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----------------------- REVIEW 1 ---------------------
PAPER: 46
TITLE: Refreshing Coins for Giving Change and Refunds in Chaum-style Anonymous Payment Systems
AUTHORS: Florian Dold, Sree Harsha Totakura, Benedikt Müller, Jeff Burdges and Christian Grothoff
Overall evaluation: -2
----------- Overall evaluation -----------
This paper proposes an anonymous payment system called Taler, based on the Chaum’s blind signature scheme. Taler employs a new refresh protocol that allows fractional payments and refunds while providing the unlinkability and untraceability. The refresh protocol uses the cut-and-choose technique to assure that the protocol is not abused for evading taxation.
Comment: The correctness of the refresh protocol does not hold. The \bar{B(i)} computed by the exchange is not equal to B(i) computed by the honest customer, as \bar{Cp(i)} is not equal to FDHK(Cp(i)). This paper does not provide a security proof or even an informal security analysis for the proposed anonymous payment system Taler, such that Taler may be insecure. I find two (possible) attacks against the refresh protocol. As the exchange does not check the validity of the public key Cp′ , the attacker can send an arbitrary public key to the exchange that will accept, and obtain a fresh coin. The attacker can spend partially a coin multiple times via refreshing the coin and obtaining a fresh coin in turn, as the refresh protocol only transforms a dirty coin into a fresh coin with the same denomination. The misbehavior will not be detected by the exchange, as the fresh coin is unlinkable to the original coin. The implementation of Taler in this paper is unclear. For example!
, the security level, the RSA modulus, and the elliptic curve etc. are not described. Moreover, the average time of the withdrawal, spending, refreshing protocols are not provided. The authors also do not compare Taler with other known anonymous payment systems. Thus, the efficiency of Taler is unclear.
Additional Comment: The description of the protocols of Taler omits many details. In particular, the authors should describe in detail how the refunds are executed using the refresh protocol, as the authors claim that the refresh protocol allows refunds as a contribution. Furthermore, the authors should interpret the notation FDHK, and cite the reference for EdDSA. The title of Subsection 3.1 may be misleading, as this subsection does not describe the security model. The authors should rename the title. The “We have computed Li…” in Subsection 4.3 should be L(i).
----------------------- REVIEW 2 ---------------------
PAPER: 46
TITLE: Refreshing Coins for Giving Change and Refunds in Chaum-style Anonymous Payment Systems
AUTHORS: Florian Dold, Sree Harsha Totakura, Benedikt Müller, Jeff Burdges and Christian Grothoff
Overall evaluation: -2
----------- Overall evaluation -----------
This paper proposes a new e-cash, named Taler, where the bank (or else called exchange) is online during the spending protocol to allow for double-spending detection. Taler allows for spending coins of various denominations by allowing a user to only spend a value v1<V (where V is the value of the withdrawn coin) and then exchange the remaining value for a new, fresh coin, of value V-v1. The proposed scheme is different compared to Chaum e-cash: in Taler coins are pairs of pk/sk keys where the public key has been signed by the bank/exchange while in typical Chaum e-cash coins are represented by unique serial numbers.
Although the proposed system is hiding some interesting ideas, I think it cannot be accepted for publication at the moment. First and most importantly the current version of the paper lacks any level of analysis (not even informal) of the security level that system achieves. In fact, what security means has not been defined even in an informal lever. Moreover, as I better explain in my specific comments below there seem to be some issues with both security and anonymity (linking different uses of same coin, ensuring coin refreshing happens for the correct value).
Finally, the description of the protocols has quite a few inconsistencies (details below): there are parts that seem unnecessary and others that are not properly defined/explained, notation is also very overloaded (there is a 2 page notation table!).
Specific comments:
- I would expect the “Security Model” section (Section 1.3) to actually explain (even in an informal way) the desired properties of the proposed scheme. These should include double-spending detection security, unforgeability, user anonymity and more importantly the new type of security introduced by coin refresh (this should be a property that guarantees that a user cannot re-fresh a coin for value more than the one that the “dirty” coin is carrying) Instead it just mentions some of the tools used in the proposed scheme (i.e. FDH signatures, cut-and-choose and what kind of security they offer).
- Related work missing: there has been previous work in “payments with refunds”. Please look at Rupp et al “P4R: Privacy-Preserving Pre-Payments with Refunds for Transportation Systems” where instead of refreshing coins, the unused amount is accumulated in some token that can later be used. How would you compare with that system?
-Found the discussion on Bitcoin too long and unnecessary - the proposed system is not decentralized anyway
-Referencing a system (Goldberg’s HINDE) that is not published makes impossible for the reviewer to check any arguments.
-Section 4.1, step 1: is W_p = w_s * G? Also where is blinding factor b selected from? What does it mean to “commit to disk”? The customer commits and keeps the commitment local? Where is this used?
-Section 4.1, step 3, what is the key K used in FDH? Also is S_w(B) a standard signature?
-Section 4.1, step 4, How can the exchange know that this was indeed a new withdrawal request? If a new blinding factor b is used, then a customer can create multiple “freshly” looking requests for the same C_p. (Also a minor point: 2nd line also reads as “if the same withdrawal request was issued before the exchange will send S_K(B)”
-Section 4.2, it seems that a customer can use a coin of value say $10 to multiple transactions of <= $10 in total. I.e. it can first a pay a merchant M1 $2 and then a merchant M2 another $5 dollars. In that case the exchange can link those two payments together. Sure, it might not know who is the owner of the coin (i.e. cannot link with withdrawal) but this is still an anonymity problem.
-Section 4.3, doesn’t seem very fair to compare with Zcash or at least it should be highlighted that a quite weaker level of anonymity is achieved.
-Section 4.3, step 1, where is the key t_s^(i) selected from? What does S_{C’} denotes? Is that a commitment (as noted in the text) or a signature (as noted in notation table?).
-Section 4.3 In this protocol I would expect the customer to somehow “prove” to the exchange what is the remaining value of the dirty coin. I do not see this happening. How does this part of the protocol ensure that a user cannot just refresh a coin for one of a much bigger value than the remaining one?
Typos
Sec. 3.1 “cryptographily” -> cryptographically
Sec 4.2, Step 1 “a exchange” -> an exchange
Sec 4.3, 3rd line should be -> at the same time
----------------------- REVIEW 3 ---------------------
PAPER: 46
TITLE: Refreshing Coins for Giving Change and Refunds in Chaum-style Anonymous Payment Systems
AUTHORS: Florian Dold, Sree Harsha Totakura, Benedikt Müller, Jeff Burdges and Christian Grothoff
Overall evaluation: -1
----------- Overall evaluation -----------
The paper introduces a variant's of Chaum's e-cash scheme (with an
on-line bank); the main novelty is a "refresh" protocol which enables
a user to exchange a coin for a new blinded one. The reason for
wanting this features is that it enables refunds from a merchant that
later can be refreshed into "clean" coins that are unlinkable to the
refunded coins. The protocol is based on what appears to be a standard
cut-and-choose approach, which does not appear to be particularly
novel. On the postive side, the problem appears a natural and if it
hasn't been done before certainly useful. On the negative side, since
the paper does not contain any formal definitions, or even semi-formal
specifications of the desiderata, it is very hard to understand what
actually is acheived. Furthermore, no proofs of security are given,
and even the protocol is hard to fully understand. As such, I would
suggest the authors to first formalize their approach and
resubmitting.
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