Bitcoin is an online distributed ledger in which coins are distributed according to the unspent t... more Bitcoin is an online distributed ledger in which coins are distributed according to the unspent transaction output (UTXO) set, and transactions describe changes to this set. Every UTXO has associated to it an amount and signature verification key, representing the quantity that can be spent and the entity authorized to do so, respectively. Because the ledger is distributed and publicly verifiable, every UTXO (and the history of all changes) is publicly available and may be used for analysis of all users’ payment history. Although this history is not directly linked to users in any way, it exposes enough structure that even small amounts of personally identifiable information may completely break users’ privacy. Further, the ability to trace coin history creates a market for “clean” coins, harming the fungibility of the underlying asset. In this paper we describe a scheme, confidential transactions, which blinds the amounts of all UTXOs, while preserving public verifiability that no ...
Optimistic execution techniques are widely used in the field of parallel discrete event simulatio... more Optimistic execution techniques are widely used in the field of parallel discrete event simulation. In this paper we discuss the use of optimism as a technique for parallelising programs written in a general purpose programming language. We present the design and implementation of a compiler system which uses optimistic simulation techniques to execute sequential C ++ programs. The use of optimistic techniques is seen as a new direction in parallelisation technology: conventional parallelising compilers are based on static (compile-time) data dependency analysis. The reliance on static information imposes an overly restrictive view of the parallelism available in a program. The static view must always be the worst case view: if it is possible for a data dependency to occur, then it must be assumed always to occur.
In this short note we show that the Bitcoin network can allow remote parties to gamble with their... more In this short note we show that the Bitcoin network can allow remote parties to gamble with their bitcoins by tossing a fair or biased coin, with no need for a trusted party, and without the possibility of extortion by dishonest parties who try to abort. The superfluousness of having a trusted party implies that there is no house edge, as is the case with centralized services that are supposed to generate a profit.
Hashcash was originally proposed as a mechanism to throttle systematic abuse of un-metered intern... more Hashcash was originally proposed as a mechanism to throttle systematic abuse of un-metered internet resources such as email, and anonymous remailers in May 1997. Five years on, this paper captures in one place the various applications, improvements suggested and related subsequent publications, and describes initial experience from experiments using hashcash. The hashcash CPU cost-function computes a token which can be used as a proof-of-work. Interactive and noninteractive variants of cost-functions can be constructed which can be used in situations where the server can issue a challenge (connection oriented interactive protocol), and where it can not (where the communication is store–and– forward, or packet oriented) respectively.
We discuss problems and trade-offs with systems providing anonymity for web browsing (or more gen... more We discuss problems and trade-offs with systems providing anonymity for web browsing (or more generally any communication system that requires low latency interaction). We focus on two main systems: the Freedom network [12] and PipeNet [8]. Although Freedom is efficient and reasonably secure against denial of service attacks, it is vulnerable to some generic traffic analysis attacks, which we describe. On the other hand, we look at PipeNet, a simple theoretical model which protects against the traffic analysis attacks we point out, but is vulnerable to denial of services attacks and has efficiency problems. In light of these observations, we discuss the trade-offs that one faces when trying to construct an efficient low latency communication system that protects users anonymity.
In this paper we discuss our research into the use of optimistic methods for general purpose para... more In this paper we discuss our research into the use of optimistic methods for general purpose parallel computing. The optimistic execution of a program can allow code to be run in parallel which static program analysis might indicate was sequential. However, this also means that it may do some work which is later found to be wrong because of a causality violation. Optimistic methods use a detection and recovery approach: causality errors are detected, and a roll-back mechanism is invoked to recover. These techniques have been used very successfully in parallel discrete event simulation, but are not as yet widely used in general purpose computing. Our research involves the development of a compiler which converts a conventional objectoriented program into a form which has calls to an optimistic run time system. The generation of time-stamps which allow for loops with an unknown number of iterations is discussed. We also describe some of the portability issues which we have addressed in this project: in particular, our use of p4, a cluster based communications library which provides the basis for portable, heterogeneous parallel computing. We describe our implementation of p4 on the transputer architecture, including some extensions to support the optimistic execution of programs, and also an outline of our work in retargetting the GNU C and C++ compilers for the transputer.
We describe attacks to which Freedom, or Freedom users, may be vulnerable. These attacks are thos... more We describe attacks to which Freedom, or Freedom users, may be vulnerable. These attacks are those that reduce the privacy of a Freedom user, through exploiting cryptographic, design or implementation issues. We include issues which may not be Freedom security issues which arise when the system is not properly used. This disclosure includes all known design or implementation flaws, as well as places where various trade-offs made while creating the system have privacy implications. We also discuss cryptographic points that are needed for a complete understanding of how Freedom works, including ones we don't believe can be used to reduce anyone's privacy.
