[tor-bugs] #21871 [Core Tor/Tor]: prop224: Change descriptor format for legacy encryption key

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#21871: prop224: Change descriptor format for legacy encryption key
-------------------------------+--------------------------------
     Reporter:  dgoulet        |      Owner:
         Type:  enhancement    |     Status:  new
     Priority:  Medium         |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor   |    Version:
     Severity:  Normal         |   Keywords:  tor-hs, prop224
Actual Points:                 |  Parent ID:
       Points:  1              |   Reviewer:
      Sponsor:  SponsorR-must  |
-------------------------------+--------------------------------
 It turns out that we might have miscalculated the legacy feature for
 introduction point.

 Currently, proposal 224 looks like this for legacy encryption keys:

 {{{
         Encryption key is specified as follow:

         [Exactly once enc-key per introduction point]

            "enc-key" SP "ntor" SP key NL

              The key is a base64 encoded curve25519 public key used to
 encrypt
              the introduction request to service.

            "enc-key" SP "legacy" NL key NL

              Base64 encoded RSA key, wrapped in "----BEGIN RSA PUBLIC
              KEY-----" armor, for use with a legacy introduction point as
              described in [LEGACY_EST_INTRO] and [LEGACY-INTRODUCE1]
 below.
 }}}

 This doesn't make much sense because this encryption key is used to
 encrypt the `ENCRYPTED` section of the INTRODUCE1 cell (section 3.2.1. and
 3.2.2.). That section can only be decrypted by the service so the
 introduction point, being legacy or not, doesn't touch it at all, it just
 passes along the bytes.

 So, the descriptor should always contain a ntor key per intro point
 because we still want the ntor handshake to happen since both client and
 service do speak the prop224 protocol.

 If the intro point is a legacy one, it should also have a "legacy key"
 which is an extra RSA public key needed in the INTRODUCE1 legacy cell and
 used by the intro point to relay the cell on the right circuit (used in
 the ESTABLISH_INTRO):

 {{{
    LEGACY_KEY_ID        [20 bytes]
    [...]
    Here, LEGACY_KEY_ID is the hash of the introduction point legacy
    encryption key that was included in the hidden service descriptor.
 }}}

 In the legacy `ESTABLISH_INTRO`:
 {{{
     PK   Bob's public key or service key        [KL octets]
 }}}

 In the current legacy code, the intro point validates that this PK field
 is an ASN.1 encoded RSA key (`rend_mid_establish_intro_legacy()`).

 Fortunately for us, this code is getting release in 030 *but* only be
 actually used in 032 (#12424).

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21871>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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