DOWNLOAD NEW ENCRYPTION SHEET

 

A friend of ours came over to our office one day and started talking about the possibility giving people a chance to encrypt messages without computers, just with a pen and paper. They would write a message, encrypt it by hand, burn/eat/melt the encryption tool (i.e., a sheet of paper), and send the message.

We thought it would be a really good fun to create something useful and so we, eventually, created a procedure and some tools. It was quite easy from the cryptography point of view as we used a one-time pad - the only encryption scheme that is unconditionally secure (no-one can break it - with a small caveat). The real challenge was to create an encryption table that would make it easy for anyone to encrypt and decrypt a message.

We ended up with paper encryption sheets:

  • one needs a pre-printed sheet of paper that can be downloaded from this page;
  • one sheet can be used for one message and never re-used (unless you are just testing to see whether it works);
  • one sheet allows encryption of a message of up to 50 characters (you need 3 sheets for a 140 character SMS);
  • each sheet contains a code number that needs to be sent with the message - 5 letters that are in large font should be enough - so that the recipient can find the right decoding sheet; and
  • they MUST NOT be disclosed to a third party, as that would compromise the message.

How to Use Sheets - Fun

  • Print some sheets from our web site;
  • On each sheet name will keep the decrypting part of the sheet (1) and who will be encrypting (2).
  • Cut the sheet into two - the upper part goes to the sender, the lower part goes to the receiver.
  • Sending
  1. Write the message in the top row of empty boxes - maximum is 50 characters - skip spaces or replace them with 'X's;
  2. For each filled box, find its letter in the left-hand column beneath it and write the character next to it in the box at the bottom;
  3. Destroy everything above the line "CUT HERE - DESTROY UPPER PART"; and
  4. Send the remaining piece to the recipient directly or type it into an SMS or email together with 5 letters of the sheet code.
  • Receiving
  1. Find the sheet with the code as received with the message.
  2. Write the received message in the top row of boxes;
  3. For each box, find its letter in the left column beneath and write the character next to it in the box at the bottom;
  4. When you've read the message (spaces are missing or replaced with 'X's) destroy the whole sheet.

How to Use Sheets - Professional

  • Make a list of people who want to encrypt messages with our sheets and note who will want to talk to who.
  • Estimate how many messages each pair will send in a month (or other suitable time interval) and write it all down.
    Note: each message requires one sheet and it is a good idea to have a small stock of sheets (too large a stock increases the danger of someone else making a copy and decrypting your messages).
  • Switch your web browser to privacy mode so that no copies of sheets remains on your computer - Firefox can do it, IE8 can allegedly do it - see Wikipedia for the list of browsers supporting this mode.
  • Print as many sheets as you need and close the browser when finished. (Get in touch if you want us to print some sheets for you and we will see what we can do. )
  • On each sheet, name who will keep the decrypting part of the sheet (1) and who will be encrypting (2).
  • Write the date after which the sheet should not be used (0).
  • Cut the sheet into two - the upper part goes to the sender, the lower part goes to the receiver (as you've just written on the sheet). Ideally, all steps up to now happen when senders and receivers are present.
  • Sending
  1. Write the message in the top row of empty boxes - maximum is 50 characters - skip spaces or replace them with 'X's;
  2. For each filled box find its letter in the left column beneath it and write the character next to it in the box at the bottom;
  3. Destroy everything above the line "CUT HERE - DESTROY UPPER PART"; and
  4. Send the remaining piece to the recipient directly or type it into an SMS or email together with 5 letters of the sheet code.
  • Receiving
  1. Find the sheet with the sheet code as received with the message.
  2. Write received message in the top line of boxes;
  3. For each box, find its letter in the left-hand column beneath it and write the character next to it in the box at the bottom;
  4. When you've read the message (spaces are missing or replaced with 'X's) destroy the whole sheet.
  • Compromise - "Disaster Recovery (DR) Plan"
    • If you have a suspicion that some sheets have been compromised, senders and recipients must destroy all of their unused copies.
  • Management of sheets
    • Define maximum time for keeping printed sheets and destroy them if you do not use them by then.
    • You can define further rules for keeping sheets safe and for their destruction in particular circumstances.

Security Caveat

The unconditional security depends on true randomness of encryption sheets. At the moment, this is ensured by the fact that the generation algorithm is on our server and we make sure that everything is erased when a new encryption sheet is downloaded.

This however can be improved by:

  1. using secure hardware, moving server abroad, ... Let us know if you are interested in a hardened version; or
  2. generate sheets in Javascript on your computer instead on our server - we may do a Javascript version if we feel idle.

Printing sheets

Follow the link below and print the pdf file - make sure the file is deleted once printed especially if an external PDF viewer is opened (e.g., Acrobat Reader).

Press the "Reload" button on your browser to generate new sheet (check the sheet code changes!).

DOWNLOAD NEW ENCRYPTION SHEET