Lazarus

Free Pascal => Beginners => Topic started by: TomTom on August 14, 2019, 01:06:39 pm

Title: How to modify file checksum?
Post by: TomTom on August 14, 2019, 01:06:39 pm

Hi :)
I have many files that need to have their checksums changed. How can I do this? Is this even possible?
These are tif images. I know I could add single pixel at random position and checksum will change but Im wondering if there is some more elegant way (not changing image).

Title: Re: How to modify file checksum?
Post by: howardpc on August 14, 2019, 01:42:23 pm

A checksum is a derived statistic.
Unless the binary representation of the file changes in some way, its checksum will not change (unless the checksum calculation algorithm is substandard).

Title: Re: How to modify file checksum?
Post by: TomTom on August 14, 2019, 01:49:54 pm

So... I need to change something in the file like in my case, set random pixel to random color for example.. or change some meta-data... Am I right?

Title: Re: How to modify file checksum?
Post by: marcov on August 14, 2019, 01:51:51 pm

Some checksums have certain properties. E.g. you make the file 4 bytes longer to n+4, and take the checksum of the first n bytes, and craft a value to put in the four extra bytes so that the value over n+4 bytes is what you want.

Title: Re: How to modify file checksum?
Post by: Thaddy on August 14, 2019, 02:35:23 pm

If the hash is over the full TTiffIFD record, you can possibly modify the date, or one of the document properties, like the model of the scanner  8-)