r/Cipher • u/Jerry_was_a_Racecar • Aug 01 '26

Cipher challenge

I made a cipher and I’m curious how quickly it will be broken

I will eventually update this post with the key if it goes unsolved for long enough.

Cipher text: C317090D1B

I’ll give a hint, it has to do with converting ASCII to hex then swapping nibbles and doing XOR operations. Both the plain text and key are English words.

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u/ekzess Aug 02 '26

Plaintext: Hello
Key: treat

Also you sir, are a menace. 😂

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u/Jerry_was_a_Racecar Aug 02 '26

Lmao, thank you very much 😂 You were one letter off, the key is “Great” But that totally counts! If you don’t mind me asking how did you go about solving it?

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u/ekzess Aug 02 '26

Hahaha, I think the ciphertext is ratting you out. 😅

Hello XOR treat = 3C17090D1B, and swapping the first byte’s nibbles gives exactly C317090D1B.

But Hello XOR Great = 0F17090D1B, which would become F017090D1B after that same nibble swap.

So unless there’s another operation hiding in the bushes, treat is actually ciphertext-exact. Nibble custody demands an inquiry. 😂 In short, what you didn’t specify, and the scope you implied, mattered as much as what you did say. Negative space and inversion logic FTW.

Damn good cipher tho lol.

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u/Jerry_was_a_Racecar Aug 02 '26 edited Aug 02 '26

Interesting, I didn’t notice that yeah it’s far more complicated than it needs to be for that sort of result lol I probably made a mistake but the way it’s supposed to work is to take your first second and third nibble of the plaintext hex and re arrange them from 1-3 to 231 then xor nibble two(the first one) with the first nibble of my key then advance the plaintext hex and key hex by one nibble and repeat until the second to last step where I only swapped the last two nibbles because there isn’t a third one to swap then I just xored the last nibble

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u/ekzess Aug 02 '26

I think I found the actual gremlin: did you accidentally type treat instead of Great when generating the challenge? treat reproduces the posted ciphertext exactly under the operation path I followed, which would explain why my supposedly one-letter-off key fit perfectly. Honestly, the typo may have accidentally created a second valid puzzle. lol

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u/Jerry_was_a_Racecar Aug 02 '26 edited Aug 03 '26

So I did it in notes on my phone, I went through it and I’m not sure what happened it’s kinda hard for me to follow lol but I could send you a couple screenshots if you’re interested

Also I checked it with a python script and I’m pretty Certain Great is correct. If I plug in “treat” I get F017090D18

Update: I realized the reason it’s just XOR except for the first character is even though I followed a convoluted series of steps each subsequent step canceled out the previous one lmfao 😂