code: public static Certificate getCertificate(KeyStore keyStore, String alias) {
  		try {
  			return keyStore.getCertificate(alias);
  		} catch (Exception e) {
  			throw new CryptoException(e);
  		}
  	}
docs: 获得 Certification
  
  @param keyStore {@link KeyStore}
  @param alias 别名
  @return {@link Certificate}