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[Cfrg] CPACE: what the "session id" is for?

Loup Vaillant-David <loup@loup-vaillant.fr> Fri, 19 June 2020 16:33 UTC

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From: Loup Vaillant-David <loup@loup-vaillant.fr>
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Date: Fri, 19 Jun 2020 18:33:19 +0200
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Subject: [Cfrg] CPACE: what the "session id" is for?
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>From the latest draft:

  https://tools.ietf.org/html/draft-haase-cpace-01

""" Let sid be a session id byte string chosen for each protocol
""" session before protocol execution; The length len(sid) SHOULD be
""" larger or equal to 16 bytes.

""" It is RECOMMENDED sid, is generated by sampling ephemeral random
""" strings.

Unlike ZPAD, The draft doesn't explain this recommendation.
What problem may occur if we omit sid altogether?

Even if G ends up being reused across several sessions, I don't believe
there's any way to tell, because Ya and Yb are uniformly distributed if
ya and yb are indeed random. I feel like I'm missing something.

Loup.

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