One Time Pad (again).

I did a video 7 years ago: Uncrackable Pen & Paper Cryptography

It had several comments, many of which seemed to think there were ways of "cracking" it. There are not. Having got a comment recently, I did another video, OTP.

Wikipedia explains its quite well. It explains your cannot crack it if :-

• The key must be at least as long as the plaintext.
• The key must be truly random.
• The key must never be reused in whole or in part.
• The key must be kept completely secret by the communicating parties.

Even so, people still comment, and a friend of mine just posted...

"Rev, set a challenge on your blog first to decode message without a key wins a firebrick dragon?"

"I would have thought brittle[sic] force decode first 4 chars and wait till it looks like a word."

I really thought I had explained it, but clearly not.

The short version is YOU CANNOT CRACK IT!

In my second video I tried to explain this by dumbing down the message to just 0 or 1, with two possible keys, 0 or 1, giving a possible encrypted message 0 or 1.

Now, imagine you intercept my message and it is a 1

You can BRUTE FORCE try every possible key:

• Key 0 means message was 1
• Key 1 means message was 0

So what was the message? You don't know. You have no way to tell.

You don't know because every possible message, no matter how meaningful or meaningless is possible with a key, with equal probability of being the actual message.

The "I would have thought brute force decode first 4 chars" misses the point - you can brute force to make every possible first 4 letters, with no clue which of those is correct or even more likely.

And just to be clear, even if you know the first 3 letters are ANX (Ref Enigma) that does not help you because the key for those letters has no impact on the next letters (unlike Enigma).

So let me try and say it again, YOU CANNOT CRACK IT! I suggest reading the Wikipedia article.