Bitcoin is an online distributed ledger in which coins are distributed according to the unspent t... more Bitcoin is an online distributed ledger in which coins are distributed according to the unspent transaction output (UTXO) set, and transactions describe changes to this set. Every UTXO has associated to it an amount and signature verification key, representing the quantity that can be spent and the entity authorized to do so, respectively. Because the ledger is distributed and publicly verifiable, every UTXO (and the history of all changes) is publicly available and may be used for analysis of all users’ payment history. Although this history is not directly linked to users in any way, it exposes enough structure that even small amounts of personally identifiable information may completely break users’ privacy. Further, the ability to trace coin history creates a market for “clean” coins, harming the fungibility of the underlying asset. In this paper we describe a scheme, confidential transactions, which blinds the amounts of all UTXOs, while preserving public verifiability that no ...
Optimistic execution techniques are widely used in the field of parallel discrete event simulatio... more Optimistic execution techniques are widely used in the field of parallel discrete event simulation. In this paper we discuss the use of optimism as a technique for parallelising programs written in a general purpose programming language. We present the design and implementation of a compiler system which uses optimistic simulation techniques to execute sequential C ++ programs. The use of optimistic techniques is seen as a new direction in parallelisation technology: conventional parallelising compilers are based on static (compile-time) data dependency analysis. The reliance on static information imposes an overly restrictive view of the parallelism available in a program. The static view must always be the worst case view: if it is possible for a data dependency to occur, then it must be assumed always to occur.
In this short note we show that the Bitcoin network can allow remote parties to gamble with their... more In this short note we show that the Bitcoin network can allow remote parties to gamble with their bitcoins by tossing a fair or biased coin, with no need for a trusted party, and without the possibility of extortion by dishonest parties who try to abort. The superfluousness of having a trusted party implies that there is no house edge, as is the case with centralized services that are supposed to generate a profit.
Hashcash was originally proposed as a mechanism to throttle systematic abuse of un-metered intern... more Hashcash was originally proposed as a mechanism to throttle systematic abuse of un-metered internet resources such as email, and anonymous remailers in May 1997. Five years on, this paper captures in one place the various applications, improvements suggested and related subsequent publications, and describes initial experience from experiments using hashcash. The hashcash CPU cost-function computes a token which can be used as a proof-of-work. Interactive and noninteractive variants of cost-functions can be constructed which can be used in situations where the server can issue a challenge (connection oriented interactive protocol), and where it can not (where the communication is store–and– forward, or packet oriented) respectively.
We discuss problems and trade-offs with systems providing anonymity for web browsing (or more gen... more We discuss problems and trade-offs with systems providing anonymity for web browsing (or more generally any communication system that requires low latency interaction). We focus on two main systems: the Freedom network [12] and PipeNet [8]. Although Freedom is efficient and reasonably secure against denial of service attacks, it is vulnerable to some generic traffic analysis attacks, which we describe. On the other hand, we look at PipeNet, a simple theoretical model which protects against the traffic analysis attacks we point out, but is vulnerable to denial of services attacks and has efficiency problems. In light of these observations, we discuss the trade-offs that one faces when trying to construct an efficient low latency communication system that protects users anonymity.
In this paper we discuss our research into the use of optimistic methods for general purpose para... more In this paper we discuss our research into the use of optimistic methods for general purpose parallel computing. The optimistic execution of a program can allow code to be run in parallel which static program analysis might indicate was sequential. However, this also means that it may do some work which is later found to be wrong because of a causality violation. Optimistic methods use a detection and recovery approach: causality errors are detected, and a roll-back mechanism is invoked to recover. These techniques have been used very successfully in parallel discrete event simulation, but are not as yet widely used in general purpose computing. Our research involves the development of a compiler which converts a conventional objectoriented program into a form which has calls to an optimistic run time system. The generation of time-stamps which allow for loops with an unknown number of iterations is discussed. We also describe some of the portability issues which we have addressed in this project: in particular, our use of p4, a cluster based communications library which provides the basis for portable, heterogeneous parallel computing. We describe our implementation of p4 on the transputer architecture, including some extensions to support the optimistic execution of programs, and also an outline of our work in retargetting the GNU C and C++ compilers for the transputer.
We describe attacks to which Freedom, or Freedom users, may be vulnerable. These attacks are thos... more We describe attacks to which Freedom, or Freedom users, may be vulnerable. These attacks are those that reduce the privacy of a Freedom user, through exploiting cryptographic, design or implementation issues. We include issues which may not be Freedom security issues which arise when the system is not properly used. This disclosure includes all known design or implementation flaws, as well as places where various trade-offs made while creating the system have privacy implications. We also discuss cryptographic points that are needed for a complete understanding of how Freedom works, including ones we don't believe can be used to reduce anyone's privacy.
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