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I have a js client, web API

Js client send some data in JSON format, API prepare a transaction to be send to a smart contract, than it returns transaction raw to client to be signed locally,

and finally the client sign the transaction (using crypto) and post it again to API which send it to blockchain.

So how to sign transaction with standart crypto library (client will not use web3) ?
Sequence diagram

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  • You might have to define "standard crypto library". (What libraries would meet that definition, and why is web3.js not one of them? Would copy/pasting code from web3.js be acceptable? If not, why specifically?)
    – user19510
    Commented Mar 7, 2019 at 12:14
  • I meant something general, because client isnot supposed to have an idea about web3, so I need a way to make him sign a transaction as any crypto sign using elliptic curve
    – maroodb
    Commented Mar 7, 2019 at 14:27
  • I am asking if it faisable to implement this or not, I am not looking for an implementation
    – maroodb
    Commented Mar 7, 2019 at 14:34
  • Yes, this is feasible.
    – user19510
    Commented Mar 7, 2019 at 16:05

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The main point, I think, is that you have to use the right elliptic curve for Ethereum. That is secp256k1. I'm not sure they have implemented other curves yet, although there was talk of supporting Curve25519 (see here)

You could also take a look at this post and this for a tutorial on signatures in Solidity.

The other thing you have to be careful about is the format of the signature. If you use Zeppelin to verify, it will expect the plain message to be prefixed with this string: "\x19Ethereum Signed Message:\n", where \x19 is actually the length of the prefix (25 characters).

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