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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.
F. OpenSSL's random number generator uses SHA-1, I believe.
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
A. Hidden servers use SHA-1 to establish introduction points on relays,
and relays use SHA-1 to check incoming introduction point
establishment requests.
B. Hidden servers use SHA-1 in multiple places when generating hidden
service descriptors.
C. Hidden servers performing basic-type client authorization for their
services use SHA-1 when encrypting introduction points contained in
hidden service descriptors.
D. Hidden service directories use SHA-1 to check whether a given hidden
service descriptor may be published under a given descriptor
identifier or not.
E. Hidden servers use SHA-1 to derive .onion addresses of their
services.
F. Clients use SHA-1 to generate the current hidden service descriptor
identifiers for a given .onion address.
G. Hidden servers use SHA-1 to remember digests of the first parts of
Diffie-Hellman handshakes contained in introduction requests in order
to detect replays.
H. Hidden servers use SHA-1 during the Diffie-Hellman key exchange with
a connecting client.
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
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