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import java.util.Arrays;
import javax.crypto.*;
import javax.crypto.spec.*;
import java.security.*;

class Msc61 {
    public static final int GCM_TAG_LENGTH = 16;
    public static final int GCM_IV_LENGTH = 12;

    public static SecretKey generateKey() {
        try {
            KeyGenerator kgen = KeyGenerator.getInstance("AES");
            kgen.init(128);
            return kgen.generateKey();
        } catch (NoSuchAlgorithmException e) {
            throw new IllegalStateException(e.toString());
        }
    }

    public static byte[] encrypt_gcm(SecretKey skey, String plaintext)  {
        /* Precond: skey is valid and GCM mode is available in the JRE;
         * otherwise IllegalStateException will be thrown. */
        try {
            byte[] ciphertext = null;
            Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");            
            byte[] initVector = new byte[GCM_IV_LENGTH];
            (new SecureRandom()).nextBytes(initVector);
            GCMParameterSpec spec = new GCMParameterSpec(GCM_TAG_LENGTH * java.lang.Byte.SIZE, initVector);
            cipher.init(Cipher.ENCRYPT_MODE, skey, spec);
            byte[] encoded = plaintext.getBytes(java.nio.charset.StandardCharsets.UTF_8);
            ciphertext = new byte[initVector.length + cipher.getOutputSize(encoded.length)];
            for (int i=0; i < initVector.length; i++) {
                ciphertext[i] = initVector[i];
            }
            // Perform encryption
            cipher.doFinal(encoded, 0, encoded.length, ciphertext, initVector.length);
            return ciphertext;
        } catch (NoSuchPaddingException | InvalidAlgorithmParameterException | ShortBufferException |
            BadPaddingException | IllegalBlockSizeException | InvalidKeyException | NoSuchAlgorithmException e) 
        {
            /* None of these exceptions should be possible if precond is met. */
            throw new IllegalStateException(e.toString());
        }
    }

    public static String decrypt_gcm(SecretKey skey, byte[] ciphertext) 
        throws BadPaddingException, IllegalBlockSizeException /* these indicate corrupt or malicious ciphertext */
        /* Note that AEADBadTagException may be thrown in GCM mode; this is a subclass of BadPaddingException */
    {
        /* Precond: skey is valid and GCM mode is available in the JRE;
         * otherwise IllegalStateException will be thrown. */
        try {
            Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");            
            byte[] initVector = Arrays.copyOfRange(ciphertext, 0, GCM_IV_LENGTH);
            GCMParameterSpec spec = new GCMParameterSpec(GCM_TAG_LENGTH * java.lang.Byte.SIZE, initVector);
            cipher.init(Cipher.DECRYPT_MODE, skey, spec);
            byte[] plaintext = cipher.doFinal(ciphertext, GCM_IV_LENGTH, ciphertext.length - GCM_IV_LENGTH);
            return new String(plaintext);
        } catch (NoSuchPaddingException | InvalidAlgorithmParameterException |
            InvalidKeyException | NoSuchAlgorithmException e) 
        {
            /* None of these exceptions should be possible if precond is met. */
            throw new IllegalStateException(e.toString());
        }
    }
}