ACCENTS Transactions on Information Security, Vol 2(6) ISSN (Online): 2455-7196 http://dx.doi.org/10.19101/TIS.2017.26003 43 Weighted threshold ECDSA for securing bitcoin wallet Pratyush Dikshit * and Kunwar Singh Department of Computer Science and Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India ©2017 ACCENTS 1.Introduction Bitcoin was introduced in a self-published paper by Satoshi Nakamoto in October, 2008[1, 2]. Bitcoin is a decentralized system which requires no central authority. In recent years, bitcoin has become increasingly accepted and used in many fields in place of physical cash. Bitcoin is a peer-to-peer network of nodes that distribute and record transactions [3]. Bitcoin transaction is a statement that Player 1 (address 1) would like to transfer some bitcoin values  to Player 2 (address 2), signed by Player 1 by his private key. Transactions are verified by network nodes and confirmed in a public distributed ledger called the block chain. The block chain consists of a series of blocks in which each block contains the hashed value of subsequent block. Every bitcoin block contains a set of verified transactions that are collected from the bitcoin broadcast network. It is assumed that the majority of nodes in the bitcoin network are honest. This makes the verification done by the nodes is correct with high probability. More technically, bitcoin is an electronic-cash system based on cryptographic algorithms. *Author for correspondence Although, similar in functionality and purposes, bitcoin transactions are different from traditional banking system in many ways, such as-  Irreversibility: Once a bitcoin transaction is updated in the ledger of block chain, that transaction is irreversible even if it is shown later that the transaction is not correct. (e.g., a stolen private key was used).  Automation: Unlike traditional banking transaction system, bitcoin transaction of any size can be fully automated and can be authorized only with a digital signature.  Pseudonymity: Traditional transaction system carries same name of the user for multiple transactions. In bitcoin context, users transact with different addresses that make them pseudonym. And to achieve privacy, bitcoin addresses transactions do not link together.  Negligible transaction fees: On traditional transaction system, merchants can charge transaction fee that can range from 0:5% to 5%, for each transaction made. Bitcoin transaction can be made at a negligible cost or none at all, as bitcoin fees are based on the amount of bitcoin sent. Research Article Abstract Bitcoin is a digital currency based on cryptographic algorithms. All the transactions of this currency are recorded and stored in a publically available database called block chain. Since, these transactions are available to everyone; bitcoins must be stored in a secured wallet. These bitcoin wallets can be opened only by its secret key. And if once the secret key of the wallet is lost, it cannot be recovered because of the irreversible nature of bitcoin transaction. To root out this problem, researchers have proposed a solution of threshold signature scheme compatible with bitcoins signature by using elliptic curve digital signature algorithm (ECDSA) providing security policy of shared control of a wallet. In that scheme, the number of players in reconstruction phase was important for recovering the signature. Our contribution is to present a weighted threshold scheme with bitcoins ECDSA signature, in which all the players/participants do not have the same weight. More exactly, a positive weight is associated to each player and the signature can be reconstructed if and only if the sum of the weights of all shares is greater than or equal to a fixed threshold. In our scheme, the number of shares in reconstruction phase is important for recovering the signature. We extend the threshold ECDSA scheme to weighted threshold ECDSA scheme. Keywords Cryptography, Shamir secret sharing, Bitcoin, Threshold signature scheme, ECDSA, Bitcoin wallet.