Questions tagged [blockchain]

The Ethereum blockchain is a distributed database based on the Ethereum whitepaper specification and stores both the full transaction list and the most recent state.

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How to find out in blockchain ledger the current ownership of asset being transacted

I am new to blockchain and smart contract as well. In smart contract we can only save the current state of any transaction but if we need to know the previously expunged transactional data ie, the ...
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How simulate blockchain and smart contract in VANET?

I am a beginner in the blokchain, I am blocked how I start simulate a blockchain and smart contract in vanet. Can you help me and inform me how I have to simulate blockchain and smart contract in the ...
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validation of data integrity in block

I am learning ethreum dapps development and have developed some dapps. I was thinking about supply chain dapp which i think , is very good application of blockchain as it ensures all the history of a ...
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How Merkle Patricia Trie updates the account balance?

This is my understanding of the Merkle Patricia Trees in Ethereum so far: There are three kinds of merkle trees storing diff info - state, transaction and receipt. When a new block is added it ...
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How to transfer ERC20 token from one account to another account but paying the transaction fee from third account?

I want to transfer token from account A to account B by paying transaction fee from account C.I have tried the following logic but token transfer from contract to contract mean contract transfer token ...
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How to build an ethereum based wallet with custom UI?

I have been interested in ethereum for quite some time now and have always wanted to learn ethereum development. I decided to create my custom erc20 tokens i was able to create 2,00,000 tokens. Now i ...
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256 views

web3.exceptions.BadFunctionCallOutput: Could not decode contract function call getEvidence return data b'' for output_types ['string']

Solidity Code: function getEvidence(string memory _evidenceID) view public returns (string memory) { require(evidenceOwner[_evidenceID] == msg.sender); string memory evidenceHash = ...
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198 views

How do I deposit an Ether amount to the smart contract function, namely deposit ? (Metamask version 0.20.7)

I have a problem with using the deposit () function on a smart contract. How do you use metamask 0.20.7 ? const contractInstance = web3.eth.contract(contractAbi).at(contractAddress); contractInstance....
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38 views

Two smart contracts interact can't pass value when I using web3.js

I have two different smart contracts interaction_1 and interaction_2 I input the data(team) from remix ide. What I want to accomplish is when user input team in user_input function at interaction_1,...
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83 views

How to print the value of a Struct ?(mapping)

If the set function receives two inputs 1.["0xca35b7d915458ef540ade6068dfe2f44e8fa733c",50,"ROC","park"] 2.["0x14723a09acff6d2a60dcdf7aa4aff308fddc160c",60,"AU","school"] How do I display the ...
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1answer
162 views

About send Ethereum Classic (ETC) coin

Need guideline for send ETC coin using web3js,As per documented flow I understand we can access all methods same as ETH but I have query regarding set provider for ETC after that I can connect to ETC ...
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118 views

Contract address is not shown in rinkeby testnet etherscan

I am using Getch with truffle framework and i created an account. I have Migrations and Election.sol. After i can migrate, i couldnt see it in etherscan website. Although truffle migrate --reset --...
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283 views

Code:const web3 = new Web3(window.web3.currentProvider) Error: window.web3 is undefined

I am going to connect my contract with the website.However,there was something wrong with the web3? I'm not sure. import Web3 from "web3"; const web3 = new Web3(window.web3.currentProvider); const ...
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1answer
14 views

Which Protocol is suitable if I want a validated person to authenticate the block?

I want to create a system in which Every user will have some specific authorities. Like Examination will validate the block containnig Student Marks. Finance should have authority to add block ...
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32 views

How to copy the data from a running private network to another

I have two seperately running private ethereum networks,i want to copy the data from the first private network to the second one without stopping.I have tried multiple ways but was not successful.I ...
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1answer
37 views

Data encryption

Since all data on the Blockchain is public. Presume I have multiple users submitting data which I want to hide, is there a way I can encrypt that data on the Blockchain ? Or do I have to encrypt off-...
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31 views

Get Submitted Data from Blockchain

I'm using a html code connected to my smart contract via testrpc node, where users submit temperature data. How can I view that data from the smart contract, each time I view from the contract I see ...
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2answers
67 views

do users should be aware of smart contract of the dApp they are using?

