Securinets Beginner CTF — Vigenère

Challenge Description

“We intercepted this strange message from an old-school cryptographer. Can you recover the flag?”

This challenge tests knowledge of classical ciphers, specifically the Vigenère cipher. Participants are given a ciphertext that follows the standard CTF flag format, allowing them to perform a known-plaintext attack to recover the encryption key and decrypt the full message.

Solution Walkthrough

🧠 Step 1: Observe the Ciphertext Structure

The provided ciphertext is: “TEOVFJNQUG{WISFBFRQ_JG_FAEZ}”

The format strongly resembles SECURINETS{...}, which is the expected flag prefix. This suggests:

TEOVFJNQUG → SECURINETS
The rest inside the braces is the encrypted payload.

🔑 Step 2: Recover the Vigenère Key

There are two practical ways to recover the key:

✅ Method 1: Using the Vigenère Table (Manual)

Refer to the Vigenère square (also called tabula recta), as explained on Bibmath.net:

To find the key letter: locate the plaintext letter on the top row, then find the ciphertext letter in that column. The row header is the key letter.

For example:

Plaintext S, Ciphertext T → Key = B
Plaintext E, Ciphertext E → Key = A
Plaintext C, Ciphertext O → Key = M

Repeating this for all 10 letters:

Plain	S	E	C	U	R	I	N	E	T	S
Cipher	T	E	O	V	F	J	N	Q	U	G
Key	B	A	M	B	O	B	A	M	B	O

→ Key = BAMBO

📎

✅ Method 2: Using Modular Arithmetic (Programmatic)

As described on GeeksforGeeks – Vigenère Cipher, the Vigenère cipher follows:

Encryption: C[i] = (P[i] + K[i]) mod 26
Decryption: P[i] = (C[i] - K[i]) mod 26
Key recovery: K[i] = (C[i] - P[i]) mod 26

Convert letters to numbers (A=0, B=1, ..., Z=25):

Plain	S(18)	E(4)	C(2)	U(20)	R(17)	I(8)	N(13)	E(4)	T(19)	S(18)
Cipher	T(19)	E(4)	O(14)	V(21)	F(5)	J(9)	N(13)	Q(16)	U(20)	G(6)
Key	(19−18)=1 → B	(4−4)=0 → A	(14−2)=12 → M	…	→ BAMBO

This confirms the same key: BAMBO.

🔓 Step 3: Decrypt the Full Ciphertext

Using the key BAMBO, decrypt the entire ciphertext (ignoring non-alphabetic characters like {, _, }):

Ciphertext: WISFBFRQ_JG_FAEZ
Key (repeated): BAMBOBAM_BO_MBAM

Apply Vigenère decryption:

🏁 Final Flag

Putting it all together: Flag: SECURINETS{VIGENERE_IS_EASY}

📚 References

💡 Tip: For CTFs, always check if the ciphertext matches the expected flag format — it often gives you the plaintext needed to recover the key!