[tor-bugs] #19020 [Core Tor/Tor]: RSA cross-certification of ed25519 keys differs from spec

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#19020: RSA cross-certification of ed25519 keys differs from spec
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     Reporter:  special       |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.9.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  prop-220, tor-ed25519-proto
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Proposal 220 section 4.2 defines a means of certifying an ed25519 key
 using an RSA key:

    Certificate type [07] (Cross-certification of Ed25519 identity
    with RSA key) contains the following data:
        ED25519_KEY                       [32 bytes]
        EXPIRATION_DATE                   [4 bytes]
        SIGNATURE                         [128 bytes]
    Here, the Ed25519 identity key is signed with router's RSA
    identity key, to indicate that authenticating with a key
    certified by the Ed25519 key counts as certifying with RSA
    identity key.  (The signature is computed on the SHA256 hash of
    the non-signature parts of the certificate, prefixed with the
    string "Tor TLS RSA/Ed25519 cross-certificate".)

 We implement this in the rsa_ed_crosscert_t trunnel structure and the
 tor_make_rsa_ed25519_crosscert function. There are two issues with this
 implementation, compared to the proposal:

 Firstly, this code includes a 1 byte SIG_LEN field before the signature,
 and a signature of variable size. We should just change this in the
 proposal.

 More significantly, this code signs the 36 byte structure directly rather
 than a SHA256 digest of the structure, and of course also doesn't have the
 prefix string in that signature. I doubt we can change this format easily
 now.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19020>
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