> For the complete documentation index, see [llms.txt](https://ctf.zeyu2001.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ctf.zeyu2001.com/2021/umdctf-2021/whose-base-is-it-anyway.md).

# Whose Base Is It Anyway

## Problem

Someone gave this ciphertext but I'm not sure how to solve it or what they based it off of. Seems tricky but can you figure it out?

## Solution

From first glance, we can see that the cipher is full of `A`s and `B`s. This is binary, i.e. base 2. After we decode this, we will get another string with `A`s, `B`s, `C`s and `D`s. This would be base 4.

This continues until base 64, where we are finally able to obtain our flag (base 64 decode).

![](/files/-MYc5mUQau98l5_KxXYd)

Note that for base 32, we are using a custom alphabet.

![](/files/-MYc5pOKceMrrgIb0Neu)

Base 64 decode the obtained string:

![](/files/-MYc5roIsXTOU2mkCkiL)

Script:

```python
from Crypto.Util.number import long_to_bytes
import base64

cipher = "    @                @            A    @                @        @    @    @        @        @            A    @            @    @        @        @                @                @        @        @        @    @    @                @        @    A    @            A    @        @    @    @                @            A    @        @    @    @        @    @    @                @        @    A    @            @    @        @    A    @                @            A    @            @    @        @    @    @        @        @            A    @            @    @        @        @                @            A    @                @        @    @    @        @        @            A    @            @    @        @        @                @            @    @        @        @        @    @    @                @        @    A    @            A    @            @    @                @            A    @                @        @    @    @        @        @        @    A    @            @    @        @    A    @                @                @        @        @        @    @    @                @        @    A    @            @    @        @        @                @            A    @                @        @    A    @            @    @            A    @            @    @        @        @                @            A    @                @        @    @    @                @        @    A    @            @    @        @    @    @                @            A    @        @    @    @        @    @    @        @    A    @            A    @            @    @        @    A    @                @        @    @    @            @    @        @    @    @                @        @    A    @            A    @        A        @                @            A    @        @        @        @    @    @                @            A    @            @    @        @    A    @                @                @            @    @        @    @    @                @        @    A    @            A    @                @                @            A    @        @    @    @        @    @    @            A    @        @    A    @            @    @        @        @                @            @    @        @        @        @    @    @        @        @            A    @            A    @        @        @                @            A    @                @        @    @    @        @    @    @            A    @            @    @        @    A    @                @        @        @        @        @        @    @    @                @        @    A    @            A    @                @                @            A    @        @    @    @        @    @    @        @        @            A    @            @    @        @        @                @            @    @        @    @    @        @    @    @        @        @            A    @            @    @        @    A    @                @            A    @        @    @    @        @    @    @                @        @    A    @            @    @        @    A    @                @                @            @    @        @    @    @                @        @    A    @            A    @        @        @                @            A    @                @        @    @    @        @    A    @        @    A    @            @    @        @    A    @                @        @        @        @    @    @        @    @    @                @        @    A    @            A    @            @    @                @            A    @        @    @    @        @    @    @        @    A    @            A    @            @    @        @        @                @                @        @        @        @    @    @        @        @            A    @            @    @        @    @    @                @            A    @        @    @    @        @    @    @        @    A    @        @    A    @            @    @        @    A    @                @        @        @        @        @        @    @    @                @        @    A    @            A    @        A        @                @            A    @                @        @    @    @            A    @            A    @            @    @        @        @                @            A    @                @        @    @    @                @        @    A    @            @    @        @    @    @                @            A    @        @    @    @        @    @    @                @        @    A    @            @    @        @    A    @                @        @        @                @        @    @    @        @        @        @    A    @            @    @                @                @            A    @                @        @    @    @        @        @            A    @            @    @        @        @                @                @        @    @    @        @    @    @        @        @        @    A    @            @    @        @    @    @                @            A    @        @    @    @        @    @    @            A    @            @"

def converter(figures, base):

    #invert oder of figures (lowest count first)
    figures = figures[::-1]
    result = 0
    #loop over all figures
    for i in range(len(figures)):
        #add the contirbution of the i-th figure
        result += figures[i]*base**i
    return result

curr_base = 2

print(cipher)

done = False
while not done:

    figures = []
    for char in cipher:

        if char.islower() :
            figures.append(26 + ord(char) - ord('a'))

        elif ord(char) >= ord('A'):
            diff = ord(char) - ord('A')
            figures.append(diff)

    while len(figures) % 8:
        figures.append(0)

    converted = converter(figures, curr_base)

    result = long_to_bytes(converted)
    # print(result)
    cipher = result.decode()

    print(curr_base)
    print(cipher)

    curr_base *= 2

    if curr_base == 64:
        break
```


---

# 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://ctf.zeyu2001.com/2021/umdctf-2021/whose-base-is-it-anyway.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.