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An associate of mine was recently arrested and is being detained in a prison in Ohio until his trial in a couple of weeks. He and I need to be able to communicate some, ummm, compromising information with each other while he's in prison. We know the telephones are monitored, and since I live in a different state, I can't visit him regularly in person. The prison does allow him occasional access to a computer that is restricted to email only, and he only gets a short amount of time to use it. Since we don't want any record of the information we're sending back and forth, we needed to come up with a simple encryption / decryption scheme that would hide the details of our communication, but allow us to discuss the situation easily. He has indicated he developed an appropriate scheme that he and I could both manage to work with for both encrypting and decrypting.

He said the name of the main witness was ywcbl95d and the evidence was in ecwryepug5. Can you help me determine what this means so I can figure out what to do? There was some additional specifics in the email I received from him, but I am thinking once I figure out how to decrypt it, I can figure out the rest myself.

Time is short, so once I take care of business, I need to respond immediately with a message indicating I've understood and that I'll meet up with him in Fort Wayne after he is released. We don't want the authorities to know where we're meeting though, so can you provide an encryption for "Fort Wayne" that I could send via email using the same scheme?

I can provide more text (i.e. more encryptions) from the email I received as needed. With the speed at which you all answer so many of the questions on this site, I'm trying to provide only minimal information to start. Hope this is an acceptable first puzzle from me! Feedback is welcome!

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One possible encryption of Fort Wayne would be:

rvk0ey 3aqztumb4s

The witness's name is ...

... Trevor.

The evidence is in ...

... Detroit.

The cipher ...

... uses the QWERTY keyboard to encode stretches between pairs of letters. Usually, these stretches are just single letters, but the cipher can also encode longer passages like TRE, which is encoded as going from Y to W in a straight line:

y → TRE → w     c → V → b     l → O → 9     5 → R → d    
e → D → c     w → E → r     y → TR → e     p → OI → u     g → T → 5    

This cipher is easy to apply when encoding and decoding, because sender and recipient have a keyboard right in front of them. It also has the nice property that there are several possible encodings for a letter. For example, F can be encoded as any of rv, tc, gd, vr, ct or dg. The cipher has drawbacks around the fringes of the letter block of the keyboard, though. P can be encoded as *ℓ or as 0; and M could be ,n or <n, but how do you encode a Z?

The bad thing about this cipher is that the encoded words stand out as being code. Surely the prison guards will know that you are hiding something when there are "words" like ywcbl95d and ecwryepug5 in the messages?

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  • $\begingroup$ Excellent answer! Yes, I played around with trying to make the encoded information human-readable words, but it just didn't pan out.. I wasn't sure how much of a hint to put in the title, but I'm assuming you would have come up with the answer even without the title? $\endgroup$
    – daroo
    Aug 24, 2018 at 10:31
  • $\begingroup$ Yes, that would work with only a few words, if at all. And it would do away with the nice prerequisite that the involved parties must be able encode everything on the spot. I like how the need for a simple cipher is enforced by the story. Good puzzle! $\endgroup$
    – M Oehm
    Aug 24, 2018 at 11:06
  • $\begingroup$ A hint I was going to give if it was needed was to indicate that a given word typically has many encryptions, but an encrypted string will always decrypt to a specific word, as you implied in your answer. I thought that was kind of a cool aspect of the cipher that could help defeat frequency analysis, etc.. $\endgroup$
    – daroo
    Aug 24, 2018 at 12:04

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