You wake up in a room. In front of you, there is some paper:

It's the day, not the year or the month! It's very useful to abecedarians. 639407344. But never forget the time, for its another key.

Jsz amxroc tt ioatblq gstk A.

There are also 26 doors, labeled A through to Z. One door leads to freedom while the others lead to a pit of acid.

Which door should you go through?


He jumped on the number! UNIX!!!


The word 'abecedarian' is used to describe a person that is learning the alphabet.


There are no clues as to which cipher is used, so you've got to work that out from the keys. If the time is a key, then that is presumably a number. And the used cipher is similar to a Vigenere cipher. Anything come to mind, cryptologists?


The only important part of the word 'abecedarian' is the meaning. A person who is learning the alphabet.


2 Answers 2


As Cristian Marian figured out, the number 639407344 is

The time/date Fri, 06 Apr 1990 13:09:04 GMT.

The question says that the day is very useful to abecedarians, which means that

The day (Friday) is going to be our alphabet key for our cipher.

Since we have the time (which is a number) as a second key, we know we're using

A Gronsfeld cipher, which is like a Vigenere but uses a number key instead of a letter key alongside the modified alphabet.

Putting it all together,

We input the cipher text with an alphabet of FRIDAYBCEGHJKLMNOPQSTUVWXZ and a key of 130904, giving us the result "You should go through door F".

So, I think I'll head through

Door F, of course!

  • 1
    $\begingroup$ @Kslkgh thanks, as always, a fun cipher puzzle! :) $\endgroup$
    – Bailey M
    Aug 14, 2015 at 13:35
  • 1
    $\begingroup$ I'm keeping myself busy until someone does Room 10 of The Twenty Doors. Here's a hint, because I'm getting bored of waiting: ahhltomot eiXect fcu eutnsr ee lnfhqso SPUN THREE TIMES! :P $\endgroup$
    – user9377
    Aug 14, 2015 at 13:40
  • 1
    $\begingroup$ So, it was door F after all :) . Well done Bailey M! I really need to expand my list of ciphers. $\endgroup$ Aug 14, 2015 at 13:49
  • 3
    $\begingroup$ @CristianMarian check out rumkin.com/tools/cipher for a bunch of 'em. That's my primary resource. :) $\endgroup$
    – Bailey M
    Aug 14, 2015 at 13:51
  • 1
    $\begingroup$ @CristianMarian no problem. See if you can figure out that weird hint Kslkgh just dropped above. :P $\endgroup$
    – Bailey M
    Aug 14, 2015 at 13:54

Ok, so I am going to start a partial answer.

After finding out what was the problem with my „number to date” conversion, I managed to find out that the number 639407344 (actually) translates to

Fri, 06 Apr 1990 13:09:04 GMT

I think it would bee too easy for the door to be the 6th (F), so I tried a Caesar cipher with 6 shifts (As OP was stating, only the day counts), but that didn't get me anywhere.

I also tried to do a vigenere cipher decoding using Fri / Friday as a key, with no luck.

Unfortunately, my encryption knowledge is very, very poor. So I got stuck.


So, the word abecedarian is important, but so far I don't see how. I recall learning the letters of the alphabet in a certain way, but I haven't found a standard for the recommended order. So, I am thinking that my approach with another order for the letters of the alphabet is not the way to go.

  • 1
    $\begingroup$ I don't think a Caesar shift is going to help, all 26 possibilities (including the original sentence) don't produce a readable output. If I had to guess, one of the four-letter words is 'door', but neither of them have a repeated character for 'o'. I don't think this is a simple cipher at all. $\endgroup$
    – Kade
    Aug 13, 2015 at 15:20
  • $\begingroup$ After trying all 26 possibilities myself, I thought of trying a vigenere cipher which doesn't have a problem with the repeating „o” not appearing as a repeating character in the encrypted text. Like I said, I have very poor knowledge of other encryption techniques, so I don't even know what I could do next $\endgroup$ Aug 13, 2015 at 15:27
  • $\begingroup$ @CristianMarian Your quite close with a Vigenere cipher - it uses a similar cipher. $\endgroup$
    – user9377
    Aug 13, 2015 at 15:58

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