Decryptor.java

import javax.crypto.*;
import javax.crypto.spec.*;
import java.io.*;
import java.nio.file.*;
import java.security.*;
import java.security.spec.*;
import java.util.Base64;

public class Decryptor {

    // ===== FOLDERS =====
    static final Path ENCRYPTED_DIR = Paths.get("encrypted");
    static final Path OUTPUT_DIR = Paths.get("output");
    static final Path USER_KEYS = Paths.get("keys/User");

    static final String USER_PRIVATE_KEY = "private.key";

    // ===== CRYPTO CONSTANTS =====
    static final int GCM_IV_SIZE = 12;
    static final int GCM_TAG_SIZE = 128;

    public static void main(String[] args) throws Exception {
        Files.createDirectories(OUTPUT_DIR);
        PrivateKey userPriv = loadPrivateKey(USER_KEYS.resolve(USER_PRIVATE_KEY));

        Files.walk(ENCRYPTED_DIR)
                .filter(p -> p.toString().endsWith(".enc"))
                .forEach(encFile -> {
                    try {
                        decryptFile(encFile, userPriv);
                        System.out.println("✔ Decrypted: " + encFile);
                    } catch (Exception e) {
                        System.out.println("❌ Failed: " + encFile + " (" + e.getMessage() + ")");
                    }
                });

        System.out.println("Decryption complete. Exiting...");
    }

    static void decryptFile(Path encFile, PrivateKey rsaPriv) throws Exception {
        Path keyFile = encFile.resolveSibling(encFile.getFileName() + ".key");
        if (!Files.exists(keyFile))
            throw new SecurityException("Missing key file: " + keyFile);

        byte[] encKey = Files.readAllBytes(keyFile);
        Cipher rsa = Cipher.getInstance("RSA/ECB/OAEPWithSHA-256AndMGF1Padding");
        rsa.init(Cipher.DECRYPT_MODE, rsaPriv);
        byte[] aesBytes = rsa.doFinal(encKey);
        SecretKey aesKey = new SecretKeySpec(aesBytes, "AES");

        Path relative = ENCRYPTED_DIR.relativize(encFile);
        Path outFile = OUTPUT_DIR.resolve(relative.toString().replace(".enc", ""));
        Files.createDirectories(outFile.getParent());

        try (DataInputStream in = new DataInputStream(new BufferedInputStream(Files.newInputStream(encFile)));
                OutputStream out = Files.newOutputStream(outFile)) {

            while (true) {
                int len;
                try {
                    len = in.readInt();
                } catch (EOFException e) {
                    break; // finished
                }

                byte[] iv = in.readNBytes(GCM_IV_SIZE);
                byte[] encrypted = in.readNBytes(len);

                Cipher aes = Cipher.getInstance("AES/GCM/NoPadding");
                aes.init(Cipher.DECRYPT_MODE, aesKey, new GCMParameterSpec(GCM_TAG_SIZE, iv));

                byte[] plain = aes.doFinal(encrypted);
                out.write(plain);
            }
        }

        Files.delete(encFile);
        Files.delete(keyFile);
    }

    static PrivateKey loadPrivateKey(Path path) throws Exception {
        byte[] b = Base64.getDecoder().decode(Files.readAllBytes(path));
        return KeyFactory.getInstance("RSA").generatePrivate(new PKCS8EncodedKeySpec(b));
    }
}