code: public static byte[] generateAesKey(final int keysize) {
  		try {
  			KeyGenerator keyGenerator = KeyGenerator.getInstance(AES_NAME);
  			keyGenerator.init(keysize);
  			SecretKey secretKey = keyGenerator.generateKey();
  			return secretKey.getEncoded();
  		} catch (NoSuchAlgorithmException e) {
  			throw new ImpossibleException(e);
  		}
  	}
docs: 生成 AES 密钥，可选长度为 128,192,256 位。
  
  @param keysize
  密钥长度
  @return 密钥