rsa-compat.js

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JavaScript RSA utils that work on Windows, Mac, and Linux with or without C compiler

In order to provide a module that "just works" everywhere, we mix and match methods from node.js core, ursa, forge, and others.

This is useful for certbot and letsencrypt.

(in the future we'd like to provide the same API to the browser)

Install

node.js

npm install --save rsa-compat

For more efficient RSA key generation: (I dropped ursa as an "optional dependency" because the non-fatal error messages on unsupported platforms and node versions were confusing people, but I still recommend installing it)

npm install --save ursa

Node < v6 support:

npm install --save buffer-v6-polyfill

CLI

npm install --global rsa-compat

Usage

CLI

You can generate keypairs on Windows, Mac, and Linux using rsa-keygen-js:

# generates a new keypair in the current directory
rsa-keypiar-js

Examples

Generate an RSA Keypair:

var RSA = require('rsa-compat').RSA;

var options = { bitlen: 2048, exp: 65537, public: true, pem: true, internal: true };

RSA.generateKeypair(options, function (err, keypair) {
  console.log(keypair);
});

Here's what the object might look like:

console.log(keypair):

{ publicKeyPem: '-----BEGIN RSA PUBLIC KEY-----\n/*base64 pem-encoded string*/'
, privateKeyPem: '-----BEGIN RSA PRIVATE KEY-----\n/*base64 pem-encoded string*/'
, privateKeyJwk: {
    kty: "RSA"
  , n: '/*base64 modulus n = pq*/'
  , e: '/*base64 exponent (usually 65537)*/'
  , d: '/*base64 private exponent (d = e^−1 (mod ϕ(n))/'
  , p: '/*base64 first prime*/'
  , q: '/*base64 second prime*/'
  , dp: '/*base64 first exponent for Chinese remainder theorem (dP = d (mod p−1))*/'
  , dq: '/*base64 Second exponent, used for CRT (dQ = d (mod q−1))/'
  , qi: '/*base64 Coefficient, used for CRT (qinv = q^−1 (mod p))*/'
  }
, publicKeyJwk: {
    kty: "RSA"
  , n: '/*base64 modulus n = pq*/'
  , e: '/*base64 exponent (usually 65537)*/'
  }

, _ursa: '/*undefined or intermediate ursa object*/'
, _ursaPublic: '/*undefined or intermediate ursa object*/'
, _forge: '/*undefined or intermediate forge object*/'
, _forgePublic: '/*undefined or intermediate forge object*/'
}

NOTE: this object is JSON safe as _ursa and _forge will be ignored

See http://crypto.stackexchange.com/questions/6593/what-data-is-saved-in-rsa-private-key to learn a little more about the meaning of the specific fields in the JWK.

Security and Compatibility

TL;DR: Use the default values 2048 and 65537 unless you have a really, really good reason to do otherwise.

Various platforms require these values.

Most security experts agree that 4096-bit is no more "secure" than 2048-bit - a fundamental vulnerability in the RSA algorithm which causes 2048 to be broken will most likely also cause 4096 to be broken (i.e. if someone can prove mathematically prove P=NP or a way to predict prime numbers). Also, many platforms only support 2048 bit keys due to the insecurity of 1024-bit keys (which are not 1/2 secure but rather 1/(2^1028) less secure) and the excess computational cost of 4096-bit keys (it's not a 2x increase, it's more like a 2^2048 increase).

As to why 65537 is even optional as a prime exponent or why it matters... no idea, but it does matter.

