keyfetch

Lightweight support for fetching JWKs.

Fetches JSON native JWKs and exposes them as PEMs that can be consumed by the jsonwebtoken package (and node's native RSA and ECDSA crypto APIs).

Features

Works great for

• jsonwebtoken (Auth0)
• OIDC (OpenID Connect)
• .well-known/jwks.json (Auth0, Okta)
• Other JWKs URLs

Crypto Support

• JWT verification
• RSA (all variants)
• EC / ECDSA (NIST variants P-256, P-384)
• Sane error codes
• esoteric variants (excluded to keep the code featherweight and secure)

Table of Contents

• Install
• Usage
• API
  • Auth0 / Okta
  • OIDC
• Errors
• Change Log

Install

npm install --save keyfetch

Usage

Retrieve a key list of keys:

var keyfetch = require("keyfetch");

keyfetch.oidcJwks("https://example.com/").then(function (results) {
    results.forEach(function (result) {
        console.log(result.jwk);
        console.log(result.thumprint);
        console.log(result.pem);
    });
});

Quick JWT verification (for authentication):

var keyfetch = require("keyfetch");
var jwt = "...";

keyfetch.jwt.verify(jwt).then(function (decoded) {
    console.log(decoded);
});

JWT verification (for authorization):

var options = { issuers: ["https://example.com/"], claims: { role: "admin" } };
keyfetch.jwt.verify(jwt, options).then(function (decoded) {
    console.log(decoded);
});

Verify a JWT with jsonwebtoken:

var keyfetch = require("keyfetch");
var jwt = require("jsonwebtoken");
var auth = "..."; // some JWT
var token = jwt.decode(auth, { json: true, complete: true });

if (!isTrustedIssuer(token.payload.iss)) {
    throw new Error("untrusted issuer");
}

keyfetch.oidcJwk(token.header.kid, token.payload.iss).then(function (result) {
    console.log(result.jwk);
    console.log(result.thumprint);
    console.log(result.pem);

    jwt.jwt.verify(jwt, { jwk: result.jwk });
});

Note: You might implement isTrustedIssuer one of these:

function isTrustedIssuer(iss) {
    return -1 !== ["https://partner.com/", "https://auth0.com/"].indexOf(iss);
}

function isTrustedIssuer(iss) {
    return (
        /^https:/.test(iss) && /(\.|^)example\.com$/.test(iss) // must be a secure domain
    ); // can be example.com or any subdomain
}

API

All API calls will return the RFC standard JWK SHA256 thumbprint as well as a PEM version of the key.

Note: When specifying id, it may be either kid (as in token.header.kid) or thumbprint (as in result.thumbprint).

JWKs URLs

Retrieves keys from a URL such as https://example.com/jwks/ with the format { keys: [ { kid, kty, exp, ... } ] } and returns the array of keys (as well as thumbprint and jwk-to-pem).

keyfetch.jwks(jwksUrl);
// Promises [ { jwk, thumbprint, pem } ] or fails

keyfetch.jwk(id, jwksUrl);
// Promises { jwk, thumbprint, pem } or fails

Auth0

If https://example.com/ is used as issuerUrl it will resolve to https://example.com/.well-known/jwks.json and return the keys.

keyfetch.wellKnownJwks(issuerUrl);
// Promises [ { jwk, thumbprint, pem } ] or fails

keyfetch.wellKnownJwk(id, issuerUrl);
// Promises { jwk, thumbprint, pem } or fails

OIDC

If https://example.com/ is used as issuerUrl then it will first resolve to https://example.com/.well-known/openid-configuration and then follow jwks_uri to return the keys.

keyfetch.oidcJwks(issuerUrl);
// Promises [ { jwk, thumbprint, pem } ] or fails

keyfetch.oidcJwk(id, issuerUrl);
// Promises { jwk, thumbprint, pem } or fails

Verify JWT

This can accept a JWT string (compact JWS) or a decoded JWT object (JWS).

This can be used purely for verifying pure authentication tokens, as well as authorization tokens.

keyfetch.jwt.verify(jwt, { strategy: "oidc" }).then(function (verified) {
    /*
    { protected: '...'  // base64 header
    , payload: '...'    // base64 payload
    , signature: '...'  // base64 signature
    , header: {...}     // decoded header
    , claims: {...}     // decoded payload
    }
  */
});

When used for authorization, it's important to specify a limited set of trusted issuers.
When using for federated authentication you may set issuers = ["*"] - but DO NOT trust claims such as email and email_verified.

