> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/reverse-engineering/lets-get-dynamic.md).

# Let's get Dynamic

We are given a binary (`chall`) and need to find the correct input (flag). Static inspection shows complex operations, so we use **dynamic analysis with GDB**.

***

### Step 1: Analyze the binary

Opened the binary in GDB:

```bash
gdb ./chall
```

Disassembled `main`:

```bash
disas main
```

Observed:

* Input is taken using `fgets`
* Some transformations (XOR operations) are applied
* Final comparison is done using `memcmp`

Key instruction:

```asm
0x0000000000400623 <+381>: call   0x4003a0 <memcmp@plt>
```

***

### Step 2: Set breakpoint at correct location

Instead of breaking on all `memcmp` calls (which includes libc noise), set breakpoint at the exact instruction:

```bash
b *0x400623
```

***

### Step 3: Run the program

```bash
r
```

Entered any input:

```
a
```

***

### Step 4: Inspect registers at breakpoint

When execution stops at the breakpoint, inspect arguments passed to `memcmp`.

On x86\_64:

* `$rdi` → first argument (input)
* `$rsi` → second argument (expected value)
* `$rdx` → length

Printed the second argument:

```bash
printf "%s\n", $rsi
```

***

### Step 5: Extract the flag

Output:

```
picoCTF{dyn4m1c_4n4ly1s_1s_5up3r_us3ful_039b10e}
```

This is the correct flag.

***

### Flag

```
picoCTF{dyn4m1c_4n4ly1s_1s_5up3r_us3ful_039b10e}
```


---

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