[Cryptography] SHA1 collisions make Git vulnerable to attakcs by third-parties, not just repo maintainers

Peter Gutmann pgut001 at cs.auckland.ac.nz
Fri Feb 24 00:42:36 EST 2017

After sitting through an endless flood of headless-chicken messages on
multiple media about SHA-1 being fatally broken, I thought I'd do a quick 
writeup about what this actually means.  In short:

  Reports of SHA-1's demise are considerably exaggerated.

What CWI/Google have done is confirmed what we've known for a long time, that 
SHA-1 is shaky.  Using a nation-state's worth of resources and a year of time 
they've shown that, with a very carefully-crafted document, you can create a 
collision.  Their presentation of the results is detailed and accurate, it's
the panicked misinterpretation of those results that are the problem.

Overall, this is a neat piece of work.  However, before everyone joins the 
headless-chicken rally, let's look at its effect on real-world protocols 
that use SHA-1.  Which ones are affected by this?

SSL: Nope.
SSH: Nope.
PGP: Nope (when used for email).
S/MIME: Nope (see above).
OCSP: Nope.
IPsec: Nope.
OpenVPN: Nope.
<lots of others>: Nope.

So what is actually affected?

Situations where you're creating signatures that need to be valid for a long 
time, and where the enormous latency between legitimate signature creation 
and forgery isn't an issue (this is why email won't be affected, having to 
wait a year between email being sent and the forgery being received will
probably raise at least some suspicions of foul play).  

What's left is long-term document signing and certificates, as pointed out
by the shattered.io FAQ.  With certificates the chances of it being 
exploitable in practice are fairly low, through a combination of CAs having 
moved away from SHA-1, the fact that certificates are only valid for a year
which means you have to race to forge before it expires, and the fact that
any CAs that weren't already randomising serial numbers before the earlier 
MD5 forged-cert attack will be doing it now.

Even for long-term document signing, these are frequently countersigned by
a TSA to deal with the fact that the original signing certificate will 
expire after a year, in which case they're safe as well.

Finally, with other stuff (software updates, ISOs, and others), (a) why were 
you still using SHA-1, and (b) you now have about 6-12 months to finally 
move to SHA-256, and this time we mean it.

For everything else, you really do need to plan the move to SHA-256.  Think
of this as a practical application of Wright's Principle, "Security won't 
get better until tools for practical exploration of the attack surface are 
made available".

Peter (who's at the tail end of a conference and only half awake, so I'll
       need to go through the paper in more detail tomorrow in case there's
       something I missed).

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