You're given a executable file match_me

Let's see what it does

$ ./match_me
12 <input_guess>
Nope <output>

Lets have a look at dynamic library calls for no input.

$ ltrace ./match_me

__libc_start_main(0x400a03, 1, 0x7ffcd5f460a8, 0x400ad0 <unfinished ...>
malloc(1000)                                                                                = 0x133c010
scanf(0x400b6d, 0x133c010, 0x133c010, 0x7f217ad32b20)                                       = 0xffffffff
malloc(1000)                                                                                = 0x133c810
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strncmp("firhfgferfibbqlkdfhh", "[[[[[[[[[[[[[[[[[[[[", 20)                                 = 11
puts("Nope"Nope
)                                                                                = 5
+++ exited (status 0) +++

Interesting. strcmp is being called to compare a hardcoded string "firhfgferfibbqlkdfhh" with some other string. Lets run again the ltrace with input "12" (without quotes)

$ ltrace ./match_me

__libc_start_main(0x400a03, 1, 0x7ffcd5f460a8, 0x400ad0 <unfinished ...>
malloc(1000)                                                                                = 0x133c010
scanf(0x400b6d, 0x133c010, 0x133c010, 0x7f217ad32b20)                                       = 0xffffffff
malloc(1000)                                                                                = 0x133c810
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strlen("firhfgferfibbqlkdfhh")                                                              = 20
strncmp("firhfgferfibbqlkdfhh", "S[[[[[[[[[[[[[[[[[[[", 20)                                 = 11
puts("Nope"Nope
)                                                                                = 5
+++ exited (status 0) +++

Nice, so we see that integer 12 is mapped to "S" (See the change of compared string from "[[[[[[..." to "S[[[..."). So, all we have to do is write a simple script to find mappings of all integers from 1-100 to corresponding characters.

So, we get 59:f, 76:i, 65:r and so on.

Key with all mappings done = firhfgferfibbqlkdfhh: 5976657559745958655976555564937857597575 Let's check our executable with above key

$ ./match_me
5976657559745958655976555564937857597575
Match

Yay! Now we need to send this key to defcon.org.in:8082

$ echo "5976657559745958655976555564937857597575" | nc defcon.org.in 8082

Flag: d4rk{595c7f5b595a59587f595c55557e5f5e57595b5b}c0de