Imagine two people who share an important secret have to split up. This requires them to communicate private information from a distance. However, an eavesdropper named Eve also wants this information, and has the ability to intercept their messages. So, Alice decides to communicate using letters written in some kind of secret code. The following analogy is helpful. First, Alice locks her message in a box, using a lock that only she and Bob know the combination to. This is known as ‘encryption.’ Then, the locked message is sent to Bob. When Bob receives the box, he opens it using the code they shared in advance. This is called ‘decryption.’ Cryptography begins when we abandon physical locks and use ‘ciphers’ instead. Think of [ciphers] as virtual locks. Ciphers allow Alice and Bob to scramble and descramble their messages so that they would appear meaningless if Eve intercepted them. Cryptography has been around for thousands of years. It has decided wars, and is at the heart of the worldwide communication network today. The fascinating story of cryptography requires us to understand two very old ideas related to number theory and probability theory.

What is cryptography? | Journey into cryptography | Computer Science | Khan Academy

94 thoughts on “What is cryptography? | Journey into cryptography | Computer Science | Khan Academy

  • March 28, 2012 at 12:35 am
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    this will be neato burrito

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  • March 28, 2012 at 12:36 am
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    so cool

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  • March 28, 2012 at 12:37 am
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    I am bad at these puzzles, but it will be an awesome thing to learn. Bring them on 😀

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  • March 28, 2012 at 12:38 am
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    I want more..

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  • March 28, 2012 at 12:41 am
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    Can't wait for the videos! I am always fascinated by cryptography. I suggest reading The Code Book by Simon Singh because it's what got me into this stuff!

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  • March 28, 2012 at 12:41 am
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    OMFG !! There's no blackboard 🙁

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  • March 28, 2012 at 12:47 am
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    i wanna learn how to make a crypter!!

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  • March 28, 2012 at 12:58 am
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    I've wanted to be a cryptographer since the age of 11.

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  • March 28, 2012 at 1:25 am
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    Sweet!

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  • March 28, 2012 at 1:27 am
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    finally Number theory!

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  • March 28, 2012 at 1:36 am
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    let's go…

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  • March 28, 2012 at 1:51 am
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    I can't wait for this series to take off.

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  • March 28, 2012 at 3:17 am
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    4:3, we meet again

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  • March 28, 2012 at 3:30 am
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    Yesh! I need more interesting explanation! I beg you!

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  • March 28, 2012 at 3:40 am
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    CMTET COBHIM HELIE OEAUA GFONX guess it can work . nice video

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  • March 28, 2012 at 3:55 am
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    These videos are really well done. I'm looking forward for the next ones.

    Reply
  • March 28, 2012 at 4:15 am
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    gracias x compartir,

    Reply
  • March 28, 2012 at 5:09 am
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    TALK ABOUT TURING!

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  • March 28, 2012 at 5:36 am
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    One of his contributions (bombe) will appear later in this series

    Reply
  • March 28, 2012 at 5:39 am
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    awesome, I recently heard about Alan Turing and have been so interested by him. His story is truly unbelievable.

    Reply
  • March 28, 2012 at 7:23 am
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    What ever happened to the good old days? Get box – put lock on box – send – have other person put lock on box – send – take lock off box – send – take lock off box – open?

    Reply
  • March 28, 2012 at 8:43 am
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    BY GOD, IS THERE ANYTHING YOU CAN'T TEACH? Silly me, BY KHAN, IS THERE ANYTHING YOU CAN'T TEACH?

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  • March 28, 2012 at 10:09 am
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    Noooo… one of my favorite skill sets is being described in an approachable way

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  • March 28, 2012 at 12:25 pm
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    For those that don't know Alan Turing, he was a scholar that worked in the UK during WW2 and decripted the Nazi messages at Bletchley Park etc. Look up his name on Wikipedia and its got everything on him.

    Im looking forward to the rest of the series on codes.

    Reply
  • March 28, 2012 at 3:08 pm
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    For anyone interested, I can only recommend you the software "Cryptool". It's pretty awesome and you can learn a lot from it (Free & Open Source afaik). Maybe khan should show it in his videos to teach things?

    Reply
  • March 28, 2012 at 3:11 pm
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    Cryptography is soooo AWWWESSOOOOME!

    Reply
  • March 28, 2012 at 3:33 pm
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    Cryptonomicon

    Reply
  • March 28, 2012 at 3:50 pm
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    I will check this out!

    Reply
  • March 28, 2012 at 8:41 pm
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    Why are they always named Alice, Bob and Eve in every cryptography example?

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  • March 28, 2012 at 11:30 pm
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    Cool. Will you be covering the basics of how once can crack codes in future? I'm intrigued about how mathematicians cracked codes, say, during WWII.

    Reply
  • March 29, 2012 at 12:14 am
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    Enjoy computer programming?

    Reply
  • March 29, 2012 at 1:39 am
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    Nice intro into public key encryption, but it's too short. :/

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  • March 30, 2012 at 10:27 am
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    EVE is evil:D

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  • March 30, 2012 at 10:46 pm
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    Keep going! Brit you sound slightly scripted, loosen up. Make it conversational!

