Ethereum: How to “encrypt” a message using a Bitcoin *public key* and use its private key to decrypt it?

Encrypting a Message with a Bitcoin Public Key and Private Key

In this article, we will explore how to use a Bitcoin public key and the corresponding private key to encrypt and decrypt messages. We will focus on Ethereum as an example, but similar techniques can be applied to other blockchain networks.

Public Key Encryption

A public key in the context of cryptography refers to a pair of keys: a public key (often represented by the hexadecimal string xpub) and a corresponding private key (usually represented by y). In this scenario, we will use the Ethereum Public Key (EPK) format, which is commonly used for public key encryption.

To encrypt a message using a Bitcoin EPK:

• Generate the Private Key: First, generate a private key for your Bitcoin wallet. You can do this on the Bitcoin website or via the Electrum wallet software.

• Create a message: Create a text string that you want to encrypt with the public key and the corresponding private key.

Using the private key for decryption

To decrypt the message using the private key:

• Convert EPK to JSON-Web Token (JWT): Convert the Bitcoin EPK to a JSON Web Token (JWT) using the ethers.js library.

• Decrypt with public key: Use the decrypted JWT as input for the public key in the form of xpub.

• Convert JWT back to Message

  

  : Finally, convert the decrypted message from the public key back to its original text format.

Code example

Here is an example code snippet using the ethers.js library:

const ethers = require('ethers');
// Define the private key and message
const privateKey = 'private_key_here';
const message = 'This is a test message.';
const encryptedMessage = 'encrypted_message';
// Convert EPK to JWT
asynchronous function convertEPKtoJWT(epk) {
const jwt = await ethers.utils.fromJsonWebToken(epk);
return jwt;
}
// Decrypt with public key
function decryptWithPublicKey(jwt, publicKey) {
// Convert JWT back to message
const messageFromJwt = await ethers.utils.recoverMessage(jwt, publicKey);
return messageFromJwt;
}
// Example usage:
asynchronous function main() {
const epk = '0x... your_EPK_here ...';
const jwt = await convertEPKtoJWT(epk);
const decryptedMessage = await decryptWithPublicKey(jwt, epk);
console.log(decryptedMessage); // This should print the original message
}
main();

Note: Make sure to replace your_private_key_here and 0x... your_EPK_here ... with actual values ​​for your private key and Ethereum public key. Also, make sure you have the necessary libraries (ethers.js) installed before running this code.

In conclusion, using a Bitcoin EPK and its corresponding private key to encrypt and decrypt messages is a secure method in the context of blockchain cryptography. The example provided demonstrates how to convert an encrypted message from an Ethereum public key format to a JSON web token (JWT) and then back to a message that can be decrypted with the same public key.

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