One Time Cipher Pads

One time Cipher Pads are one of the hardest ciphers to solve, and yes – sorry – they do crop up in cosykiller.

One Time Ciphers rely on the sender and receiver having the same pre-shared key that is longer than any message likely to be sent.  Usually after a single message is sent, the key is changed, but in Cosykiller we make things slightly easier by keeping the same encryption key throughout – which means you just need to find what it is both sender and receiver are using as their shared pad.

To make them easier to work with, most agencies use blocks of letters or numbers so that less transcription errors occur.  So if you spot blocks of letters, the chances are you’re looking at a one time cipher.

How do they work?

One time ciphers work by converting letters into numbers and back again.

So… first of all you need to convert the letters of your one time cipher pad into a number.  a=1, b=2 etc.

So if your one time cipher code was going to be a message like:


Then your numerical code would be:

15 14 5 20 9 13 5 3 9 16 8 5 18 19 19 20 9 14 11

Then you need to do the same for your actual message.

So if you wanted to send a message to a friend that read:

I hate ciphers

Your Number to letter cipher would read:

9 8 1 20 5 3 9 16 8 5 18 19

Now you engage in a spot of maths!

If you write all your numbers directly above each other, then you simply add them together, which means:

15+9 gives you your first NEW letter of the encrypted message.

15+9 is 24 – or in otherwords the letter X

14 +8 gives you your second encrypted letter – V

And so on.

If you end up with a letter pair that is greater then the alphabet e.g. 20+20 then all you do is subtract 26 from the number.  so your encrypted letter becomes 14, or N.

To decrypt you just do the same in reverse, but in this time subtracting your cipher from the encrypted message.

This will occasionally give you a minus number, but all you need to do is add on 26, to get back to the number it should be and convert back into letters.

You can find electronic help  converting between letters and numbers HERE which will speed you up as you’re trying to find out fast whether you’ve found the right cipher pad.

When it comes to a shared pad – it helps to find something that both people may own.  Sometimes this might be a bible, or a book of the same imprint.  For cosykiller – just remember it’s likely to be something you’ve seen before… and something that both individuals will have.


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