> 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/pico-bank.md).

# Pico Bank

A banking Android app that hides a flag across two locations: binary-encoded transaction values and a server OTP endpoint.

### Step 1 — Decompile the APK

Open JADX-GUI and drag in `picobank.apk`. Navigate to:

```
com.example.picobank → Login
```

You'll find hardcoded credentials:

* **Username:** `johnson`
* **Password:** `tricky1990`

### Step 2 — Part 1: Binary Transactions

In `MainActivity`, transaction amounts are suspiciously binary-looking. As the notification hints:

![Notification hint](https://github.com/user-attachments/assets/d56d7880-4cc9-4110-b499-1ef18b2392fb)

Convert each binary amount to decimal → ASCII:

```python
transactions = [
    "1110000",  "1101001", "1100011", "1101111",
    "1000011",  "1010100", "1000110", "1111011",
    "110001",   "1011111", "1101100", "110001",
    "110011",   "1100100", "1011111", "110100",
    "1100010",  "110000",  "1110101", "1110100",
    "1011111",  "1100010", "110011",  "110001",
    "1101110",  "1100111", "1011111"
]
result = ''.join(chr(int(b, 2)) for b in transactions)
print(result)
# → picoCTF{1_l13d_4b0ut_b31ng_
```

### Step 3 — Part 2: OTP Endpoint

In the `OTP` class, find the hardcoded OTP `9673` and the `/verify-otp` endpoint. Send:

```bash
curl -X POST http://amiable-citadel.picoctf.net:58539/verify-otp \
  -H "Content-Type: application/json" \
  -d '{"otp":"9673"}'
```

Response:

![Server response](https://github.com/user-attachments/assets/90b0c23c-83ba-4cd7-b43e-0c3fee90ba65)

```json
{"success":true,"flag":"s3cur3d_m0b1l3_l0g1n_1ff8ddb7}"}
```

### Full Flag

Combining both parts:

```
picoCTF{1_l13d_4b0ut_b31ng_s3cur3d_m0b1l3_l0g1n_1ff8ddb7}
```


---

# 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/pico-bank.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.