Generate key pairs and PEM-encode them using vanilla JS and browser Crypto API

keypairs.js

// 

// Convert an ArrayBuffer into a string.
// From https://developers.google.com/web/updates/2012/06/How-to-convert-ArrayBuffer-to-and-from-String
function arrayBufToString(buf) {
	return String.fromCharCode.apply(null, new Uint8Array(buf));
}

function pemEncode(label, data) {
	const base64encoded = window.btoa(data);
	const base64encodedWrapped = base64encoded.replace(/(.{64})/g, "$1\n");
	return `-----BEGIN ${label}-----\n${base64encodedWrapped}\n-----END ${label}-----`;
}

async function exportKeyAsString(format, key) {
	const exported = await window.crypto.subtle.exportKey(format, key);
	return arrayBufToString(exported);
}

async function pemEncodedPrivateKey(keyPair) {
	const exported = await exportKeyAsString("pkcs8", keyPair.privateKey);
	return pemEncode("PRIVATE KEY", exported);
}

async function pemEncodedPublicKey(keyPair) {
	const exported = await exportKeyAsString("spki", keyPair.publicKey);
	return pemEncode("PUBLIC KEY", exported);
}

window.crypto.subtle.generateKey(
	{
		name: "ECDSA",
		namedCurve: "P-256"
	},
	// if using RSA, use something like this instead (https://developer.mozilla.org/en-US/docs/Web/API/RsaHashedKeyGenParams):
	// {
	// 	name: "RSA-PSS",
	// 	modulusLength: 4096,
	// 	publicExponent: new Uint8Array([1, 0, 1]),
	// 	hash: "SHA-256",
	// },
	true,
	["sign", "verify"]
).then(keyPair => {
	pemEncodedPrivateKey(keyPair).then((privateKeyPEM) => {
		console.log(privateKeyPEM);
	});

	pemEncodedPublicKey(keyPair).then((publicKeyPEM) => {
		console.log(publicKeyPEM);
	});
});