Summary#

This challenge introduces the concept of modulo (mod) arithmetic, a fundamental concept in cryptography.

It can be solved either manually or using a simple Python script.

Analysis#

So we got a file messgae.txt:

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$ file message.txt
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message.txt: ASCII text, with no line terminators

Its content is:

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350 63 353 198 114 369 346 184 202 322 94 235 114 110 185 188 225 212 366 374 261 213

What is modulo?#

Modulo is a mathematical operation that finds the remainder when one number is divided by another. For example:

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10 mod 3 = 1  (because 10 ÷ 3 = 3 remainder 1)
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350 mod 37 = 17

In cryptography, modulo is often used to wrap numbers into a fixed range, such as mapping numbers to letters or digits.

Challenge#

According to the challenge instructions:

1. Take each number mod 37
2. Map the result to a character using this mapping:

• 0–25 → A–Z
• 26–35 → 0–9
• 36 → _

3. Combine the decoded characters.
4. Wrap the result in the format: picoCTF{decrypted_message}

Decryption#

We calculate each number modulo 37 and map it to the corresponding character:

Decrypted Message#

Putting it together:

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R0UND_N_R0UND_ADD17EC2

Similar can be done with Python:

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numbers = [350, 63, 353, 198, 114, 369, 346, 184, 202, 322, 94, 235, 114, 110, 185, 188, 225, 212, 366, 374, 261, 213]
2

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def decode(n):
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    r = n % 37
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    if 0 <= r <= 25:
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        return chr(ord('A') + r)
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    elif 26 <= r <= 35:
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        return chr(ord('0') + (r - 26))
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    else:
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        return "_"
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flag = "".join(decode(n) for n in numbers)
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print("picoCTF{" + flag + "}")

⚡ Raikiri

🎉 Flag pwned!

Wrapped in picoCTF flag format:

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picoCTF{R0UND_N_R0UND_ADD17EC2}

💡 TL;DR / Lesson Learned

This challenge demonstrates how the modulo operation can be used in cryptography to map numbers to a fixed set of symbols, such as letters, digits, or special characters.