Thursday, 29 January 2015

God save the queen

Having explained the basic process of how you might send secret messages using nothing more than pen and paper (and dice to make the keys) [here], I have made a further video which explains how one might comply with future legislation that requires a "back door entry" to be added.

See the video [here].

The basic change to the process is as follows. After composing the message and encoding it and sending it, but before destroying the evidence...

  • Write down a benign message alone side the coded message.
  • This benign message could be anything, my video uses "GOD SAVE THE QUEEN", but you could put anything. Ideally something that looks like it should be private but is not incriminating - perhaps something about a planned sexual encounter :-)
  • You write it in the same way you would your covert message, so as per my previous instructions you put 4 spaces at the start to allow the key to be identified.
  • You then use the calculator to subtract each letter in the benign message from the coded message, continuing to the end of the sheet.
  • This gives a sequence of gibberish, as you would expect.
  • You then write that sequence on a separate sheet, and put the date and time of your message sending.
  • Now, destroy the original key sheet and message.
  • You then send this new sheet as the "Key used to send message at date/time" to the key escrow trusted third party to which you have been required to deposit keys (the only logical "back door entry" for a one time pad system)

This means that if ever the powers that be want to check what you sent, they can get this "key" from the key escrow trusted third party with an appropriate court order or whatever, and use it to decode your intercepted message. The problem is that when they decode the message, all they get is your benign message "GOD SAVE THE QUEEN" or whatever. They do not see the real message and have no evidence that any other message exists.

The whole point of a one time pad is that every possible message is equally likely. A key could be provided to decode the coded message to any plain text message you like!

A further step would be to pre-agree the benign message (have it on the key sheets you originally share) so that the recipient can do the same. That way if they ask either end for copies of keys later, you have keys to hand over and they will actually agree.

An interesting point on all of this is that I know of at least one person who has had great fun coding this all in C since my last blog on this. Making a million keys and putting on a USB stick, and making tools to allow coding and decoding messages. Obviously these tools could do this extra step as well, overwriting the original key on the USB stick with the new benign message key. He is not a programmer normally, and is using this to help learn more C coding, but he is the end user in this - not a "tech company" or someone that can easily be identified and targeted with some requirement to add a back door. He is running software that he did not even download from the Internet, but made from scratch. The best bit is that he could be seen to be apparently complying with requirements for a "back door entry" by key escrow or a requirement to retain keys and still have private messages!

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