caesar picoCTF 2019 Solution

Description

1. Step 1

   Brute-force all 26 Caesar shifts

   Observation

   I noticed the challenge title explicitly names the Caesar cipher and the ciphertext is a short lowercase string inside the flag braces, which meant the key space is only 26 possible shifts and trying every one is faster and more reliable than deducing the correct shift analytically.

   A Caesar cipher shifts each letter by a fixed amount. With only 26 possible keys, brute-force is trivial. Run the Python snippet below to print every possible decryption - the shift that produces readable English inside the braces is the answer.

   python

   python3 -c "
   s='tifjjzexkyvilsztfehnahooda'
   for i in range(26):
       print(i,''.join(chr((ord(c)-97+i)%26+97) if c.isalpha() else c for c in s))
   "

   Copied!

   What didn't work first

   Tried: Decode the ciphertext using ROT13 only, assuming that is the standard Caesar shift.

   ROT13 is a fixed shift of 13 and is common online, but the challenge uses an arbitrary shift between 0 and 25. Applying ROT13 produces one specific output that will almost certainly not be readable English. The brute-force loop is necessary because you do not know the shift in advance - only one of the 26 outputs will form recognizable words.

   Tried: Treat the entire picoCTF{...} string including the braces as input to the shift loop.

   The braces, underscore, and digits are not alphabetic characters, so the ord() arithmetic will mishandle them or produce garbage output for those positions. The challenge description states the ciphertext appears only inside the braces, so pass only the inner string 'tifjjzexkyvilsztfehnahooda' to the loop - the chr() branch for c.isalpha() passes non-alpha characters through unchanged, but curly braces still confuse the index when the wrapper string is not stripped first.

   Learn more

   The Caesar cipher is a monoalphabetic substitution cipher named after Julius Caesar, who reportedly used a shift of 3 to protect military communications. Every letter in the plaintext is replaced by the letter a fixed number of positions later in the alphabet, wrapping around from Z back to A.

   Because there are only 26 possible shifts (including 0, which does nothing), the Caesar cipher has an exhaustively small key space. A brute-force attack simply tries all 26 possibilities and shows the results - a human can immediately spot the one that reads as English. The Python one-liner above does exactly this: for each shift i, it maps every lowercase letter c by computing (ord(c) - 97 + i) % 26 + 97. Subtracting 97 converts the ASCII code to 0-25, adding i and taking mod 26 wraps the shift, and adding 97 converts back to ASCII.

   In real cryptanalysis, the Caesar cipher is also trivially broken by frequency analysis: in English the most common letter is 'e'. If you find the most frequent letter in the ciphertext, the difference from 'e' gives you the shift. This generalizes into more powerful attacks on simple substitution ciphers.

   The hint "salting it won't help" is a red herring - salt is a concept from password hashing (a random value added before hashing to prevent rainbow table attacks), which has nothing to do with the Caesar cipher. Recognizing such misdirection is a useful CTF skill.

Flag

With only 26 possible keys, brute-force is trivial - the shift that produces recognizable English words is the answer.