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Scratch.java

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Base64;

class Scratch {
    public static void main(String[] args) throws IOException {
        String pathPrefix = "C:\\Users\\Administrator\\AppData\\Roaming\\Google\\AndroidStudio2023.1\\scratches";
        String filename = "20240101-123사4567-X-10-1234678.TXT";

        byte key = getKey(filename, Charset.forName("EUC-KR")); // 0x5A

        System.out.printf("key: 0x%02X\n", key);

        byte[] in = new String(Files.readAllBytes(Path.of(pathPrefix, "src.txt"))).replaceAll("\r\n", "").getBytes(Charset.forName("EUC-KR"));
        byte[] out = encrypt(in, key);

        Files.write(Path.of(pathPrefix, filename), out);
        Files.write(Path.of(pathPrefix, "base64_" + filename), Base64.getEncoder().encode(out));
    }

    public static byte getKey(String password, Charset charset) {
        int acc = 0;
        byte[] bytes = password.getBytes(charset);

        for (byte aByte : bytes) {
            acc += aByte & 255;
        }

        byte key = (byte) (acc & 255);
        if (key == 0) {
            key = 1;
        }

        return key;
    }

    public static byte[] encrypt(byte[] var1, byte var2) {
        return a(var1, var2);
    }

    public static byte[] decrypt(byte[] var1, byte var2) {
        return a(var1, var2);
    }

    public static byte[] a(byte[] in, byte key) {
        int len;
        byte[] out = new byte[len = in.length];

        for (int i = 0; i < len; ++i) {
            out[i] = (byte) (in[i] ^ key);
        }

        return out;
    }
}