XML signature HMAC truncation authentication bypass

[Jon Callas]

On Jul 17, 2009, at 8:39 PM, Peter Gutmann wrote:

> PGP Desktop 9 uses as its default an iteration count of four
> million (!!) for its password hashing, which looks like a DoS to  
> anything that
> does sanity-checking of input.

That's precisely what it is -- a denial of service to password crackers.

There are a couple of things I'll add, one in the OpenPGP standard,  
and one in that implementation.

In the standard, the iteration count is not a count of hash iterations  
as in (e.g.) PKCS#5, but a length of output. So four million is four  
million bytes of output. For SHA-1, that's a count of 200,000, and for  
SHA-256 125,000 iterations. While this is a bit eccentric, it allows  
you to use any size hash and any block size cipher. Even more  
eccentric is the way it's encoded, as an 8-bit floating point value.

In the implementation, we upped the default because of more password  
cracking, but also added a twist in it. We time the number of  
iterations take 1/10 of a second on the computer you're using, and use  
that value. The goal is to have the iteration count scale as computers  
get faster without having to make software changes.

The downsides of this are left as an exercise for the reader (as are  
the obvious workarounds).

	Jon

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