Get RSA Public Key from a Private Key Using the Web Crypto API

#Public Key From Private Key - Readme.md

See https://putzisan.com/articles/derive-rsa-public-key-from-private-key-web-crypto for more information.

publicKeyFromPrivateKey.js

function base64ToUint8Array(base64) {
  return new Uint8Array(
    atob(base64)
      .split("")
      .map((c) => c.charCodeAt(0)),
  );
}

function pemToBase64(pem) {
  return pem
    .replace(/-----BEGIN (.*)-----/, "")
    .replace(/-----END (.*)-----/, "")
    .replace(/\s/g, ""); // Remove all whitespace characters
}

async function publicKeyFromPrivateKey(privateKeyPem) {
  const base64 = pemToBase64(privateKeyPem);
  const keyData = base64ToUint8Array(base64);

  const privateKey = await crypto.subtle.importKey(
    "pkcs8",
    keyData,
    { name: "RSASSA-PKCS1-v1_5", hash: "SHA-256" },
    true, // extractable
    ["sign"],
  );

  const privateKeyJwk = await crypto.subtle.exportKey("jwk", privateKey);
  const { n, e } = privateKeyJwk;
  const publicKeyJwk = { kty: "RSA", e, n };

  return crypto.subtle.importKey(
    "jwk",
    publicKeyJwk,
    { name: "RSASSA-PKCS1-v1_5", hash: "SHA-256" },
    true,
    ["verify"],
  );
}

publicKeyFromPrivateKeyExampleUsage.js

function arrayBufferToBase64(arrayBuffer) {
  return btoa(String.fromCharCode(...new Uint8Array(arrayBuffer)));
}

function arrayBufferToPublicKeyPem(buffer) {
  const base64 = arrayBufferToBase64(buffer);
  const pemFormattedBlocks = base64.match(/.{1,64}/g)?.join("\n") || ""; // Format the Base64 string into PEM blocks
  return `-----BEGIN PUBLIC KEY-----\n${pemFormattedBlocks}\n-----END PUBLIC KEY-----`;
}


const privateKeyPemExample = `-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----`;

const publicKey = await publicKeyFromPrivateKey(privateKeyPemExample);

const publicKeySpki = await crypto.subtle.exportKey("spki", publicKey);
const publicKeyPem = arrayBufferToPublicKeyPem(publicKeySpki);

console.log(publicKeyPem);