[TLS] ESNI padding the Certificate message

Stephen Farrell <stephen.farrell@cs.tcd.ie> Thu, 13 December 2018 13:10 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Thu, 13 Dec 2018 13:10:07 +0000
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Subject: [TLS] ESNI padding the Certificate message
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Hiya,

Was just adding code for this and I noticed that the draft says
a server: "SHOULD pad the Certificate message, via padding at
the record layer, such that its length equals the size of the
largest possible Certificate (message) covered by the same ESNI
key."

I think that ought also mention the CertificateVerify as that
could also tell you something about the ESNI being used if the
key lengths in the various certificates differ.

More specific to openssl though, there isn't really a terribly
easy way to do such fine-grained padding (that I've found so
far), so to cover the above, and as there are likely many
other ways that the ESNI could be exposed, it might be good to
add that clients and servers could reasonably decide to pad all
handshake messages or even all records. (That last being what
is currently easiest with openssl, and hence what I've done
for now with my proof-of-concept code:-)

Cheers,
S.