An ancient encryption method guards the flag. Master the art of frequency analysis and crack the Caesar cipher to reveal the hidden message.
Decrypt the Caesar cipher encrypted message to reveal the flag.
You intercept an encrypted message that appears to use classical cryptography. The encryption is weak by modern standards - can you break it?
Caesar cipher shifts each letter by a fixed number
There are only 25 possible shifts to try
Look for common English words like "THE" or "FLAG"
Frequency analysis can help identify the shift
Spoiler Warning: The solution below contains the complete walkthrough. Try to solve the challenge yourself first!
Caesar cipher is a simple substitution cipher where each letter is shifted by a fixed number of positions.
Caesar cipher shifts each letter in the alphabet by a constant value. With only 26 letters, there are only 25 possible keys.
Use a tool or script to try all 25 possible shifts.
Look through the outputs for readable text. The flag format will be obvious.
CTF{C4ES4R_CIPHER_IS_TOO_EASY}Classical ciphers are vulnerable to brute force attacks
Frequency analysis can reveal patterns in encrypted text
Modern encryption uses much stronger algorithms
Never use Caesar cipher for real-world encryption
1# Python example
2def caesar_decrypt(text, shift):
3 result = ""
4 for char in text:
5 if char.isalpha():
6 ascii_offset = 65 if char.isupper() else 97
7 result += chr((ord(char) - ascii_offset - shift) % 26 + ascii_offset)
8 else:
9 result += char
10 return result
11
12encrypted = "FGS{F4HV4U_FLSKHU_LV_WRR_HDVB}"
13for shift in range(26):
14 print(f"Shift {shift}: {caesar_decrypt(encrypted, shift)}")