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Initial division of SHA1 uses by collision/preimage needs.

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More thought is needed on each collision opportunity to figure out how
exploitable it is.
This commit is contained in:
Nick Mathewson 2009-05-02 11:52:38 -05:00
parent 08e4a07720
commit 7e411f4b30
1 changed files with 51 additions and 2 deletions
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53
doc/spec/proposals/ideas/xxx-what-uses-sha1.txt
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@ -60,35 +60,81 @@ Why now?
one look silly.
Triage
How severe are these problems? Let's divide them into these
categories:
PREIMAGE -- A SHA-1 usage that only depends on preimage
resistance
COLLISION<role> -- A SHA-1 usage that depends on collision
resistance, but the only party who could mount a
collision-based attack is already in a trusted role
(like a distribution signer or a directory authority).
COLLISION -- A SHA-1 usage that depends on collision resistance
and doesn't need the attacker to have any special keys.
There is no need to put much effort into fixing PREIMAGE usages in
the near-term: even _MD5_ is still preimage resistant. To fix
COLLISION<code-signing> usages is not too important either, since
anyone who has the key to sign the code can mount far worse
attacks. It would be good to fix COLLISION<authority> usages,
since we try to resist. The COLLISION usages are the most
important to fix.
Additionally, we need to consider the impact of a successful attack
in each of these cases. SHA-1 collisions are still expensive even
if recent results are verified, and anybody with the resources to
compute one also has the resources to mount a decent Sybil attack.
Let be pessimistic, and not assume that producing collisions of a given
format is actually any harder than producing collisions at all.
What Tor uses hashes for today:
1. Infrastructure.
A. Our X.509 certificates are signed with SHA-1.
COLLSION
B. TLS uses SHA-1 (and MD5) internally to generate keys.
PREIMAGE?
C. Some of the TLS ciphersuites we allow use SHA-1.
PREIMAGE?
D. When we sign our code with GPG, it might be using SHA-1.
COLLISION<code-signing>
E. Our GPG keys might be authenticated with SHA-1.
COLLISION<code-signing-key-signing>
F. OpenSSL's random number generator uses SHA-1, I believe.
PREIMAGE
2. The Tor protocol
A. Everything we sign, we sign using SHA-1-based OAEP-MGF1.
PREIMAGE?
B. Our CREATE cell format uses SHA-1 for: OAEP padding.
PREIMAGE?
C. Our EXTEND cells use SHA-1 to hash the identity key of the
target server.
COLLISION
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.
NONE
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.
NONE
G. We use SHA-1 to derive our circuit keys from the negotiated g^xy
value.
NONE
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.
NONE
3. Directory services
[All are COLLISION or COLLISION<authority> ]
A. All signatures are generated on the SHA-1 of their corresponding
documents, using PKCS1 padding.
* In dir-spec.txt, section 1.3, it states,
@ -159,9 +205,12 @@ What Tor uses hashes for today:
A. We log information about servers based on SHA-1 hashes of their
identity keys.
COLLISION
B. The controller identifies servers based on SHA-1 hashes of their
identity keys.
COLLISION
C. Nearly all of our configuration options that list servers allow SHA-1
hashes of their identity keys.
COLLISION
E. The deprecated .exit notation uses SHA-1 hashes of identity keys
COLLISION