There is no check for third block which means it can be any random characters of length four.ġ4. Then it takes the second block 0x1, get the int value and adds it to the block 1 esi value from above step and place it into esi register itself.ġ3. Next it takes the first block 0x0 gets the int value and place it into esi register mov esi, eax.ġ2. If all the above passes, it gets each block (chunk separated by dash) and check if length is four.ġ1. Second check is mov dword, 0x2034 which then checks the number of components separated by - and compares it with cmp eax, 0x4, which means, the serial key has four dashes in it. The first conditional check is cmp eax, 0x13 which checks if the serial number is 19 decimals long.ĩ. Go back to the running program and enter some serial number and click validate, which will make the debugger stop at the breakpoint.Ĩ. So add to the initial address 00001b2d 55 push ebp under that section.ħ. Now is the time to add breakpoints so that we can step and check register values as the program runs. Check the pseudo code to get an idea of the algorithm.Ħ. Check the left pane for the available methods. Click the Play icon under controls which will run the Sandwich.app under the debugger.ĥ. The path will be auto filled to something like /Users/./Sandwich.app/Contents/MacOS/Sandwich. Double click to launch the server window.Ĥ. and the server will be listed under local servers section. Now in the Hopper, navigate to Debug->Select Debugger. Download, install and run the app which will launch the debug server locally.ģ. Instead of remote server, we will connect to the current system using Hopper Debugger Server. Load the Sandwich.app in Hopper Disassembler, choose the default and proceed to disassembly screen.Ģ. Patching is just easy as changing one instruction.ġ. The goal is to produce a keygen as opposed to patching. This is a small app (by HAWKE, 2009) which requires us to find the correct serial key. This is an explanation of these techniques as well as a tutorial on how to reverse OS X crackme challenge ( 1-Sandwitch.zip | mirror). But we can have an isolated machine where the program can be run and analysed remotely. However when we analyse malware, running them is dangerous. This is very useful to understand the program's algorithm and values much easier than static binary analysis. Dynamic code analysis in reverse engineering binaries is where we execute the binary for analysis, adding breakpoints, inspecting processor state and such.
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