Cryptography: Crash Course Computer Science #33

Today we’re going to talk about how to keep information secret, and this isn’t a new goal. From as early as Julius Caesar’s Caesar cipher to Mary, Queen of …


  1. Hey guys, there’s been a ton of news the past week about a vulnerability in the WPA2 protocol that protects our Wi-Fi networks, and since we say that AES is secure in this video, we thought it would be helpful to explain how it all relates. In October of 2017, researchers released a viable hack against WPA2, known as KRACK Attack, which uses AES to ensure secure communication between computers and network routers. The problem isn't with AES, which is provably secure, but with the communication protocol between router and computer. In order to set up secure communication, the computer and router have to agree through what's called a "handshake". If this handshake is interrupted in just the right way, an attacker can cause the handshake to fault to an insecure state and reveal critical information which makes the connection insecure. As is often the case with these situations, the problem is with an implementation, not the secure algorithm itself. Our friends over at Computerphile have a great video on the topic:

  2. "For a 128-bit keys, you'd need trillions of years to try every combination, even if you used every single computer on the planet today. So you better get started" XD

  3. I loved cryptography as a kid so much that I actually used to entertain myself with frequency analysis. I had a lot of books on cryptography, and I used to try to crack the examples before the book explained them, the way people try to solve mystery books before they reach the end. Now I'm falling in love with cryptography again, but with a more modern, computerized version. Thanks Crash Course!

  4. hey carrie (or anyone else if you would like to help) , I wanted to understand that private key sharing( at 10:00) , so i thought of making myself a simple practical example out of it, but my calculations say your equation from 10:00 is not correct . can you please have a look?

    So If i have to transmit character '"P' (ascii 80 )from Boy A to B:

    >> I assumed x=2 would be private key of A and y=3 would be private key of B)
    >> I assumed my public key function as cipher(x,y)= (3^x % 7)^y
    – A would first Transfer cipher_a = 3^2%7 = 2 to B

    – B would first Transfer cipher_ a= 3^3%7 = 6 to A

    >> I assumed my publically available encryptor function as encrypted_text = cipher(B) ^x
    + ascii(character)
    – Thus, A would transfer the encrypted character 'P' to be as e_t = (6)^2+80 = 106

    >> I assumed my publically available decryptor function as ascii(character) = encrypted_text – cipher(A) ^y

    – Thus, B would recieve the value as val = 106- (2)^3 = 97 , which is not equal to our original value of 80

    Thus (B^y mod M)^x != (B^x mod M)^y != (B^xy mod M)

    But rather
    (B^yx mod M^x) == (B^xy mod M^y) but != B^xy %M

    Is this right?

  5. Hot oil in the medieval ages wasn't used like you see it in Hollywood.

    Oil was an expensive resources so what they would actually do is pour boiling water or extremely hot sand. Yes there was some occasions where it happened but that's an exception not the norm as it's portrayed.

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