WEBVTT

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Hey, everybody, this is Paul.

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Welcome to your first lesson in
cryptography.

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So to explain
what cryptography is,

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I'm just going to start out
with a little example.

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So let's say
that we've got some person.

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So let's just go ahead and name
this person Paul

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since that's a pretty neat name.

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And then let's say that Paul has
a friend.

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Let's pretend like Paul
and his friend are

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on different sides of the world,

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but Paul has an important
message to send his friend.

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But because Paul is on the other
side of the world,

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he can't just hand his friend
this message.

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So now let's pretend like this
message that Paul has

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has some secret information
that he only wants his friend

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to know about and nobody else.

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Well, if Paul sends his letter
to the friend around the world,

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there's going to be a whole
bunch of people

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that will be handling
this letter.

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So there's a risk
that as Paul sends this letter,

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somewhere along the way,
the letter may be intercepted

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by somebody
other than Paul's friend.

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And if this letter falls into
the hands of the wrong person,

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that person may read
the secret message.

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Sometimes that person will keep
the message,

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or he could take note of
that information

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and then continue to pass
the letter on to Paul's friend.

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So how can Paul and his friend
be sure that nobody in between

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has uncovered
the secret message.

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One thing Paul could do is he
could use cryptography

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to encrypt the message.

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That way, if somebody
in the middle got the message,

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they would be unable to
understand its contents.

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So when Paul first writes
the letter,

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he writes it in English

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because that's the language
he speaks.

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So if Paul wrote his letter
in English, Spanish, Chinese,

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German, French, or any of
the common languages

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that a multitude of people
understand, we would consider

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that this message
is written in plaintext.

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So if the message is written
in plaintext,

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that means it's very easy for
anybody who speaks that language

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to understand its contents.

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So what Paul needs to do is

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he needs to encrypt his message

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before he sends it.

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So what does it mean

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to encrypt a message?

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Let's pretend like

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Paul's secret message is

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the word "fun."

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He doesn't want anybody other
than his friend

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to read this secret message.

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Well, anybody that speaks
English

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can read the letters f-u-n

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and understand that Paul is
trying to send the message "fun"

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to his friend.

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So in order for Paul to encrypt
the plaintext "fun,"

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he converts the letters f-u-n
to numbers.

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Then once his message is
in numerical form,

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he can use a key to modify
the values of this number.

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You can think of the key
as mathematical operations

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that we can perform

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on the numerical value
of Paul's message.

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After applying the key
to this numerical value,

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we end up with
a new numerical value,

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which is known as ciphertext.

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So now if Paul puts his
ciphertext in an envelope

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and sends the envelope
to his friend,

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Paul won't have to worry
about somebody in the middle

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discovering the secret message.

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If somebody in the middle
discovers the message,

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they won't be able
to decrypt the message

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without a proper key.

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Now if the message continues
to Paul's friend,

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Paul's friend will need a way
to decrypt the message

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and recover the original
plaintext.

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So in order to convert
the ciphertext into plaintext,

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Paul's friend needs to decrypt
the ciphertext.

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In order
to decrypt the ciphertext,

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the friend will need
a decryption key.

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Using the decryption key,

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the friend will be able
to convert the ciphertext

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into the numerical value
corresponding to the plaintext.

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Once the friend has
the plaintext value,

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he can convert it back
into the original message.

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Now that the numerical value
is converted

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back to
the English language,

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the friend has retrieved
the original plaintext

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that was sent in Paul's message.

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So in a nutshell,

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that's how cryptography works.