code: private byte[] resetAndRepeatDigest(byte[] digestData) {
  		final int digestCount = Math.max(1, this.digestCount);
  		reset();
  		for (int i = 0; i < digestCount - 1; i++) {
  			digestData = doDigest(digestData);
  			reset();
  		}
  		return digestData;
  	}
docs: 重复计算摘要，取决于{@link #digestCount} 值<br>
  每次计算摘要前都会重置{@link #digest}
  
  @param digestData 第一次摘要过的数据
  @return 摘要