使用MessageDigest SHA-256的POI XSSF / XLSX散列不确定性 [英] POI XSSF / XLSX hashing indeterminism with MessageDigest SHA-256

问题描述

使用MessageDigest SHA-256实现获取POI XLSX格式的确定性哈希值似乎存在问题，即使对于空的ByteArray流也是如此。这种情况在数百次甚至数千次迭代后随机发生。

There seems to be a problem with getting deterministic hash values for the POI XLSX format, with MessageDigest SHA-256 implementation, even for empty ByteArray streams. This happens randomly, after several hundreds or even only thousands of iterations.

用于重现问题的相关代码片段：

The relevant code snippets used to reproduce the problem:

// TestNG FileTest:
@Test(enabled = true) // indeterminism at random iterations, such as 400 or 1290
public void emptyXLSXTest() throws IOException, NoSuchAlgorithmException {
    final Hasher hasher = new HasherImpl();
    boolean differentSHA256Hash = false;
    for (int i = 0; i < 10000; i++) {
        final ByteArrayOutputStream excelAdHoc1 = BusinessPlanInMemory.getEmptyExcel("xlsx");
        final ByteArrayOutputStream excelAdHoc2 = BusinessPlanInMemory.getEmptyExcel("xlsx");

        byte[] expectedByteArray = excelAdHoc1.toByteArray();
String expectedSha256 = hasher.sha256(expectedByteArray);
byte[] actualByteArray = excelAdHoc2.toByteArray();
String actualSha256 = hasher.sha256(actualByteArray);

if (!expectedSha256.equals(actualSha256)) {
            differentSHA256Hash = true;
            System.out.println("ITERATION: " + i);
            System.out.println("EXPECTED HASH: " + expectedSha256);
            System.out.println("ACTUAL HASH: " + actualSha256);
            break;
        }
    }
    Assert.assertTrue(differentSHA256Hash, "Indeterminism did not occur");
}

参考Hasher和POI代码：

Referenced Hasher and POI code:

// HasherImpl class:
public String sha256(final InputStream stream) throws IOException, NoSuchAlgorithmException {
    final MessageDigest digest = MessageDigest.getInstance("SHA-256");
    final byte[] bytesBuffer = new byte[300000]; 
    int bytesRead = -1;
    while ((bytesRead = stream.read(bytesBuffer)) != -1) {
        digest.update(bytesBuffer, 0, bytesRead);
    }
    final byte[] hashedBytes = digest.digest();
    return bytesToHex(hashedBytes);
}

试图消除因创建时间等元数据导致的不确定性，但无济于事：

Tried to eliminate indeterminism due to meta data like creation time, to no avail:

// POI BusinessPlanInMemory helper class:
public static ByteArrayOutputStream getEmptyExcel(final String fileextension) throws IOException {
    Workbook wb;

    if (fileextension.equals("xls")) {
        wb = new HSSFWorkbook();
    }
    else {
        wb = new XSSFWorkbook();
        final POIXMLProperties props = ((XSSFWorkbook) wb).getProperties();
        final POIXMLProperties.CoreProperties coreProp = props.getCoreProperties();
        coreProp.setCreated("");
        coreProp.setIdentifier("1");
        coreProp.setModified("");
    }

    wb.createSheet();

    final ByteArrayOutputStream excelStream = new ByteArrayOutputStream();
    wb.write(excelStream);
    wb.close();
    return excelStream;
}

HSSF / XLS格式似乎不受所述问题的影响。
是否有人有线索，如果不是POI本身的错误，可能是什么导致这种情况？基本上，上面的代码是指
https://poi.apache.org/spreadsheet/ examples.html BusinessPlan示例

The HSSF / XLS format seems not to be affected by the problem described. Does anybody have a clue, what could be causing this, if not a bug in POI itself? Basically, the code above refers to https://poi.apache.org/spreadsheet/examples.htmlBusinessPlan example

感谢您的投入！

推荐答案

这不是一个明确的答案，但这是我的怀疑会发生什么：

This is not a definitive answer but this is my suspicion what happens:

docx和xlsx文件格式基本上是一堆压缩的xml文件。将它们重命名为.zip并使用您喜欢的zip工具打开时很容易看到。

docx and xlsx file formats are basically a bunch of zipped-up xml-files. This can easily be seen when renaming them to .zip and opening with your favorite zip-tool.

当检查由word创建的文件时，我注意到更改时间戳存档中包含的所有文件始终为 1980-01-01 00:00:00 ，而在使用POI创建的文件中，它将显示文件的实际时间戳。

When examining a file created by word I noticed that the change-timestamp of all files contained in the archive is always 1980-01-01 00:00:00 while in those created with POI it will show the actual timestamp the file was created.

所以我怀疑当 excelAdHoc1 中的一个或多个文件之间存在时间戳差异时，会出现问题和 excelAdHoc2 。当创建一个或另一个文件时，时钟切换到下一秒可能会发生这种情况。

So my I suspect that your problem occurs when there is a timestamp-difference between one or more of the files in excelAdHoc1 and excelAdHoc2. This might happen when the clock switches to the next second while creating one or the other file.

这不会影响XLS文件，因为HSSF格式不属于zipped xml-type因此不包含任何可能具有不同时间戳的嵌套文件。

This would not affect XLS-files since the HSSF-format is not of the "zipped xml"-type and thus does not contain any nested files that might have different timestamps.

要在写入文件后更改时间戳，可以尝试使用`java .util.zip``包。我没有测试过，但这应该可以解决问题：

To change the timestamps after writing the file you could try using the `java.util.zip``-package. I haven't tested it but this should do the trick:

ZipFile file = new ZipFile(pathToFile);
Enumeration<ZipEntry> e = file.entries();
while(e.hasMoreElements()) {
    ZipEntry entry = e.nextElement();
    entry.setTime(0L);
}

这篇关于使用MessageDigest SHA-256的POI XSSF / XLSX散列不确定性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！