Re: [CFRG] Domain separation in HPKE
Benjamin Lipp <benjamin.lipp@inria.fr> Tue, 02 March 2021 15:20 UTC
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From: Benjamin Lipp <benjamin.lipp@inria.fr>
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Subject: Re: [CFRG] Domain separation in HPKE
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Dear Martin, Thanks for your remarks. > 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. The first paragraph of Section 8.5 Domain Separation [1] contains > […] This is achieved by the different prefix-free label parameters in the calls to LabeledExtract() and LabeledExpand(). […] Do you think that is sufficient? > 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. Indeed. I think the advice should instead just be to use LabeledExtract and LabeledExpand with prefix-free labels, respecting the suite_id as defined in Section 4 and using a proper kem_id. I drafted a suggestion in a pull request: https://github.com/cfrg/draft-irtf-cfrg-hpke/pull/212 Best regards, Benjamin [1] https://cfrg.github.io/draft-irtf-cfrg-hpke/draft-irtf-cfrg-hpke.html#section-8.5
- [CFRG] Domain separation in HPKE Martin Thomson
- Re: [CFRG] Domain separation in HPKE Benjamin Lipp