public static SslClientAuth forConfig(String key) {
        if (key == null) {
            return SslClientAuth.NONE;
        }
        String upperCaseKey = key.toUpperCase(Locale.ROOT);
        for (SslClientAuth auth : VALUES) {
            if (auth.name().equals(upperCaseKey)) {
                return auth;
            }
        }
        return null;
    }
Describes whether the server should require or request client authentication.
clients/src/main/java/org/apache/kafka/common/config/SslClientAuth.java