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The Bitcoin Backbone Protocol with Chains of Variable Difficulty

  • Juan GarayEmail author
  • Aggelos Kiayias
  • Nikos Leonardos
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 10401)

Abstract

Bitcoin’s innovative and distributedly maintained blockchain data structure hinges on the adequate degree of difficulty of so-called “proofs of work,” which miners have to produce in order for transactions to be inserted. Importantly, these proofs of work have to be hard enough so that miners have an opportunity to unify their views in the presence of an adversary who interferes but has bounded computational power, but easy enough to be solvable regularly and enable the miners to make progress. As such, as the miners’ population evolves over time, so should the difficulty of these proofs. Bitcoin provides this adjustment mechanism, with empirical evidence of a constant block generation rate against such population changes.

In this paper we provide the first formal analysis of Bitcoin’s target (re)calculation function in the cryptographic setting, i.e., against all possible adversaries aiming to subvert the protocol’s properties. We extend the q-bounded synchronous model of the Bitcoin backbone protocol [Eurocrypt 2015], which posed the basic properties of Bitcoin’s underlying blockchain data structure and shows how a robust public transaction ledger can be built on top of them, to environments that may introduce or suspend parties in each round.

We provide a set of necessary conditions with respect to the way the population evolves under which the “Bitcoin backbone with chains of variable difficulty” provides a robust transaction ledger in the presence of an actively malicious adversary controlling a fraction of the miners strictly below \(50\%\) at each instant of the execution. Our work introduces new analysis techniques and tools to the area of blockchain systems that may prove useful in analyzing other blockchain protocols.

1 Introduction

The Bitcoin backbone [11] extracts and analyzes the basic properties of Bitcoin’s underlying blockchain data structure, such as “common prefix” and “chain quality,” which parties (“miners”) maintain and try to extend by generating “proofs of work” (POW, aka “cryptographic puzzles” [1, 8, 14, 23])1. It is then formally shown in [11] how fundamental applications including consensus [17, 22] and a robust public transaction ledger realizing a decentralized cryptocurrency (e.g., Bitcoin [20]) can be built on top of them, assuming that the hashing power of an adversary controlling a fraction of the parties is strictly less than 1/2.

The results in [11], however, hold for a static setting, where the protocol is executed by a fixed number of parties (albeit not necessarily known to the participants), and therefore with POWs (and hence blockchains) of fixed difficulty. This is in contrast to the actual deployment of the Bitcoin protocol where a “target (re)calculation” mechanism adjusts the hardness level of POWs as the number of parties varies during the protocol execution. In more detail, in [11] the target T that the hash function output must not exceed, is set and hardcoded at the beginning of the protocol, and in such a way that a specific relation to the number of parties running the protocol is satisfied, namely, that a ratio f roughly equal to \(q n T/2^{\kappa }\) is small, where q is the number of queries to the hash function that each party is allowed per round, n is the number of parties, and \(\kappa \) is the length of the hash function output. Security was only proven when the number of parties is n and the choice of target T is never recalculated, thus leaving as open question the full analysis of the protocol in a setting where, as in the real world, parties change dynamically over time.

In this paper, we abstract for the first time the target recalculation algorithm from the Bitcoin system, and present a generalization and analysis of the Bitcoin backbone protocol with chains of variable difficulty, as produced by an evolving population of parties, thus answering the aforementioned open question.

In this setting, there is a parameter m which determines the length of an “epoch” in number of blocks.2 When a party prepares to compute the j-th block of a chain with \(j \bmod m = 1\), it uses a target calculation algorithm that determines the proper target value to use, based on the party’s local view about the total number of parties that are present in the system, as reflected by the rate of blocks that have been created so far and are part of the party’s chain. (Each block contains a timestamp of when it was created; in our synchronous setting, timestamps will correspond to the round numbers when blocks are created—see Sect. 2.) To accomodate the evolving population of parties, we extend the model of [11] to environments that are free to introduce and suspend parties in each round. In other respects, we follow the model of [11], where all parties have the same “hashing power,” with each one allowed to pose q queries to the hash function that is modeled as a “random oracle” [3]. We refer to our setting as the dynamic q -bounded synchronous setting.

In order to give an idea of the issues involved, we note that without a target calculation mechanism, in the dynamic setting the backbone protocol is not secure even if all parties are honest and follow the protocol faithfully. Indeed, it is easy to see that a combination of an environment that increases the number of parties and adversarial network conditions can lead to substantial divergence (a.k.a. “forks”) in the chains of the honest parties, leading to the violation of the agreement-type properties that are needed for the applications of the protocol, such as maintaining a robust transaction ledger. The attack is simple: the environment increases the number of parties constantly so that the block production rate per round increases (which is roughly the parameter f mentioned above); then, adversarial network conditions may divide the parties into two sets, A and B, and schedule message delivery so that parties in set A receive blocks produced by parties in A first, and similarly for set B. According to the Bitcoin protocol, parties adopt the block they see first, and thus the two sets will maintain two separate blockchains.

While this specific attack could in principle be thwarted by modifying the Bitcoin backbone (e.g., by randomizing which block a party adopts when they receive in the same round two blocks of the same index in the chain), it certainly would not cope with all possible attacks in the presence of a full-blown adversary and target recalculation mechanism. Indeed, such an attack was shown in [2], where by mining “privately” with timestamps in rapid succession, corrupt miners are able to induce artificially high targets in their private chain; even though such chain may grow slower than the main chain, it will still make progress and, via an anti-concentration argument, a sudden adversarial advance that can break agreement amongst honest parties cannot be ruled out.

Given the above, our main goal is to show that the backbone protocol with a Bitcoin-like target recalculation function satisfies the common prefix and chain quality properties, as an intermediate step towards proving that the protocol implements a robust transaction ledger. Expectedly, the class of protocols we will analyze will not preserve its properties for arbitrary ways in which the number of parties may change over time. In order to bound the error in the calibration of the block generation rate that the target recalculation function attempts, we will need some bounds on the way the number of parties may vary. For \(\gamma \in \mathbb {R}^+\) and \(s\in \mathbb {N}\), we will call a sequence \((n_r)_{r \in \mathbb {N}}\) of parties \((\gamma ,s)\) -respecting if it holds that in a sequence of rounds S with \(|S|\le s\), \(\max _{r\in S} n_r \le \gamma \cdot \min _{r\in S} n_r\), and will determine for what values of these parameters the backbone protocol is secure.

After formally describing blockchains of variable difficulty and the Bitcoin backbone protocol in this setting, at a high level our analysis goes as follows. We first introduce the notion of goodness regarding the approximation that is performed on f in an epoch. In more detail, we call a round r \((\eta ,\theta )\) -good, for some parameters \(\eta ,\theta \in \mathbb {R}^+\), if the value \(f_r\) computed for the actual number of parties and target used in round r by some honest party, falls in the range \([\eta f, \theta f]\), where f is the initial block production rate (note that the first round is always assumed good). Together with “goodness” we introduce the notion of typical executions, in which, informally, for any set S of consecutive rounds the successes of the adversary and the honest parties do not deviate too much from their expectations as well as no “bad” event concerning the hash function occurs (such as a collision). Using a martingale bound we demonstrate that almost all polynomially bounded (in \(\kappa \)) executions are typical.

Next, we proceed to show that in a typical execution any chain that an honest party adopts (1) contains timestamps that are approximately accurate (i.e., no adversarial block has a timestamp that differs too much from its real creation time), and (2) it has a target such that the probability of block production remains near the fixed constant f, i.e., it is “good.” Finally, these properties allow us to demonstrate that a typical execution enjoys the common prefix and chain quality properties, which is a stepping stone towards the ultimate goal, that of establishing that the backbone protocol with variable difficulty implements a robust transaction ledger. Specifically, we show the following:

Main Result. (Informal—see Theorems 4 and 5). The Bitcoin backbone protocol with chains of variable difficulty, suitably parameterized, satisfies with overwhelming probability in m and \(\kappa \) the properties of (1) persistence—if a transaction \( tx \) is confirmed by an honest party, no honest party will ever disagree about the position of \( tx \) in the ledger, and (2) liveness—if a transaction \( tx \) is broadcast, it will eventually become confirmed by all honest parties.

Remark. Regarding the actual parameterization of the Bitcoin system (that uses epochs of \(m=2016\) blocks), even though it is consistent with all the constraints of our theorems (cf. Remark 3 in Sect. 6.1), it cannot be justified by our martingale analysis. In fact, our probabilistic analysis would require much longer epochs to provide a sufficiently small probability of attack. Tightening the analysis or discovering attacks for parameterizations beyond our security theorems is an interesting open question.

Finally, we note that various extensions to our model are relevant to the Bitcoin system and constitute interesting directions for further research. Importantly, a security analysis in the “rational” setting (see, e.g., [9, 15, 24]), and in the “partially synchronous,” or “bounded-delay” network model [7, 21]3.

2 Model and Definitions

We describe our protocols in a model that extends the synchronous communication network model presented in [10, 11] for the analysis of the Bitcoin backbone protocol in the static setting with a fixed number of parties (which in turn is based on Canetti’s formulation of “real world” notion of protocol execution [4, 5, 6] for multi-party protocols) to the dynamic setting with a varying number of parties. In this section we provide a high-level overview of the model, highlighting the differences that are intrinsic to our dynamic setting.

Round Structure and Protocol Execution. As in [10], the protocol execution proceeds in rounds with inputs provided by an environment program denoted by \(\mathcal {Z} \) to parties that execute the protocol \(\varPi \), and our adversarial model in the network is “adaptive,” meaning that the adversary \(\mathcal {A} \) is allowed to take control of parties on the fly, and “rushing,” meaning that in any given round the adversary gets to see all honest players’ messages before deciding his strategy. The parties’ access to the hash function and their communication mechanism are captured by a joint random oracle/diffusion functionality which reflects Bitcoin’s peer structure. The diffusion functionality, [10], allows the order of messages to be controlled by \(\mathcal {A} \), i.e., there is no atomicity guarantees in message broadcast [13], and, furthermore, the adversary is allowed to spoof the source information on every message (i.e., communication is not authenticated). Still, the adversary cannot change the contents of the messages nor prevent them from being delivered. We will use Diffuse as the message transmission command that captures this “send-to-all” functionality.

The parties that may become active in a protocol execution are encoded as part of a control program C and come from a universe \(\mathcal {U}\) of parties.

The protocol execution is driven by an environment program \(\mathcal {Z} \) that interacts with other instances of programs that it spawns at the discretion of the control program C. The pair \((\mathcal {Z}, C)\) forms of a system of interactive Turing machines (ITM’s) in the sense of [5]. The execution is with respect to a program \(\varPi \), an adversary \(\mathcal {A} \) (which is another ITM) and the universe of parties \(\mathcal {U}\). Assuming the control program C allows it, the environment \(\mathcal {Z} \) can activate a party by writing to its input tape. Note that the environment \(\mathcal {Z} \) also receives the parties’ outputs when they are produced in a standard subroutine-like interaction. Additionally, the control program maintains a flag for each instance of an ITM, (abbreviated as ITI in the terminology of [5]), that is called the \(\mathtt {ready}\) flag and is initially set to false for all parties.

The environment \(\mathcal {Z} \), initially is restricted by C to spawn the adversary \(\mathcal {A} \). Each time the adversary is activated, it may send one or more messages of the form \((\mathsf {Corrupt}, P_i)\) to C and C will mark the corresponding party as corrupted.

Functionalities Available to the Protocol. The ITI’s of protocol \(\varPi \) will have access to a joint ideal functionality capturing the random oracle and the diffusion mechanism which is defined in a similar way as [10] and is explained below.

  • The random oracle functionality. Given a query with a value x marked for “calculation” for the function \(H(\cdot )\) from an honest party \(P_i\) and assuming x has not been queried before, the functionality returns a value y which is selected at random from \(\{0,1\}^\kappa \); furthermore, it stores the pair (xy) in the table of \(H(\cdot )\), in case the same value x is queried in the future. Each honest party \(P_i\) is allowed to ask q queries in each round as determined by the diffusion functionality (see below). On the other hand, each honest party is given unlimited queries for “verification” for the function \(H(\cdot )\). The adversary \(\mathcal {A} \), on the other hand, is given a bounded number queries in each round as determined by diffusion functionality with a bound that is initialized to 0 and determined as follows: whenever a corrupted party is activated, the party can ask the bound to be increased by q; each time a query is asked by the adversary the bound is decreased by 1. No verification queries are provided to \(\mathcal {A} \). Note that the value q is a polynomial function of \(\kappa \), the security parameter. The functionality can maintain tables for functions other than \(H(\cdot )\) but, by convention, the functionality will impose query quotas to function \(H(\cdot )\) only.

