caesar picoCTF 2019 Solution

Description

1. Step 1Brute-force all 26 Caesar shifts

   Observation

   The title names the Caesar cipher, and the ciphertext is a short lowercase string inside the braces. There are only 26 possible shifts, so trying all of them is faster and more reliable than reasoning out the right one.

   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 everywhere online, but this challenge uses some arbitrary shift from 0 to 25. Applying ROT13 gives one output that almost certainly is not English. You do not know the shift in advance, so loop through all 26 and read the one that forms words.

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

   Braces, underscores, and digits are not letters, so the ord() arithmetic mangles them. The ciphertext lives only inside the braces, so pass just the inner string 'tifjjzexkyvilsztfehnahooda' to the loop. The isalpha() branch leaves non-letters untouched, but keeping the wrapper on still throws off the indexing.

   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.