I've decided to solve the reversing challenges using radare2, a free and open source reverse engineering framework. I have first learned about r2 back in 2011. during a huge project, where I had to reverse a massive, 11MB statically linked ELF. I simply needed something that I could easily patch Linux ELFs with. Granted, back then I've used r2 alongside IDA, and only for smaller tasks, but I loved the whole concept at first sight. Since then, radare2 evolved a lot, and I was planning for some time now to solve some crackmes with the framework, and write writeups about them. Well, this CTF gave me the perfect opportunity :)

Because this writeup aims to show some of r2's features besides how the crackmes can be solved, I will explain every r2 command I use in blockquote paragraphs like this one:

r2 tip: Always use ? or -h to get more information!

If you know r2, and just interested in the crackme, feel free to skip those parts! Also keep in mind please, that because of this tutorial style I'm going to do a lot of stuff that you just don't do during a CTF, because there is no time for proper bookkeeping (e.g. flag every memory area according to its purpose), and with such small executables you can succeed without doing these stuff.

A few advice if you are interested in learning radare2 (and frankly, if you are into RE, you should be interested in learning r2 :) ):

The framework has a lot of supplementary executables and a vast amount of functionality - and they are very well documented. I encourage you to read the available docs, and use the built-in help (by appending a ? to any command) extensively! E.g.:

[0x00000000]> ?
Usage: [.][times][cmd][~grep][@[@iter]addr!size][|>pipe] ; ...
Append '?' to any char command to get detailed help
Prefix with number to repeat command N times (f.ex: 3x)
|%var =valueAlias for 'env' command
| *off[=[0x]value]     Pointer read/write data/values (see ?v, wx, wv)
| (macro arg0 arg1)    Manage scripting macros
| .[-|(m)|f|!sh|cmd]   Define macro or load r2, cparse or rlang file
| = [cmd]              Run this command via rap://
| /                    Search for bytes, regexps, patterns, ..
| ! [cmd]              Run given command as in system(3)
| # [algo] [len]       Calculate hash checksum of current block
| #!lang [..]          Hashbang to run an rlang script
| a                    Perform analysis of code
| b                    Get or change block size


[0x00000000]> a?
|Usage: a[abdefFghoprxstc] [...]
| ab [hexpairs]     analyze bytes
| aa                analyze all (fcns + bbs) (aa0 to avoid sub renaming)
| ac [cycles]       analyze which op could be executed in [cycles]
| ad                analyze data trampoline (wip)
| ad [from] [to]    analyze data pointers to (from-to)
| ae [expr]         analyze opcode eval expression (see ao)
| af[rnbcsl?+-*]    analyze Functions
| aF                same as above, but using anal.depth=1


Also, the project is under heavy development, there is no day without commits to the GitHub repo. So, as the readme says, you should always use the git version!

Some highly recommended reading materials: