使用纯 PHP 验证两个文件是否相同?

When an user uploads a new file to the system, I compute SHA-1 hash of the uploaded file contents and then check if a file with identical hash already exists in the storage backend. PHP puts the uploaded file in /tmp before my code gets to run and then I run sha1sum against the uploaded file to get SHA-1 hash of the file contents. I then compute fanout from the computed SHA-1 hash and decide storage directory under NFS mounted directory hierarchy. (For example, if the SHA-1 hash for a file contents is 37aefc1e145992f2cc16fabadcfe23eede5fb094 the permanent file name is /nfs/data/files/37/ae/fc1e145992f2cc16fabadcfe23eede5fb094.) In addition to saving the actual file contents, I INSERT a new line into a SQL database for the user submitted meta data (e.g. Content-Type, original filename, datestamp, etc).

我目前想出的极端情况是，新上传的文件具有 SHA-1 哈希值，该哈希值与存储后端中的现有哈希值相匹配.我知道这种意外发生的变化是天文数字低，但我想确定.(对于故意案例，请参阅 https://shattered.io/)

The corner case I'm currently figuring out is the case where a new uploaded file has SHA-1 hash that matches existing hash in the storage backend. I know that the changes for this happening by accident are astronomically low, but I'd like to be sure. (For on purpose case, see https://shattered.io/)

给定两个文件名$file_a和$file_b，如何快速检查两个文件的内容是否相同?假设文件太大加载到内存中.对于 Python，我会使用 filecmp.cmp() 但 PHP 似乎没有任何类似的东西.我知道这可以通过 fread() 完成并在找到不匹配的字节时中止，但我宁愿不编写该代码.

Given two filenames $file_a and $file_b, how to quickly check if both files have identical contents? Assume that files are too big to be loaded into memory. With Python, I'd use filecmp.cmp() but PHP does not seem to have anything similar. I know that this can be done with fread() and aborting if a non-matching byte is found, but I'd rather not write that code.

推荐答案

如果你已经有一个 SHA1 和，你可以简单地做:

If you already have one SHA1 sum, you can simply do:

if ($known_sha1 == sha1_file($new_file))

否则

if (filesize($file_a) == filesize($file_b)
    && md5_file($file_a) == md5_file($file_b)
)

也检查文件大小，以在一定程度上防止散列冲突(这已经非常不可能了).也使用 MD5，因为它比 SHA 算法快得多(但不那么独特).

Checking file size too, to somewhat prevent a hash collision (which is already very unlikely). Also using MD5 because it's significantly faster than the SHA algorithms (but a little less unique).

更新:

这是如何准确地相互比较两个文件.

This is how to exactly compare two files against each other.

function compareFiles($file_a, $file_b)
{
    if (filesize($file_a) != filesize($file_b))
        return false;

    $chunksize = 4096;
    $fp_a = fopen($file_a, 'rb');
    $fp_b = fopen($file_b, 'rb');
        
    while (!feof($fp_a) && !feof($fp_b))
    {
        $d_a = fread($fp_a, $chunksize)
        $d_b = fread($fp_b, $chunksize);
        if ($d_a === false || $d_b === false || $d_a !== $d_b)
        {
            fclose($fp_a);
            fclose($fp_b);
            return false;
        }
    }
 
    fclose($fp_a);
    fclose($fp_b);
          
    return true;
}

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