King Caesar

Overview

We are given a ciphertext string Whnqrfrp{Wk1v_H4jx4w_1u_4_K0l3_V1pso3u_Ym4s_A0w} under the challenge title "King Caesar". The description explains that this secret message has been shifted using a repeating letter-shift pattern, namely 3, 5, 2, 4, 1. The challenge is to find the real flag, because applying a uniform ROT13 or even a uniform ROT23 only produces a fake flag.

Analysis

The first step is to observe the flag format, which is usually of the form {...} with words separated by underscores. Each word inside the flag is treated as a single unit, which is then shifted according to the pattern. So the first word is shifted by 3 letters, the second word by 5, the third word by 2, the fourth word by 4, the fifth word by 1, and then the pattern repeats for the following words.

Exploitation

We apply this pattern to the ciphertext:

• Word 1 Wk1v, shifted back 3 letters, becomes Th1s.
• Word 2 H4jx4w, shifted back 5 letters, becomes C4es4r.
• Word 3 1u, shifted back 2 letters, becomes 1s.
• Word 4 4, shifted by 4, stays 4 because it is a digit.
• Word 5 K0l3, shifted back 1 letter, becomes J0k3.
• Word 6 V1pso3u, shifted back 3 letters, becomes S1mpl3r.
• Word 7 Ym4s, shifted back 5 letters, becomes Th4n.
• Word 8 A0w, shifted back 2 letters, becomes Y0u.

After processing every word according to the pattern, the final result is the hidden real flag.

Flag

Teknocom{Th1s_C4es4r_1s_4_J0k3_S1mpl3r_Th4n_Y0u}