[tor-dev] Proposal 248: Remove all RSA identity keys

[Nick Mathewson]

Filename: 248-removing-rsa-identities.txt
Title: Remove all RSA identity keys
Authors: Nick Mathewson
Created: 15 August 2015
Status: Draft

1. Summary

   With 0.2.7.2-alpha, all relays will have Ed25519 identity keys.  Old
   identity keys are 1024-bit RSA, which should not really be considered
   adequate.  In proposal 220, we describe a migration path to start
   using Ed25519 keys.  This proposal describes an additional migration
   path, for finally removing our old Ed25519 keys.

   See also proposal 245, which describes a migration path away from the
   old TAP RSA1024-based circuit extension protocol.

1.1. Steps of migration

   Phase 1. Prepare for routers that do not advertise their RSA
     identities, by teaching clients and relays and other dependent
     software how to handle them.  Reject such routers at the authority
     level.

   Phase 2. Once all supported routers and clients are updated to phase
     1, we can accept routers at the authority level which lack RSA
     keys.

   Phase 3. Once all authorities accept routers without RSA keys, we can
     finally remove RSA keys from relays.

2. Accepting descriptors without RSA identities

   We make the following changes to the descriptor format:

   If an ed25519 key and signature are present, then these elements may
   be omitted: "fignerprint", "signing-key", "router-signature".  They
   must either be all present or all absent.  If they are all absent,
   then the router has no RSA identity key.

   Authorities MUST NOT accept routers descriptors of this form in phase
   1.

3. Accepting handshakes without RSA identities

   When performing a new version of our link handshake, only the Ed25519
   key and certificates and authentication need to be performed.  If the
   link handshake is performed this way, it is accepted as
   authenticating the route with an ed25519 key but no RSA key.

   A circuit extension EXTEND2 cell may contain an Ed25519 identity but
   not an RSA identity.  In this case, the relay should connect the
   circuit to any connection with the correct ed25519 identity,
   regardless of RSA identity.  If an EXTEND2 cell contains an RSA
   identity fingerprint, however, its the relay receiving it should not
   connect to any relay that has a different RSA identity or that has no
   identity, even if the Ed25519 identity does match.

4. UI updates

   In phase 1 we can update our UIs to refer to all relays that have
   Ed25519 keys by their Ed25519 keys.  We can update our configuration
   and control port interfaces so that they accept Ed keys as well as
   RSA keys.

   During phase 1, we should warn about identifying any dual-identity
   relays by their Ed identity alone.

   For backward compatibility, we should consider a default that refers
   to referring to Ed25519 relays by the first 160 bits of their key.
   This would allow many controller-based tools to work transparently
   with the new key types.

5. Changes to external tools

   This is the big one.  We need a relatively comprehensive list of
   tools we can break with the above changes.  Anything that refers to
   relays by SHA1(RSA1024_id) will need to be able to remember and use
   an Ed25519 key instead.

5. Testing

   Before going forward with phase 2 and phase 3, we need to verify that
   we did phase 1 correctly.  To do so, we should create a small
   temporary testing network, and verify that it works correctly as we
   make the phase 2 and phase 3 changes.