Go で JWK を使用して JWT 署名を検証する方法

type myClaims struct {
    jwt.StandardClaims
}

func main() {
    claims := &myClaims{}

    _, err := jwt.ParseWithClaims("ey...", claims, getKey)
    if err != nil {
        log.Fatal(err)
    }
}

const jwksURL = "https://companyx.okta.com/oauth2/v1/keys"

func getKey(token *jwt.Token) (interface{}, error) {
    // TODO: Cache the response to avoid repeated HTTP requests
    set, err := jwk.FetchHTTP(jwksURL)
    if err != nil {
        return nil, err
    }
    keyID, ok := token.Header["kid"].(string)
    if !ok {
        return nil, errors.New("expecting JWT header to have string kid")
    }
    if key := set.LookupKeyID(keyID); len(key) == 1 {
        return key[0].Materialize()
    }
    return nil, fmt.Errorf("unable to find key %q", keyID)
}