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

Eric Rescorla <ekr@rtfm.com> Wed, 15 March 2017 20:29 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 15 Mar 2017 13:29:12 -0700
Message-ID: <CABcZeBMg7Ln_4Wgj-vc3g4Xpxy=091gjHi8m8BEugWPvffWRPQ@mail.gmail.com>
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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This seems like compelling logic. The fingerprinting concern doesn't seem
that serious in any case.

-Ekr


On Wed, Mar 15, 2017 at 1:25 PM, 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
>
> Since then, RFC 7748 has been published and X25519 in TLS has been shipped
> by several implementations. From my testing, none of BoringSSL, Cloudflare
> servers (tls-tris?), Go, NSS, or OpenSSL implement this check. They all
> mask off that bit, as RFC 7748 prescribes. (These were all the
> implementations I tested.)
>
> RFC 7748 provides a function on bytes, so, barring strong reasons not to,
> I think it is best for TLS to simply reference RFC 7748 as-is. The stated
> motivation is preventing implementation fingerprinting, yet all the
> implementations listed mask. This paragraph also does not align with the
> current TLS 1.3 draft.
>
> Thus, I would suggest the paragraph be removed from the document. It would
> then prescribe RFC 7748's behavior which is to mask off that bit. Do others
> agree?
>
> David
>
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