How to implement authorization using Keycloak

Question

I created a REST API in Go that is necessary an authorization layer, for this layer I am trying use Keycloak. The API will be consumed by a third-party backend service, anyone knows the workflow to integrate Go client and keycloak or already implemented it? I figured out an adapter called Gocloak but in its documentation there is not any example for this purpose.

Answer 1

Authorization is typically application specific, so I can't help much there, but here's some information on authenticating JWTs from Keycloak. After JWTs are authenticated, you can use their claims to authorize the request.

Keycloak exposes what's known as a JSON Web Key Set (JWKS). This resource should be used to authenticate JWTs. I've wrote a package for this purpose. It's an extension of github.com/golang-jwt/jwt/v4.

The package is called github.com/MicahParks/keyfunc. I've pasted the code example for Keycloak below.

package main

import (
    "log"
    "time"

    "github.com/golang-jwt/jwt/v4"

    "github.com/MicahParks/keyfunc"
)

func main() {
    // Get the JWKS URL.
    //
    // This is a local Keycloak JWKS endpoint for the master realm.
    jwksURL := "http://localhost:8080/auth/realms/master/protocol/openid-connect/certs"

    // Create the keyfunc options. Use an error handler that logs. Refresh the JWKS when a JWT signed by an unknown KID
    // is found or at the specified interval. Rate limit these refreshes. Timeout the initial JWKS refresh request after
    // 10 seconds. This timeout is also used to create the initial context.Context for keyfunc.Get.
    options := keyfunc.Options{
        RefreshErrorHandler: func(err error) {
            log.Printf("There was an error with the jwt.Keyfunc\nError: %s", err.Error())
        },
        RefreshInterval:   time.Hour,
        RefreshRateLimit:  time.Minute * 5,
        RefreshTimeout:    time.Second * 10,
        RefreshUnknownKID: true,
    }

    // Create the JWKS from the resource at the given URL.
    jwks, err := keyfunc.Get(jwksURL, options)
    if err != nil {
        log.Fatalf("Failed to create JWKS from resource at the given URL.\nError: %s", err.Error())
    }

    // Get a JWT to parse.
    jwtB64 := "eyJhbGciOiJQUzM4NCIsInR5cCIgOiAiSldUIiwia2lkIiA6ICJMeDFGbWF5UDJZQnR4YXFTMVNLSlJKR2lYUktudzJvdjVXbVlJTUctQkxFIn0.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.Rxrq41AxbWKIQHWv-Tkb7rqwel3sKT_R_AGvn9mPIHqhw1m7nsQWcL9t2a_8MI2hCwgWtYdgTF1xxBNmb2IW3CZkML5nGfcRrFvNaBHd3UQEqbFKZgnIX29h5VoxekyiwFaGD-0RXL83jF7k39hytEzTatwoVjZ-frga0KFl-nLce3OwncRXVCGmxoFzUsyu9TQFS2Mm_p0AMX1y1MAX1JmLC3WFhH3BohhRqpzBtjSfs_f46nE1-HKjqZ1ERrAc2fmiVJjmG7sT702JRuuzrgUpHlMy2juBG4DkVcMlj4neJUmCD1vZyZBRggfaIxNkwUhHtmS2Cp9tOcwNu47tSg"

    // Parse the JWT.
    token, err := jwt.Parse(jwtB64, jwks.Keyfunc)
    if err != nil {
        log.Fatalf("Failed to parse the JWT.\nError: %s", err.Error())
    }

    // Check if the token is valid.
    if !token.Valid {
        log.Fatalf("The token is not valid.")
    }
    log.Println("The token is valid.")

    // End the background refresh goroutine when it's no longer needed.
    jwks.EndBackground()
}