This is part of The Twenty Doors series.
The previous one is The Twenty Doors! (ROOM 1)
The next one is The Twenty Doors! (ROOM 3)


After entering the first door, you find yourself in a bigger room. You look at the keypad.

[+] [-] [X]
[÷] [∞] [±]

You look around the room. Then, you see the paper.


And, as before, there is a hint carved into the wall - A to Z Alphabet

This time, the answer seems to be a maths symbol. Or symbols. Still, you've got this far, so you may as well try.


There is only one symbol involved.


A to Z was not required. It could have been A to B, I to C or anything. Each one changes the encrypted text. Now, which cipher is that?


It is an eight-letter word.




The cipher used above is called Baconian.

The next door will be added when the current door is solved!

  • 1
    $\begingroup$ Since these are now going to be separate puzzles, I don't think this is a semi-interactive puzzle anymore, is it? $\endgroup$
    – Bailey M
    Commented Jun 30, 2015 at 19:11
  • 1
    $\begingroup$ I did the same thing myself with my current not-actually-a-semi-interactive puzzle. :) These look like a lot of fun, by the way! $\endgroup$
    – Bailey M
    Commented Jun 30, 2015 at 19:13
  • 4
    $\begingroup$ No freebies here: ADDITION, SUBTRACT, MULTIPLY, DIVIDING, INFINITY all have eight letters and are all plausible plaintext. $\endgroup$
    – lorimer
    Commented Jun 30, 2015 at 19:23
  • 2
    $\begingroup$ Maybe also noteworthy, Alphabet is eight letters too $\endgroup$
    – Cain
    Commented Jun 30, 2015 at 19:38
  • 1
    $\begingroup$ So when this puzzle was first posted, I used a dictionary of 8 letter words and tried to find any word that matched the pattern of the ciphertext (possibly reversed, rotated, etc). I didn't really see a word that matched the symbols, but since the hints seem like subsitution, maybe someone else wants to dig through ~800 words. gist.github.com/ehotinger/88d31cbe18e29e89f489 It's possible that I'm missing a word or the word is two words, compressed/scrambled, etc. though. $\endgroup$ Commented Jul 1, 2015 at 16:23

4 Answers 4


We should press



BIFID using the Baconian Cipher

Deciphering KDKGEOVB:

Using Bifid cipher with Alphabet as a key and Translate A to Z gives INFINITY

  • 1
    $\begingroup$ Could you explain this a little? My understanding of a Bifid cipher is that the key couldn't have repeated letters? en.wikipedia.org/wiki/Bifid_cipher $\endgroup$
    – Cain
    Commented Jul 1, 2015 at 19:35
  • $\begingroup$ @Cain - You are correct. The second A in "alphabet" is just ignored when you use a deciphering page like this one. $\endgroup$
    – Len
    Commented Jul 2, 2015 at 1:30
  • 2
    $\begingroup$ I thought this was the answer, I just had no reason besides "OMIGOSH IT'S AN INFINITY BUTTON I WANNA PRESS ITTTTT" $\endgroup$ Commented Jul 5, 2015 at 22:00
  • $\begingroup$ can you help me with it: I use rumkin.com/tools/cipher/bifid.php to decipher the BIFID-encoded message KDKGEOVB. The key is : Alphabet which changes to Zlphabet (or Zlphzbet?). The decrypted message wouldn't be infinity. Which point am I missing? $\endgroup$
    – Davood
    Commented Sep 27, 2019 at 1:34

Here's an attempt:

Convert each letter to it's numerical place in the alphabet. A=1, B=2, Z=26. That gives the following numbers: [11,4,11,7,5,15,22,2]

I'm not sure which direction is correct here but I suspect what we want is

$11\times4\div11+7\times5-15-22-2$ because that's $0$ if you use infix notation that takes into account the order of operations.

If that's true, then the button presses would be:

$\times\div+\times---\ \mathrm{ENTER}$

Interestingly, If instead we use A=0 and the calculator uses immediate execution, the solution is

The same! $\left(\left(\left(\left(10\times3\right)\div10\right)+6\right)\times4\right)-14-21-1=0$
Unless, of course, the intended answer is $2$ because this is the second room. In that case, we could change it slightly at the end: $\left(\left(\left(\left(10\times3\right)\div10\right)+6\right)\times4\right)-14-21+1=2$

  • 1
    $\begingroup$ But what if it's similar to computers, where A = 0 :O! $\endgroup$
    – Mark N
    Commented Jun 30, 2015 at 19:49
  • 1
    $\begingroup$ Also, what if it's in RPN? en.wikipedia.org/wiki/Reverse_Polish_notation $\endgroup$
    – Cain
    Commented Jun 30, 2015 at 19:56
  • 1
    $\begingroup$ @MarkN How about this for mindblowing: Using $E$ for enter, the buttons are $\times E \div E + E \times E - - - E $. Does that look familiar? It's the same operations with an Enter after each one. $\endgroup$ Commented Jun 30, 2015 at 19:58
  • $\begingroup$ @Cain There are no means that I can detect for determining if what I'm only assuming is a calculator uses RPN. Therefore, I choose to go with infix notation instead as it is more common so far as I know. $\endgroup$ Commented Jun 30, 2015 at 20:01
  • 1
    $\begingroup$ @GentlePurpleRain I use scientific calculators that obey the order or operations. However, given that this is found in a dungeon, it may not be so advanced. $\endgroup$ Commented Jun 30, 2015 at 20:05

Maybe this is a very simplistic guess, but here goes.

Perhaps the piece of paper with KDKGEOVB is a red herring and the clue is just the words 'A to Z Alphabet'. Among the symbols we have, only one of the symbols is also an alphabet, i.e. 'X'. We read it as multiplication, but the correct symbol is the only one which is a part of the English alphabet.

So I would press 'X', then 'ENTER'.

  • 1
    $\begingroup$ It looks like a capital X, in a capital A to Z Alphabet. $\endgroup$
    – mmking
    Commented Jul 1, 2015 at 10:27
  • $\begingroup$ Nice answer, but no. $\endgroup$
    – user9377
    Commented Jul 1, 2015 at 15:04

The lack of food, water and day light, forced me to make this desperate attempt and so I'll probably die

Deciphering KDKGEOVB using a this key pzsbotsl (which came out of thin air) with the vigenere cipher you get ADDITION, so I'd press the pluss sign and hit enter


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