Re: [TLS] draft-ietf-tls-rfc4492bis-15 and the X25519 significant bit.

Brian Smith <brian@briansmith.org> Wed, 15 March 2017 21:33 UTC

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From: Brian Smith <brian@briansmith.org>
Date: Wed, 15 Mar 2017 11:32:47 -1000
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To: David Benjamin <davidben@chromium.org>
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Subject: Re: [TLS] draft-ietf-tls-rfc4492bis-15 and the X25519 significant bit.
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David Benjamin <davidben@chromium.org> wrote:
> draft-ietf-tls-rfc4492bis-15, section 5.11, contains the following text:
>
>    Since there are some implementation of the X25519 function that
>    impose this restriction on their input and others that don't,
>    implementations of X25519 in TLS SHOULD reject public keys when the
>    high-order bit of the final byte is set (in other words, when the
>    value of the rightmost byte is greater than 0x7F) in order to prevent
>    implementation fingerprinting.  Note that this deviates from RFC 7748
>    which suggests that This value be masked.
>
> https://tools.ietf.org/html/draft-ietf-tls-rfc4492bis-15#section-5.11
>
> There was a thread about this way back, but it seems not to have come to a
> clear conclusion:
> https://www.ietf.org/mail-archive/web/tls/current/msg18821.html

Although I argued in favor of a different alternative in that previous
thread, given how RFC 7748 ended up, I agree with your suggested
change, to align with RFC 7748.

Cheers,
Brian
-- 
https://briansmith.org/