> 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/otp-implementation.md).

# OTP implementation

This challenge is a great example of how to use **GDB Python scripting** to "brute force" a flag when the math is too annoying to reverse by hand.

### 1. The Goal

We are given a program (`otp`) and an encrypted string. To get the flag, we need to find a **100-character key** that the program likes. Once we have that key, we XOR it with the encrypted string to reveal the flag.

### 2. The Problem: The "Jumble"

When you type a key into the program, it doesn't just check it. It "jumbles" it:

* The first character you type is scrambled.
* Every character after that is scrambled **based on the previous character’s result**.
* Finally, the program adds the letter `'a'` to everything and checks if it matches a hardcoded 100-character string: `mlaebfkoib...`

Because each character depends on the one before it, we can't just guess the whole 100-character key at once. But we **can** guess them one by one.

### 3. The Strategy: The "Checkpoint"

Instead of doing the math, we use GDB (the debugger) to do the work for us. We set a **breakpoint** at the exact moment the program finishes jumbling a character.

#### The Breakpoint Logic (`0x55555555499d`)

We tell GDB: *"Stop right here. Look at the memory where the jumbled character is stored, and tell me what it is."*

#### The Script Logic

We wrote a Python script that runs inside GDB:

1. **Try a character:** It runs the program with `key + '0'`.
2. **Check the result:** The breakpoint triggers and tells the script: *"The jumbled version of that character is 'x'."*
3. **Compare:** If 'x' matches the character we need (from `mlaebfkoib...`), we keep it and move to the next position.
4. **Repeat:** If it doesn't match, we try `'1'`, then `'2'`, and so on.

### 4. The Final Step: XOR

After the script runs for a few minutes, it finds all 100 characters of the key.

* **Key found:** `6fa2e2a25c3b5869...`
* **Ciphertext:** `1fcb81cd1f6f1e12...`

When you XOR these two hex strings together, the "jumble" cancels out, and the ASCII text appears.

### Final Result

**Flag:** `picoCTF{cust0m_jumbl3s_4r3nt_4_g0Od_1d3A_ca692500}`


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/reverse-engineering/otp-implementation.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.