Re: [tor-bugs] #22995 [Core Tor/Tor]: prop224 should say we use SHA3-256 for rend circuit digests

["Tor Bug Tracker & Wiki" <blackhole@xxxxxxxxxxxxxx>]

#22995: prop224 should say we use SHA3-256 for rend circuit digests
------------------------------------+------------------------------------
 Reporter:  teor                    |          Owner:  asn
     Type:  defect                  |         Status:  assigned
 Priority:  Medium                  |      Milestone:  Tor: 0.3.2.x-final
Component:  Core Tor/Tor            |        Version:
 Severity:  Normal                  |     Resolution:
 Keywords:  prop224, tor-spec, doc  |  Actual Points:
Parent ID:                          |         Points:  0.5
 Reviewer:                          |        Sponsor:
------------------------------------+------------------------------------

Comment (by asn):

 What's the problem here? The spec does say that SHA3-256 should be used:
 {{{
 4.2.1. Key expansion

    The hidden service and its client need to derive crypto keys from the
    NTOR_KEY_SEED part of the handshake output. To do so, they use the KDF
    construction as follows:

        K = KDF(NTOR_KEY_SEED | m_hsexpand,    HASH_LEN * 2 + S_KEY_LEN *
 2)

    The first HASH_LEN bytes of K form the forward digest Df; the next
 HASH_LEN
    bytes form the backward digest Db; the next S_KEY_LEN bytes form Kf,
 and the
    final S_KEY_LEN bytes form Kb.  Excess bytes from K are discarded.
 }}}

 Do you think we should make it clearer in section 5 that those keys are
 from `KDF()` which:
 {{{
       * Instantiate KDF with SHAKE-256.
 }}}

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22995#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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