
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Date: Thu, 13 Feb 2020 06:48:21 -0500
Cc: Shay Gueron <shay.gueron@gmail.com>, "<tls@ietf.org>" <TLS@ietf.org>
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To: Rob Sayre <sayrer@gmail.com>
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Subject: Re: [TLS] Requesting working group adoption of draft-stebila-tls-hybrid-design
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On Feb 12, 2020, at 11:24 PM, Rob Sayre <sayrer@gmail.com> wrote:
> 
> Would it be ok to add a rationale to the "Goals" section around backward compatibility? I'm not sure how the compatibility points will interact with downgrade attacks.

For now I don't think we're envisioning anything different on downgrade compared to current DH group negotiation.  For example, a client that prefers curve25519 but also is willing to use nistp256 should be able to talk to a server that only supports nistp256.  If the server also supports curve25519, an adversary who removes curve25519 from a hello message to try to trick them into 'downgrading' to nistp256 would eventually be caught by the handshake transcript authentication.  The same holds in this setting.

Douglas

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