  • The diffusion functionality. This functionality keeps track of rounds in the protocol execution; for this purpose it initially sets a variable round to be 1. It also maintains a Receive() string defined for each party \(P_i\) in \(\mathcal {U}\). A party that is activated is allowed to query the functionality and fetch the contents of its personal Receive() string. Moreover, when the functionality receives a message \((\mathsf {Diffuse}, m)\) from party \(P_i\) it records the message m. A party \(P_i\) can signal when it is complete for the round by sending a special message \((\mathsf {RoundComplete})\). With respect to the adversary \(\mathcal {A} \), the functionality allows it to receive the contents of all contents sent in \(\mathsf {Diffuse}\) messages for the round and specify the contents of the Receive() string for each party \(P_i\). The adversary has to specify when it is complete for the current round. When all parties are complete for the current round, the functionality inspects the contents of all Receive() strings and includes any messages m that were diffused by the parties in the current round but not contributed by the adversary to the Receive() tapes (in this way guaranteeing message delivery). It also flushes any old messages that were diffused in previous rounds and not diffused again. The variable round is then incremented.

The Dynamic \({\varvec{q}}\) -Bounded Synchronous Setting. Consider \(\mathbf {n} = \{n_r\}_{r\in \mathbb {N}}\) and \(\mathbf {t} = \{t_r\}_{r\in \mathbb {N}}\) two series of natural numbers. As mentioned, the first instance that is spawned by \(\mathcal {Z} \) is the adversary \(\mathcal {A} \). Subsequently the environment may spawn (or activate if they are already spawned) parties \(P_i\in \mathcal {U}\). The control program maintains a counter in each sequence of activations and matches it with the current round that is maintained by the diffusion functionality. Each time an honest party diffuses a message containing the label “\(\mathtt {ready}\)” the control program C increases the ready counter for the round. In round r, the control program C will enable the adversary \(\mathcal {A} \) to complete the round, only provided that (i) exactly \(n_r\) parties have transmitted \(\mathtt {ready}\) message, (ii) the number of (“corrupt”) parties controlled by \(\mathcal {A} \) should match \(t_r\).

Parties, when activated, are able to read their input tape \(\mathrm{I}\textsc {nput}()\) and communication tape \(\mathrm{R}\textsc {eceive}()\) from the diffusion functionality. Observe that parties are unaware of the set of activated parties. The Bitcoin backbone protocol requires from parties (miners) to calculate a POW. This is modeled in [11] as parties having access to the oracle \(H(\cdot )\). The fact that (active) parties have limited ability to produce such POWs, is captured as in [11] by the random oracle functionality and the fact that it paces parties to query a limited number of queries per round. The bound, q, is a function of the security parameter \(\kappa \); in this sense the parties may be called q-bounded4. We refer to the above restrictions on the environment, the parties and the adversary as the dynamic q -bounded synchronous setting.

The term \(\{\textsc {view} ^{P, \mathbf {t},\mathbf {n}}_{\varPi , \mathcal {A},\mathcal {Z}}(z)\}_{z\in \{0,1\}^*}\) denotes the random variable ensemble describing the view of party P after the completion of an execution running protocol \(\varPi \) with environment \(\mathcal {Z} \) and adversary \(\mathcal {A} \), on input \(z\in \{0,1\}^*\). We will only consider a “standalone” execution without any auxiliary information and we will thus restrict ourselves to executions with \(z = 1^\kappa \). For this reason we will simply refer to the ensemble by \(\textsc {view} ^{P,\mathbf {t},\mathbf {n}}_{\varPi ,\mathcal {A},\mathcal {Z}}\). The concatenation of the view of all parties ever activated in the execution is denoted by \(\textsc {view} ^{\mathbf {t},\mathbf {n}}_{\varPi , \mathcal {A},\mathcal {Z}}\).

Properties of Protocols. In our theorems we will be concerned with properties of protocols \(\varPi \) running in the above setting. Such properties will be defined as predicates over the random variable \(\textsc {view} ^{ \mathbf {t},\mathbf {n} }_{\varPi , \mathcal {A},\mathcal {Z}}\) by quantifying over all possible adversaries \(\mathcal {A} \) and environments \(\mathcal {Z} \). Note that all our protocols will only satisfy properties with a small probability of error in \(\kappa \) as well as in a parameter k that is selected from \(\{1,\ldots ,\kappa \}\) (with foresight we note that in practice would be able to choose k to be much smaller than \(\kappa \), e.g., \(k=6\)).

The protocol class that we will analyze will not be able to preserve its properties for arbitrary sequences of parties. To restrict the way the sequence \(\mathbf {n}\) is fluctuating we will introduce the following class of sequences.

Definition 1

For \(\gamma \in \mathbb {R}^+\), we call a sequence \((n_r)_{r \in \mathbb {N}}\) \((\gamma ,s)\) -respecting if for any set S of at most s consecutive rounds, \(\max _{r\in S} n_r \le \gamma \cdot \min _{r\in S} n_r\).

Observe that the above definition is fairly general and also can capture exponential growth; e.g., by setting \(\gamma =2 \) and \(s=10\), it follows that every 10 rounds the number of ready parties may double. Note that this will not lead to an exponential running time overall since the total run time is bounded by a polynomial in \(\kappa \), (due to the fact that \((\mathcal {Z}, C)\) is a system of ITM’s, \(\mathcal {Z} \) is locally polynomial bounded, C is a polynomial-time program, and thus [5, Proposition 3] applies).

More formally, a protocol \(\varPi \) would satisfy a property Q for a certain class of sequences \(\mathbf {n}, \mathbf {t}\), provided that for all PPT \(\mathcal {A} \) and locally polynomial bounded \(\mathcal {Z} \), it holds that \(Q(\textsc {view} ^{\mathbf {t},\mathbf {n}}_{\varPi , \mathcal {A},\mathcal {Z}})\) is true with overwhelming probability of the coins of \(\mathcal {A},\mathcal {Z} \) and the random oracle functionality.

In this paper, we will be interested in \((\gamma , s)\)-respecting sequences \(\mathbf {n}\), sequences \(\mathbf {t}\) suitably restricted by \(\mathbf {n}\), and protocols \(\varPi \) suitably parameterized given \(\mathbf {n}, \mathbf {t}\).

3 Blockchains of Variable Difficulty

We start by introducing blockchain notation; we use similar notation to [10], and expand the notion of blockchain to explicitly include timestamps (in the form of a round indicator). Let \(G(\cdot )\) and \(H(\cdot )\) be cryptographic hash functions with output in \(\{0,1\}^\kappa \). A block with target \(T \in \mathbb {N}\) is a quadruple of the form \(B=\langle r, st, x, ctr\rangle \) where \(st\in \{0,1\}^\kappa , x \in \{0,1\}^*\), and \(r,ctr\in \mathbb {N}\) are such that they satisfy the predicate \(\mathsf {validblock}^T_q(B)\) defined as
$$\begin{aligned} ( H( ctr, G(r, st, x)) < T ) \wedge (ctr\le q). \end{aligned}$$
The parameter \(q \in \mathbb {N}\) is a bound that in the Bitcoin implementation determines the size of the register ctr; as in [10], in our treatment we allow q to be arbitrary, and use it to denote the maximum allowed number of hash queries in a round (cf. Sect. 2). We do this for convenience and our analysis applies in a straightforward manner to the case that ctr is restricted to the range \(0 \le ctr <2^{32}\) and q is independent of ctr.

A blockchain, or simply a chain is a sequence of blocks. The rightmost block is the head of the chain, denoted \(\mathrm {head}(\mathcal {C})\). Note that the empty string \(\varepsilon \) is also a chain; by convention we set \(\mathrm {head}(\varepsilon ) = \varepsilon \). A chain \(\mathcal {C}\) with \(\mathrm {head}(\mathcal {C}) = \langle r, st,x,ctr\rangle \) can be extended to a longer chain by appending a valid block \(B = \langle r', st', x', ctr' \rangle \) that satisfies \(st' = H( ctr, G(r , st,x) )\) and \(r'>r\), where \(r'\) is called the timestamp of block B. In case \(\mathcal {C}=\varepsilon \), by convention any valid block of the form \(\langle r', st',x', ctr'\rangle \) may extend it. In either case we have an extended chain \(\mathcal {C}_\mathsf {new} = \mathcal {C}B\) that satisfies \(\mathrm {head}(\mathcal {C}_\mathsf {new}) = B\).

The length of a chain \(\mathop {\mathrm {len}}(\mathcal {C})\) is its number of blocks. Consider a chain \(\mathcal {C}\) of length \(\ell \) and any nonnegative integer k. We denote by \(\mathcal {C}^{\lceil k}\) the chain resulting from “pruning” the k rightmost blocks. Note that for \(k\ge \mathop {\mathrm {len}}(\mathcal {C})\), \(\mathcal {C}^{\lceil k}=\varepsilon \). If \(\mathcal {C}_1\) is a prefix of \(\mathcal {C}_2\) we write \(\mathcal {C}_1 \preceq \mathcal {C}_2\).

Given a chain \(\mathcal {C}\) of length \(\mathop {\mathrm {len}}(\mathcal {C}) = \ell \), we let \(\mathbf x_\mathcal {C}\) denote the vector of \(\ell \) values that is stored in \(\mathcal {C}\) and starts with the value of the first block. Similarly, \(\mathbf r_\mathcal {C}\) is the vector that contains the timestamps of the blockchain \(\mathcal {C}\).

For a chain of variable difficulty, the target T is recalculated for each block based on the round timestamps of the previous blocks. Specifically, there is a function \(D: \mathbb {Z}^* \rightarrow \mathbb {R}\) which receives an arbitrary vector of round timestamps and produces the next target. The value \(D(\varepsilon )\) is the initial target of the system. The difficulty of each block is measured in terms of how many times the block is harder to obtain than a block of target \(T_0\). In more detail, the difficulty of a block with target T is equal to \(T_0/T\); without loss of generality we will adopt the simpler expression 1 / T (as \(T_0\) will be a constant across all executions). We will use \(\mathrm {diff}(\mathcal {C})\) to denote the difficulty of a chain. This is equal to the sum of the difficulties of all the blocks that comprise the chain.

The Target Calculation Function. Intuitively, the target calculation function \(D(\cdot )\) aims at maintaining the block production rate constant. It is parameterized by \(m\in \mathbb {N}\) and \(f\in (0,1)\); Its goal is that m blocks will be produced every m / f rounds. We will see in Sect. 6 that the probability f(Tn) with which n parties produce a new block with target T is approximated by
$$\begin{aligned} f(T,n)\approx \frac{qTn}{2^\kappa }. \end{aligned}$$
(Note that \(T/2^{\kappa }\) is the probability that a single player produces a block in a single query.)
To achieve the above goal Bitcoin tries to keep \({qTn}/{2^\kappa }\) close to f. To that end, Bitcoin waits for m blocks to be produced and based on their difficulty and how fast these blocks were computed it computes the next target. More specifically, say the last m blocks of a chain \(\mathcal {C}\) are for target T and were produced in \(\varDelta \) rounds. Consider the case where a number of players
$$\begin{aligned} n(T,\varDelta )=\frac{2^\kappa m}{qT\varDelta } \end{aligned}$$
attempts to produce m blocks of target T; note that it will take them approximately \(\varDelta \) rounds in expectation. Intuitively, the number of players at the point when m blocks were produced is estimated by \(n(T,\varDelta )\); then the next target \(T'\) is set so that \(n(T,\varDelta )\) players would need m / f rounds in expectation to produce m blocks of target \(T'\). Therefore, it makes sense to set
$$\begin{aligned} T'=\frac{\varDelta }{m/f}\cdot T, \end{aligned}$$
because if the number of players is indeed \(n(T,\varDelta )\) and remains unchanged, it will take them m / f rounds in expectation to produce m blocks. If the initial estimate of the number parties is \(n_0\), we will assume \(T_0\) is appropriately set so that \(f\approx q T_0 n_0/2^\kappa \) and then
$$\begin{aligned} T'=\frac{n_0}{n(T,\varDelta )}\cdot T_0. \end{aligned}$$

Remark 1

Recall that in the flat q-bounded setting all parties have the same hashing power (q-queries per round). It follows that \(n_0\) represents the estimated initial hashing power while \(n(T,\varDelta )\) the estimated hashing power during the last m blocks of the chain \(\mathcal {C}\). As a result the new target is equal to the initial target \(T_0\) multiplied by the factor \(n_0/n(T,\varDelta )\), reflecting the change of hashing power in the last m blocks.