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  • March 31, 2012 at 9:40 pm
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    nonsense

    Reply
  • April 4, 2012 at 2:57 am
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    hey khan, can you please do a series on neuroscience?

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  • April 4, 2012 at 6:22 pm
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    This is correct but the whole preshared cipher/key is rare when this is applied modernly. I like the box example but with TWO locks, one for Alice and one for Bob. A inserts msg and locks with her lock. Sends box to Bob where he cant open A's lock but attaches his own lock. Sends it back to A where she unlocks her lock and keeps B lock on. Send back to Bob and he unlocks and reads the msg. Anyone in the middle would always have a locked box they couldn't open and read the msg.

    Reply
  • April 5, 2012 at 11:17 am
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    Pls also post some more videos on crytography as i find it very interesting. Pls come up with advanced as well as beginner topics

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  • April 5, 2012 at 6:46 pm
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    I will get to this later on 🙂

    Reply
  • April 5, 2012 at 6:47 pm
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    More on the way, it will gradually get more advanced as we go!

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  • April 5, 2012 at 6:47 pm
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    Good questions.The next 5 or so videos should clear this up for you!

    Reply
  • April 12, 2012 at 4:07 pm
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    When are we going to do invisible ink? 🙂

    Reply
  • April 12, 2012 at 10:19 pm
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    its not to late

    Reply
  • April 23, 2012 at 1:33 am
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    you mean "it's not too late"

    Reply
  • May 9, 2012 at 1:49 am
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    why

    Reply
  • May 22, 2012 at 11:10 am
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    Very nice job I hope it will expand into more dailed series.

    Reply
  • June 1, 2012 at 12:26 am
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    wow !

    Reply
  • August 3, 2012 at 4:11 am
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    There is invisible ink and has been around for many many years.

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  • August 4, 2012 at 4:11 pm
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    Idiot. I was asking when will the video about invisible ink be.

    Reply
  • August 26, 2012 at 4:21 am
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    Well, another idiot here. Selenography is a branch of astronomy that deals with the charting of the moon's surface. How in the hell is that related to the invisible ink?

    Stop talking non-sense people.

    Reply
  • September 4, 2012 at 11:52 pm
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    1:06 Meet me at elephant lake!
    YOUR ENCRYPTION SUCKS

    Reply
  • September 27, 2012 at 10:35 am
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    Cryptography. Why not talk about bitcoins

    Reply
  • September 27, 2012 at 10:25 pm
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    what is the song name

    Reply
  • October 26, 2012 at 3:30 am
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    Code shared in the past….

    Reply
  • December 7, 2012 at 1:04 am
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    Very much an established standard. Eve = Eavesdropper. Alice and Bob = People sending and receiving messages.

    Reply
  • December 7, 2012 at 1:04 am
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    They probably mean stenography

    Reply
  • December 11, 2012 at 2:28 am
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    Only *she and Bob….

    Reply
  • January 5, 2013 at 2:55 pm
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    How did you know what to do to get that message?

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  • January 14, 2013 at 3:02 pm
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    How does Bob get the code without Eve intercepting it? Do they make the code together?

    Reply
  • February 19, 2013 at 3:37 pm
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    I'm seeing again I don't understand shit of this lol

    Reply
  • February 28, 2013 at 8:03 am
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    i hope it doesnt continue like this and goes back to only a black background and commentary and some doodling basically

    Reply
  • March 20, 2013 at 10:42 pm
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    Make a video about choosing better lottery numbers!

    Reply
  • May 13, 2013 at 6:39 pm
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    Average Cryptographer's Password of Choice: &!*sgv=afk"S'8L/;!!>.#^fB;_HY
    Average Person's Password of Choice: password123

    Reply
  • June 21, 2013 at 10:29 am
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    One way is frequency analysis, if the poeple hid the secret just by shifting the letters it still doesn't change their patterns for example what are letters that commonly occur in pairs when we write words? Examples include o and e get it? After you break part of the secret message then you can try figuring out the rest.

    Reply
  • July 29, 2013 at 8:41 am
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    Eve 2.0 is a hacker. She pulls out a saw and cuts the box in half, circumventing the encryption entirely. Eve is now root 😀

    Reply
  • August 11, 2013 at 5:40 am
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    super helpful !

    Reply
  • August 29, 2013 at 12:49 am
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    Actually the truth is somewhere in between, as while your example of a good password is (probably) impossible to crack it is also impossible to remember. A better choice would be a nonsense sentence, easily remembered, vanishingly unlikely to be cracked by machine. e.g. alicef0ll0wedthera88it

    Reply
  • September 16, 2013 at 11:39 pm
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    hello

    Reply
  • September 24, 2013 at 6:46 pm
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    I'm so sad to have discovered that this series are actually a copy of another channel.
    watch?v=M23z5Ch8NSk
    Just look at the upload dates… Can't believe this…

    Reply
  • October 28, 2013 at 8:54 am
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    That guy works for Khan Academy, so they use his vids.

