Hidden service descriptors: outer wrapper [DESC-OUTER]

The format for a hidden service descriptor is as follows, using the meta-format from dir-spec.txt.

"hs-descriptor" SP version-number NL

[At start, exactly once.]

       The version-number is a 32 bit unsigned integer indicating the version
       of the descriptor. Current version is "3".

     "descriptor-lifetime" SP LifetimeMinutes NL

       [Exactly once]

       The lifetime of a descriptor in minutes. An HSDir SHOULD expire the
       hidden service descriptor at least LifetimeMinutes after it was
       uploaded.

       The LifetimeMinutes field can take values between 30 and 720 (12
       hours).

    "descriptor-signing-key-cert" NL certificate NL

       [Exactly once.]

       The 'certificate' field contains a certificate in the format from
       proposal 220, wrapped with "-----BEGIN ED25519 CERT-----".  The
       certificate cross-certifies the short-term descriptor signing key with
       the blinded public key.  The certificate type must be [08], and the
       blinded public key must be present as the signing-key extension.

     "revision-counter" SP Integer NL

       [Exactly once.]

       The revision number of the descriptor. If an HSDir receives a
       second descriptor for a key that it already has a descriptor for,
       it should retain and serve the descriptor with the higher
       revision-counter.

       (Checking for monotonically increasing revision-counter values
       prevents an attacker from replacing a newer descriptor signed by
       a given key with a copy of an older version.)

       Implementations MUST be able to parse 64-bit values for these
       counters.

     "superencrypted" NL encrypted-string

       [Exactly once.]

       An encrypted blob, whose format is discussed in [HS-DESC-ENC] below. The
       blob is base64 encoded and enclosed in -----BEGIN MESSAGE---- and
       ----END MESSAGE---- wrappers. (The resulting document does not end with
       a newline character.)

     "signature" SP signature NL

       [exactly once, at end.]

       A signature of all previous fields, using the signing key in the
       descriptor-signing-key-cert line, prefixed by the string "Tor onion
       service descriptor sig v3". We use a separate key for signing, so that
       the hidden service host does not need to have its private blinded key
       online.

HSDirs accept hidden service descriptors of up to 50k bytes (a consensus parameter should also be introduced to control this value).