On 4 September 2014, an unusual and novel attack was executed against the Monero cryptocurrency network. This attack partitioned the network into two distinct subsets which refused to accept the legitimacy of the other subset. This had myriad effects, not all of which are yet known. The attacker had a short window of time during which a sort of counterfeiting could occur, for example. This research bulletin describes deficiencies in the CryptoNote reference code allowing for this attack, describes the solution initially put forth by Rafal Freeman from Tigusoft.pl and subsequently by the CryptoNote team, describes the current fix in the Monero code base, and elaborates upon exactly what the offending block did to the network. This research bulletin has not undergone peer review, and reflects only the results of internal investigation.
Recently, there have been some vague fears about the CryptoNote source code and protocol floating around the internet based on the fact that it is a more complicated protocol than, for instance, Bitcoin. The purpose of this note is to try and clear up some misconceptions, and hopefully remove some of the mystery surrounding Monero Ring Signatures. I will start by comparing the mathematics involved in CryptoNote ring signatures (as described in [CN]) to the mathematics in [FS], on which CryptoNote is based. After this, I will compare the mathematics of the ring signature to what is actually in the CryptoNote codebase.
We identify several blockchain analysis attacks available to degrade the untraceability of the CryptoNote 2.0 protocol. We analyze possible solutions, discuss the relative merits and drawbacks to those solutions, and recommend improvements to the Monero protocol that will hopefully provide long-term resistance of the cryptocurrency against blockchain analysis. Our recommended improvements to Monero include a protocol-level network-wide minimum mix-in policy of n = 2 foreign outputs per ring signature, a protocol-level increase of this value to n = 4 after two years, and a wallet-level default value of n = 4 in the interim. We also recommend a torrent-style method of sending Monero output. We also discuss a non-uniform, age-dependent mix-in selection method to mitigate the other forms of blockchain analysis identified herein, but we make no formal recommendations on implementation for a variety of reasons. The ramifications following these improvements are also discussed in some detail. This research bulletin has not undergone peer review, and reflects only the results of internal investigation.
This article introduces a method of hiding transaction amounts in the strongly decentralized anonymous cryptocurrency Monero. Similar to Bitcoin, Monero is a cryptocurrency which is distributed through a proof of work “mining” process. The original Monero protocol was based on CryptoNote, which uses ring signatures and one-time keys to hide the destination and origin of transactions. Recently the technique of using a commitment scheme to hide the amount of a transaction has been discussed and implemented by Bitcoin Core Developer Gregory Maxwell. In this article, a new type of ring signature, A Multi-layered Linkable Spontaneous Anonymous Group signature is described which allows for hidden amounts, origins and destinations of transactions with reasonable efficiency and verifiable, trustless coin generation. Some extensions of the protocol are provided, such as Aggregate Schnorr Range Proofs, and Ring Multisignature. The author would like to note that early drafts of this were publicized in the Monero Community and on the bitcoin research irc channel. Blockchain hashed drafts are available in [14] showing that this work was started in Summer 2015, and completed in early October 2015. An eprint is also available at http://eprint.iacr.org/2015/1098.
This technical note generalizes the concept of spend outputs using basic set theory. The definition captures a variety of earlier work on identifying such outputs. We quantify the effects of this analysis on the Monero blockchain and give a brief overview of mitigations.
This bulletin describes a modification to Monero's linkable ring signature scheme that permits dual-key outputs as ring members. Key images are tied to both output one-time public keys in a dual, preventing both keys in that transaction from being spent separately. This method has applications to non-interactive refund transactions. We discuss the security implications of the scheme.
We present threshold ring multi-signatures (thring signatures) for collaborative computation of ring signatures, present a game of existential forgery for thring signatures, and discuss uses of thring signatures in digital currencies that include spender-ambiguous cross-chain atomic swaps for confidential amounts without a trusted setup. We present an implementation of thring signatures that we call linkable spontaneous threshold anonymous group signatures, and prove the implementation existentially unforgeable.
This technical note describes an algorithm used to prove knowledge of the same discrete logarithm across different groups. The scheme expresses the common value as a scalar representation of bits, and uses a set of ring signatures to prove each bit is a valid value that is the same (up to an equivalence) across both scalar groups.
Ring signatures are a common construction used to provide signer ambiguity among a non-interactive set of public keys specified at the time of signing. Unlike early approaches where signature size is linear in the size of the signer anonymity set, current optimal solutions either require centralized trusted setups or produce signatures logarithmic in size. However, few also provide linkability, a property used to determine whether the signer of a message has signed any previous message, possibly with restrictions on the anonymity set choice. Here we introduce Triptych, a family of linkable ring signatures without trusted setup that is based on generalizations of zero-knowledge proofs of knowledge of commitment openings to zero. We demonstrate applications of Triptych in signer-ambiguous transaction protocols by extending the construction to openings of parallel commitments in independent anonymity sets. Signatures are logarithmic in the anonymity set size and, while verification complexity is linear, collections of proofs can be efficiently verified in batches. We show that for anonymity set sizes practical for use in distributed protocols, Triptych offers competitive performance with a straightforward construction.
