Field slide attacks and how to avoid them.

[Amir Herzberg]

John says, 

> I've been noticing a lot of ways you can mess up a cryptographic
> protocol due to the "sliding around" of fields within a 
> signed or MACed
> message.  The classic example of this is the old attack on PGP
> fingerprints, which let you use some odd keysize, and thus get two
> different keys (with different keysizes) with the same hash, without
> breaking the hash function.  (The raw bits of the two keys 
> are the same,
> but the fields are broken up differently.)

Use MAC function properly designed to prevent such attacks, such as HMAC
http://www.ietf.org/rfc/rfc2104.txt. 

Best, Amir Herzberg



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