Category: Reverse Engineering, 30 points
What integer does this program print with arguments 182476535 and 3742084308? File: chall.S Flag format: picoCTF{XXXXXXXX} -> (hex, lowercase, no 0x, and 32 bits. ex. 5614267 would be picoCTF{0055aabb})
Since this is a ARMv8 assembly we can't run it in most desktop computers so we have to think (reverse engineer) our way through this one.
Reading the calling convention, adding comments next to the assembly lines and
We notice that there are 2 calls to atoi which converts char* to int, and we
know that the arguments passed to main are found in argv, which of type
pointer to char*, which fits to the usage of atoi to convert the arguments
from char* to int.
Afterwards there's a call to func1, which checks: w1 <= w0, which given
w0 = 182476535 and w1 = 3742084308, the CMP is false and there's no jump
in the next instruction bls, which means then that w0 = w1, and the
function returns, and from there we maintain w0 and which is then copied to
w1 using mov w1, w0 and is printed by the call to printf with first
argument "Result: %ld\n", and w1 as second argument.
We convert that value, 3742084308 to hex, and we get the contents of the flag
as specified in the description of the file.
picoCTF{df0bacd4}