취약하거나 위험한 암호화 알고리즘 사용

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import java.security.MessageDigest;
import java.security.PrivateKey;
import java.security.Signature;

public class LegacyCrypto {
    // Insecure: ECB exposes plaintext patterns
    public byte[] encryptEcb(byte[] plaintext, SecretKey key) throws Exception {
        Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE, key);
        return cipher.doFinal(plaintext);
    }

    // Insecure: MD5 is collision-prone and fast to brute-force
    public byte[] weakDigest(byte[] data) throws Exception {
        MessageDigest md = MessageDigest.getInstance("MD5");
        return md.digest(data);
    }

    // Insecure: SHA1-based signature is deprecated
    public byte[] signSha1(byte[] data, PrivateKey privKey) throws Exception {
        Signature sig = Signature.getInstance("SHA1withRSA");
        sig.initSign(privKey);
        sig.update(data);
        return sig.sign();
    }
}

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import java.nio.ByteBuffer;
import java.security.PrivateKey;
import java.security.SecureRandom;
import java.security.Signature;
import java.util.Base64;

public class ModernCrypto {
    // Secure: AES-GCM (AEAD) with random 96-bit nonce, 128-bit tag
    public byte[] encryptGcm(byte[] plaintext, SecretKey key) throws Exception {
        byte[] nonce = new byte[12];
        SecureRandom.getInstanceStrong().nextBytes(nonce);
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec spec = new GCMParameterSpec(128, nonce);
        cipher.init(Cipher.ENCRYPT_MODE, key, spec);
        byte[] ct = cipher.doFinal(plaintext);
        return ByteBuffer.allocate(nonce.length + ct.length).put(nonce).put(ct).array();
    }

    // Secure password hashing with PBKDF2 (salt + high iteration)
    public String hashPassword(char[] password) throws Exception {
        byte[] salt = new byte[16];
        SecureRandom.getInstanceStrong().nextBytes(salt);
        PBEKeySpec spec = new PBEKeySpec(password, salt, 600_000, 256);
        SecretKeyFactory skf = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
        byte[] hash = skf.generateSecret(spec).getEncoded();
        return Base64.getEncoder().encodeToString(salt) + ":" + Base64.getEncoder().encodeToString(hash);
    }

    // Strong signature with SHA-256
    public byte[] signStrong(byte[] data, PrivateKey privKey) throws Exception {
        Signature sig = Signature.getInstance("SHA256withRSA");
        sig.initSign(privKey);
        sig.update(data);
        return sig.sign();
    }
}