1 Filename: xxx-what-uses-sha1.txt
2 Title: Where does Tor use SHA-1 today?
12 Tor uses SHA-1 as a message digest. SHA-1 is showing its age:
13 theoretical attacks for finding collisions against it get better
14 every year or two, and it will likely be broken in practice before
17 According to smart crypto people, the SHA-2 functions (SHA-256, etc)
18 share too much of SHA-1's structure to be very good. Some people
19 like other hash functions; most of these have not seen enough
20 analysis to be widely regarded as an extra-good idea.
22 By 2012, the NIST SHA-3 competition will be done, and with luck we'll
23 have something good to switch too. But it's probably a bad idea to
24 wait until 2012 to figure out _how_ to migrate to a new hash
25 function, for two reasons:
26 1) It's not inconceivable we'll want to migrate in a hurry
27 some time before then.
28 2) It's likely that migrating to a new hash function will
29 require protocol changes, and it's easiest to make protocol
30 changes backward compatible if we lay the groundwork in
31 advance. It would suck to have to break compatibility with
32 a big hard-to-test "flag day" protocol change.
34 This document attempts to list everything Tor uses SHA-1 for today.
35 This is the first step in getting all the design work done to switch
38 This document SHOULD NOT be a clearinghouse of what to do about our
39 use of SHA-1. That's better left for other individual proposals.
44 The recent publication of "MD5 considered harmful today: Creating a
45 rogue CA certificate" by Alexander Sotirov, Marc Stevens, Jacob
46 Appelbaum, Arjen Lenstra, David Molnar, Dag Arne Osvik, and Benne de
47 Weger has reminded me that:
49 * You can't rely on theoretical attacks to stay theoretical.
50 * It's quite unpleasant when theoretical attacks become practical
51 and public on days you were planning to leave for vacation.
52 * Broken hash functions (which SHA-1 is not quite yet AFAIU)
53 should be dropped like hot potatoes. Failure to do so can make
58 What Tor uses hashes for today:
62 A. Our X.509 certificates are signed with SHA-1.
63 B. TLS uses SHA-1 (and MD5) internally to generate keys.
64 C. Some of the TLS ciphersuites we allow use SHA-1.
65 D. When we sign our code with GPG, it might be using SHA-1.
66 E. Our GPG keys might be authenticated with SHA-1.
67 F. OpenSSL's random number generator uses SHA-1, I believe.
71 A. Everything we sign, we sign using SHA-1-based OAEP-MGF1.
72 B. Our CREATE cell format uses SHA-1 for: OAEP padding.
73 C. Our EXTEND cells use SHA-1 to hash the identity key of the
75 D. Our CREATED cells use SHA-1 to hash the derived key data.
76 E. The data we use in CREATE_FAST cells to generate a key is the
78 F. The data we send back in a CREATED/CREATED_FAST cell is the length
80 G. We use SHA-1 to derive our circuit keys from the negotiated g^xy value.
81 H. We use SHA-1 to derive the digest field of each RELAY cell, but that's
82 used more as a checksum than as a strong digest.
86 A. All signatures are generated on the SHA-1 of their corresponding
87 documents, using PKCS1 padding.
88 B. Router descriptors identify their corresponding extra-info documents
89 by their SHA-1 digest.
90 C. Fingerprints in router descriptors are taken using SHA-1.
91 D. Fingerprints in authority certs are taken using SHA-1.
92 E. Fingerprints in dir-source lines of votes and consensuses are taken
94 F. Networkstatuses refer to routers identity keys and descriptors by their
96 G. Directory-signature lines identify which key is doing the signing by
97 the SHA-1 digests of the authority's signing key and its identity key.
98 H. The following items are downloaded by the SHA-1 of their contents:
100 I. The following items are downloaded by the SHA-1 of an identity key:
103 4. The rendezvous protocol
107 5. The bridge protocol
111 6. The Tor user interface
113 A. We log information about servers based on SHA-1 hashes of their
115 B. The controller identifies servers based on SHA-1 hashes of their
117 C. Nearly all of our configuration options that list servers allow SHA-1
118 hashes of their identity keys.
119 E. The deprecated .exit notation uses SHA-1 hashes of identity keys