# Cryptography, Perfect Secrecy and One Time Pads | Two Minute Papers #25 ## Метаданные - **Канал:** Two Minute Papers - **YouTube:** https://www.youtube.com/watch?v=Q-XKOPNIDAg - **Дата:** 15.11.2015 - **Длительность:** 5:43 - **Просмотры:** 14,603 ## Описание Cryptography helps us to communicate securely with someone in the presence of third parties. We use this when we do for instance, online banking or even as mundane tasks as reading our gmail. In this episode, we review some cipher techniques such as the Caesar cipher, rot13, and as we find out how easy they are to break, we transition to the only known technique to yield perfect secrecy: one time pads. Are they practical enough for everyday use? How do our findings relate to extraterrestrial communications? Both questions get answered in the video. Additional comment: "In modern certification cryptanalysis, if a cipher output can be distinguished from a PRF (pseudo random functions), it's enough to deem it broken." - Source: https://twitter.com/cryptoland/status/666721478675668993 ______________________ The paper "Cipher printing telegraph systems: For secret wire and radio telegraphic communications" is available here: http://math.boisestate.edu/~liljanab/Math509Spring10/vernam.pdf You can try encrypting your own messages on these websites: http://practicalcryptography.com/ciphers/caesar-cipher/ http://rot13.com/index.php http://www.braingle.com/brainteasers/codes/onetimepad.php Subscribe if you would like to see more of these! - http://www.youtube.com/subscription_center?add_user=keeroyz The thumbnail background was created by Adam Foster (CC BY 2.0) - https://flic.kr/p/b99vsi Splash screen/thumbnail design: Felícia Fehér - http://felicia.hu Károly Zsolnai-Fehér's links: Patreon → https://www.patreon.com/TwoMinutePapers Facebook → https://www.facebook.com/TwoMinutePapers/ Twitter → https://twitter.com/karoly_zsolnai Web → https://cg.tuwien.ac.at/~zsolnai/ ## Содержание ### [0:00](https://www.youtube.com/watch?v=Q-XKOPNIDAg) dear fellow Scholars this is 2 minute papers with car cryptography helps us communicate securely with someone in the presence of third parties we use this ### [0:10](https://www.youtube.com/watch?v=Q-XKOPNIDAg&t=10s) Caesar Cipher when we do for instance online banking or even as mundane tasks as reading our Gmail one of the simplest ways of doing cryptography is using the Caesar Cipher we have a message and each letter we shift with the same amount okay wait what does shifting mean Shifting The Letter A by one becomes B e and shifting e by one becomes F and so on the amount of Shifting doesn't have to be exactly one it can be anything as long as we shift all letters in the message with the same amount if we would run out of the alphabet for instance by Shifting the last letter Z by one we get a the first letter back there's a special case of Caesar ciphers that we call rth 13 ### [0:52](https://www.youtube.com/watch?v=Q-XKOPNIDAg&t=52s) Rot13 that has an interesting property it means that we shift the entirety of the message by 13 letters let's encrypt a message with Rod 13 we obtained some gibberish okay now let's pretend that this gibberish is again a message that we would like to encrypt we get the original message back why is that since there is 26 letters in the basic Latin alphabet we first shift by 13 then doing it again we shift by 13 letters which is a total of 26 therefore we went around the clock and ended up where we start started mathematicians like to describe this concisely by saying that the inverse of the rod 13 function is itself if you call it again you end up with the same message we know the statistical probabilities of different letters in the English language for instance we know that the letter e is relatively common and Z is pretty rare if we shift our Alphabet by a fixed amount the probabilities will remain the same only for different letters therefore this Cipher is quite easy to break even automatically with a computer this is anything but secure communication the ### [2:03](https://www.youtube.com/watch?v=Q-XKOPNIDAg&t=123s) The One-Time Pad Encryption onetime pad encryption is One Step Beyond this where we don't shift each letter with the same amount but with different amounts this list of numbers to use for shifting is called a pad because it can be written on a pad of paper and it has to be as long as the message itself why one time why paper no worries we're going to find out soon enough if we use this technique we'll enjoy a number of benef beneficial properties for instance take a look at this example with a onetime pad we have two v's in the encrypted output but the first V corresponds to an h and the second V corresponds to a p therefore if I see a v in the encrypted output I have no idea which letter it was in the input Computing statistical probabilities doesn't make any sense here and we're powerless in Breaking this so even if you can intercept this message as a third party you have no idea what it is about it's very easy to prove mathematically that the probability of the message being happy is the very same probability as hello or ABCDE e or actually any ### [3:16](https://www.youtube.com/watch?v=Q-XKOPNIDAg&t=196s) One-Time Pad gibberish the onetime pad is the only known technique that has optimal perfect secrecy meaning that it is impossible to crack as long as it is used correctly this is mathematically proven it is not a surprise that it had seen plenty of use during the second world war so what does it mean to use it correctly several things pads need to be delivered separately from the message itself for instance you walk up to the recipient and give them the pad in person the exchange of the pads is a huge problem if you're on the Internet or at War now you must also be worried that the pad must not be damaged if you lose just one number the remainder of your message is going to be completely gone are build up you're done the key in the pad needs perfectly random numbers no shortcuts generating perfectly random numbers is anything but a trivial task and is subject to lots of discussion one time pads have actually been broken because of this there's an excellent episode on a well-known Channel called Vsauce on what random really means make sure to check it out the pad has to be destroyed upon use and should never be reused so if you do all this you're using it correctly corly in the age of the internet it is not really practical because you cannot send a delivery guy with the secret pad next to every message you send on the internet so in a nutshell onetime pad is great but it is not practical for large scale real-time communication from AAR and as crazy as it sounds if a civilization can find a method to do practical communication with perfect cryptography their communication will look indistinguishable from noise this is amazing there's tons of ongoing debates on the fact that we're being exposed to tons of radio signals around the earth why can we still not find any signs of extraterrestrial communication well there you have the answer and this is going to blow your mind if practical perfect cryptography is mathematically possible the communication of any sufficiently advanced civilization is indistinguishable from noise they may be transmitting their diabolical plans through us this very moment and all we would here is White Noise crazy isn't it thanks for watching and for your generous support and I'll see you next time --- *Источник: https://ekstraktznaniy.ru/video/14921*