algorithm - How does one make a Zip bomb?

Question

This question about zip bombs naturally led me to the Wikipedia page on the topic. The article mentions an example of a 45.1 kb zip file that decompresses to 1.3 exabytes.

What are the principles/techniques that would be used to create such a file in the first place? I don't want to actually do this, more interested in a simplified "how-stuff-works" explanation of the concepts involved.

The article mentions 9 layers of zip files, so it's not a simple case of zipping a bunch of zeros. Why 9, why 10 files in each?

Answer 1

Citing from the Wikipedia page:

One example of a Zip bomb is the file 45.1.zip which was 45.1 kilobytes of compressed data, containing nine layers of nested zip files in sets of 10, each bottom layer archive containing a 1.30 gigabyte file for a total of 1.30 exabytes of uncompressed data.

So all you need is one single 1.3GB file full of zeroes, compress that into a ZIP file, make 10 copies, pack those into a ZIP file, and repeat this process 9 times.

This way, you get a file which, when uncompressed completely, produces an absurd amount of data without requiring you to start out with that amount.

Additionally, the nested archives make it much harder for programs like virus scanners (the main target of these "bombs") to be smart and refuse to unpack archives that are "too large", because until the last level the total amount of data is not that much, you don't "see" how large the files at the lowest level are until you have reached that level, and each individual file is not "too large" - only the huge number is problematic.