xxx-choosing-crypto-in-tor-protocol.txt

Filename: xxx-choosing-crypto-in-tor-protocol.txt
Title: Picking cryptographic standards in the Tor wire protocol
Author: Marian
Created: 2009-05-16
Status: Draft

Motivation:

  SHA-1 is horribly outdated and not suited for security critical
  purposes. SHA-2, RIPEMD-160, Whirlpool and Tigerare good options
  for a short-term replacement, but in the long run, we will
  probably want to upgrade to the winner or a semi-finalist of the
  SHA-3 competition.

  For a 2006 comparison of different hash algorithms, read:
  http://www.sane.nl/sane2006/program/final-papers/R10.pdf

  Other reading about SHA-1:
  http://www.schneier.com/blog/archives/2005/02/sha1_broken.html
  http://www.schneier.com/blog/archives/2005/08/new_cryptanalyt.html
  http://www.schneier.com/paper-preimages.html

  Additionally, AES has been theoretically broken for years. While
  the attack is still not efficient enough that the public sector
  has been able to prove that it works, we should probably consider
  the time between a theoretical attack and a practical attack as an
  opportunity to figure out how to upgrade to a better algorithm,
  such as Twofish.

  See:
  http://schneier.com/crypto-gram-0209.html#1

Design:

  I suggest that nodes should publish in directories which
  cryptographic standards, such as hash algorithms and ciphers,
  they support. Clients communicating with nodes will then
  pick whichever of those cryptographic standards they prefer
  the most. In the case that the node does not publish which
  cryptographic standards it supports, the client should assume
  that the server supports the older standards, such as SHA-1
  and AES, until such time as we choose to desupport those
  standards.

  Node to node communications could work similarly. However, in
  case they both support a set of algorithms but have different
  preferences, the disagreement would have to be resolved
  somehow. Two possibilities include:
  * the node requesting communications presents which
    cryptographic standards it supports in the request. The
    other node picks.
  * both nodes send each other lists of what they support and
    what version of Tor they are using. The newer node picks,
    based on the assumption that the newer node has the most up
    to date information about which hash algorithm is the best.
    Of course, the node could lie about its version, but then
    again, it could also maliciously choose only to support older
    algorithms.

  Using this method, we could potentially add server side support
  to hash algorithms and ciphers before we instruct clients to
  begin preferring those hash algorithms and ciphers. In this way,
  the clients could upgrade and the servers would already support
  the newly preferred hash algorithms and ciphers, even if the
  servers were still using older versions of Tor, so long as the
  older versions of Tor were at least new enough to have server
  side support.

  This would make quickly upgrading to new hash algorithms and
  ciphers easier. This could be very useful when new attacks
  are published.

  One concern is that client preferences could expose the client
  to segmentation attacks. To mitigate this, we suggest hardcoding
  preferences in the client, to prevent the client from choosing
  to use a new hash algorithm or cipher that no one else is using
  yet. While offering a preference might be useful in case a client
  with an older version of Tor wants to start using the newer hash
  algorithm or cipher that everyone else is using, if the client
  cares enough, he or she can just upgrade Tor.

  We may also have to worry about nodes which, through laziness or
  maliciousness, refuse to start supporting new hash algorithms or
  ciphers. This must be balanced with the need to maintain
  backward compatibility so the client will have a large selection
  of nodes to pick from. Adding new hash algorithms and ciphers
  long before we suggest nodes start using them can help mitigate
  this. However, eventually, once sufficient nodes support new
  standards, client side support for older standards should be
  disabled, particularly if there are practical rather than merely
  theoretical attacks.

  Server side support for older standards can be kept much longer
  than client side support, since clients using older hashes and
  ciphers are really only hurting theirselvse.

  If server side support for a hash algorithm or cipher is added
  but never preferred before we decide we don't really want it,
  support can be removed without having to worry about backward
  compatibility.

Security implications:
  Improving cryptography will improve Tor's security. However, if
  clients pick different cryptographic standards, they could be
  partitioned based on their cryptographic preferences. We also
  need to worry about nodes refusing to support new standards.
  These issues are detailed above.

Specification:

  Todo. Need better understanding of how Tor currently works or
  help from someone who does.

Compatibility:

  This idea is intended to allow easier upgrading of cryptographic
  hash algorithms and ciphers while maintaining backwards
  compatibility. However, at some point, backwards compatibility
  with very old hashes and ciphers should be dropped for security
  reasons.

Implementation:

  Todo.

Performance and scalability nodes:

  Better hashes and cipher are someimes a little more CPU intensive
  than weaker ones. For instance, on most computers AES is a little
  faster than Twofish. However, in that example, I consider Twofish's
  additional security worth the tradeoff.

Acknowledgements:

  Discussed this on IRC with a few people, mostly Nick Mathewson.
  Nick was particularly helpful in explaining how Tor works,
  explaining goals, and  providing various links to Tor
  specifications.