[TLS] draft-ietf-tls-curve25519-01 and the X25519 significant bit.

Adam Langley <agl@imperialviolet.org> Tue, 22 December 2015 18:52 UTC

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From: Adam Langley <agl@imperialviolet.org>
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Cc: Simon Josefsson <simon@josefsson.org>
Subject: [TLS] draft-ietf-tls-curve25519-01 and the X25519 significant bit.
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Curve25519, as the name suggests, operates on 255-bit numbers. When
encoded as bytes, there's obviously a 256th bit that needs to be
specified.

Curve25519 implementations didn't set the bit but did used to vary on
how they parsed it. Some would take a 256-bit number and reduce it
while others would ignore the bit completely.

However, I believe that implementations have converged on ignoring it.
That behaviour is specified in draft-irtf-cfrg-curves and tested via
the test vectors.

Currently https://tools.ietf.org/html/draft-ietf-tls-curve25519-01#section-2.3
says that implementations SHOULD reject inputs with the high-bit set.
I think that should be dropped. The X25519 function is specified in
terms of bytes in draft-irtf-cfrg-curves and I think the TLS spec
should just use that draft.


Cheers

AGL

-- 
Adam Langley agl@imperialviolet.org https://www.imperialviolet.org