So dApps are frontend + contract. Users interact with smart contracts via frontend. Do users i.e. people should be aware of how the contract is designed while using any dApp? are their any risks of ...
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1answer
2k views

html:82 Uncaught ReferenceError: App is not defined at HTMLButtonElement.onclick (html:82)

It's showing me uncaught reference error in my index.html file <div id="candidate-box"></div> <button class="btn btn-primary" onclick="App.vote()">Vote</button> ...
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30 views

Interaction smart contract and third-party app : How to make sure the request comes from the original contract?

I'm working on a blockchain project where I need to access data stored outside the smart contract. The smart contract own a RSA keypair (one public and one private key). We want the private key to ...
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1answer
71 views

Create multiple tokens for a unique property

here is the idea: we assign a set of unique tokens to a company and then we allow people to buy some tokens and get partial ownership of that company. How can I achieve this? I was thinking of ...
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49 views

Geth Sync failed

I started to sync Ethereum using command geth --syncmode "fast" on my windows system. It was taking so much time so i have stopped sync and used the following command. geth --syncmode "fast" --cache=...
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1answer
65 views

totalSupply doesn't pass test

I'm starting programming. I wanted to do my first contract and I assigned totalSupply to 1000000. pragma solidity ^0.5.8; /** * The DappToken contract Constructor Set the total number of tokens ...
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1answer
27 views

can't get struct in return

I am trying to execute unit tests for a solidity smart contract: I have: mapping (bytes16=>Device) public Device_Table struct Device { bytes16 UUID; bytes16 hardware_UUID; string ...
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191 views

How to compile solidity code in Kali Linux

I have tried to install the remix IDE for compile my codes in solidity, since for some reason the remix IDE don't open online. I executed the codes bellow and give that errors. Sorry anything ...
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54 views

Can't call contract function in Remix in Web3 provider mode, but it works in Javascript VM

A simple contract for addition, an error is reported when calling the function add in Web3 provider mode: "Gas estimation errored with the following message (see below). The transaction execution ...
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61 views

Why this error occurring when i running my dapp?

I create a dapp using Solidity,Web3Js,Javascript and Html with metamask i always getting this error when i am trying to submit my transition i am getting this error at c (inpage.js:1) at ...
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1answer
800 views

Error: The data field must be HEX encoded data

I am trying to make a simple dApp by using a tutorial. I am using the following command ; npm install ganache-cli web3 so that we can use web3 to get fake ethereum nodes/accounts by connecting it ...
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1answer
268 views

Can't connect to localhost:8545 while using ethereumjs-testrpc web3?

I am developing a simple Voting dApp by following a tutorial. In this tutorial we are using npm install ethereumjs-testrpc web3 so that we can get the fake nodes of Ethereum. For that i first ...
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250 views

How to get the value from the frontend form and save value to the blockchain using +web3 1.0.0-beta.37 + reactjs

I am writing my first Dapp and want to save the value of the form to the blockchain. I am Using React and web3 js 1.0.0-beta.37 on the frontend. Also I am new to react. Following is my App.js ...
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45 views

How to handle password for unlock Account in web3?

In web3 there is function web3.eth.personal.unlockAccount(address [, callback]) to unlock the account so whenever we call the smart contract function we need to unlock the account in geth client ...
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1answer
35 views

Where I can find the response in the blockchain (transaction inputs) or storage file in smart contract (resultat of transaction)?

when a user sends a tarnsaction to define the access control of a resource to a device s1,then it will be executed by smart contract and the result of the transaction is stored in the storage file of ...
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21 views

Parity synced at not existing block 4839212

I have problem with parity, cant get synchronization in archive mode to end, anything, changing arguments, max-, min-, snapshot-peers, parity verisions do not help. I have blockchain in archive mode ...
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2answers
47 views

