Name: 弁護士のためのCS50 2019 - 暗号化 (CS50 for Lawyers 2019 - Cryptography)
Uploaded: 2021-01-14T10:19:48.000Z
Duration: 1 h 27 min 35 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

We're going to answer those two questions in exactly that order.

It's the art and science of obscuring, and ideally protecting, information.

Now it's an art and a science because there's math involved with it.

It's pretty straightforward to manipulate characters in some way

by adding some constant number to them or to change them

But it's an art, because doing so in a way to defend against potential attacks

There's a lot of guesswork and calculation

that needs to go into play to find a really strong cipher.

Cryptography gives us the opportunity to have

a basic level of security against an adversary who might

We usually contrast, in cipher information,

with information that is presented in the clear, which

is to say there's no protection surrounding it at all.

And it's generally considered better to protect information using cryptography

than to have information just freely available out there.

Now a cipher, we're going to start by talking about cryptography

We'll lead up to more modern forms of cryptography,

which are derived from more ancient forms of cryptography.

But a cipher is one of the most fundamental forms of cryptography.

And recall that an algorithm is just a step-by-step set of instructions

And in case, the task is to obscure or encipher information.

And ciphers can also be used in reverse to unobscure, or decipher,

that same information that was previously encoded or enciphered.

Now there are many different ciphers out there

that have varying levels of security potential.

Some of the more ancient ciphers that we're going to start with

should be [INAUDIBLE] be considered to have no security potential at all

But again, this leads into the more modern approach to cryptography,

which is much more secure than some of these basic ones.

And now let's start by imagining that we have possession of this device.

Now if you're looking at this device and it seems somewhat familiar to you,

it may be because you've recently seen the movie A Christmas Story,

where Ralphie, the character there, obtains

one of these, which is a little orphan Annie's secret society decoder pin.

And this decoder pin has a set of numbers going sequentially one

through 26 around the inner edge, and a set

of letters, which is not presented in any particular order,

And what would happen is the radio announcer would provide,

And then it would read off some secret message

that, ostensibly, only individuals who possessed this pin,

or many of the duplicate versions of this pin that were distributed

to children around the country, could then decipher

by taking the numbers that were given over the radio

and transforming them back into letters so that it makes sense.

So if you can, if you zoom in on this image,

it might be a little difficult to see, but you

can see that the 3 corresponds to the letter L, and the 4 corresponds to an M

based on this particular setting of this decoder pin.

So this is one potential, what we would call a substitution cipher,

where we're changing, we're substituting a letter in this case for a number,

and that number will henceforth represent that letter

But what is the problem with this cipher?

Or more generally, when we think about issues in computer science

where we have adversaries who are trying to penetrate some system,

or break a code, or break in, or hack into anything,

hack your password, we sometimes frame this in terms of asking the question,

Where is the vulnerability that is potentially

And in this case, it's that anybody who has access to this pin

is able to break any cipher that is made with this pin.

And again, this pin was distributed pretty extensively in 1930s and 40s

to children who listened to this very popular radio program.

So these pins were in the hands of many people.

And anybody who had access to the pin would

And so that is, how we might frame this attack vector,

is the key, in this case, the pin, which we will call a key for this purpose,

It's pretty well known how to use this key and manipulate this key.

But that's just one example of a substitution cipher.

We have many different examples of substitution ciphers that we could use.

Let's just take another very simple, straightforward one,

which is imagine we have all of the letters of the alphabet

and we're just going to assign the ordinal position of that letter

So with the secret society pin, there was this sort of random element

There wasn't a rhyme or reason to them, although the numbers were sequential.

Let's use sequential letters and map them to their sequential numbers.

Both of these things are increasing linearly.

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弁護士のためのCS50 2019 - 暗号化 (CS50 for Lawyers 2019 - Cryptography)

potential

context

pattern

arbitrary