当我使用MemoryStream时，为什么这两个文件哈希值相同? [英] Why do these two files hash to the same value when I use MemoryStream?

问题描述

我正在编写一个c#例程，该例程从jpg文件创建哈希.如果我将字节数组传递给SHA512对象，则会得到预期的行为，但是，如果我传递内存流，则两个文件始终会哈希为相同的值.

I'm writing a c# routine that creates hashes from jpg files. If I pass in a byte array to my SHA512 object then I get the expected behavior, however, if I pass in a memory stream the two files always hash to the same value.

示例1:

        SHA512 mySHA512 = SHA512.Create();

        Image img1 = Image.FromFile(@"d:\img1.jpg");
        Image img2 = Image.FromFile(@"d:\img2.jpg");
        MemoryStream ms1 = new MemoryStream();
        MemoryStream ms2 = new MemoryStream();

        img1.Save(ms1, ImageFormat.Jpeg);
        byte[] buf1 = ms1.GetBuffer();
        byte[] hash1 = mySHA512.ComputeHash(buf1);

        img2.Save(ms2, ImageFormat.Jpeg);
        byte[] buf2 = ms2.GetBuffer();
        byte[] hash2 = mySHA512.ComputeHash(buf2);

        if (Convert.ToBase64String(hash1) == Convert.ToBase64String(hash2))
            MessageBox.Show("Hashed the same");
        else
            MessageBox.Show("Different hashes");

产生不同的散列".但是ComputeHash方法的重载之一将流对象引入其中，而我宁愿使用它.当我这样做时:

That produces "Different hashes". But one of the overloads of the ComputeHash method takes a stream object in and I'd rather use that. When I do:

        SHA512 mySHA512 = SHA512.Create();

        Image img1 = Image.FromFile(@"d:\img1.jpg");
        Image img2 = Image.FromFile(@"d:\img2.jpg");
        MemoryStream ms1 = new MemoryStream();
        MemoryStream ms2 = new MemoryStream();

        img1.Save(ms1, ImageFormat.Jpeg);
        byte[] hash1 = mySHA512.ComputeHash(ms1);

        img2.Save(ms2, ImageFormat.Jpeg);
        byte[] hash2 = mySHA512.ComputeHash(ms2);

        if (Convert.ToBase64String(hash1) == Convert.ToBase64String(hash2))
            MessageBox.Show("Hashed the same");
        else
            MessageBox.Show("Different hashes");

这会产生哈希相同".

我在想什么呢?

推荐答案

您不会回退MemoryStream，因此哈希是从空字节序列中计算出来的.使用

You're not rewinding your MemoryStreams, so the hash is computed from an empty sequence of bytes. Use

ms1.Position = 0;
ms2.Position = 0;

在调用Save之后.

还有一点要注意:不要以这种方式使用GetBuffer.使用ToArray可以为您提供一个字节流，其大小与流的长度相同-GetBuffer返回原始缓冲区，该缓冲区通常(通常)具有一些填充，您不希望意外使用该填充.当然，如果您确保只使用它的相关部分，则可以使用GetBuffer-这样可以避免创建新的数据副本.

One further note: don't use GetBuffer in this way. Use ToArray which will give you a byte array the same size as the stream's length - GetBuffer returns the raw buffer which will (usually) have some padding, which you wouldn't want to use accidentally. You can use GetBuffer if you then make sure you only use the relevant portion of it, of course - this avoids creating a new copy of the data.

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