Ethereum: How would one « 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 look at how to use a Bitcoin public key and its 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 denoted 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:

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

• Create Message

  

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

Use Private Key to Decrypt

To decrypt the message using the private key:

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

• Decrypt with Public Key: Use the decoded JWT as input to the public key in xpub' form.

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

Sample Code

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

const ethers = request('ethers');
// Provide the private key and message
const privateKey = 'your_private_key_here';
const message = 'This is a test message.';
const encryptedMessage = 'encrypted_message';
// Convert EPK to JWT
async function convertEPKtoJWT(epk) {
const jwt = await ethers.utils.fromJsonWebToken(epk);
return jwt;
}
// Decrypt with public key
function decryptWithPublicKey(jwt, publicKey) {
// Recover JWT to message
const messageFromJwt = await ethers.utils.recoverMessage(jwt, publicKey);
return messageFromJwt;
}
// Example usage:
async 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_hereand0x… your_EPK_here…with the actual values ​​of your private key and Ethereum public key. Also, make sure you have installed the required libraries (ethers.js`) before running this code.

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