Security.java

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.SecureRandom;
import java.util.Random;

import lombok.NonNull;
import lombok.SneakyThrows;
import lombok.experimental.UtilityClass;

@UtilityClass
public class Security {
	private final Random RANDOM = new SecureRandom();
	
	public String bytesToHexNibbles(@NonNull byte[] in, boolean spaceSeparated) {
		if (in.length == 0) return "";
		StringBuilder out = new StringBuilder(in.length * 2 + (spaceSeparated ? in.length - 1 : 0));
		for (int i = 0; i < in.length; i++) {
			if (spaceSeparated && out.length() > 0) out.append(' ');
			out.append(String.format("%02X", in[i]));
		}
		return out.toString();
	}
	
	public byte[] hexNibblesToBytes(@NonNull String in) {
		in = in.replaceAll("\\s+", "");
		if (in.length() == 0) return new byte[0];
		if ((in.length() & 1) != 0) throw new IllegalArgumentException("Expected an even number of (non-space) characters.");
		byte[] out = new byte[in.length() / 2];
		int s = 0;
		for (int i = 0; i < in.length(); i++) {
			int v = in.charAt(i);
			
			if (v >= '0' && v <= '9') v -= '0';
			else if (v >= 'A' && v <= 'F') v = v - 'A' + 10;
			else if (v >= 'a' && v <= 'f') v = v - 'a' + 10;
			else throw new IllegalArgumentException("Expected a hex nibble (0-9, A-F).");
			
			if ((i & 1) == 0) s = v << 4;
			else out[i / 2] = (byte) (s + v);
		}
		return out;
	}
	
	private final class Storage {
		private static byte[] SYSTEM_KEY;
		
		static {
			SYSTEM_KEY = loadSystemKey();
		}
		
		@SneakyThrows(IOException.class)
		private static byte[] loadSystemKey() {
			Path keyFile = Paths.get(HomeFinder.homeDir(Security.class), "server.key");
			if (!Files.isRegularFile(keyFile)) throw throwServerKeyFileProblem(keyFile, "file is missing");
			String raw = new String(Files.readAllBytes(keyFile), StandardCharsets.UTF_8);
			byte[] res = hexNibblesToBytes(raw);
			if (res.length != 32) throw throwServerKeyFileProblem(keyFile, "file is not properly formatted (expected 64 hex digits).");
			return  res;
		}
		
		private static RuntimeException throwServerKeyFileProblem(Path keyFile, String msg) throws IOException {
			byte[] rnd = new byte[32];
			RANDOM.nextBytes(rnd);
			throw new IOException(String.format("Problem with server.key: %s\nPaste this content into file %s and restart the server:\n%s",
				msg,
				keyFile.toAbsolutePath().toString(),
				bytesToHexNibbles(rnd, true)
			));
		}
	}
	
	@SneakyThrows
	public final byte[] getSystemKey() throws IOException {
		try {
			return Storage.SYSTEM_KEY;
		} catch (ExceptionInInitializerError e) {
			Throwable cause = e.getCause();
			throw cause == null ? e : cause;
		}
	}

	public void main(String[] args) throws IOException {
		System.out.println("The system key is: " + bytesToHexNibbles(getSystemKey(), true));
	}
}