Margaret,
It is actually my (weak) understanding that taking more inputs
does not actually result in more "uniqueness", at least for
random number generation.
Does anyone know how that works for hashing?
AFAIK, the bigest problem with random number generation is
non-random seed data. Adding more sources of randomness
helps in that. In this case, relying in just one MAC address
as a seed for a hash function is probably not good enough, but
e.g. taking the MAC address *and* the machine's current idea
of date & time in millisecond precision probably is.
As another issue, just picking a cryptographically strong
hash function and using it as a random number generator is
typically *not* good enough for many uses of random numbers,
but IMHO it is OK for generating these kinds of identifiers.
Thus, if we want a method for automatically generating
prefixes for globally unique enough site local addresses,
a decent method might be
sha1( current date and time in ms | interface MAC )
where the date & time would be a 64 bit integer and the
MAC address either 48 or 64 bit MAC, the 48 bit version
enlarged to 64 bits. Note that there is no need for time
synchronization. If there are more implementations of MD5
than SHA-1, MD5 would be good here, too.
In the case of a collision, the algorithm can be simply
rerun. The new result will be completely different.
--Pekka Nikander
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