Generating custom .onion names with the WebCrypto API

## onion-gen.js
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

/*
 * thirty-two
 * https://github.com/chrisumbel/thirty-two
 *
 * Copyright (c) 2011, Chris Umbel
 */
function base32(plain) {
  var charTable = "abcdefghijklmnopqrstuvwxyz234567";
  plain = new Uint8Array(plain);

  var shiftIndex = 0;
  var digit = 0;
  var encoded = "";

  for (var i=0; i<plain.length;) {
    var current = plain[i];

    if (shiftIndex > 3) {
      digit = current & (0xff >> shiftIndex);
      shiftIndex = (shiftIndex + 5) % 8;
      digit = (digit << shiftIndex) | ((i + 1 < plain.length) ?
        plain[i + 1] : 0) >> (8 - shiftIndex);
      i++;
    } else {
      digit = (current >> (8 - (shiftIndex + 5))) & 0x1f;
      shiftIndex = (shiftIndex + 5) % 8;
      if (shiftIndex == 0) i++;
    }

    encoded += charTable[digit];
  }

  return encoded;
}

/*
 * base64-arraybuffer
 * https://github.com/niklasvh/base64-arraybuffer
 *
 * Copyright (c) 2012 Niklas von Hertzen
 * Licensed under the MIT license.
 */
function base64(arraybuffer) {
  var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  var bytes = new Uint8Array(arraybuffer),
  i, len = bytes.length, base64 = "";

  for (i = 0; i < len; i+=3) {
    base64 += chars[bytes[i] >> 2];
    base64 += chars[((bytes[i] & 3) << 4) | (bytes[i + 1] >> 4)];
    base64 += chars[((bytes[i + 1] & 15) << 2) | (bytes[i + 2] >> 6)];
    base64 += chars[bytes[i + 2] & 63];
  }

  if ((len % 3) === 2) {
    base64 = base64.substring(0, base64.length - 1) + "=";
  } else if (len % 3 === 1) {
    base64 = base64.substring(0, base64.length - 2) + "==";
  }

  return base64;
}

function formatKey(key) {
  // Export the DER-encoded ASN.1 private key information.
  var promise = crypto.subtle.exportKey("pkcs8", key);

  return promise.then(function (pkcs8) {
    var encoded = base64(pkcs8);

    // Wrap lines after 64 characters.
    var formatted = encoded.match(/.{1,64}/g).join("\n");

    // Wrap the formatted key in a header and footer.
    return "-----BEGIN PRIVATE KEY-----\n" + formatted +
           "\n-----END PRIVATE KEY-----";
  });
}

function generateRSAKey() {
  var alg = {
    // This could be any supported RSA* algorithm.
    name: "RSASSA-PKCS1-v1_5",
    // We won't actually use the hash function.
    hash: {name: "SHA-1"},
    // Tor hidden services use 1024 bit keys.
    modulusLength: 1024,
    // We will use a fixed public exponent for now.
    publicExponent: new Uint8Array([0x03])
  };

  return crypto.subtle.generateKey(alg, true, ["sign", "verify"]);
}

function computeOnionHash(publicKey) {
  // Export the DER encoding of the SubjectPublicKeyInfo structure.
  var promise = crypto.subtle.exportKey("spki", publicKey);

  promise = promise.then(function (spki) {
    // Compute the SHA-1 digest of the SPKI.
    // Skip 22 bytes (the SPKI header) that are ignored by Tor.
    return crypto.subtle.digest({name: "SHA-1"}, spki.slice(22));
  });

  return promise.then(function (digest) {
    // Base32-encode the first half of the digest.
    return base32(digest.slice(0, 10));
  });
}

function findOnionName(pattern) {
  var key;

  // Start by generating a random key pair.
  var promise = generateRSAKey().then(function (pair) {
    key = pair.privateKey;

    // Generate the .onion hash of the public key.
    return computeOnionHash(pair.publicKey);
  });

  return promise.then(function (hash) {
    // Try again if the pattern doesn't match.
    if (!pattern.test(hash)) {
      return findOnionName(pattern);
    }

    // Key matches! Export and format it.
    return formatKey(key).then(function (formatted) {
      return {key: formatted, hash: hash};
    });
  });
}

findOnionName(/ab/).then(function (result) {
  console.log(result.hash + ".onion", result.key);
}, function (err) {
  console.log("An error occurred, please reload the page.");
});