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0.7.3 • Public • Published

Universal Module for Cryptographic Key Utilities in JavaScript

npm version Dependencies License: MIT

WARNING: At this time this solution should be considered suitable for research and experimentation, further code and security review is needed before utilization in a production application.

Introduction and Overview

This library is designed to 'universally' provide several functions for a cryptographic key handling, which means it works both on most browsers and on Node.js just by importing from npm/source code. This key utility library provides converters for EC/RSA keys in PEM/DER<->JWK, octet form of EC keys <-> JWK, and computation of JWK thumbprints. Especially for the conversion PEM/DER <->JWK, encryption and decryption of private key in DER/PEM are supported.


At your project directory, do either one of the following.

  • From npm/yarn:
    $ npm install --save js-crypto-key-utils // npm
    $ yarn add js-crypto-key-utils // yarn
  • From GitHub:
    $ git clone https://github.com/junkurihara/jscu.git
    cd js-crypto-utils/packages/js-crypto-key-utils
    & yarn build

Then you should import the package as follows.

import keyutil from 'js-crypto-key-utils'; // for npm
import keyutil from 'path/to/js-crypto-key-utils/dist/index.js'; // for github

The bundled file is also given as js-crypto-key-utils/dist/jsckey.bundle.js for a use case where the module is imported as a window.jsckey object via script tags.


Supported key types are Json Web Key (JWK, RFC7517), and PEM/DER. Octet-Formatted Key (SECG SEC1 2.3.3 and 2.3.4, link to PDF) is also available for elliptic curve cryptography keys. Note that for PEM/DER, public keys are encoded to the form of SubjectPublicKeyInfo (SPKI) defined as a part of X.509 public key certificate (RFC5280). The detailed encoding rule for elliptic curve cryptographic keys is given in RFC5480. On the other hand, private keys are encoded to hte form of PrivateKeyInfo defined in PKCS#8 (RFC5958). The detailed encoding rule for elliptic curve cryptographic keys is given in RFC5915 as well as SPKI. Please refer to RFC3447 for the detailed encoding rule of RSA public and private keys.


At first, you need to instantiate Key object by importing various type of keys.

const yourStringPemKey = '------BEGIN PRIVATE...'; // SPKI (public key) or PKCS8 (either encrypted or plaintext private key)
const yourBinaryDerKey = new Uint8Array([...]); // SPKI (public key) or PKCS8 (either encrypted or plaintext private key)
const yourJasonWebKey = {kty: 'EC', ... };
const yourOctetFormKey = new Uint8Array([0x04, ...]) // only for Elliptic Curve Crypto Key.
const keyObjFromPem = new keyutil.Key('pem', yourStringPemKey);
const keyObjFromDer = new keyutil.Key('der', yourBinaryDerKey);
const keyObjFromJwk = new keyutil.Key('jwk', yourJasonWebKey); 
const keyObjFromOct = new keyutil.Key('oct', yourOctetFormKey, {namedCurve: '...'}); //namedCurve like 'P-256K' is required.

Handling key objects

In a case where the imported key is encrypted (pem/der), it needs to be decrypted before getting exported.

const keyObj = new keyutil.Key('pem', encryptedPemKey);
const yourPassphrase = '...';
// first now you can check the status
const isEncrypted = keyObj.isEncrypted;
// before using the key object, decrypt the key object with the passphrase.
if(isEncrypted) await keyObj.decrypt(yourPassphrase);
const thisMustBeFalse = keyObj.isEncrypted; // false
// you can lock the key object by encrypting it as well.
if(!thisMustBeFalse) await keyObj.encrypt(yourPassphrase);
const thisMustBeTrue = keyObj.isEncrypted; // true;

Exporting keys in desired format

From instantiated key objects, various types of keys can be exported.

const keyObj = new keyutil.Key('pem', pemKey);
// jwk
let jwk;
if(!keyObj.isEncrypted) jwk = await keyObj.export('jwk');
if(keyObj.isPrivate) jwk = await keyObj.export('jwk', {outputPublic: true}); // export public key from private key.
// pem
let pem;
if(!keyObj.isEncrypted) pem = await keyObj.export('pem');
if(keyObj.isPrivate) pem = await keyObj.export('pem', {outputPublic: true}); // export public key from private key.
// Only ECC Key: export compressed form of public key from private/public key.
pem = await keyObj.export('pem', {outputPublic: true, compact: true});
// der
let der;
if(!keyObj.isEncrypted) der = await keyObj.export('der');
if(keyObj.isPrivate) der = await keyObj.export('der', {outputPublic: true}); // export public key from private key.
// Only ECC Keys: export compressed form of public key from private/public key.
der = await keyObj.export('der', {outputPublic: true, compact: true});
// Only ECC Keys
// octet from
let oct;
if(!keyObj.isEncrypted) oct = await keyObj.export('oct');
if(keyObj.isPrivate) oct = await keyObj.export('oct', {outputPublic: true}); // export public key from private key.
// export compressed form of public key from private/public key.
oct = await keyObj.export('oct', {outputPublic: true, compact: true});

Exporting encrypted keys with arbitrary specified passphrase.

All you need to export encrypted private keys in PEM/DER is just putting passphrase in the API. The default encryption algorithm follows PKCS#5 v2.1 (RFC8018) and uses AES256-CBC and HMAC-with-SHA-256 to encrypt your private key.

const keyObj = new keyutil.Key('pem', pemKey);
// encrypt with default params
let encryptedPem;
if(!keyObj.isEncrypted && keyObj.isPrivate)
  encryptedPem = await keyObj.export('pem', {encryptParams: {passphrase: 'top secret'}});
// encrypt with intended params
if(!keyObj.isEncrypted && keyObj.isPrivate)
  encryptedPem = await keyObj.export('pem', {
    encryptParams: {
      passphrase: 'top secret',
      algorithm: 'pbes2', // default. 'pbeWith...' is also available, i.e., pbes1.
      cipher: 'aes256-cbc', // default. for encryption. 'aes128-cbc', 'aes192-cbc'(only node), 'des-ede3-cbc' are available as well.
      prf: 'hmacWithSHA256' // default. for key derivation

Getting JWK Thumbprint

You can obtain the JWK Thumbprint defined in RFC7638 from instantiated key object. The API can be invoked as follows.

const keyObj = new keyutil.Key('pem', pemKey);
let thumbprint;
// with default params (hash is SHA-256, output is in Uint8Array)
if(!keyObj.isEncrypted) thumbprint = await keyObj.getJwkThumbprint();
// with intented params
if(!keyObj.isEncrypted) thumbprint = await keyObj.getJwkThumbprint(
  'hex' // output is hex string

Note that the thumbprint generated from a public key is exactly same as that from its paired private key.


Now this library supports the following curve for elliptic curve cryptography.

  • P-256 (secp256r1)
  • P-384 (secp384r1)
  • P-521 (secp521r1)
  • P-256K (secp256k1)


Licensed under the MIT license, see LICENSE file.


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