How to Patch Binary with IDA Pro

Last night I received a couple of emails from friends of mine asking how to patch binaries through IDA Pro. I was pretty tired for writing a full answering email so I decided to send them a good link... I wasn't able to find out a good link on the argument. :O !!

For this reason today I want to show you how to patch a binary using IDA Pro. The patching process is maybe the most difficult part of reversing engineering since you need to modify directly the binary code without compromising the program control flow. You need to figure out what the binary does, where it does what, and how it performs the actions you want to modify. For example, if we consider a key-generator design process what is mainly needed is the key generation function. In order to build the generation procedure (in the key-generator)we need to copy the "generator function" (in the binary) and paste it on a generic assembler "print out" template. Running the compiled template you will see as output the generated keys. If you prefer to patch the original key-generator you need inject/modify/delete instructions directly on the binary without altering the control flow and without triggering exception handlers.