# Decrypting a Cesar Cipher puzzle from a hex

I have the following hex to ascii conversion:

70656c636762207a726e61662070656c636762207a726e = pelcgb zrnaf pelcgb zrn

Applied with ROT13 I get: crypto means crypto mea

I'm also provided with the following 2 hex converted to ascii. i took these from the existing ciphers. I'm not sure if it means anything at all:

656c67627a72616670656367207a6e = elgbzrafpecg zn

7063206e206c6272 = pc n lbr

My final piece of the puzzle is the following HEX:

200000000000471f0411060a0c43061115541f520c061507104543131714541c541801184e1c0552111910064e0a450800154f111f18041d004e

But it translate to this?

?????G??????C???T?R?????EC???T?T???N??R????N?E???O??????N

I'm not sure what is required to solve this further as I'm just stuck after applying the ROT13.

• Where do these things come from? The context might be relevant. – Gareth McCaughan Aug 11 '18 at 16:31
• updated with image – Patoshi パトシ Aug 11 '18 at 17:21
• Well, from the picture, the first text is clearly a key. (Either the whole text, or the word "caesar": mea is Latin for "my".) Maybe try to use the key on the other texts (Vigenere cipher is basically a Caesar cipher with a key) and see if that gets you anywhere. – Bass Aug 11 '18 at 18:09
• i converted the long hex int ascii but its most ??? marks. I also tried to convert it to binary then ascii, but i get weird characters. Vigenere cipher wont work as I can't convert the other long hex into ascii. – Patoshi パトシ Aug 11 '18 at 21:13
• Ah, so the couple of other strings are from somewhere else, then. The key also has the caret "^" on it. That usually symbolises the bitwise XOR operation, maybe that could help? – Bass Aug 11 '18 at 21:34

We have two pieces of information (the other fragmentary bits are, I think, irrelevant): the key given as 70656c636762207a726e61662070656c636762207a726e, which interpreted as a string of ASCII characters is "pelcgb zrnaf pelcgb zrn", whose ROT13 is "crypto means crypto mea"; and the ciphertext given as 200000000000471f0411060a0c43061115541f520c061507104543131714541c541801184e1c0552111910064e0a450800154f111f18041d004e. And, as Bass points out in comments, there is a caret suggestive of XORing.

So, suppose we

begin by XORing the key we have with the first part of the ciphertext (note: the key is shorter than the ciphertext). We get "Cryptography, the pract". Looks promising.

It's rather obvious that

the key seems like it might be the first portion of a repeating cycle: "crypto means crypto means crypto means crypto mea...". If we XOR that byte-by-byte with the ciphertext, we get this: "Cryptography, the practice and study o%1cr!*(ma{<1|j}mt!"

Clearly we're doing something right here, but clearly something's gone a little wrong. But

we have enough now to suspect that this might be a quotation. So put "cryptography the practice and study" into your favourite search engine. You'll find a few plausible continuations, but the one that jumps out as most likely ends "... of hiding information", which has the not-immediately-obvious merit of being the right length.

If that's the plaintext, what was the key?

Well, if plain = cipher XOR key then key = plain XOR cipher, and from this we find that the actual key was "crypto means crypto means crypto meanscrypto means crypto ". It seems that a space has been misplaced for some reason.

That's how I happened to find it, but

you might think googling likely answers isn't in the spirit of the thing, in which case I commend to you the following alternative approach. Our slightly-garbled plaintext gets as far as "Cryptography, the practice and study o" before descending into chaos, and it's natural to guess that the next plaintext character is an "f" and the one after that a space. This tells us that the key begins "crypto means crypto means crypto meanscr", and that's surely enough to make the same guess about a missing space.

Anyway, the plaintext is:

Cryptography, the practice and study of hiding information"

• What are the required skill sets in order to solve this puzzle? I tried my best and only got so far to only know that it was a HEX format. But I'm not familar with XOR at all so it might not have even been possible for more to solve something like this as I don't have the fundamental knowledge in that area. – Patoshi パトシ Aug 13 '18 at 19:33
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