Check if same image has already been uploaded by comparing BASE64?

Question

My question is concerning an idea I had, where I could check if an image has already been uploaded by comparing their base64-encoded strings...

Example use-case would be to find duplicates in your database...

The operation would be pretty big i guess - first converting the image to base64and then using something like "strcmp()" to compare..

Not sure if this would make a lot of sense but what do you think of the idea?

Would it be too big of an operation? How accurate would it be? Does the idea make any sense?

A hash is smaller and this is what it is for: http://php.net/md5_file — AbraCadaver, May 07 '15 at 17:14

score 2 · Accepted Answer · answered May 08 '15 at 01:10

Here's a function that can help you compare files faster.

Aside from checking an obvious thing like file size, you can play more with comparing binary chunks.
For example, check the last n bytes as well as a chunk of a random offset.

I used checksum comparison as a last resort.

When optimizing check order, you can also take into account if you are generally expecting files to be different or not.

function areEqual($firstPath, $secondPath, $chunkSize = 500){

    // First check if file are not the same size as the fastest method
    if(filesize($firstPath) !== filesize($secondPath)){
        return false;
    }

    // Compare the first ${chunkSize} bytes
    // This is fast and binary files will most likely be different 
    $fp1 = fopen($firstPath, 'r');
    $fp2 = fopen($secondPath, 'r');
    $chunksAreEqual = fread($fp1, $chunkSize) == fread($fp2, $chunkSize);
    fclose($fp1);
    fclose($fp2);

    if(!$chunksAreEqual){
        return false;
    }

    // Compare hashes
    // SHA1 calculates a bit faster than MD5
    $firstChecksum = sha1_file($firstPath);
    $secondChecksum = sha1_file($secondPath);
    if($firstChecksum != $secondChecksum){
        return false;
    }

    return true;
}

I like the idea! :) Thank you for the function! – der-lukas May 09 '15 at 06:17 — der-lukas, May 09 '15 at 06:17

score 1 · Answer 2 · answered May 07 '15 at 17:08

1

If I would do something like this I would use md5 hash instead of base64_encode.

$equal = ( md5($image1) == md5($image2)) ? true : false;

answered May 07 '15 at 17:08

gotha

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Hm, could you explain why? – der-lukas May 07 '15 at 17:09
Probably `md5_file()` – AbraCadaver May 07 '15 at 17:16
Several megabytes encoded image will be quite big string to compare. [md5_file](http://php.net/md5_file) will be faster. If it is stored in database, its a little bit more complicated, but I think it still will be faster. I havent checked so the best solution will be to try them both and see which one performs better. – gotha May 07 '15 at 17:16
This worked well for comparing byte arrays (not base64 string) – Nickson Yap Oct 08 '19 at 04:11

Check if same image has already been uploaded by comparing BASE64?

2 Answers2