0xcpu/elementary.md

## elementary.md

      
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              elementary.md
            
          
    Search in IDA for mov     eax, 0 and exclude from results those occuring in main function. (Let's save all this in a file named step1)
Then filter the information from step1 to extract only addresses, For example, using this command:
awk '{ print $1 }' step1 | cut -d':' -f2 > avoid_addr
Then use angr script to find a solution.
import angr
import claripy

# angr uses 0x400000 as base address for PIE executables

START = 0x40071a # start of main
FIND  = 0x40077f # Good job message basic block
AVOID = [0x400786] # Wrong messages bassic block
with open("avoid_addr", "r") as fin:
    for l in fin:
        # add other addresses to avoid, all those "mov eax, 0"
        AVOID.append(0x400000 + int(l.strip(), 16))

BUF_LEN = 104


def char(state, c):
    return state.solver.And(c <= '~', c >= ' ')


def main():
    p = angr.Project("elementary")

    flag = claripy.BVS('flag', BUF_LEN * 8)
    state = p.factory.blank_state(addr=START, stdin=flag)

    for c in flag.chop(8):
        state.solver.add(char(state, c))

    ex = p.factory.simulation_manager(state)
    ex.use_technique(angr.exploration_techniques.Explorer(find=FIND, avoid=AVOID))

    ex.run()

    return ex.found[0].posix.dumps(0).decode("utf-8")


if __name__ == '__main__':
    print("flag: {}".format(main()))
Profit: flag: p4{I_really_hope_you_automated_this_somehow_otherwise_it_might_be_a_bit_frustrating_to_do_this_manually}