Added user login and account creation

backend/src/main/java/achievements/misc/Password.java

@@ -0,0 +1,95 @@
+package achievements.misc;
+
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.Random;
+
+public class Password {
+
+	private static final Random RANDOM = new SecureRandom();
+
+	public final String salt;
+	public final String hash;
+
+	private Password(String salt, String hash) {
+		this.salt = salt;
+		this.hash = hash;
+	}
+
+	public static Password generate(String password) {
+		// Generate the salt
+		var salt = new byte[16]; // 128 bits
+		RANDOM.nextBytes(salt);
+
+		return new Password(
+			encode(salt),
+			encode(hash(salt, password.getBytes()))
+		);
+	}
+
+	public static boolean validate(String salt, String password, String hash) {
+		System.out.println(salt + ", " + password);
+		var srcHash    = hash(decode(salt), password.getBytes());
+		var targetHash = decode(hash);
+		for (int i = 0; i < srcHash.length; ++i) {
+			if (srcHash[i] != targetHash[i]) {
+				return false;
+			}
+		}
+		return true;
+	}
+
+	private static byte[] hash(byte[] salt, byte[] password) {
+		try {
+			var concat = new byte[salt.length + password.length];
+			int i = 0;
+			for (; i < salt.length; ++i) {
+				concat[i] = salt[i];
+			}
+			for (int j = 0; j < password.length; ++j) {
+				concat[i + j] = password[j];
+			}
+
+			var md = MessageDigest.getInstance("SHA-256");
+			return md.digest(concat);
+		} catch (NoSuchAlgorithmException e) {
+			e.printStackTrace();
+		}
+		return null;
+	}
+
+	private static String encode(byte[] bytes) {
+		var chars = new char[bytes.length << 1];
+		for (int i = 0; i < bytes.length; ++i) {
+			chars[(i << 1)    ] = toHex(bytes[i] >> 0);
+			chars[(i << 1) + 1] = toHex(bytes[i] >> 4);
+		}
+		return new String(chars);
+	}
+
+	private static byte[] decode(String data) {
+		var decoded = new byte[data.length() >> 1];
+		for (int i = 0; i < data.length(); i += 2) {
+			int currentByte =
+				(fromHex(data.charAt(i    ))     ) |
+				(fromHex(data.charAt(i + 1)) << 4);
+			decoded[i >> 1] = (byte) (currentByte & 0xFF);
+		}
+		return decoded;
+	}
+
+	private static char toHex(int halfByte) {
+		halfByte = halfByte & 0xF;
+		if (0  <= halfByte && halfByte <= 9 ) return (char) (halfByte + '0'     );
+		if (10 <= halfByte && halfByte <= 15) return (char) (halfByte + 'a' - 10);
+		return '0';
+	}
+
+	private static int fromHex(char c) {
+		if ('0' <= c && c <= '9') return c - '0';
+		if ('A' <= c && c <= 'F') return c - 'A' + 10;
+		if ('a' <= c && c <= 'f') return c - 'a' + 10;
+		return 0;
+	}
+}