Re: draft-klensin-emailaddr-i18n-00

[John Cowan <jcowan@xxxxxxxxxxxxxxxxx>]

Adam M. Costello scripsit:

> The receiver will be fooled only if it blindly assumes that the text
> is normalized.  I don't think receivers should make that assumption.
> They can't be fooled if they force the text to be normalized whenever it
> matters.

Oh yes they can, and all the worse.  Consider the classical birthday attack:
see http://www.x5.net/faqs/crypto/q96.html for details if you need them.
It depends on the ability to generate 2^(n/2) variants (where n is the
number of bits in the n-bit signature hash function) of a message to be
used for the spoof.  Typically this involves things like altering whitespace,
but a close inspection will detect these.

If we play with Unicode canonical equivalence in a world where receivers
normalize, however, we can create variants that are quite undetectable by
the receiver.  Typical German text contains about 5% accented characters,
so a 20K message can be given 2^1000 variants, more than enough to break
reasonable hash functions.

-- 
John Cowan  jcowan@xxxxxxxxxxxxxxxxx  www.ccil.org/~cowan  www.reutershealth.com
"The competent programmer is fully aware of the strictly limited size of his own
skull; therefore he approaches the programming task in full humility, and among
other things he avoids clever tricks like the plague."  --Edsger Dijkstra