func base64url_decode(s string) ([]byte, error) {
    base64Str := strings.Map(func(r rune) rune {
        switch r {
        case ‘-‘:
            return ‘+‘
        case ‘_‘:
            return ‘/‘
        }

        return r
    }, s)

    if pad := len(base64Str) % 4; pad > 0 {
        base64Str += strings.Repeat("=", 4-pad)
    }

    return base64.StdEncoding.DecodeString(base64Str)
}