How to Encrypt Using A1Z26#

1. Take the plaintext letter.
2. Find its position in the alphabet.
3. Replace the letter with its corresponding number.

For example, “A” becomes 1, “B” becomes 2, and so on up to “Z” = 26.
Decoding is simply the reverse process: convert numbers back into letters to recover the original text.

Now let’s decrypt our flag:

Split into the part before the braces and the part inside:

• Before braces: 16 9 3 15 3 20 6
• Inside braces: 20 8 5 14 21 13 2 5 18 19 13 1 19 15 14

Convert each number using A1Z26 (A=1, B=2, …, Z=26):

Before braces

• 16 → P
• 9 → I
• 3 → C
• 15 → O
• 3 → C
• 20 → T
• 6 → F

Assembled: P I C O C T F → PICOCTF

Inside braces

• 20 → T
• 8 → H
• 5 → E
• 14 → N
• 21 → U
• 13 → M
• 2 → B
• 5 → E
• 18 → R
• 19 → S
• 13 → M
• 1 → A
• 19 → S
• 15 → O
• 14 → N

Assembled: T H E N U M B E R S M A S O N → THENUMBERSMASON

Using the standard flag format, the final flag is: PICOCTF{THENUMBERSMASON}

⚡ Raikiri

🎉 Flag pwned! The numeric ciphertext has been decoded successfully.

This can also be solved using online tools like dcode.fr.

💡 TL;DR / Lesson Learned

Number-to-Letter (A1Z26) ciphers are simple yet effective for learning about encoding.