Based on the above we give the formal definition of the target (re)calculation function, which is as follows.

Definition 2

For fixed constants \(\kappa ,\tau ,m,n_0,T_0\), the target calculation function \(D:\mathbb {Z}^*\rightarrow \mathbb {R}\) is defined as
$$ D(\varepsilon )=T_0\quad \text {and}\quad D(r_1,\dots ,r_v)= {\left\{ \begin{array}{ll} \frac{1}{\tau }\cdot T &{}\hbox {if } \frac{n_0}{n(T,\varDelta )}\cdot T_0<\frac{1}{\tau }\cdot T \hbox {;}\\ \tau \cdot T&{}\hbox {if } \frac{n_0}{n(T,\varDelta )}\cdot T_0>\tau \cdot T \hbox {;}\\ \frac{n_0}{n(T,\varDelta )}\cdot T_0&{}\hbox {otherwise,}\\ \end{array}\right. } $$
where \(n(T,\varDelta )=2^\kappa m /qT\varDelta \), with \(\varDelta =r_{m'}-r_{m'-m}\), \(T=D(r_1,\dots ,r_{m'-1})\), and \(m'={m\cdot \lfloor v/m\rfloor }\).

In the definition, \((r_1,\dots ,r_v)\) corresponds to a chain of v blocks with \(r_i\) the timestamp of the ith block; \(m',\varDelta ,\) and T correspond to the last block, duration, and target of the last completed epoch, respectively.

Remark 2

A remark is in order about the case \(\frac{n_0}{n(T,\varDelta )}\cdot T_0\notin [\frac{1}{\tau }T,\tau T]\), since this aspect of the definition is not justified by the discussion preceeding Definition 2. At first there may seem to be no reason to introduce such a “dampening filter” in Bitcoin’s target recalculation function and one should let the parties to try collectively to approximate the proper target. Interestingly, in the absence of such dampening, an efficient attack is known [2] (against the common-prefix property). As we will see, this dampening is sufficient for us to prove security against all attackers, including those considered in [2] (with foresight, we can say that the attack still holds but it will take exponential time to mount).

4 The Bitcoin Backbone Protocol with Variable Difficulty

In this section we give a high-level description of the Bitcoin backbone protocol with chains of variable difficulty; a more detailed description, including the pseudocode of the algorithms, is given in the full version. The presentation is based on the description in [11]. We then formulate two desired properties of the blockchain—common prefix and chain quality—for the dynamic setting.

4.1 The Protocol

As in [11], in our description of the backbone protocol we intentionally avoid specifying the type of values/content that parties try to insert in the chain, the type of chain validation they perform (beyond checking for its structural properties with respect to the hash functions \(G(\cdot ),H(\cdot )\)), and the way they interpret the chain. These checks and operations are handled by the external functions \(V(\cdot ), I(\cdot )\) and \(R(\cdot )\) (the content validation function, the input contribution function and the chain reading function, resp.) which are specified by the application that runs “on top” of the backbone protocol. The Bitcoin backbone protocol in the dynamic setting comprises three algorithms.

Chain Validation. The \(\mathsf {validate}\) algorithm performs a validation of the structural properties of a given chain \(\mathcal {C}\). It is given as input the value q, as well as hash functions \(H(\cdot ), G(\cdot )\). It is parameterized by the content validation predicate predicate \(V(\cdot )\) as well as by \(D(\cdot )\), the target calculation function (Sect. 3). For each block of the chain, the algorithm checks that the proof of work is properly solved (with a target that is suitable as determined by the target calculation function), and that the counter ctr does not exceed q. Furthermore it collects the inputs from all blocks, \({\mathbf x}_\mathcal {C}\), and tests them via the predicate \(V(\mathbf x_\mathcal {C})\). Chains that fail these validation procedure are rejected.

Chain Comparison. The objective of the second algorithm, called \(\mathsf {maxvalid}\), is to find the “best possible” chain when given a set of chains. The algorithm is straightforward and is parameterized by a \(\mathsf {max} (\cdot )\) function that applies some ordering to the space of blockchains. The most important aspect is the chains’ difficulty in which case \(\mathsf {max} ( \mathcal {C}_1, \mathcal {C}_2 )\) will return the most difficult of the two. In case \(\mathrm {diff}(\mathcal {C}_1) = \mathrm {diff}(\mathcal {C}_2)\), some other characteristic can be used to break the tie. In our case, \(\mathsf {max} (\cdot , \cdot )\) will always return the first operand to reflect the fact that parties adopt the first chain they obtain from the network.

Proof of Work. The third algorithm, called \(\mathsf {pow}\), is the proof of work-finding procedure. It takes as input a chain and attempts to extend it via solving a proof of work. This algorithm is parameterized by two hash functions \(H(\cdot ),G(\cdot )\) as well as the parameter q. Moreover, the algorithm calls the target calculation function \(D(\cdot )\) in order to determine the value T that will be used for the proof of work. The procedure, given a chain \(\mathcal {C}\) and a value x to be inserted in the chain, hashes these values to obtain h and initializes a counter ctr. Subsequently, it increments ctr and checks to see whether \(H(ctr, h) < T\); in case a suitable ctr is found then the algorithm succeeds in solving the POW and extends chain \(\mathcal {C}\) by one block.

The Bitcoin Backbone Protocol. The core of the backbone protocol with variable difficulty is similar to that in [11], with several important distinctions. First is the procedure to follow when the parties become active. Parties check the \(\mathtt {ready}\) flag they possess, which is false if and only if they have been inactive in the previous round. In case the \(\mathtt {ready}\) flag is false, they diffuse a special message ‘\(\mathbf {Join}\)’ to request the most recent version of the blockchain(s). Similarly, parties that receive the special request message in their \(\mathrm{R}\textsc {eceive}()\) tape broadcast their chains. As before parties, run “indefinitely” (our security analysis will apply when the total running time is polynomial in \(\kappa \)). The input contribution function \(I(\cdot )\) and the chain reading function \(R(\cdot )\) are applied to the values stored in the chain. Parties check their communication tape \(\mathrm{R}\textsc {eceive}()\) to see whether any necessary update of their local chain is due; then they attempt to extend it via the POW algorithm \(\mathsf {pow}\). The function \(I(\cdot )\) determines the input to be added in the chain given the party’s state st, the current chain \(\mathcal {C}\), the contents of the party’s input tape \(\mathrm{I}\textsc {nput}()\) and communication tape Receive(). The input tape contains two types of symbols, \(\mathrm{R}\textsc {ead}\) and \((\mathrm{I}\textsc {nsert}, value)\); other inputs are ignored. In case the local chain \(\mathcal {C}\) is extended the new chain is diffused to the other parties. Finally, in case a Read symbol is present in the communication tape, the protocol applies function \(R(\cdot )\) to its current chain and writes the result onto the output tape Output().

4.2 Properties of the Backbone Protocol with Variable Difficulty

Next, we define the two properties of the backbone protocol that the protocol will establish. They are close variants of the properties in [11], suitably modified for the dynamic q-bounded synchronous setting.

The common prefix property essentially remains the same. It is parameterized by a value \(k\in \mathbb {N}\), considers an arbitrary environment and adversary, and it holds as long as any two parties’ chains are different only in their most recent k blocks. It is actually helpful to define the property between an honest party’s chain and another chain that may be adversarial. The definition is as follows.

Definition 3

(Common-Prefix Property). The common-prefix property \(Q_\mathsf {cp}\) with parameter \(k\in \mathbb {N}\) states that, at any round of the execution, if a chain \(\mathcal {C}\) belongs to an honest party, then for any valid chain \(\mathcal {C}'\) in the same round such that either \(\mathrm {diff}(\mathcal {C}')>\mathrm {diff}(\mathcal {C})\), or \(\mathrm {diff}(\mathcal {C}')=\mathrm {diff}(\mathcal {C})\) and \(\mathrm {head}(\mathcal {C}')\) was computed no later than \(\mathrm {head}(\mathcal {C})\), it holds that \(\mathcal {C}^{\lceil k}\preceq \mathcal {C}'\hbox { and } \mathcal {C}'^{\lceil k}\preceq \mathcal {C}\).

The second property, called chain quality, expresses the number of honest-party contributions that are contained in a sufficiently long and continuous part of a party’s chain. Because we consider chains of variable difficulty it is more convenient to think of parties’ contributions in terms of the total difficulty they add to the chain as opposed to the number of blocks they add (as done in [11]). The property states that adversarial parties are bounded in the amount of difficulty they can contribute to any sufficiently long segment of the chain.

Definition 4

(Chain-Quality Property). The chain quality property \(Q_\mathsf {cq}\) with parameters \(\mu \in \mathbb {R}\) and \(\ell \in \mathbb {N}\) states that for any party P with chain \(\mathcal {C}\) in \(\textsc {view} ^{\mathbf {t},\mathbf {n}}_{\varPi , \mathcal {A},\mathcal {Z}}\), and any segment of that chain of difficulty d such that the timestamp of the first block of the segment is at least \(\ell \) smaller than the timestamp of the last block, the blocks the adversary has contributed in the segment have a total difficulty that is at most \(\mu \cdot d\).

4.3 Application: Robust Transaction Ledger

We now come to the (main) application the Bitcoin backbone protocol was designed to solve. A robust transaction ledger is a protocol maintaining a ledger of transactions organized in the form of a chain \(\mathcal {C}\), satisfying the following two properties.

  • Persistence: Parameterized by \(k\in \mathbb {N}\) (the “depth” parameter), if an honest party P, maintaining a chain \(\mathcal {C}\), reports that a transaction tx is in \(\mathcal {C}^{\lceil k}\), then it holds for every other honest party \(P'\) maintaining a chain \(\mathcal {C}'\) that if \(\mathcal {C}'^{\lceil k}\) contains tx, then it is in exactly the same position.

  • Liveness: Parameterized by \(u,k\in \mathbb {N}\) (the “wait time” and “depth” parameters, resp.), if a transaction tx is provided to all honest parties for u consecutive rounds, then it holds that for any player P, maintaining a chain \(\mathcal {C}\), tx will be in \(\mathcal {C}^{\lceil k}\).

We note that, as in [11], Liveness is applicable to either “neutral” transactions (i.e., those that they are never in “conflict” with other transactions in the ledger), or transactions that are produced by an oracle \(\mathsf {Txgen}\) that produces honestly generated transactions.

5 Overview of the Analysis

Our main goal is to show that the backbone protocol satisfies the properties common prefix and chain quality (Sect. 4.2) in a \((\gamma ,s)\)-respecting environment as an intermediate step towards proving, eventually, that the protocol implements a robust transaction ledger. In this section we present a high-level overview of our approach; the full analysis is then presented in Sect. 6. To prove the aforementioned properties we first characterize the set of typical executions. Informally, an execution is typical if for any set S of consecutive rounds the successes of the adversary and the honest parties do not deviate too much from their expectations and no bad event occurs with respect to the hash function (which we model as a “random oracle”). Using the martingale bound of Theorem 6 we demonstrate that almost all polynomially bounded executions are typical. We then proceed to show that in a typical execution any chain that an honest party adopts (1) contains timestamps that are approximately accurate (i.e., no adversarial block has a timestamp that differs too much by its real creation time) and (2) has a target such that the probability of block production remains near a fixed constant f. Finally, these properties of a typical execution will bring us to our ultimate goal: to demonstrate that a typical execution enjoys the common prefix and the chain quality properties, and therefore one can build on the blockchain a robust transaction ledger (Sect. 4.3). Here we highlight the main steps and the novel concepts that we introduce.

“Good” Executions. In order to be able to talk quantitatively about typical executions, we first introduce the notion of \((\eta ,\theta )\) -good executions, which expresses how well the parties approximate f. Suppose at round r exactly n parties query the oracle with target T. The probability at least one of them will succeed is
$$\begin{aligned} f(T,n)=1-\Bigl (1-\frac{T}{2^\kappa }\Bigr )^{qn}. \end{aligned}$$
For the initial target \(T_0\) and the initial estimate of the number of parties \(n_0\), we denote \(f_0 = f(T_0, n_0)\). Looking ahead, the objective of the target recalculation mechanism is to maintain a target T for each party such that \(f(T, n_r)\approx f_0\) for all rounds r. (For succintness, we will drop the subscript and simply refer to it as f.)

