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

David Benjamin <davidben@chromium.org> Wed, 15 March 2017 20:25 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 15 Mar 2017 20:25:34 +0000
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Subject: [TLS] draft-ietf-tls-rfc4492bis-15 and the X25519 significant bit.
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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