Try this. :)

https://kel.bz/post/lll/

(If you work through the prerequisites and use "understanding this post" as a sort of roadmap of what you actually need to know, this gets you about 2/3rds through undergraduate linear algebra, and you can skim through nullspaces --- all in the service of learning a generally useful tool for attacking cryptosystems).

Thanks for the notes. This is marvellous. I do not work on, or have interest in cryptography algorithms, but this is such an interesting read.