Now, at a round r of an execution E the honest parties might be querying the random oracle for various targets. We denote by \(T_r^{\min }(E)\) and \(T_r^{\max }(E)\) the minimum and maximum over those targets. We say r is a target-recalculation point of a valid chain \(\mathcal {C}\), if there is a block with timestamp r and m exactly divides the number of blocks up to (and including) this block. Consider constants \(\eta \in (0,1]\) and \(\theta \in [1,\infty )\) and an execution E:

Definition 5 (Abridged). A round r is \((\eta ,\theta )\) -good in E if \(\eta f \le f(T_r^{\min }(E),n_r)\) and \(f(T_r^{\max }(E),n_r) \le \theta f\). An execution E is \((\eta ,\theta )\) -good if every round of E was \((\eta ,\theta )\)-good.

We are going to study the progress of the honest parties only when their targets lie in a reasonable range. It will turn out that, with high probability, the honest parties always work with reasonable targets. The following bound will be useful because it gives an estimate of the progress the honest parties have made in an \((\eta ,\theta )\)-good execution. We will be interested in the progress coming from uniquely successful rounds, where exactly one honest party computed a POW. Let \(Q_r\) be the random variable equal to the (maximum) difficulty of such rounds (recall a block with target T has difficulty 1 / T); 0 otherwise. We refer to \(Q_r\) also as “unique” difficulty. We are able to show the following.

Proposition 2 (Informal). If r is an \((\eta ,\theta )\)-good round in an execution E, then \(\mathbf {E}[Q_r(E_{r-1})]\ge (1-\theta f){pn_r}\), where \(Q_r(E_{r-1})\) is the unique difficulty conditioned on the execution so far, and \(p =\frac{q}{2^\kappa }\).

“Per round” arguments regarding relevant random variables are not sufficient, as we need executions with “good” behavior over a sequence of rounds—i.e., variables should be concentrated around their means. It turns out that this is not easy to get, as the probabilities of the experiments performed per round depend on the history (due to target recalculation). To deal with this lack of concentration/variance problem, we introduce the following measure.

Typical Executions. Intuitively, the idea that this notion captures is as follows. Note that at each round of a given execution E the parties perform Bernoulli trials with success probabilities possibly affected by the adversary. Given the execution, these trials are determined and we may calculate the expected progress the parties make given the corresponding probabilities. We then compare this value to the actual progress and if the difference is “reasonable” we declare E typical. Note, however, that considering this difference by itself will not always suffice, because the variance of the process might be too high. Our definition, in view of Theorem 6 (Appendix A), says that either the variance is high with respect to the set of rounds we are considering, or the parties have made progress during these rounds as expected. A bit more formally, for a given random oracle query in an execution E, the history of the execution just before the query takes place, determines the parameters of the distribution that the outcome of this query follows as a POW (a Bernoulli trial). For the queries performed in a set of rounds S, let V(S) denote the sum of the variances of these trials.

Definition 8 (Abridged). An execution E is \((\epsilon ,\eta ,\theta )\)-typical if, for any given set S of consecutive rounds such that V(S) is appropriately bounded from above:
  • The average unique difficulty is lower-bounded by \(\frac{1}{|S|}(\sum _{r\in S}\mathbf {E}[Q_r(E_{r-1})] -\epsilon (1-\theta f)p\sum _{r\in S}n_r)\);

  • the average maximum difficulty is upper-bounded by \(\frac{1}{|S|} (1+\epsilon )p\sum _{r\in S}n_r\);

  • the adversary’s average difficulty of blocks with “easy” targets is upper-bounded by \(\frac{1}{|S|} (1+\epsilon )p\sum _{r\in S}t_r\), while the number of blocks with “hard” targets is bounded below m by a suitable constant; and

  • no “bad events” with respect to the hash function occur (e.g., collisions).

The following is one of the main steps in our analysis.

Proposition 4 (Informal). Almost all polynomially bounded executions (in \(\kappa \)) are typical. The probability of an execution not being typical is bounded by \(\exp (-\varOmega ( \min \{ m, \kappa \}) + \ln L)\) where L is the total run-time.

Recall (Remark 2) that the dynamic setting (specifically, the use of target recalculation functions) offers more opportunities for adversarial attacks [2]. The following important intermediate lemma shows that if a typical execution is good up to a certain point, chains that are privately mined for long periods of time by the adversary will not be adopted by honest parties.

Lemma 2 (Informal). Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. If \(E_{r}\) is \((\eta ,\theta )\)-good, then, no honest party adopts at round \(r+1\) a chain that has not been extended by an honest party for at least \(O(\frac{m}{\tau f})\) consecutive rounds.

An easy corollary of the above is that in typical executions, the honest parties’ chains cannot contain blocks with timestamps that differ too much from the blocks’ actual creation times.

Corollary 1 (Informal). Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. If \(E_{r-1}\) is \((\eta ,\theta )\)-good, then the timestamp of any block in \(E_{r}\) is at most \(O(\frac{m}{\tau f})\) away from its actual creation time (cf. the notion of accuracy in Definition 6).

Additional important results we obtain regarding \((\eta ,\theta )\)-good executions are that their epochs last about as much as they should (Lemma 3), as well as a “self-correcting” property, which essentially says that if every chain adopted by an honest party is \((\eta \gamma ,\smash {\frac{\theta }{\gamma }})\)-good in \(E_{r-1}\) (cf. the notion of a good chain in Definition 5), then \(E_r\) is \((\eta ,\theta )\)-good (Corollary 2). The above (together with several smaller intermediate steps that we omit from this high-level overview) allow us to conclude:

Theorem 1 (Informal). A typical execution in a \((\gamma ,s)\)-respecting environment is \(O(\frac{m}{\tau f})\)-accurate and \((\eta ,\theta )\)-good.

Common Prefix and Chain Quality. Typical executions give us the two desired low-level properties of the blockchain:

Theorems 2 and 3 (Informal). Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Under the requirements of Table 1 (Sect. 6.1), common prefix holds for any \(k\ge \theta \gamma m/ 8 \tau \) and chain quality holds for \(\ell = m/16\tau f\) and \(\mu \le 1-\delta /2\), where for all r, \(t_r < n_r( 1-\delta )\).

Robust Transaction Ledger. Given the above we then prove the properties of the robust transaction ledger:

Theorems 4 and 5 (Informal). Under the requirements of Table 1, the backbone protocol satisfies persistence with parameter \(k=\varTheta (m)\) and liveness with wait time \(u=\varOmega (m+k)\) for depth k.

We refer to Sect. 6 for the full analysis of the protocol.

6 Full Analysis

In this section we present the full analysis and proofs of the backbone protocol and robust transaction ledger application with chains of variable difficulty. The analysis follows at a high level the roadmap presented in Sect. 5.

6.1 Additional Notation, Definitions, and Preliminary Propositions

Our probability space is over all executions of length at most some polynomial in \(\kappa \). Formally, the set of elementary outcomes can be defined as a set of strings that encode every variable of every party during each round of a polynomially bounded execution. We won’t delve into such formalism and leave the details unspecified. We will denote by \(\mathrm{Pr}\) the probability measure of this space. Define also the random variable \(\mathcal {E}\) taking values on this space and with distribution induced by the random coins of all entities (adversary, environment, parties) and the random oracle.

Suppose at round r exactly n parties query the oracle with target T. The probability at least one of them will succeed is
$$\begin{aligned} f(T,n)=1-\Bigl (1-\frac{T}{2^\kappa }\Bigr )^{qn}. \end{aligned}$$
For the initial target \(T_0\) and the initial estimate of the number of parties \(n_0\), we denote \(f_0 = f(T_0, n_0)\). Looking ahead, the objective of the target recalculation mechanism would be to maintain a target T for each party such that \(f(T, n_r)\approx f_0\) for all rounds r. For this reason, we will drop the subscript from \(f_0\) and simply refer to it as f; to avoid confusion, whenever we refer to the function \(f(\cdot ,\cdot )\), we will specify its two operands.

Note that f(Tn) is concave and increasing in n and T. In particular, Fact 2 applies. The following proposition provides useful bounds on f(Tn). For convenience, define \(p=q/2^{\kappa }\).

Proposition 1

For positive integers \(\kappa ,q,T,n\) and f(Tn) defined as above,
$$\frac{pTn}{1+pTn}\le f(T,n)\le {pTn}\le \frac{f(T,n)}{1-f(T,n)},\ \, \hbox {where}\ \,p=\frac{q}{2^\kappa }.$$

Proof

The bounds can be obtained using the inequalities \((1-x)^\alpha \ge 1-x\alpha \), valid for \(x\le 1\) and \(\alpha \ge 1\), and \(e^{-x}\le \frac{1}{1+x}\), valid for \(x\ge 0\).    \(\square \)

At a round r of an execution E the honest parties might be querying the random oracle for various targets. We denote by \(T_r^{\min }(E)\) and \(T_r^{\max }(E)\) the minimum and maximum over those targets. We say r is a target-recalculation point of a valid chain \(\mathcal {C}\), if there is a block with timestamp r and m exactly divides the number of blocks up to (and including) this block.

We now define two desirable properties of executions which will be crucial in the analysis. We will show later that most executions have these properties.

Definition 5

Consider an execution E and constants \(\eta \in (0,1]\) and \(\theta \in [1,\infty )\). A target-recalculation point r in a chain \(\mathcal {C}\) in E is \((\eta ,\theta )\) -good if the new target T satisfies \(\eta f\le f(T,n_r)\le \theta f\). A chain \(\mathcal {C}\) in E is \((\eta ,\theta )\) -good if all its target-recalculation points are \((\eta ,\theta )\) -good. A round r is \((\eta ,\theta )\) -good in E if \(\eta f\le f(T_r^{\min }(E),n_r)\) and \(f(T_r^{\max }(E),n_r)\le \theta f\). We say that E is \((\eta ,\theta )\) -good if every round of E was \((\eta ,\theta )\)-good.

For a round r, the following set of chains is of interest. It contains, besides the chains that the honest parties have, those chains that could potentially belong to an honest party.where \(\mathcal {C}\in E_r\) means that \(\mathcal {C}\) exists and is valid at round r.

Definition 6

Consider an execution E. For \(\epsilon \in [0,\infty )\), a block created at round r is \(\epsilon \) -accurate if it has a timestamp \(r'\) such that \(|r'-r|\le \epsilon \frac{ m}{f}\). We say that \(E_r\) is \(\epsilon \) -accurate if no chain in \(\mathcal {S}_r\) contains a block that is not \(\epsilon \)-accurate. We say that E is \(\epsilon \) -accurate if for every round r in the execution, \(E_r\) is \(\epsilon \)-accurate.

Our next step is to define the typical set of executions. To this end we define a few more quantities and random variables.

In an actual execution E the honest parties may be split across different chains with possibly different targets. We are going to study the progress of the honest parties only when their targets lie in a reasonable range. It will turn out that, with high probability, the honest parties always work with reasonable targets. For a round r, a set of consecutive rounds S, and constant \(\eta \in (0,1)\), let
$$\begin{aligned} T^{(r,\eta )}=\frac{\eta f}{pn_r}\quad \hbox {and}\quad T^{(S,\eta )}=\min _{r\in S}T^{(r,\eta )}. \end{aligned}$$
To expunge the mystery from the definition of \(T^{(r,\eta )}\), note that in an \((\eta ,\theta )\)-good round all honest parties query for target at least \(T^{(r,\eta )}\). We now define for each round r a real random variable \(D_r\) equal to the maximum difficulty among all blocks with targets at least \(T^{(r,\eta )}\) computed by honest parties at round r. Define also \(Q_r\) to equal \(D_r\) when exactly one block was computed by an honest party and 0 otherwise.

Regarding the adversary, we are going to be interested in periods of time during which he has gathered a number of blocks in the order of m. Given that the targets of blocks are variable themselves, it is appropriate to consider the difficulty acquired by the adversary not in a set of consecutive rounds but rather in a set of consecutive adversarial queries that may span a number of rounds but do are not necessarily a multiple of q.

