Instruction Counting

Instruction counting can be a useful technique to solve some password checkers and similar binaries. When verification operations are run character by character, counting instructions can find the validity of a password character by character. That way, it is sometimes possible to dynamically solve these binaries without even looking at the assembly.

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Some password checking and similar binaries (e.g. a binary accepting a value as input and returning whether that value is valid) check input validity character by character. When the execution paths for valid and invalid characters differ, so will the number of executed instructions. So you can run the binary with all possible inputs for the first character and compare the number of instructions run for each. The outlier might be the valid character. Repeat this for each character until you have a fully valid password.

$ qemu-x86_64 -d exec,nochain ./pwd-checker foobar 2>&1 | wc -l

450302

Caution

The same technique works by using perf or similar tools. However, this introduces a lot of kernel (scheduling, etc.) noise into the measurements. So, when possible, use instruction counting instead!