If your authorization claims can be expressed as exact string matches, you can specify those too.

keyfetch.jwt.verify(jwt, {
  strategy: 'oidc',
  issuers: [ 'https://example.com/' ],
  //iss: 'https://example.com/',
  claims: { role: 'admin', sub: 'abc', group: 'xyz' }
}).then(function (verified) {

• strategy may be oidc (default) , auth0, or a direct JWKs url.
• issuers must be a list of https urls (though http is allowed for things like Docker swarm), or '*'
• iss is like issuers, but only one
• claims is an object with arbitrary keys (i.e. everything except for the standard iat, exp, jti, etc)
• exp may be set to false if you're validating on your own (i.e. allowing time drift leeway)
• jwks can be used to specify a list of allowed public key rather than fetching them (i.e. for offline unit tests)
• jwk same as above, but a single key rather than a list

Decode JWT

try {
  console.log( keyfetch.jwt.decode(jwt) );
} catch(e) {
  console.error(e);
}

{ protected: '...'  // base64 header
, payload: '...'    // base64 payload
, signature: '...'  // base64 signature
, header: {...}     // decoded header
, claims: {...}     // decoded payload

It's easier just to show the code than to explain the example.

keyfetch.jwt.decode = function (jwt) {
    // Unpack JWS from "compact" form
    var parts = jwt.split(".");
    var obj = {
        protected: parts[0],
        payload: parts[1],
        signature: parts[2]
    };

    // Decode JWT properties from JWS as unordered objects
    obj.header = JSON.parse(Buffer.from(obj.protected, "base64"));
    obj.claims = JSON.parse(Buffer.from(obj.payload, "base64"));

    return obj;
};

Cache Settings

keyfetch.init({
    // set all keys at least 1 hour (regardless of jwk.exp)
    mincache: 1 * 60 * 60,

    // expire each key after 3 days (regardless of jwk.exp)
    maxcache: 3 * 24 * 60 * 60,

    // re-fetch a key up to 15 minutes before it expires (only if used)
    staletime: 15 * 60
});

There is no background task to cleanup expired keys as of yet. For now you can limit the number of keys fetched by having a simple whitelist.

Errors

JSON.stringify()d errors look like this:

{
  code: "INVALID_JWT",
  status: 401,
  details: [ "jwt.claims.exp = 1634804500", "DEBUG: helpful message" ]
  message: "token's 'exp' has passed or could not parsed: 1634804500"
}

SemVer Compatibility:

• code & status will remain the same.
• message is NOT included in the semver compatibility guarantee (we intend to make them more client-friendly), neither is detail at this time (but it will be once we decide on what it should be).
• details may be added to, but not subtracted from

Hint	Code	Status	Message (truncated)
bad gateway	BAD_GATEWAY	502	The auth token could not be verified because our se...
insecure issuer	MALFORMED_JWT	400	The auth token could not be verified because our se...
parse error	MALFORMED_JWT	400	The auth token could not be verified because it is ...
no issuer	MALFORMED_JWT	400	The auth token could not be verified because it doe...
malformed exp	MALFORMED_JWT	400	The auth token could not be verified because it's e...
expired	INVALID_JWT	401	The auth token is expired. To try again, go to the ...
inactive	INVALID_JWT	401	The auth token isn't valid yet. It's activation dat...
bad signature	INVALID_JWT	401	The auth token did not pass verification because it...
jwk not found old	INVALID_JWT	401	The auth token did not pass verification because ou...
jwk not found	INVALID_JWT	401	The auth token did not pass verification because ou...
no jwkws uri	INVALID_JWT	401	The auth token did not pass verification because it...
unknown issuer	INVALID_JWT	401	The auth token did not pass verification because it...
failed claims	INVALID_JWT	401	The auth token did not pass verification because it...

Change Log

Minor Breaking changes (with a major version bump):

• v3.0.0
  • reworked error messages (also available in v2.1.0 as client_message)
  • started using let and template strings (drops really old node compat)
• v2.0.0
  • changes from the default issuers = ["*"] to requiring that an issuer (or public jwk for verification) is specified

See other changes in CHANGELOG.md.