For a set of consecutive queries indexed by a set J, we define the following value that will act as a threshold for targets of blocks that are attempted adversary.
$$\begin{aligned} T^{(J)}=\frac{\eta (1-\delta )(1-2\epsilon )(1-\theta f)}{32\tau ^3\gamma } \cdot \frac{m}{|J|}\cdot 2^\kappa . \end{aligned}$$
Given the above threshold, for \(j\in J\), if the adversary computed at his j-th query a block of difficulty at most \(1/T^{\smash {(J)}}\), then let the random variable \(A^{\smash {(J)}}_j\) be equal to the difficulty of this block; otherwise, let \(A^{\smash {(J)}}_j=0\). The above definition suggests that we collect in \(A^{\smash {(J)}}_j\) the difficulty acquired by the adversary as long as it corresponds to blocks that are not too difficult (i.e., those with targets less than \(T^{(J)}\)). With foresight we note that this will enable a concentration argument for random variable \(A^{\smash {(J)}}_j\). We will usually drop the superscript (J) from A.

Let \(\mathcal {E}_{r-1}\) contain the information of the execution just before round r. In particular, a value \(E_{r-1}\) of \(\mathcal {E}_{r-1}\) determines the targets against which every party will query the oracle at round r, but it does not determine \(D_r\) or \(Q_r\). If E is a fixed execution (i.e., \(\mathcal {E}=E\)), denote by \(D_r(E)\) and \(Q_r(E)\) the value of \(D_r\) and \(Q_r\) in E. If a set of consecutive queries J is considered, then, for \(j\in J\), \(A^{\smash {(J)}}_j(E)\) is defined analogously. In this case we will also write \(\mathcal {E}^{\smash {(J)}}_j\) for the execution just before the j-th query of the adversary.

With respect to the random variables defined above, the following bound will be useful because it gives an estimate of the progress the honest parties have made in an \((\eta ,\theta )\)-good execution. Note that we are interested in the progress coming from uniquely successful rounds, where exactly one honest party computed a POW. The expected difficulty that will be computed by the \(n_r\) honest parties at round r is \(pn_r\). However, the easier the POW computation is, the smaller \(\mathbf {E}[Q_r|\mathcal {E}_{r-1}=E_{r-1}]\) will be with respect to this value. Since the execution is \((\eta ,\theta )\)-good, a POW is computed by the honest parties with probability at most \(\theta f\). This justifies the appearance of \((1-\theta f)\) in the bound.

Proposition 2

If round r is \((\eta ,\theta )\)-good in E, then  \(\mathbf {E}[Q_r|\mathcal {E}_{r-1}=E_{r-1}]\ge (1-\theta f){pn_r}\).

Proof

Let us drop the subscript r for convenience. Suppose that the honest parties were split into k chains with corresponding targets \(T_1\le T_2\le \cdots \le T_k=T^{\max }\). Let also \(n_1,n_2,\dots ,n_k\), with \(n_1+\cdots +n_k=n\), be the corresponding number of parties with each chain. First note that
$$\prod _{j\in [k]}\bigl [1-f(T_j,n_j)\bigr ] \ge \prod _{j\in [k]}\bigl [1-f(T^{\max },n_j)\bigr ] =1-f(T^{\max },n)\ge 1-\theta f,$$
where the first inequality holds because f(Tn) is increasing in T. Proposition 1 now gives
$$ \mathbf {E}[Q_r|\mathcal {E}_{r-1}=E_{r-1}] =\sum _{i\in [k]}\frac{f(T_i,n_i)/T_i}{1-f(T_i,n_i)}\cdot \prod _{j\in [k]}\bigl [1-f(T_j,n_j)\bigr ] \ge (1-\theta f)\sum _{i\in [k]}pn_i.$$
   \(\square \)

The properties we have defined will be shown to hold in a \((\gamma ,s)\)-respecting environment, for suitable \(\gamma \) and s. The following simple fact is a consequence of the definition.

Fact 1

In a \((\gamma ,s)\)-respecting environment, for any set S of consecutive rounds with \(|S|\le s\), any \(S'\subseteq S\), and any \(n\in \{n_r:r\in S\}\),
$$\begin{aligned} \frac{1}{\gamma }\cdot n\le \frac{1}{|S'|}\cdot \sum _{r\in S'}n_r\le \gamma \cdot n. \end{aligned}$$

Proof

The average of several numbers is bounded by their \(\min \) and \(\max \). Furthermore, the definition of \((\gamma ,s)\)-respecting implies \(\min _{r\in S}n_r\ge \frac{1}{\gamma }\max _{r\in S}n_r\ge \frac{1}{\gamma }n\) and \(\max _{r\in S}n_r\le \gamma \min _{r\in S}\le \gamma n\). Thus,
$$\frac{1}{\gamma }\cdot n\le \min _{r\in S}n_r\le \min _{r\in S'}n_r\le \frac{1}{|S'|}\cdot \sum _{r\in S'}n_r \le \max _{r\in S'}n_r\le \max _{r\in S}n_r\le \gamma \cdot n.$$
   \(\square \)

Our analysis involves a number of parameters that are suitably related. Table 1 summarizes them, recalls their definitions and lists all the constraints that they should satisfy.

Remark 3

We remark that for the actual parameterization of the parameters \(\tau ,m,f\) of Bitcoin5, i.e., \(\tau =4,m=2016,f=0.03\), vis-à-vis the constraints of Table 1, they can be satisfied for \(\delta = 0.99, \eta =0.268, \theta =1.995,\epsilon = 2.93\cdot 10^{-8}\), for \(\gamma =1.281\) and \(s = 2.71\cdot 10^{5}\). Given that s measures the number of rounds within which a fluctuation of \(\gamma \) may take place, we have that the constraints are satisfiable for a fluctuation of up to \(28\%\) every approximately 2 months (considering a round to last 18 s).

Table 1.

System parameters and requirements on them. The parameters are as follows: positive integers smL; positive reals \(f,\gamma ,\delta ,\epsilon ,\tau ,\eta ,\theta \), where \(f,\epsilon ,\delta \in (0,1),\) and \(0<\eta \le 1\le \theta \).

\(n_r\): number of honest parties mining in round r

\(t_r\): number of activated parties that are corrupted

\(\delta \): advantage of honest parties, \(\forall r (t_r/n_r<1-\delta )\)

\((\gamma , s)\): determines how the number of parties fluctuates across rounds, cf. Definition 1

f: probability at least one honest party succeeds in a round assuming \(n_0\) parties and target \(T_0\) (the protocol’s initialization parameters)

\(\tau \): the dampening filter, see Definition 2

\((\eta ,\theta )\): lower and upper bound determining the goodness of an execution, cf. Definition 5

\(\epsilon \): quality of concentration of random variables in typical executions, cf. Definition 8

m: the length of an epoch in number of blocks

L: the total run-time of the system

[(R0)] \(\forall r : t_r < (1-\delta ) n_r\)

[(R1)] \(s\ge \frac{\tau m}{f}+\frac{m}{8\tau f}\)

[(R2)] \(\frac{\delta }{2}\ge 2\epsilon +\theta f\)

[(R3)] Open image in new window

[(R4)] \(17(1+\epsilon )\theta \le 8\tau (\gamma -{\theta f})\)

[(R5)] \(9(1+\epsilon )\eta \gamma ^2\le 4(1-\eta \gamma f)\)

[(R6)] \(7\theta (1-\epsilon )(1-\theta f)\ge 8\gamma ^2\)

6.2 Chain-Growth Lemma

We now prove the Chain-growth lemma. This lemma appears already in [11], but it refers to number of blocks instead of difficulty. In [16] the name “chain growth” appears for the first time and the authors explicitly state a chain-growth property.

Informally, this lemma says that honest parties will make as much progress as how many POWs they obtain. Although simple to prove, the chain-growth lemma is very important, because it shows that no matter what the adversary does the honest parties will advance (in terms of accumulated difficulty) by at least the difficulty of the POWs they have acquired.

Lemma 1

Let E be any execution. Suppose that at round u an honest party has a chain of difficulty d. Then, by round \(v+1\ge u\), every honest party will have received a chain of difficulty at least \(\,d+\sum _{r=u}^vD_r(E)\).

Proof

By induction on \(v-u\). For the basis, \(v+1=u\) and \(\,d+\sum _{r=u}^vD_r(E)=d\). Observe that if at round u an honest party has a chain \(\mathcal {C}\) of difficulty d, then that party broadcast \(\mathcal {C}\) at a round earlier than u. It follows that every honest party will receive \(\mathcal {C}\) by round u.

For the inductive step, note that by the inductive hypothesis every honest party has received a chain of difficulty at least \(d'=d+\sum _{r=u}^{v-1}D_r\) by round v. When \(D_v=0\) the statement follows directly, so assume \(D_v>0\). Since every honest party queried the oracle with a chain of difficulty at least \(d'\) at round v, if follows that an honest party successful at round v broadcast a chain of difficulty at least \(d'+D_v=d+\sum _{r=u}^vD_r\).    \(\square \)

6.3 Typical Executions: Definition and Related Proofs

We can now define formally our notion of typical executions. Intuitively, the idea that this definition captures is as follows. Suppose that we examine a certain execution E. Note that at each round of E the parties perform Bernoulli trials with success probabilities possibly affected by the adversary. Given the execution, these trials are determined and we may calculate the expected progress the parties make given the corresponding probabilities. We then compare this value to the actual progress and if the difference is reasonable we declare E typical. Note, however, that considering this difference by itself will not always suffice, because the variance of the process might be too high. Our definition, in view of Theorem 6, says that either the variance is high with respect to the set of rounds we are considering, or the parties have made progress during these rounds as expected.

Beyond the behavior of random variables described above, a typical execution will also be characterized by the absence of a number of bad events about the underlying hash function \(H(\cdot )\) which is used in proofs of work and is modeled as a random oracle. The bad events that are of concern to us are defined as follows; (recall that a block’s creation time is the round that it has been successfully produced by a query to the random oracle either by the adversary or an honest party).

Definition 7

An insertion occurs when, given a chain \(\mathcal {C}\) with two consecutive blocks B and \(B'\), a block \(B^*\) created after \(B'\) is such that \(B,B^*,B'\) form three consecutive blocks of a valid chain. A copy occurs if the same block exists in two different positions. A prediction occurs when a block extends one with later creation time.

Given the above we are now ready to specify what is a typical execution.

Definition 8

(Typical execution). An execution E is \((\epsilon ,\eta ,\theta )\)-typical if the following hold:
  1. (a)
    If, for any set S of consecutive rounds, \(pT^{(S,\eta )}\sum _{r\in S}n_r\ge \frac{\eta m}{16\tau \gamma }\), then
    $$\begin{aligned}\begin{gathered} \sum _{r\in S}Q_r(E)\ge \sum _{r\in S}\mathbf {E}[Q_r|\mathcal {E}_{r-1}=E_{r-1}] -\epsilon (1-\theta f)p\sum _{r\in S}n_r \\ \text { and } \sum _{r\in S}D_r(E)\le (1+\epsilon )p\sum _{r\in S}n_r. \end{gathered}\end{aligned}$$
     
  2. (b)
    For any set J indexing a set of consecutive queries of the adversary we have
    $$\begin{aligned} \sum _{j\in J}A_j(E)\le (1+\epsilon )2^{-\kappa }|J| \end{aligned}$$
    and during these queries the blocks with targets (strictly) less than \(\tau T^{\smash {(J)}}\) that the adversary has acquired are (strictly) less than \(\frac{\eta (1-\epsilon )(1-\theta f)}{32\tau ^2\gamma }\cdot m\).
     
  3. (c)

    No insertions, no copies, and no predictions occurred in E.

     

Remark 4

Note that if J indexes the queries of the adversary in a set S of consecutive rounds, then \(|J|=q\sum _{r\in S}t_r\) and the inequality in Definition 8(b) reads \(\sum _{j\in J}A_j(E)\le (1+\epsilon )p\sum _{r\in S}t_r\).

The next proposition simplify our applications of Definition 8(a).

Proposition 3

Assume E is a typical execution in a \((\gamma ,s)\)-respecting environment. For any set S of consecutive rounds with \(|S|\ge \frac{m}{16\tau f}\),
$$ \sum _{r\in S}D_r\le (1+\epsilon )p\sum _{r\in S}n_r .$$
If in addition, E is \((\eta ,\theta )\)-good, then
$$ \sum _{r\in S}Q_r\ge (1-\epsilon )(1-\theta f)p\sum _{r\in S}n_r $$
and any block computed by an honest party at any round r corresponds to target at least \(T^{(r,\eta )}\), and so contributes to the random variables \(D_r\) and \(Q_r\) (if the r was uniquely successful).

