tor/doc/spec/proposals/103-multilevel-keys.txt

Filename: 103-multilevel-keys.txt
Title: Splitting identity key from regularly used signing key.
Version: $Revision$
Last-Modified: $Date$
Author: Nick Mathewson
Created:
Status: Open

Overview:

  This document proposes a change in the way identity keys are used, so that
  highly sensitive keys can be password-protected and seldom loaded into RAM.

  It presents options; it is not yet a complete proposal.

Proposal:

  Replacing a directory authority's identity key in the event of a compromise
  would be tremendously annoying.  We'd need to tell every client to switch
  their configuration, or update to a new version with an uploaded list.  So
  long as some weren't upgraded, they'd be at risk from whoever had
  compromised the key.

  With this in mind, it's a shame that our current protocol forces us to
  store identity keys unencrypted in RAM.  We need some kind of signing key
  stored unencrypted, since we need to generate new descriptors/directories
  and rotate link and onion keys regularly.  (And since, of course, we can't
  ask server operators to be on-hand to enter a passphrase every time we
  want to rotate keys or sign a descriptor.)

  The obvious solution seems to be to have a signing-only key that lives
  indefinitely (months or longer) and signs descriptors and link keys, and a
  separate identity key that's used to sign the signing key.  Tor servers
  could run in one of several modes:
    1. Identity key stored encrypted.  You need to pick a passphrase when
       you enable this mode, and re-enter this passphrase every time you
       rotate the signing key.
    1'. Identity key stored separate.  You save your identity key to a
       floppy, and use the floppy when you need to rotate the signing key.
    2. All keys stored unencrypted.  In this case, we might not want to even
       *have* a separate signing key.  (We'll need to support no-separate-
       signing-key mode anyway to keep old servers working.)
    3. All keys stored encrypted. You need to enter a passphrase to start
       Tor.
  (Of course, we might not want to implement all of these.)

  Case 1 is probably most usable and secure, if we assume that people don't
  forget their passphrases or lose their floppies.  We could mitigate this a
  bit by encouraging people to PGP-encrypt their passphrases to themselves,
  or keep a cleartext copy of their secret key secret-split into a few
  pieces, or something like that.

  Migration presents another difficulty, especially with the authorities.  If
  we use the current set of identity keys as the new identity keys, we're in
  the position of having sensitive keys that have been stored on
  media-of-dubious-encryption up to now.  Also, we need to keep old clients
  (who will expect descriptors to be signed by the identity keys they know
  and love, and who will not understand signing keys) happy.

A possible solution:

  One thing to consider is that router identity keys are not very sensitive:
  if an OR disappears and reappears with a new key, the network treats it as
  though an old router had disappeared and a new one had joined the network.
  The Tor network continues unharmed; this isn't a disaster.

  Thus, the ideas above are mostly relevant for authorities.

  The most straightforward solution for the authorities is probably to take
  advantage of the protocol transition that will come with proposal 101, and
  introduce a new set of signing _and_ identity keys used only to sign votes
  and consensus network-status documents.  Signing and identity keys could be
  delivered to users in a separate, rarely changing "keys" document, so that
  the consensus network-status documents wouldn't need to include N signing
  keys, N identity keys, and N certifications.

  Note also that there is no reason that the identity/signing keys used by
  directory authorities would necessarily have to be the same as the identity
  keys those authorities use in their capacity as routers.  Decoupling these
  keys would give directory authorities the following set of keys:

       Directory authority identity:
           Highly confidential; stored encrypted and/or offline.  Used to
           identity directory authorities.  Shipped with clients.  Used to
           sign Directory authority signing keys.

       Directory authority signing key:
           Stored online, accessible to regular Tor process.  Used to sign
           votes and consensus directories.  Downloaded as part of a "keys"
           document.

           [Administrators SHOULD rotate their signing keys every month or
           two, just to keep in practice and keep from forgetting the
           password to the authority identity.]

       V1-V2 directory authority identity:
           Stored online, never changed.  Used to sign legacy network-status
           and directory documents.

       Router identity:
           Stored online, seldom changed.  Used to sign server descriptors
           for this authority in its role as a router.  Implicitly certified
           by being listed in network-status documents.

       Onion key, link key:
           As in tor-spec.txt


Extensions to Proposal 101.

  Add the following elements to vote documents:

     "dir-identity-key": The long-term identity key for this authority.
     "dir-key-published": The time when this directory's signing key was last
         changed.
     "dir-key-certification": A signature of the fields "fingerprint",
         "dir-key-published", "dir-signing-key", and "dir-identity-key",
         concatenated, in that order.  The signed material extends from the
         beginning of "fingerprint" through the newline after
         "dir-key-certification".  The identity key is used to generate this
         signature.

      The elements "fingerprint", "dir-key-published", "dir-signing-key",
      "dir-identity-key", and "dir-key-certification" together constitute a
      "key certificate".  These are generated offline when starting a v2.1
      authority.

      The elements "dir-signing-key", "dir-key-published", and
      "dir-identity-key", "dir-key-certification" and MUST NOT appear in
      consensus documents.

      The "fingerprint" field is generated based on the identity key, not
      the signing key.

  Consensus network statuses change as follows:

      Remove dir-signing-key.

      Change "directory-signature" to take a fingerprint of the authority's
      identity key rather than the authority's nickname.

  Add a new document type:

      A "keys" document contains all currently known key certification
      certificates.  All authorities serve it at

          http://<hostname>/tor/status/keys.z

      Caches and clients download the keys document whenever they receive a
      consensus vote that uses a key they do not recognize.  Caches download
      from authorities; clients download from caches.

  Verification:

      [XXXX write me]