Reversing

next [count]

// Continue to the next source line in the current (innermost) stack frame. This is similar to step, but function calls that appear within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the original stack level that was executing when you gave the next command. This command is abbreviated n.

// An argument count is a repeat count, as for step.

004005ef <+50>:    mov    rax,QWORD PTR [rbp-0x10]
004005f3 <+54>:    add    rax,0x8
004005f7 <+58>:    mov    rax,QWORD PTR [rax]
004005fa <+61>:    mov    esi,0x4006da
004005ff <+66>:    mov    rdi,rax
00400602 <+69>:    call   0x4004b0 <strcmp@plt>

(gdb) b main

(gdb) run BBBB

(gdb) b *0x0000000000400602

(gdb) n

(gdb) x/10s $rsi
0x4006da:    "AAAA-Z10N-42-OK"
0x4006ea:    "Access Granted!"

la strcmp viene fatta tra i due registri rsi e rdi, e ritorna zero se contengono stringhe uguali;

ltrace ./license_1 AAAA
__libc_start_main(0x4005bd, 2, 0x7ffccebfc2d8, 0x400640 <unfinished ...>
printf("Checking License: %s\n", "AAAA"Checking License: AAAA
)                                                  = 23
strcmp("AAAA", "AAAA-Z10N-42-OK")                                                         = -45
puts("WRONG!"WRONG!
)                                                                            = 7
+++ exited (status 0) +++ 

C Code

#include <string.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
        if(argc==2) {
        printf("Checking License: %s\n", argv[1]);
        if(strcmp(argv[1], "AAAA-Z10N-42-OK")==0) {
            printf("Access Granted!\n");
        } else {
            printf("WRONG!\n");
        }
    } else {
        printf("Usage: <key>\n");
    }
    return 0;
}