Proof

We first partition S into several parts with size at least \(\frac{m}{16\tau f}\) and at most s. In view of Proposition 2, for both of the inequalities, we only need to verify the ‘if’ part of Definition 8(a) for each part \(S'\) of S. Indeed, by the definition of \(T^{(S',\eta )}\) and Fact 1, \(pT^{(S',\eta )}\sum _{r\in S'}n_r\ge \eta f|S'|/\gamma \ge \frac{\eta m}{16\tau \gamma }\). The last part, in view of the definition of \(T^{(r,\eta )}\), is equivalent to r being \((\eta ,\theta )\)-good.    \(\square \)

Almost all polynomially bounded executions (in \(\kappa \)) are typical:

Proposition 4

Assuming the ITM system \((\mathcal {Z},C)\) runs for L steps, the event “\(\mathcal {E}\hbox { is not typical}\)” is bounded by \(\exp (- \varOmega (\min \{m,\kappa \}) + \ln L)\). Specifically, the bound is \(\exp \bigl \{-\frac{\eta \epsilon ^2(1-2\delta )m}{64\tau ^3\gamma }+2(\ln L +\ln 2)\bigr \}+2^{-\kappa +1+2\log L}\).

Proof

See the full version.    \(\square \)

6.4 Typical Executions are Good and Accurate

Lemma 2

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. If \(E_{r}\) is \((\eta ,\theta )\)-good, then \(\mathcal {S}_{r+1}\) contains no chain that has not been extended by an honest party for at least \(\frac{m}{16\tau f}\) consecutive rounds.

Proof

Suppose—towards a contradiction—\(\mathcal {C}\in \mathcal {S}_{r+1}\) and has not been extended by an honest party for at least \(\frac{m}{16\tau f}\) rounds. Without loss of generality we may assume that \(r+1\) is the first such round.

Let \(r^*\le r\) denote the greatest timestamp among the blocks of \(\mathcal {C}\) computed by honest parties (\(r^*=0\) if none exists). Define \(S=\{r^*+1,\dots ,r\}\) with \(|S|\ge \frac{m}{16\tau f}\) and the index-set of the corresponding set of queries \(J=\{1,\dots ,q\sum _{r\in S}t_r\}\). Suppose that the blocks of \(\mathcal {C}\) with timestamps in S span k epochs with corresponding targets \(T_1,\dots ,T_k\). For \(i\in [k]\) let \(m_i\) be the number of blocks with target \(T_i\) and set \(M=m_1+\cdots +m_k\).

Our plan is to contradict the assumption that \(\mathcal {C}\in \mathcal {S}_{r+1}\), by showing that the honest parties have accumulated more difficulty than the adversary. To be precise, note that the blocks \(\mathcal {C}\) has gained in S sum to \(\sum _{i\in [k]}\frac{m_i}{T_i}\) difficulty. On the other hand, by the Chain-Growth Lemma 1, all the honest parties have advanced during the rounds in S by \(\sum _{r\in S}D_r(E)\ge \sum _{r\in S}Q_r(E)\). Since \(|S|\ge \frac{m}{16\tau f}\), Proposition 3 implies that \(\sum _{r\in S}Q_r(E)\) is at least \((1-\epsilon )(1-\theta f)p\sum _{r\in S}n_r\). Therefore, to obtain a contradiction, it suffices to show that
$$\begin{aligned} \sum _{i\in [k]}\frac{m_i}{T_i}<(1-\epsilon )(1-\theta f)p\sum _{r\in S}n_r. \end{aligned}$$
(1)
We proceed by considering cases on M.
First, suppose \(M\ge 2M'\), where \(M'=\frac{\eta (1-\epsilon )(1-\theta f)}{32\tau ^2\gamma }\cdot m\) (see Definition 8(b)). Partition the part of \(\mathcal {C}\) with these M blocks into \(\ell \) parts, so that each part has the following properties: (1) it contains at most one target-calculation point, and (2) it contains at least \(M'\) blocks with the same target. Note that such a partition exists because \(M\ge 2M'\) and \(M'<m\). For \(i\in [\ell ]\), let \(j_i\in J\) be the index of the query during which the last block of the i-th part was computed. Set \(J_i=\{j_{i-1}+1,\dots ,j_i\}\), with \(j_0=0\). Note that Definition 8(c) implies \(j_{i-1}<j_i\), and this is a partition of J. Recalling Definition 8(b), the sum of the difficulties of all the blocks in the i-th part is at most \(\sum _{j\in J_i}A_j(E)\). This holds because one of the targets is at least \(\tau T^{(J_i)}\) (since more than \(M'\) blocks have been computed in \(J_i\) with this target) and so both are at least \(\smash {T^{(J_i)}}\) (since targets with at most one calculation point between them can differ by a factor at most \(\tau \)). Thus,
$$ \sum _{i\in [k]}\frac{m_i}{T_i} \le \sum _{i\in [\ell ]\atop j\in J_i}A_j(E) \le \sum _{i\in [\ell ]}\frac{1+\epsilon }{2^\kappa }|J_i| =(1+\epsilon )p\sum _{r\in S}t_r <(1+\epsilon )(1-\delta )p\sum _{r\in S}n_r ,$$
where in the last step we used Requirement (R0). Requirement (R1) implies \((1+\epsilon )(1-\delta )\le (1-\epsilon )(1-\theta f)\)); thus, Eq. (1) holds concluding the case \(M\ge 2M'\).
Otherwise, \(k\le 2\) and \(m_1+m_2<2M'\). Let \(S'\) consist of the first \(\frac{m}{16\tau f}\) rounds of S. We are going to argue that in this case Eq. (1) holds even for \(S'\) in the place of S. Since we are in a \((\gamma ,s)\)-respecting environment, by Fact 1, \(\gamma \sum _{r\in S'}n_r\ge n_{r^*}|S'|\). Furthermore, since \(r^*\) is \((\eta ,\theta )\)-good, \(T_1\ge T^{(r^*,\eta )}=\eta f/pn_{r^*}\). Recalling also that \(T_2\ge T_1/\tau \), we have \(\frac{m_1}{T_1}+\frac{m_2}{T_2}\le \frac{m_1+\tau m_2}{T_1}\), which in turn is at most
$$ \frac{\tau M}{T^{(r^*,\eta )}} <\frac{2\tau M'pn_{r^*}}{\eta f} \le \frac{2\tau \gamma M'p\sum _{r\in S'}n_r}{\eta f|S'|} \le \frac{32\tau ^2\gamma M'p\sum _{r\in S}n_r}{\eta m} $$
and, after substituting \(M'\), Eq. (1) holds concluding this case and the proof.    \(\square \)

Corollary 1

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. If \(E_{r-1}\) is \((\eta ,\theta )\)-good, then \(E_{r}\) is \(\frac{m}{16\tau f}\)-accurate.

Proof

Suppose—towards a contradiction—that, for some \(r^*\le r\), \(\mathcal {C}\in \mathcal {S}_{r^*}\) contains a block which is not \(\frac{m}{16\tau f}\)-accurate and let \(u\le r^*\le r\) be the timestamp of this block and v its creation time. If \(u-v>\frac{m}{16\tau f}\), then every honest party would consider \(\mathcal {C}\) to be invalid during rounds \(v,v+1,\dots ,u\). If \(v-u>\frac{m}{16\tau f}\), then in order for \(\mathcal {C}\) to be valid it should not contain any honest block with timestamp in \(u,u+1,\dots ,v\). (Note that we are using Definition 8(c) here as a block could be inserted later.) In either case, \(\mathcal {C}\in \mathcal {S}_{r^*}\), but has not been extended by an honest party for at least \(\frac{m}{16\tau f}\) rounds. Since \(E_{r^*-1}\) is \((\eta ,\theta )\)-good, the statement follows from Lemma 2.    \(\square \)

Lemma 3

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment and \(r^*\) an \((\eta \gamma ,\frac{\theta }{\gamma })\)-good target-recalculation point of a valid chain \(\mathcal {C}\). For \(r>r^*+\frac{\tau m}{f}\), assume \(E_{r-1}\) is \((\eta ,\theta )\)-good. Then, either the duration \(\varDelta \) of the epoch of \(\mathcal {C}\) starting at \(r^*\) satisfies
$$\begin{aligned} \frac{m}{\tau f}\le \varDelta \le \frac{\tau m}{f}, \end{aligned}$$
or \(\mathcal {C}\notin \mathcal {S}_u\) for each \(u\in \{r^*+\frac{\tau m}{f},\ldots ,r\}\).

Proof

Let T be the target of the epoch in question.

For the upper bound, assume \(\varDelta >\frac{\tau m}{f}\). We show first that in the rounds \(S=\{r^*+\frac{m}{16\tau f},\dots ,r^*+\frac{\tau m}{f}-\frac{m}{16\tau f}\}\) the honest parties have acquired more than \(\frac{m}{T}\) difficulty. Note that the rounds of S are \((\eta ,\theta )\)-good as they come before r. Thus, by Proposition 3, the difficulty acquired in S by the honest parties is at least
$$ (1-\epsilon )(1-\theta f)p\sum _{r\in S}n_r \ge (1-\epsilon )(1-\theta f)p\cdot \frac{|S|n_{r^*}}{\gamma }\ge (1-\epsilon )(1-\theta f)|S|\frac{\eta f}{T} >\frac{m}{T}. $$
For the first inequality, we used Fact 1. For the second, recall that \(r^*\) is Open image in new window -good and so \(pTn_{r^*}\ge f(T,n_{r^*})\ge \eta \gamma f\). For the last inequality observe that Open image in new window and thus follows from Requirement (R3).

Next, we observe that chain \(\mathcal {C}\) either has a block within the epoch in question that is computed by an honest party in a round within the period \([r^*,r^*+\frac{m}{16\tau f})\), or by Lemma 2, \(\mathcal {C}\notin \mathcal {S}_u\) for each \(u\in \{r^*+\frac{m}{16\tau f},\ldots ,r\}\supseteq \{r^*+\frac{\tau m}{f},\ldots ,r\}\). Assuming the first happens, it follows that by round \(r^*+\frac{\tau m}{f}-\frac{m}{16\tau f}\) the honest parties’ chains have advanced by an amount of difficulty which exceeds the total difficulty of the epoch in question. This means that no honest party will extend \(\mathcal {C}\) during the rounds \(\{r^*+\frac{\tau m}{f}-\frac{m}{16\tau f}+1,\dots ,\varDelta \}\). Since it is assumed \(\varDelta >r^*+\frac{\tau m}{f}\), Lemma 2 can then be applied to imply that \(\mathcal {C}\notin \mathcal {S}_u\) for \(u\in \{r^*+\frac{\tau m}{f},\dots ,r\}\).

For the lower bound, we assume \(\varDelta <\frac{m}{\tau f}\) and that \(\mathcal {C}\in \mathcal {S}_u\) for some \(u\in \{r^*+\varDelta +1,\dots ,r\}\), and seek a contradiction. Clearly, the honest parties contributed only during the set of rounds \(S=\{r^*,\dots ,r^*+\varDelta \}\). The adversary, by Lemma 2, may have contributed only during \(S'=\{r^*-\frac{m}{16\tau f},\dots ,r^*+\varDelta +\frac{m}{16\tau f}\}\). Let J be the set of queries available to the adversary during the rounds in \(S'\). We show that in a typical execution the honest parties together with the adversary cannot acquire difficulty \(\frac{m}{T}\) in the rounds in the sets S and \(S'\) respectively. With respect to the honest parties, Proposition 3 applies. Regarding the adversary, assume first \(T\ge T^{(J)}\) (it is not hard to verify that the case \(T<T^{(J)}\) leads to a more favorable bound). It follows that the total difficulty contributed to the epoch is at most
$$ (1+\epsilon )p\biggl (\sum _{r\in S}n_r+\sum _{r\in S'}t_r\biggr ) \le (1+\epsilon )p\gamma n_{r^*}(|S|+|S'|) <(1+\epsilon )p\gamma n_{r^*}\cdot \frac{17m}{8\tau f} .$$
The first inequality follows from Fact 1 using \(t_r<(1-\delta )n_r\). For the second substitute the upper bounds on the sizes of S and \(S'\). Next, note that \(r^*\) is an Open image in new window -good recalculation point and so Open image in new window . By Proposition 1, Open image in new window . It follows that the last displayed quantity is at most \(\frac{17(1+\epsilon )\theta }{8\tau (\gamma -{\theta f})}\cdot \frac{m}{T}\) and recalling Requirement (R4) this less than \(\frac{m}{T}\) as desired.    \(\square \)

Proposition 5

Assume E is a typical execution in a \((\gamma ,s)\)-respecting environment. Consider a round r and a set of consecutive rounds S with \(|S|\ge \frac{m}{32\tau ^2f}\). If \(E_{r-1}\) is \((\eta ,\theta )\)-good, then the adversary, during the rounds in S, has contributed at most \((1-\delta )(1+\epsilon )p\sum _{r\in S}n_r\) difficulty to \(\mathcal {S}_r\).

