Sergio Lerner wrote:

I looking for a public-key cryptosystem that allows commutation of the operations of encription/decryption for different users keys
( Ek(Es(m)) =  Es(Ek(m)) ).

Diffie-Hellman combined with Pohlig-Hellman can do what you describe.

It's a variation on using DH (as a public key system rather than as a key agreement system) rather than El Gamal. If it's not obvious how to implement it ask offlist.

But I don't think that's what you need. PK is not the same thing as signatures. It's not "a commutative signing primitive".

That's not something which I've come across before, but maybe I could work one out ... I gather you need for the verifier to be unable to tell the order in which the signatures were applied? Can't think offhand of any other reason why you'd need one.

But I need to know exactly what you need.

Need coffee, it's raining .. dilemma, wet or unwoken?

-- Peter Fairbrother

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