[CFRG] Domain separation in HPKE

Martin Thomson <mt@lowentropy.net> Tue, 02 March 2021 03:41 UTC

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Date: Tue, 02 Mar 2021 14:41:26 +1100
From: "Martin Thomson" <mt@lowentropy.net>
To: cfrg@irtf.org
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Archived-At: <https://mailarchive.ietf.org/arch/msg/cfrg/PhsZ5LeVal1CNCvOt3OqIYkgBhw>
Subject: [CFRG] Domain separation in HPKE
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HPKE uses string labels for domain separation.  However, this only works because the labels it uses are carefully chosen so that no label is a prefix of any other.  The draft might benefit from mentioning this constraint as it is not obvious.

Also in the text on domain separation:

   One way to ensure this is by using an equal or similar
   prefixing scheme with an identifier different from "HPKE-v1".

LabeledExpand includes a 2-byte value that encodes the output length before this label, so this advice is wrong for that.