JavaScript RSA utils that work on Windows, Mac, and Linux with or without C compiler
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| A Root Project.

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

This was built for the ACME.js and Greenlock.js Let’s Encrypt clients and is particularly suitable for building certbot-like clients.

(if you’re looking for similar tools in the browser, consider Bluecrypt)



npm install --save rsa-compat

If you need compatibility with older versions of node, you may need to npm install --save ursa-optional node-forge.


npm install --global rsa-compat



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

# generates a new keypair in the current directory


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) {

Here’s what the object might look like:


{ 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)*/'

See to learn a little more about the meaning of the specific fields in the JWK.

API Summary

  • 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)
  • RSA.thumbprint(keypair)

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) { /*...*/ });

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


{ 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)


Imports keypair as JWKs and internal values _ursa and _forge.

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

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


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


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


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

keypair.publicKeyJwk = RSA.exportPublicJwk(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);


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"


Generates a JWK thumbprint.


var thumb = RSA.thumbprint(keypair);

// kK4OXp5CT1FEkHi6WkegldmeTJecSTyJN-DxZ91nQ30

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, [ '', '' ]);


web-safe base64 for certbot/letsencrypt:

RSA.generateCsrDerWeb64(keypair, names):

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


Old Node Versions

In recent versions of node >= v10.12 native RSA key generation is fairly quick for 2048-bit keys (though it may still be too slow for some applications with 4096-bit keys).

In old versions, however, and especially on ARM and/or MIPS procesors, RSA key generation can be very, very slow.

In old node versions ursa can provide faster key generation, but it must be compiled. ursa will not compile for new node versions, but they already include the same openssl bindings anyawy.

npm install --save ursa

Also, if you need Node < v6 support:

npm install --save buffer-v6-polyfill

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 ½ 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.


  • v2.0
    • remove ursa and node-forge deps
    • mark for node v10.11+
  • v1.9
    • consistently handle key generation across node crypto, ursa, and forge
    • move all other operations to rasha.js and rsa-csr.js
    • bugfix non-standard JWKs output (which mostly worked)
    • move dependencies to optional
  • v1.4.0
    • remove ursa as dependency (just causes confusion), but note in docs
    • drop node < v6 support


rsa-compat.js directly includes code from Rasha.js and RSA-CSR.js (also Root projects), retrofitted for rsa-compat.

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