commuting?/non-group cipher?

From: Peter Fairbrother (zenadsl6186_at_zen.co.uk)
Date: 10/28/04 

Date: Thu, 28 Oct 2004 06:56:03 +0100

Some ciphers have the property that a double encryption can always be
replaced by a single encryption, ie E(k3)[P] = E(k1)[E(k2)[P]]

Does anyone know the correct name for this property? If there isn't one,
does anyone know a reason why "commuting (adj.)" cipher would not be okay?

Can anyone think of an example of a cipher with this property that is not a
group? 

-- 
Peter Fairbrother

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