One-Time Pad Cipher (Perfect Security)

𝐎𝐧𝐞-𝐓𝐢𝐦𝐞 𝐏𝐚𝐝 𝐂𝐢𝐩𝐡𝐞𝐫 (𝐏𝐞𝐫𝐟𝐞𝐜𝐭 𝐒𝐞𝐜𝐮𝐫𝐢𝐭𝐲)
𝗜𝗻𝗱𝗲𝘅:
- What is the one-time cipher?
- How does it work?
- What are the rules to provide perfect security?

𝐎𝐮𝐫 𝐅𝐁 𝐏𝐚𝐠𝐞:

𝐈𝐦𝐚𝐠𝐞𝐬, 𝐚𝐧𝐢𝐦𝐚𝐭𝐢𝐨𝐧𝐬 𝐚𝐧𝐝 𝐯𝐢𝐝𝐞𝐨𝐬 𝐜𝐫𝐞𝐝𝐢𝐭𝐬:
- Pixabay

𝐃𝐚𝐯𝐢𝐝'𝐬 𝐁𝐨𝐨𝐤𝐬
- 𝗪𝗲𝗶𝗿𝗱 𝗠𝗮𝘁𝗵𝘀: 𝗔𝘁 𝘁𝗵𝗲 𝗘𝗱𝗴𝗲 𝗼𝗳 𝗜𝗻𝗳𝗶𝗻𝗶𝘁𝘆 𝗮𝗻𝗱 𝗕𝗲𝘆𝗼𝗻𝗱
- 𝗪𝗲𝗶𝗿𝗱𝗲𝗿 𝗠𝗮𝘁𝗵𝘀: 𝗔𝘁 𝘁𝗵𝗲 𝗘𝗱𝗴𝗲 𝗼𝗳 𝘁𝗵𝗲 𝗣𝗼𝘀𝘀𝗶𝗯𝗹𝗲
- 𝗪𝗲𝗶𝗿𝗱𝗲𝘀𝘁 𝗠𝗮𝘁𝗵𝘀: 𝗔𝘁 𝘁𝗵𝗲 𝗙𝗿𝗼𝗻𝘁𝗶𝗲𝗿𝘀 𝗼𝗳 𝗥𝗲𝗮𝘀𝗼𝗻
** The kindle versions are available

#one-time​​​​​​​ #pad​ #cipher

[perplexedon]

Your example is not actually a perfect one time pad, because rand(0, 99)mod26 is not the same distribution as rand(0, 25).

There is a 4% chance of a shift between 0 and 21 and a 3% chance for a shift of 22 to 25. For comparison, consider if there was a 0.01% chance of a shift between 1 and 25, and a 99.75% chance of a shift of 0. You'd be pretty sure the message was unchanged. In this way, with a sufficiently large message size, your cipher could be broken with frequency analysis. The OTP is only as good as the random distribution!

An additional rule would therefore be that the random distribution must be uniform.

[__Razer]

The problem with "Perfect Security" is the "Rules for Perfect Security". The security is only as good as your security of the key, and how do you perfectly securely store or transmit the key to the recipient of the message? I can think of a few reasonably secure ways to do it, but it's not _perfect_. You can have two identical books with hundreds of pages of keys, and include the page number of the key for the message in plain text with the message, with prior instruction to destroy a page after it is used. Of course, the book itself would still be vulnerable to compromise, if anyone were able to copy it or steal it, and intercept a message, your message is no longer secure.