I've read some papers, not that much. But I don't mind reinventing the
wheel, as long as the new protocol is simpler to explain.
Reading the literature, I couldn't find a e-cash protocol which :
- Hides the destination / source of payments.
- Hides the amount of money transferred.
- Hides the account balance of each person from the bank.
- Allows off-line payments.
- Avoids giving the same bill to two different people by design. This
means that the protocol does not need to detect the use of cloned bills.
- Gives each person a cryptographic proof of owning the money they have
in case of dispute.
I someone points me out a protocol that manages to fulfill this
requirements, I'd be delighted.
I think I can do it with a commutative signing primitive, and a special
zero-proof of knowledge.
Regards,
Sergio Lerner.
On 22/03/2010 10:25 a.m., Jonathan Katz wrote:
That paper was from 1980. A few things have changed since then. =)
In any case, my point still stands: what you actually want is some
e-cash system with some special properties. Commutative encryption is
neither necessary nor (probably) sufficient for what you want. Have
you at least looked at the literature (which must be well over 100
papers) on e-cash?
On Mon, 22 Mar 2010, Sergio Lerner wrote:
Commutativity is a beautiful and powerful property. See On the power
of Commutativity in Cryptography by Adi Shamir.
Semantic security is great and has given a new provable sense of
security, but commutative building blocks can be combined to build
the strangest protocols without going into deep mathematics, are
better suited for teaching crypto and for high-level protocol design.
They are like the Lego blocks of cryptography!
Now I'm working on an new untraceable e-cash protocol which has some
additional properties. And I'm searching for a secure commutable
signing primitive.
Best regards,
Sergio Lerner.
On 22/03/2010 09:56 a.m., Jonathan Katz wrote:
Sounds like a bad idea -- at a minimum, your encryption will be
deterministic.
What are you actually trying to achieve? Usually once you understand
that, you can find a protocol solving your problem already in the
crypto literature.
On Sun, 21 Mar 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)) ).
I haven't found a simple cryptosystem in Zp or Z/nZ.
I think the solution may be something like the RSA analogs in
elliptic curves. Maybe a scheme that allows the use of a common
modulus for all users (RSA does not).
I've read on some factoring-based cryptosystem (like Meyer-Muller
or Koyama-Maurer-Okamoto-Vantone) but the cryptosystem authors say
nothing about the possibility of using a common modulus, neither
for good nor for bad.
Anyone has a deeper knowledge on this crypto to help me?
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