Question regarding common modulus on elliptic curve cryptosystems

Peter Fairbrother zenadsl6186 at
Tue Mar 23 11:29:40 EDT 2010

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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