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131

How to get the modulus and exponent of an RSA public key in Node.js

rated 0 times [ 135] [ 4] / answers: 1 / hits: 5529 / 4 Years ago, wed, april 29, 2020, 12:00:00

I am creating an ACME client and I need to find the modulus and exponent of my RSA public key, which I generate using the following code:

crypto.generateKeyPairSync('rsa', {

    modulusLength: 4096,

    publicKeyEncoding: {

        type: 'spki',

        format: 'pem'

    },

    privateKeyEncoding: {

        type: 'pkcs8',

        format: 'pem'

    }

});

I need the modulus and exponent, so that I can use them in the JWK section of my JWS:

alg: 'RS256',

jwk: {

    kty: 'RSA',

    e: '...',

    n: '...'

},

nonce,

url: directory.newAccount

I have managed to decode the public key from base64 to hex using the following line, but I am not sure what to do next:

Buffer.from(publicKey, 'base64').toString('hex');

How do I find the modulus and exponent of an RSA public key in Node.js?

EDIT 1

I have discovered that Node.js uses the public exponent 65537 by default: Node.js documentation.

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josefn

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answered 4 Years ago leslijessalyng · Accepted Answer

It's easy as a piece of cake; it doesn't require 3rd party libraries

import { createPublicKey } from 'crypto'



const pemPublicKey = `-----BEGIN PUBLIC KEY-----

MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAphRAj+tRfbrYwnSFbWrj

...

vQIDAQAB

-----END PUBLIC KEY-----

`

const publicKey = createPublicKey(pemPublicKey)

console.log(publicKey.export({ format: 'jwk' }))

it will return object:

{

  kty: 'RSA',

  n: [modulus string],

  e: [exponent string]

}