Proof

Without loss of generality, we will assume in this proof that \(t_r=(1-\delta )n_r\) for each \(r\in S\). Furthermore, we assume \(|S|\le \frac{\tau m}{f}\). If this is not the case, then we can partition S to parts of appropriate sizes and apply the arguments that follow to each sum. The statement will follow upon summing over all parts.

By Lemma 2, for any block B in \(\mathcal {S}_r\), there is a block in the same chain and computed at most \(\frac{m}{16\tau f}\) rounds earlier than it. By Lemma 3, there is at most one recalculation point between them. Let u be the round the honest party computed this block and T its target. Note that since E is \((\eta ,\theta )\)-good, \(T\ge T^{(u,\eta )}=\frac{\eta f}{pn_u}\) and the target of B is at least \(\tau ^{(-1)}T\). We are going to show that, with J the set of queries that correspond to S, we have \(\tau ^{-1}T\ge T^{\smash {(J)}}\). This will suffice, because \((1-\delta )(1+\epsilon )p\sum _{r\in S}n_r\ge (1+\epsilon )p\sum _{r\in S}t_r\), and this is at least \(\sum _{j\in J}A_j\) in a typical execution (Definition 8(b)).

Note first that, using Fact 1 and the lower-bound on |S|,
$$ 2^{-\kappa }|J| =(1-\delta )p\sum _{r\in S}n_r \ge (1-\delta )p\frac{|S|n_u}{\gamma }\ge (1-\delta )p\frac{mn_u}{32\tau ^3f\gamma } .$$
Recalling the definition of \(T^{(J)}\) and using this bound,
$$ T^{(J)}=\frac{\eta (1-\delta )(1-2\epsilon )(1-\theta f)}{32\tau ^3\gamma }\cdot \frac{m}{|J|}\cdot 2^\kappa \le \frac{\eta f(1-2\epsilon )(1-\theta f)}{\tau pn_u} <\frac{T^{(u,\eta )}}{\tau }\le \frac{T}{\tau },$$
as desired.    \(\square \)

Lemma 4

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment and assume \(E_{r-1}\) is \((\eta ,\theta )\)-good. If \(\mathcal {C}\in \mathcal {S}_r\), then \(\mathcal {C}\) is Open image in new window -good in \(E_r\).

Proof

Note that it is our assumption that every chain is Open image in new window -good at the first round. Therefore, to prove the statement, it suffices to show that if a chain is Open image in new window -good at a recalculation point \(r^*\), then it will also be Open image in new window -good at then next recalculation point \(r^*+\varDelta \).

Let \(r^*\) and \(r^*+\varDelta \le r\) be two consecutive target-calculation points of a chain \(\mathcal {C}\) and T the target of the corresponding epoch. By Lemma 3 and Definition 2 of the target-recalculation function, the new target will be
$$\begin{aligned} T'=\frac{\varDelta }{m/f}\cdot T, \end{aligned}$$
where \(\varDelta \) is the duration of the epoch.
We wish to show that
$$\begin{aligned} \eta \gamma f\le f(T',n_{r^*+\varDelta })\le {\theta f}/\gamma . \end{aligned}$$
To this end, let \(S=\{r^*,\dots ,r^*+\varDelta \}\), \(S'=\bigl \{\max \{0,r^*-\frac{m}{16\tau f}\},\dots ,\min \{r^*+\varDelta +\frac{m}{16\tau f},r\}\bigr \}\), and let J index the queries available to the adversary in \(S'\). Note that, by Corollary 1, every block in the epoch was computed either by an honest party during a round in S or by the adversary during a round in \(S'\).
Suppose—towards a contradiction—that \(f(T',n_{r^*+\varDelta })<\eta \gamma f\). Using the definition of f(Tn), this implies \({qn_{r^*+\varDelta }}\ln \bigl (1-\frac{T'}{2^\kappa }\bigr )>\ln (1-\eta \gamma f).\) Applying the inequality \(-\frac{x}{1-x}<\ln (1-x)<-x\), valid for \(x\in (0,1)\), substituting the expression for \(T'\) above and rearranging, we obtain
$$\begin{aligned} \frac{m}{T}>\frac{1-\eta \gamma f}{\eta \gamma }\cdot p\varDelta n_{r^*+\varDelta }. \end{aligned}$$
By Propositions 3 and 5 it follows that
$$ \frac{m}{T} \le 2(1+\epsilon )p\sum _{r\in S'}n_r \le 2(1+\epsilon )p\cdot \frac{\varDelta +\frac{m}{8\tau f}}{|S'|}\cdot \sum _{r\in S'}n_r. $$
By Lemma 3, \(\varDelta \ge \frac{m}{\tau f}\). Thus, \(\frac{\varDelta +\frac{m}{8\tau f}}{\varDelta }\le \frac{9}{8}\). Using this, Requirement (R5), and combining the inequalities on \(\frac{m}{T}\),
$$ \gamma n_{r^*+\varDelta } <\frac{9(1+\epsilon )\eta \gamma ^2}{4(1-\eta \gamma f)}\cdot \frac{1}{|S'|}\sum _{r\in S'}n_r \le \frac{1}{|S'|}\sum _{r\in S'}n_r, $$
contradicting Fact 1.
For the upper bound, assume Open image in new window , which (see Proposition 1) implies
$$\begin{aligned} \frac{m}{T}<\frac{\gamma }{\theta }\cdot p\varDelta n_{r^*+\varDelta }. \end{aligned}$$
Set \(S=\{r^*+\frac{m}{16\tau f},\dots ,r^*+\varDelta -\frac{m}{16\tau f}\}\). Since an honest party posses \(\mathcal {C}\) at round r, it follows by Lemma 2 that there is a block computed by an honest party in \(\mathcal {C}\) during \(\{r^*,\dots ,r^*+\frac{m}{16\tau f}-1\}\) and one during \(\{r^*+\varDelta -\frac{m}{16\tau f}+1,\dots ,r^*+\varDelta \}\). By the Chain-Growth Lemma 1, it follows that the honest parties computed less than \(\frac{m}{T}\) difficulty during S. In particular,
$$ \frac{m}{T} >(1-\epsilon )(1-\theta f)p\sum _{r\in S}n_r \ge (1-\epsilon )(1-\theta f)p\cdot \frac{\varDelta -\frac{m}{8\tau f}}{|S|}\cdot \sum _{r\in S}n_r .$$
By Lemma 3, \(\varDelta \ge \frac{m}{\tau f}\). Thus, \(\frac{\varDelta -\frac{m}{8\tau f}}{\varDelta }\ge \frac{7}{8}\). Using this, Requirement (R6), and combining the inequalities on \(\frac{m}{T}\),
$$ \frac{n_{r^*+\varDelta }}{\gamma }>\frac{7\theta }{8\gamma ^2}(1-\epsilon )(1-\theta f)\cdot \frac{1}{|S|}\sum _{r\in S}n_r \ge \frac{1}{|S|}\sum _{r\in S}n_r ,$$
contradicting Fact 1.   \(\square \)

Corollary 2

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment and \(E_{r-1}\) be \((\eta ,\theta )\)-good. If every chain in \(\mathcal {S}_{r-1}\) is \((\eta \gamma ,\smash {\frac{\theta }{\gamma }})\)-good, then \(E_r\) is \((\eta ,\theta )\)-good.

Proof

We use notations and definitions of Lemma 3. Let \(\mathcal {C}\mathcal {S}_r\) and let \(r^*\) be its last recalculation point in \(E_{r-1}\). Let T be the target after \(r^*\) and \(T'\) the one at r. We need to show that \(f(T',n_r)\in [\eta f,\theta f]\). Note that if r is a recalculation point, this follows by Lemma 4. Otherwise, \(T'=T\) and \(\eta \gamma \le f(T,n_{r^*})\le \theta f/\gamma \). Using Lemma 3, \(r-r^*\le \varDelta \le \frac{\tau m}{f}\). Thus, \(\frac{1}{\gamma }n_{r^*}\le n_r\le \gamma n_{r^*}\). By Fact 2 we have \(f(T,n_r)\le f(T,\gamma n_{r^*})\le \gamma f(T,n_{r^*})\le \theta f\) and \(f(T,n_r)\ge f(T,{\textstyle \frac{1}{\gamma }}n_{r^*})\ge {\textstyle \frac{1}{\gamma }}f(T,n_{r^*})\ge \eta f.\)    \(\square \)

Corollary 3

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Then every round is \((\eta ,\theta )\)-good in E.

Proof

For the sake of contradiction, let r be the smallest round of E that is not \((\eta ,\theta )\)-good. This means that there is a chain \(\mathcal {C}\) and an honest party that possesses this chain in round r and the corresponding target T is such that \(f(T,n_r) \not \in [\eta f, \theta f]\). Note that \(E_{r-1}\) is \((\eta ,\theta )\)-good, and so, by Corollary 1, \(E_{r}\) is \(\frac{m}{16\tau f}\)-accurate. Let \(r^*<r\) be the last Open image in new window -good recalculation point of \(\mathcal {C}\) (let \(r^*\) be 0 in case there is no such point).

First suppose that there is another recalculation point \(r'\in (r^*,r]\). By the definition of \(r^*\), \(r'\) is not Open image in new window -good. However, the assumptions of Lemma 4 hold, implying that \(\mathcal {C}\) is Open image in new window -good. We have reached a contradiction.

We may now assume that there is no recalculation point in \((r^*,r]\) and so the points \(r^*\) and r correspond to the same target T with \(\eta \gamma \le f(T,n_{r^*})\le \theta f/\gamma \). Note that since \(r^*\) is an Open image in new window -good recalculation point and \(E_{r-1}\) is \((\eta ,\theta )\)-good, we have \(r-r^*\le \frac{\tau m}{f}\). This follows from Lemma 3, because \(\mathcal {C}\) belongs to an honest party at round r. Thus, \(\frac{1}{\gamma }n_{r^*}\le n_r\le \gamma n_{r^*}\), and so (by Fact 2) \(f(T,n_r)\le f(T,\gamma n_{r^*})\le \gamma f(T,n_{r^*})\le \theta f\) and \(f(T,n_r)\ge f(T,{\textstyle \frac{1}{\gamma }}n_{r^*})\ge {\textstyle \frac{1}{\gamma }}f(T,n_{r^*})\ge \eta f.\)    \(\square \)

Theorem 1

A typical execution in a \((\gamma ,s)\)-respecting environment is \(\frac{m}{16\tau f}\)-accurate and \((\eta ,\theta )\)-good.

Proof

This follows from Corollaries 3 and 1.    \(\square \)

6.5 Common Prefix and Chain Quality

Proposition 6

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Any \(\frac{\theta \gamma m}{8\tau }\) consecutive blocks in an epoch of a chain \(\mathcal {C}\in \mathcal {S}_r\) have been computed in at least \(\frac{m}{16\tau f}\) rounds.

Proof

Suppose—towards a contradiction—that the blocks of \(\mathcal {C}\) where computed during the rounds in \(S^*\), for some \(S^*\) such that \(|S^*|<\frac{m}{16\tau f}\). Consider an S such that \(S^*\subseteq S\) and \(|S|=\frac{m}{16\tau f}\) and the property that a block of target T in \(\mathcal {C}\) was computed by an honest party in some round \(v\in S\). Such an S exists by Lemmas 2 and 3. By Propositions 3 and 5, the number of blocks of target T computed in S is at most
$$ (1+\epsilon )(2-\delta )pT\sum _{u\in S}n_u \le (1+\epsilon )(2-\delta )pT\gamma n_v|S| \le \frac{(1+\epsilon )(2-\delta )\gamma |S|\theta f}{1-\theta f} \le \frac{\theta \gamma m}{8\tau } .$$
For the first inequality we used Fact 1, for the second Fact 1 and that round v is \((\eta ,\theta )\)-good, and for the last one Requirement (R2).    \(\square \)

Let us say that two chains \(\mathcal {C}\) and \(\mathcal {C}'\) diverge before round r, if the timestamp of the last block on their common prefix is less than r.

