Rating:
# string-cheese
## Overview
- Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆
- 644 solves / 112 points
## Background
> Author: aplet123
I'm something of a cheese connoisseur myself. If you can guess my favorite flavor of string cheese, I'll even give you a flag. Of course, since I'm lazy and socially inept, I slapped together a program to do the verification for me.
Connect to my service at `nc lac.tf 31131`
Note: The attached binary is the exact same as the one executing on the remote server.
## Find the flag
**In this challenge, we can download a [file](https://github.com/siunam321/CTF-Writeups/blob/main/LA-CTF-2023/Rev/caterpillar/string_cheese):**
```shell
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:36:34(HKT)]
└> file string_cheese
string_cheese: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=85f5294fa950449028c1ef7655304e4e873172d2, for GNU/Linux 3.2.0, not stripped
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:36:35(HKT)]
└> chmod +x string_cheese
```
It's a 64-bit ELF executable, and it's not stripped.
**Let's try to run it:**
```shell
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:36:37(HKT)]
└> ./string_cheese
What's my favorite flavor of string cheese? idk
Hmm... I don't think that's quite it. Better luck next time!
```
So, we need to provide the correct favorite flavor of string cheese.
**Now, let's use `strings` to list out all the strings inside that binary:**
```shell
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:38:22(HKT)]
└> strings string_cheese
[...]
flag.txt
Cannot read flag.txt.
What's my favorite flavor of string cheese?
blueberry
...how did you know? That isn't even a real flavor...
Well I guess I should give you the flag now...
Hmm... I don't think that's quite it. Better luck next time!
[...]
```
Found it!
- Favorite flavor of string cheese: `blueberry`
**Or, you can use `ltrace` to display calls that are made to shared libraries:**
```shell
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:38:23(HKT)]
└> ltrace ./string_cheese
printf("What's my favorite flavor of str"...) = 44
fflush(0x7f4fcbc7b760What's my favorite flavor of string cheese? ) = 0
fgets(test
"test\n", 256, 0x7f4fcbc7aa80) = 0x7ffc4d00c500
strcspn("test\n", "\n") = 4
strcmp("test", "blueberry") = 18
puts("Hmm... I don't think that's quit"...Hmm... I don't think that's quite it. Better luck next time!
) = 61
+++ exited (status 0) +++
```
**Finally, `nc` to the challenge port, and read the flag:**
```shell
┌[siunam♥earth]-(~/ctf/LA-CTF-2023/Rev/string-cheese)-[2023.02.11|15:39:42(HKT)]
└> nc lac.tf 31131
What's my favorite flavor of string cheese? blueberry
...how did you know? That isn't even a real flavor...
Well I guess I should give you the flag now...
lactf{d0n7_m4k3_fun_0f_my_t4st3_1n_ch33s3}
```
- **Flag: `lactf{d0n7_m4k3_fun_0f_my_t4st3_1n_ch33s3}`**
# Conclusion
What we've learned:
1. Using `strings` & `ltrace` To Extract Hidden String
