Filename: xxx-what-uses-sha1.txt
Title: Where does Tor use SHA-1 today?
Version: $Revision$
Last-Modified: $Date$
Author: Nick Mathewson
Created: 30-Dec-2008
Status: Meta


Introduction:

   Tor uses SHA-1 as a message digest. SHA-1 is showing its age:
   theoretical attacks for finding collisions against it get better
   every year or two, and it will likely be broken in practice before
   too long.

   According to smart crypto people, the SHA-2 functions (SHA-256, etc)
   share too much of SHA-1's structure to be very good.  Some people
   like other hash functions; most of these have not seen enough
   analysis to be widely regarded as an extra-good idea.

   By 2012, the NIST SHA-3 competition will be done, and with luck we'll
   have something good to switch too.  But it's probably a bad idea to
   wait until 2012 to figure out _how_ to migrate to a new hash
   function, for two reasons:
         1) It's not inconceivable we'll want to migrate in a hurry
            some time before then.
         2) It's likely that migrating to a new hash function will
            require protocol changes, and it's easiest to make protocol
            changes backward compatible if we lay the groundwork in
            advance.  It would suck to have to break compatibility with
            a big hard-to-test "flag day" protocol change.

   This document attempts to list everything Tor uses SHA-1 for today.
   This is the first step in getting all the design work done to switch
   to something else.

   This document SHOULD NOT be a clearinghouse of what to do about our
   use of SHA-1.  That's better left for other individual proposals.


Why now?

   The recent publication of "MD5 considered harmful today: Creating a
   rogue CA certificate" by Alexander Sotirov, Marc Stevens, Jacob
   Appelbaum, Arjen Lenstra, David Molnar, Dag Arne Osvik, and Benne de
   Weger has reminded me that:

       * You can't rely on theoretical attacks to stay theoretical.
       * It's quite unpleasant when theoretical attacks become practical
         and public on days you were planning to leave for vacation.
       * Broken hash functions (which SHA-1 is not quite yet AFAIU)
         should be dropped like hot potatoes.  Failure to do so can make
         one look silly.



What Tor uses hashes for today:

1. Infrastructure.

   A. Our X.509 certificates are signed with SHA-1.
   B. TLS uses SHA-1 (and MD5) internally to generate keys.
   C. Some of the TLS ciphersuites we allow use SHA-1.
   D. When we sign our code with GPG, it might be using SHA-1.
   E. Our GPG keys might be authenticated with SHA-1.

2. The Tor protocol

   A. Everything we sign, we sign using SHA-1-based OAEP-MGF1.
   B. Our CREATE cell format uses SHA-1 for: OAEP padding.
   C. Our EXTEND cells use SHA-1 to hash the identity key of the
      target server.
   D. Our CREATED cells use SHA-1 to hash the derived key data.
   E. The data we use in CREATE_FAST cells to generate a key is the
      length of a SHA-1.
   F. The data we send back in a CREATED/CREATED_FAST cell is the length
      of a SHA-1.
   G. We use SHA-1 to derive our circuit keys from the negotiated g^xy value.
   H. We use SHA-1 to derive the digest field of each RELAY cell, but that's
      used more as a checksum than as a strong digest.

3. Directory services

   A. All signatures are generated on the SHA-1 of their corresponding
      documents, using PKCS1 padding.
   B. Router descriptors identify their corresponding extra-info documents
      by their SHA-1 digest.
   C. Fingerprints in router descriptors are taken using SHA-1.
   D. Fingerprints in authority certs are taken using SHA-1.
   E. Fingerprints in dir-source lines of votes and consensuses are taken
      using SHA-1.
   F. Networkstatuses refer to routers identity keys and descriptors by their
      SHA-1 digests.
   G. Directory-signature lines identify which key is doing the signing by
      the SHA-1 digests of the authority's signing key and its identity key.
   H. The following items are downloaded by the SHA-1 of their contents:
      XXXX list them
   I. The following items are downloaded by the SHA-1 of an identity key:
      XXXX list them too.

4. The rendezvous protocol

   XXXX write me

5. The bridge protocol

   XXXX write me

6. The Tor user interface

   A. We log information about servers based on SHA-1 hashes of their
      identity keys.
   B. The controller identifies servers based on SHA-1 hashes of their
      identity keys.
   C. Nearly all of our configuration options that list servers allow SHA-1
      hashes of their identity keys.
   E. The deprecated .exit notation uses SHA-1 hashes of identity keys