Automating user data storage on a blockchain

I have a smart contract that contains a function to store data onto the blockchain. The function stores two integer variables to the blockchain. Now, I can manually store the values from the remix-ide ...
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2answers
35 views

Decrease the time between new blocks in Ethereum Development

I'm trying to create a DAPP (with docker-compose) which will require many transactions, and I am currently experiencing nonce problems with my Transaction Manager. However, I'm not looking for help ...
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445 views

How to decode SHA256 hash value and retrieve data in Solidity

I am working on a project in solidity where i am encode a string to SHA256 hash value, and now i want to decode SHA256 hash value and retrieve the actual data. please help hoe to do it. Below is the ...
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1answer
407 views

Store data on ethereum blockchain from a text file

I have a simple smart contract running over a local Ethereum node. It has two functions: one for sending data and another for retrieving it back. I have a text file and want to send data from it to ...
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1answer
72 views

How to store and encrypt Keys?

I am developing chrome extension like metamask for the private Chain. I have generated the keys and using localStorage to store the Keys. But I am not sure whether I am going correct or not for ...
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1answer
38 views

Getting error in Deposit function in smart contract : The constructor should be payable if you send value

I am getting the error while deposit the amount in remix. Following is my code pragma solidity ^0.4.19; contract Savings { address owner; uint256 deadline; modifier onlyOwner() { require(...
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1answer
71 views

Calling a function from another contract is not working. But calling the function from deployed contract is working

I have a problem with calling a function from another contract. I don't know why it is not working. Contract DateTime(https://github.com/pipermerriam/ethereum-datetime) is a util for converting ...
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86 views

How to auto-execute smart contract's function by Aion or other method in the private chain?

I was a little bit confused about Aion scheduling system, because I want to do it on the private chain, in the example https://github.com/ETH-Pantheon/Aion, they have both smart contract Aion and ...
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1answer
362 views

How to connect with Metamask and Smart contract?

I am ready with my frontend design. I have only 3 files HTML, CSS, and JAVASCRIPT. So, I didn't create any project or DAPP to have other files. Now next step is to connect with metamask and smart ...
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323 views

How can I display the data returned from the solidity function?

Here is a js file that will try to get the list of departments entered into the system. I wanted to display all the department's name in a html page. But nothing is displaying. Below code is the js ...
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68 views

Stop Data mining on Private Blockchain running on Etherrum platform

We have setup a private blockchain platform using Ethereum and we are using Java based UI layer for it. Can we stop mining process as it is taking lots of time and making the transactions slow?
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120 views

Unable to Synchronize raspberry pi node my private network

I have created a private network with 3 nodes - two nodes in computer and one in raspberry pi I connected computer and raspberry pi to same network through LAN computer ip: 192.168.122.1 pi ip ...
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21 views

Send Data into BlockChain Network using w3?

HI I want to store my bank information on ethereum network using w3. can anyone help me to send my data //Bank.sol pragma solidity >=0.4.21 <0.6.0; contract Bank { struct User {...
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37 views

Deployment of smart contract

How to deploy a smart contract on my own blokchain network? I have created using python and running on postman client server interface. I have already tested on ganache , solidity IDE and ethfiddle.
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1answer
100 views

how to connect my html form that is username and password to blockchin and taking the username as metamask key?

I am developing a decentralized cloud web application, when the user will login into the application he/she will generate password from blockchain, I am not getting how to connect my html form that is ...
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1answer
249 views

Query Ethereum Blockchain for functions return value of a Specified Smart Contract

I want to get smart contract functions return value sent by a specified smart contract within a range of blocks. I found a [manual] working method to achieve this: async function getTxsByAccount(...
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2answers
297 views

Blockchain Smart Contract EHR

I'm trying to do a miny EHR(Electronical Health Record) with Smart Contracts where I have a Pacient Contract. The problem is that in solidity everything is public, even the private variables, so I ...

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