    Reply
  • November 9, 2013 at 7:58 pm
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    http://youtu.be/fLaTJYY30wM here is a tutorfriendnow video,a new company that homeschools kids with 5th grade math and science.and its ALL FREE.

    Reply
  • November 17, 2013 at 11:04 pm
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    i've got an education erection

    Reply
  • December 20, 2013 at 4:27 am
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    Looks like a BC license plate to me.  No?

    Reply
  • December 22, 2013 at 12:04 am
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    Great videos!

    Reply
  • August 3, 2014 at 2:53 pm
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    Hello Khan team,

    I enjoy your channel very much and I hope I will do so for a long time. But as a former carpenter I have to say that the finger joint on the box is awful terrible. 😉

    Best regards.

    Reply
  • November 6, 2014 at 4:52 pm
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    poxa, eu queria em portugues 🙁

    Reply
  • December 25, 2014 at 6:36 am
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    HEY its a BC plate!!!

    Reply
  • January 22, 2015 at 8:49 am
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    :37 "that only her and Bob know" should be "that only she and Bob know." You would not say, "her knows", you would say "she knows" the secret code.

    Reply
  • December 10, 2015 at 4:05 pm
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    Vídeos de criptografía y seguridad informática en español en YouTube en el proyecto de píldoras formativas Thoth. Saludos Dr. J. Ramió.

    Reply
  • March 5, 2016 at 12:00 pm
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    1:08 your code has a spelling error. It's a W instead of N.

    Reply
  • October 6, 2016 at 1:50 pm
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    who came here from the little brother series?

    Reply
  • October 13, 2016 at 2:09 am
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    Why does it sound like Alice is having an affair with Bob?

    Reply
  • February 8, 2017 at 8:50 pm
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    where does the code to decrypt the encrypted data come from?
    someone tell me please

    Reply
  • March 31, 2017 at 7:26 am
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    Can we call EVE as NSA?

    Reply
  • June 23, 2017 at 5:33 am
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    KEY 10 STD 26+1

    64312 595 386146 94763 5837304 959725 8645 78 49286 9659 1622 287 843 782929 8645 78 49286 9659 1622 287 843 782929 83 5381 492 7836 86853 689248 264 6949 2414 681764 84562 99676 583 4849 65893 159382 59887 285 856294 357 553 118993 54489 771 884548 2225 49 77664 5833 988 5856 289855 77 69 868 4899 1217 77797 88 2386 719564 681 445897 83 333897 461 865 7896 488692 754 8698 69157 48648 657 438 91837 23228956 3859 4897 68 717 59176 7985 556597 844 28993 798 77 7778 466894 591864 883 6646 895 89992 66 78 488368 9993 9982 92 798 28991 97 96 51958 8893 78 68 477784 739 21957 88885 7779 882 8956 663 429 4556674 86946 357 483 8355 7787 4633894 966 2238 744 4886 86995 94 799 788 788 49684 5964 883 184 47 76791 586 7864 996 49682 487873 83838 4849 484849 461 697 366 9982 286482 1138134592 48945 882 4897 3372 474 439696

    Reply
  • August 20, 2017 at 10:03 pm
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    What`s the name of the intro music?

    Reply
  • August 22, 2017 at 9:31 pm
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    This does not explain well AT ALL!

    Reply
  • November 26, 2017 at 3:20 pm
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    Video transcript

    Imagine two people who share an important secret have to split up This requires them to communicate private information from a distance. However, an eavesdropper named Eve also wants this information, and has the ability to intercept their messages. So, Alice decides to communicate using letters written in some kind of secret code. The following analogy is helpful. First, Alice locks her message in a box, using a lock that only she and Bob know the combination to. This is known as 'encryption.' Then, the locked message is sent to Bob. When Bob receives the box, he opens it using the code they shared in advance. This is called 'decryption.' Cryptography begins when we abandon physical locks and use 'ciphers' instead. Think of [ciphers] as virtual locks. Ciphers allow Alice and Bob to scramble and descramble their messages so that they would appear meaningless if Eve intercepted them. Cryptography has been around for thousands of years. It has decided wars, and is at the heart of the worldwide communication network today. The fascinating story of cryptography requires us to understand two very old ideas related to number theory and probability theory.

    Reply
  • December 14, 2017 at 1:58 pm
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    I was searching for any Cryptography Community to join but I could'd find one. So I've created a Telegram Group all about "Cryptography" , if you are a Cryptography enthusiast , You can join community here : https://t.me/joinchat/E6oALUYaubzaKIMvAmnR9Q
    We love ♥️ Cryptography

    Reply
  • December 23, 2017 at 2:01 am
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    Something about your videos. They are so addicted. thanks

    Reply
  • January 28, 2018 at 5:46 pm
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    я хреново по английски, есть на русском?

    Reply
  • March 13, 2018 at 2:41 am
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    why this gives me the chills or something? lol

    Reply
  • November 5, 2019 at 4:57 pm
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    what is this crap

    Reply

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