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I am trying to build a dApp where a user A sends a request to a user B mentioning a certain sum of ether that B can get.

If B accepts the offer, then B get the ether proposed by A.

When I'm logged in with B's address, I can't call the smart contract using A's address, which makes sense. However, I was wondering if there is something similar when logged with B's address as :

this.state.contract.methods.myFunction().send({ from: A_Address })

To sum up, I want that A sends a certain amount of Ether to B when B clicks on a button and the button appears only if A has allowed his consent. Is it possible ?

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That's not directly possible without having A's private key. However there are other a bit more indirect ways:

1) A signs such a transaction in advance with his private key. He then sends the signed transaction to B (not via blockchain). If B wants to accept the transaction he broadcasts it to the blockchain. Or:

2) A middleman contract where A stores Ethers. When B calls some accept function in the contract the Ethers are sent to him.

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  • Thank you a lot for your answer. I wasn't sure that 1) was possible. How can A send a transaction to B ? And how can B get the transaction ? Are there any specific functions to use ?
    – Julien
    Mar 10, 2020 at 17:04
  • Think of it as A writing a letter to the EVM instructing it to send Ethers to B. A then seals the letter in an envelope and gives the envelope to B. When B wants the Ethers he just has to deliver to whole envelope to EVM. B can't change the actual letter. In web3 there's a signTransaction function: web3js.readthedocs.io/en/v1.2.0/…
    – Lauri Peltonen
    Mar 11, 2020 at 5:07
  • Thank you. I've read about the signTransaction function but it requires the private key. Is there any way to sign without using the private key (I mean using it implicitly). I read a lot about it but most of people store their private key in plain text which, in my understanding, isn't a good idea.
    – Julien
    Mar 12, 2020 at 1:46
  • No you can't sign a transaction without a private key. That's the whole point of signing a transaction. But what I'm trying to say is that nobody else has to know A's private key - he himself signs the transaction and then sends it to B. Once the transaction has been signed there's no way to see the used private key.
    – Lauri Peltonen
    Mar 12, 2020 at 7:09

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