API

• RSA.generateKeypair(options, cb)
  • (deprecated RSA.generateKeypair(bitlen, exp, options, cb))
• RSA.import(options)
  • (deprecated RSA.import(keypair, options))
• RSA.exportPrivatePem(keypair)
• RSA.exportPublicPem(keypair)
• RSA.exportPrivateJwk(keypair)
• RSA.exportPublicJwk(keypair)
• RSA.signJws(keypair, header, protect, payload)
  • (deprecated RSA.signJws(keypair, payload, nonce))
• RSA.generateCsrPem(keypair, names)
• RSA.generateCsrDerWeb64(keypair, names)

keypair can be any object with any of these keys publicKeyPem, privateKeyPem, publicKeyJwk, privateKeyJwk

RSA.generateKeypair(options, cb)

Create a private keypair and export it as PEM, JWK, and/or internal formats

RSA.generateKeypair(null, function (keypair) { /*...*/ });

RSA.generateKeypair({
  bitlen: 2048, exp: 65537, pem: false, public: false, internal: false
}, function (keypair) { /*...*/ });

options:

{ public: false       // export public keys
, pem: false          // export pems
, jwk: true           // export jwks
, internal: false     // preserve internal intermediate formats (_ursa, _forge)
, thumbprint: false   // JWK sha256 thumbprint
, fingerprint: false  // NOT IMPLEMENTED (RSA key fingerprint)
}

RSA.import(options)

Imports keypair as JWKs and internal values _ursa and _forge.

var keypair = RSA.import({ type: 'RSA', privateKeyPem: '...' });

console.log(keypair);

{ privateKeyPem: ..., privateKeyJwk: ..., _ursa: ..., _forge: ... }

RSA.export*(keypair)

You put in an object like { privateKeyPem: '...' } or { publicKeyJwk: {} } and you get back the keys in the format you requested.

Note:

• Private keys can be used to export both private and public keys
• Public keys can NOT be used to generate private keys

Example:

var keypair = { privateKeyPem: '...' };

keypair.publicKeyJwk = RSA.exportPublicJwk(keypair);

console.log(keypair);

RSA.signJws(keypair, payload, nonce)

Generates a signature in JWS format (necessary for certbot/letsencrypt).

var message = "Hello, World!"
var nonce = crypto.randomBytes(16).toString('hex');
var jws = RSA.signJws(keypair, message, nonce);

console.log(jws);

The result looks like this:

{ "header": {
    "alg": "RS256",
    "jwk": {
      "kty": "RSA",
      "n": "AMJubTfOtAarnJytLE8fhNsEI8wnpjRvBXGK/Kp0675J10ORzxyMLqzIZF3tcrUkKBrtdc79u4X0GocDUgukpfkY+2UPUS/GxehUYbYrJYWOLkoJWzxn7wfoo9X1JgvBMY6wHQnTKvnzZdkom2FMhGxkLaEUGDSfsNznTTZNBBg9",
      "e": "AQAB"
    }
  },
  "protected": "eyJub25jZSI6IjhlZjU2MjRmNWVjOWQzZWYifQ",
  "payload": "JLzF1NBNCV3kfbJ5sFaFyX94fJuL2H-IzaoBN-ciiHk",
  "signature": "Wb2al5SDyh5gjmkV79MK9m3sfNBBPjntSKor-34BBoGwr6n8qEnBmqB1Y4zbo-5rmvsoPmJsnRlP_hRiUY86zSAQyfbisTGrGBl0IQ7ditpkfYVm0rBWJ8WnYNqYNp8K3qcD7NW72tsy-XoWEjNlz4lWJeRdEG2Nt4CJgnREH4Y"
}

RSA.generateCsr*(keypair, names)

You can generate the CSR in human-readable or binary / base64 formats:

RSA.generateCsrPem(keypair, names):

var pem = RSA.generateCsrPem(keypair, [ 'example.com', 'www.example.com' ]);

console.log(pem);

web-safe base64 for certbot/letsencrypt:

RSA.generateCsrDerWeb64(keypair, names):

var web64 = RSA.generateCsrDerWeb64(keypair, [ 'example.com', 'www.example.com' ]);

console.log(web64);

ChangeLog:

• v1.4.0
  • remove ursa as dependency (just causes confusion), but note in docs
  • drop node < v6 support