Lemma 5

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Any \(\mathcal {C},\mathcal {C}'\in \mathcal {S}_r\) do not diverge before round \(r-\frac{m}{16\tau f}\).

Proof

Consider the last block on the common prefix of \(\mathcal {C}\) and \(\mathcal {C}'\) that was computed by an honest party and let \(r^*\) be the round on which it was computed (set \(r^*=0\) if no such block exists). Denote by \(\mathcal {C}^*\) the common part of \(\mathcal {C}\) and \(\mathcal {C}'\) up to (and including) this block and let \(d^*=\mathrm {diff}(\mathcal {C}^*)\) and \(S=\{i:r^*<u<r\}\). We claim that
$$\begin{aligned} (1+\epsilon )(1-\delta )p\sum _{u\in S}n_u\ge \sum _{u\in S}Q_u. \end{aligned}$$
(2)
In view of Proposition 5, it suffices to show that the difficulty which the adversary contributed to \(\mathcal {C}\) and \(\mathcal {C}'\) is at least the right-hand side of (2). The proof of this rests on the following observation.

Consider any block B extending a chain \(\mathcal {C}_1\) that was computed by an honest party in a uniquely successful round \(u\in S\). Consider also an arbitrary \(d\in \mathbb {R}\) such that \(\mathrm {diff}(\mathcal {C}_1)\le d<\mathrm {diff}(\mathcal {C}_1B)\). We are going to argue that if another chain of difficulty at least d exists, then the block that “contains” the point of difficulty d was computed by the adversary. More formally, suppose a chain \(\mathcal {C}_2B'\) exists such that \(B'\ne B\) and \(\mathrm {diff}(\mathcal {C}_2)\le d<\mathrm {diff}(\mathcal {C}_2B')\). We observe that \(B'\) was computed by the adversary. This is because no honest party would extend \(\mathcal {C}_2\) at a round later than u since \(\mathrm {diff}(\mathcal {C}_2)\le d<\mathrm {diff}(\mathcal {C}_1B)\); on the other hand, if an honest party computed \(B'\) at some round \(u'<u\), then no honest party would have extended \(\mathcal {C}_1\) at round u since \(\mathrm {diff}(\mathcal {C}_1)\le d<\mathrm {diff}(\mathcal {C}_2B')\); finally, note that u is also ruled out since it was a uniquely successful round by assumption.

Returning to the proof of (2) note that, by the Chain-Growth Lemma 1, \(\mathrm {diff}(\mathcal {C}')\) and \(\mathrm {diff}(\mathcal {C})\) are at least \(d^*+\sum _{u\in S}Q_u\). To show (2) it suffices to argue that for all \(d\in (d^*,\sum _{u\in S}Q_u]\) there is always a \(B'\) as above that lies either on \(\mathcal {C}\), or on \(\mathcal {C}'\), or on their common prefix. But this is always possible since B cannot be both on \(\mathcal {C}\) and \(\mathcal {C}'\) (note that by the definition of \(r^*\), B cannot be on their common prefix). To finish the proof note that (2) contradicts Proposition 3 for large enough S.    \(\square \)

Theorem 2

(Common Prefix). Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. For any round r and any two chains in \(\mathcal {S}_r\), the common-prefix property holds for \(k\ge \frac{\theta \gamma m}{4\tau }\).

Proof

Suppose common prefix fails for two chains \(\mathcal {C}\) and \(\mathcal {C}'\) at round r. At least k / 2 of the blocks in each chain after their common prefix, lie in a single epoch. Proposition 6 implies that \(\mathcal {C}\) and \(\mathcal {C}'\) diverge before round \(r-\frac{m}{16\tau f}\), contradicting Lemma 5.    \(\square \)

Theorem 3

(Chain Quality). Suppose E is a typical execution in a \((\gamma ,s)\)-respecting environment. For the chain of any honest party at any round in E, the chain-quality property holds with parameters \(\ell =\frac{m}{16\tau f}\) and Open image in new window , where \(\lambda =\max \{t_r/n_r\}<(1-\delta )\).

Proof

Let us denote by \(B_i\) the i-th block of \(\mathcal {C}\) so that \(\mathcal {C}=B_1 \dots B_{\mathop {\mathrm {len}}(\mathcal {C})}\) and consider L consecutive blocks \(B_u,\dots ,B_v\). Define \(L'\) as the least number of consecutive blocks \(B_{u'},\dots ,B_{v'}\) that include the L given ones (i.e., \(u'\le u\) and \(v\le v'\)) and have the properties (1) that the block \(B_{u'}\) was computed by an honest party or is \(B_1\) in case such block does not exist, and (2) that there exists a round at which an honest party was trying to extend the chain ending at block \(B_{v'}\). Observe that number \(L'\) is well defined since \(B_{\mathop {\mathrm {len}}(\mathcal {C})}\) is at the head of a chain that an honest party is trying to extend. Denote by \(d'\) the total difficulty of these \(L'\) blocks. Define also \(r_1\) as the round that \(B_{u'}\) was created (set \(r_1=0\) if \(B_{u'}\) is the genesis block), \(r_2\) as the first round that an honest party attempts to extend \(B_{v'}\), and let \(S=\{r:r_1\le r\le r_2\}\). Note that \(|S|\ge \frac{m}{16\tau f}\).

Now let x denote the total difficulty of all the blocks from honest parties that are included in the L blocks and—towards a contradiction—assume that
$$\begin{aligned} x<\Bigl [1-\Bigl (1+\frac{\delta }{2}\Bigr )\lambda \Bigr ]d \le \Bigl [1-\Bigl (1+\frac{\delta }{2}\Bigr )\lambda \Bigr ]d' .\end{aligned}$$
(3)
Suppose first that all the \(L'\) blocks \(\{B_j:u'\le j\le v'\}\) have been computed during the rounds in the set S. Recalling Proposition 5, we now argue the following sequence of inequalities.
$$\begin{aligned} (1+\epsilon )(1-\delta )p\sum _{u\in S}n_u\ge d'-x \ge \Bigl (1+\frac{\delta }{2}\Bigr )\lambda d' \ge \Bigl (1+\frac{\delta }{2}\Bigr )\lambda \sum _{u\in S}Q_u .\end{aligned}$$
(4)
The first inequality follows from the definition of x and \(d'\) and Proposition 5. The second one comes from the relation between x and \(d'\) outlined in (3). To see the last inequality, assume \(\sum _{u\in S}Q_u>d'\). But then, by the Chain-Growth Lemma 1, the assumption than an honest party is on \(B_{v'}\) at round \(r_2\) is contradicted as all honest parties should be at chains of greater length. We now observe that (4) contradicts Proposition 3, since
$$ \Bigl (1+\frac{\delta }{2}\Bigr )\lambda \sum _{u\in S}Q_u >(1-\epsilon )(1-\theta f)\Bigl (1-\frac{\delta }{2}\Bigr )p\sum _{u\in S}n_u \ge (1+\epsilon )(1-\delta )p\sum _{u\in S}n_u ,$$
where the middle inequality follows by Requirement (R2).

To finish the proof we need to consider the case in which these \(L'\) blocks contain blocks that the adversary computed in rounds outside S. It is not hard to see that this case implies either a prediction or an insertion and cannot occur in a typical execution.    \(\square \)

6.6 Persistence and Liveness

Theorem 4

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Persistence is satisfied with depth \(k\ge \frac{\theta \gamma m}{4\tau }\).

Proof

Suppose an honest party P has at round r a chain \(\mathcal {C}\) such that \(\mathcal {C}^{\lceil k}\) contains a transaction \(\mathrm {tx}\).

We first show that the \(k\ge \smash {\frac{\theta \gamma m}{4\tau }}\) blocks of \(\mathcal {C}\) cannot have been computed in less than \(\smash {\frac{m}{16\tau f}}\) rounds. Suppose—towards a contradiction—that this was the case. By Lemma 3, at least \(\smash {\frac{\theta \gamma m}{8\tau }}\) of the k blocks belong to a single epoch and Proposition 6 is contradicted.

To show persistence, note that if any party \(P'\ne P\) has a chain \(\mathcal {C}'\) at round r and \(\mathcal {C}^{\lceil k}\) is not a prefix of \(\mathcal {C}'\), then Lemma 5 is contradicted. Next, let \(r'>r\) be the first round after r such that an honest party \(P'\) has a chain \(\mathcal {C}'\) such that \({\mathcal {C}^{\lceil k}}\) is not a prefix of \(\mathcal {C}'\). By the note above and the minimality of \(r'\) it follows that no honest party had a prefix of \(\mathcal {C}'\) at round \(r'-1\). Thus, \(\mathcal {C}'\) existed at round \(r'-1\) and \(P'\) had another chain \(\mathcal {C}''\) at that round such that \(\mathcal {C}^{\lceil k}\preceq \mathcal {C}''\) and \(\mathrm {diff}(\mathcal {C}'')<\mathrm {diff}(\mathcal {C}')\). We now observe that \(\mathcal {C}'\) and \(\mathcal {C}''\) contradict Lemma 5 at round \(r'-1\).    \(\square \)

Theorem 5

Let E be a typical execution in a \((\gamma ,s)\)-respecting environment. Liveness is satisfied for depth k with wait-time \(\frac{m}{16\tau f}+\frac{\gamma k}{\eta f(1-\epsilon )(1-\theta f)}\).

Proof

Suppose a transaction \(\mathrm {tx}\) is included in any block computed by an honest party for \(\smash {\frac{m}{16\tau f}}\) consecutive rounds and let S denote the set of \(\smash {\frac{\gamma k}{\eta f(1-\epsilon )(1-\theta f)}}\) rounds that follow these rounds. Consider now the chain \(\mathcal {C}\) of an arbitrary honest party after the rounds in S. By Lemma 2, \(\mathcal {C}\) contains an honest block computed in the \(\frac{m}{16\tau f}\) rounds. This block contains \(\mathrm {tx}\). Furthermore, after the rounds in the set S, on top of this block there has been accumulated at least \(\sum _{r\in S}Q_r\) amount of difficulty. We claim that this much difficulty corresponds to at least k blocks. To show this, assume \(|S|\le s\) (or consider only the first s rounds of S). Let T be the smallest target computed by an honest party during the rounds in S and let u be such a round. It suffices to show \(T\sum _{r\in S}Q_r\ge k\). Indeed,
$$ T\sum _{r\in S}Q_r \ge (1-\epsilon )(1-\theta f)pT\sum _{r\in S}n_r \ge (1-\epsilon )(1-\theta f)\frac{pTn_u|S|}{\gamma }\ge k .$$
The first inequality follows from Proposition 3, the second by Fact 1, and for the last one we substitute the size of S and use that \(pTn_u\ge f(T,n_u)\ge \eta f\) (since u is \((\eta ,\theta )\)-good).    \(\square \)

Footnotes

  1. 1.

    In Bitcoin, solving a proof of work essentially amounts to brute-forcing a hash inequality based on SHA-256.

  2. 2.

    In Bitcoin, m is set to 2016 and roughly corresponds to 2 weeks in real time—assuming the number of parties does not change much.

  3. 3.

    In the latest version of [10], we show that in the case of fixed difficulty, the analysis of the Bitcoin backbone in the synchronous model extends with relative ease to partial synchrony. We leave the extension of the variable-difficulty case for future work.

  4. 4.

    In [11] this is referred to as the “flat-model” in terms of computational power, where all parties are assumed equal. In practice, different parties may have different “hashing power”; note that this does not sacrifice generality since one can imagine that real parties are simply clusters of some arbitrary number of flat-model parties.

  5. 5.

    Note that in order to calculate f, we can consider that a round of full interaction lasts 18 s; If this is combined with the fact that the target is set for a POW to be discovered approximately every 10 min, we have that 18/600 = 0.3 is a good estimate for f.

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Copyright information

© International Association for Cryptologic Research 2017

Authors and Affiliations

  1. 1.Yahoo ResearchSunnyvaleUSA
  2. 2.University of Edinburgh and IOHKEdinburghUK
  3. 3.National and Kapodistrian University of AthensAthensGreece

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