Why does AWS Cognito use multiple public keys for JWTs?

Question

When I download the JWT set for a particular User Pool available at: https://cognito-idp.{region}.amazonaws.com/{userPoolId}/.well-known/jwks.json

The JSON contains 2 keys. All the users that I have created for the pool seem to use just one of these keys.

What is the reason for having multiple keys per User Pool?

Answer 1

According to the documentation:

Amazon Cognito generates two pairs of RSA cryptograpic keys for each user pool. One of the private keys is used to sign the token.

Presumably this is for security reasons. From limited trial-and-error it appears that one is used to encrypt id tokens and the other is used to encrypt access tokens. Or perhaps the purpose is to support key rotation (as suggested by @Michael-sqlbot).

---

Once you understand the why (or don't understand), the question becomes what to do with the two keys.

Again referring to the documentation, to validate the JWT signature you need to:

1. Decode the ID token
2. Compare the local key ID (kid) to the public kid
3. Use the public key to verify the signature using your JWT library.

Steps 1 and 2 are necessary to figure out which RSA cryptograpic key was used to encrypt your JWT.

import jsonwebtoken from 'jsonwebtoken'
import jwkToPem from 'jwk-to-pem'

const jsonWebKeys = [  // from https://cognito-idp.us-west-2.amazonaws.com/<UserPoolId>/.well-known/jwks.json
    {
        "alg": "RS256",
        "e": "AQAB",
        "kid": "ABCDEFGHIJKLMNOPabc/1A2B3CZ5x6y7MA56Cy+6ubf=",
        "kty": "RSA",
        "n": "...",
        "use": "sig"
    },
    {
        "alg": "RS256",
        "e": "AQAB",
        "kid": "XYZAAAAAAAAAAAAAAA/1A2B3CZ5x6y7MA56Cy+6abc=",
        "kty": "RSA",
        "n": "...",
        "use": "sig"
    }
]

function validateToken(token) {
    const header = decodeTokenHeader(token)  // {"kid":"XYZAAAAAAAAAAAAAAA/1A2B3CZ5x6y7MA56Cy+6abc=", "alg": "RS256"}
    const jsonWebKey = getJsonWebKeyWithKID(header.kid)
    verifyJsonWebTokenSignature(token, jsonWebKey, function(err, decodedToken) {
        if (err) {
            console.error(err)
        } else {
            console.log(decodedToken)
        }
    })
}

function decodeTokenHeader(token) {
    const [headerEncoded] = token.split('.')[0]
    const buff = new Buffer(headerEncoded, 'base64')
    const text = buff.toString('ascii')
    return JSON.parse(text)
}

func getJsonWebKeyWithKID(kid) {
    for (let jwk of jsonWebKeys) {
        if (jwk.kid == kid) {
            return jwk
        }
    }
    return null
}

function verifyJsonWebTokenSignature(token, jsonWebKey, clbk) {
    const pem = jwkToPem(jsonWebKey)
    jsonwebtoken.verify(token, pem, { algorithms: ['RS256'] }, function(err, decodedToken) {
        return clbk(err, decodedToken)
    })
}

validateToken('xxxxxxxxx.XXXXXXXX.xxxxxxxx')