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I have the following text string:

This is a test message.

• Using my bitcoin public key (bitcoin address?), how can I encrypt this message?




• How would I decrypt the message using a bitcoin private key?

If you want to encrypt messages, you should use a proper message/file encryption tool like PGP/GPG. Homebrewed cryptography using bitcoin things is prone to having poor security properties.

Yes, this is possible.

However, I want to upfront state that this is not advisable for multiple reasons:

• Bitcoin keys are intended to be single use for privacy reasons, and leveraging them for encryption unnecessarily encourages treating them as a long-lived identity.


• There may be ugly and dangerous interactions when keys are used for multiple protocols independently.


• You're much better off using systems that were actually designed for encryption than trying to piggy-back off Bitcoin's cryptography.


• Implementing your own cryptography is very dangerous (in general, unless you know what you're doing, and get plenty of review from experts).

A scheme called ECIES exists that lets you leverage elliptic curve keys to create an encryption system.

In short, it works by:

The sender:

• generates an ephemeral private key k using a strong cryptographic random number generator, with associated public key k = kG (multiplication refers to Elliptic Curve multiplication here).


• computes an ECDH shared secret s = H(kP), where P is the public key of the recipient.


• encrypts the message m using AES, with s as the key, to obtain c = AECEnc_s(m).


• sends K and c to the recipient.

The recipient:

• also computes the ECDH shared secret, using s = H(pK), where p is his private key.


• decrypts the message using s, m' = AESDec_s(c).

Be sure to follow the warnings given previously in other answers, but for the record there is an npm module that implemented this: eccrypto (DYOR on it's security):

Install dependencies

$ npm install -g eccrypto

index.js

var crypto = require("crypto");
var eccrypto = require("eccrypto");

var privateKeyA = crypto.randomBytes(32);
var publicKeyA = eccrypto.getPublic(privateKeyA);
var privateKeyB = crypto.randomBytes(32);
var publicKeyB = eccrypto.getPublic(privateKeyB);

// Encrypting the message for B.
eccrypto.encrypt(publicKeyB, Buffer("msg to b")).then(function(encrypted) 
 // B decrypting the message.
 eccrypto.decrypt(privateKeyB, encrypted).then(function(plaintext) 
 console.log("Message to part B:", plaintext.toString());
 );
);

// Encrypting the message for A.
eccrypto.encrypt(publicKeyA, Buffer("msg to a")).then(function(encrypted) 
 // A decrypting the message.
 eccrypto.decrypt(privateKeyA, encrypted).then(function(plaintext) 
 console.log("Message to part A:", plaintext.toString());
 );
);