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Even more strongly, there is the difference between "almost
never" and "never". Even if there were an infinite number of key
id's along the real number continuum, the possibility of a
collision is mathematically 0%, but it is still possible. Heck,
the possibility of ANY id would be mathematically 0, but each
key would still have an ID.
Here, we are dealing with a discrete distribution, so there
/are/ mass points (be they VERY very small) at each ID, so yes,
it is 100% certain that eventually, not only will there be a
collision, but every key will have a collision. It may be
though, that the waiting time may be longer than the heat death
of the universe for the latter, so we don't have to worry about
that too much :).
- --Avi
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Comment: Most recent key: Click show in box @ http://is.gd/4xJrs
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----
User:Avraham
pub 3072D/F80E29F9 1/30/2009 Avi (Wikimedia-related key)
<avi(_dot_)wiki(_at_)gmail(_dot_)com
Primary key fingerprint: 167C 063F 7981 A1F6 71EC ABAA 0D62 B019 F80E
29F9
On Thu, Jan 20, 2011 at 11:00 AM, Jon Callas <jon(_at_)callas(_dot_)org> wrote:
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A side-effect of this is something that's either an obvious extension or in
the spec already.
Section 3.3 says: "Implementations SHOULD NOT assume that Key IDs are
unique."
It's said that since 2440, for the following reason...
In PGP1 days, Vinnie started working there and told me that he'd generated
a key and gotten a certain error when putting it into the keyserver. I was
thrilled, because that error was a duplicate keyid error. We'd been having
debates over this ourselves. Being a software engineer, I tend to consider
assuming that a database key is unique is bad form. I recognize that
pseudo-random 64-bit numbers don't collide easily at all, but assuming
uniqueness is something that is easily coded around. That key was my proof
that engineering-wise, don't assume uniqueness.
Sadly, he had deleted the key and just generated a new one, so that key is
the Nessie of key ids. It's a crypto-cryptologist's dream, the true random
collision. And we will never know if it was genuine. Sigh.
Nonetheless, that led to that clause in 3.3, and I assert that if an
implementation breaks because of a keyid collision, the implementation has a
bug.
So you can consider a keyid to just be a database key. The way we do it now
(truncating a fingerprint) is a fine way to do it, but the underlying
principle is that an implementor really needs to code around the eventuality
that there will be a collision and do some right thing.
Yeah, I know it's easier said than done, but that doesn't make it false.
I forget what Terry Pratchett novel has it, but in one of them he has a
discussion that anything that is a million-to-one against is a certainty.
The million-to-one thing *will* happen. Cryptographers need to keep that
principle in the back of their head, too.
Jon
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