>> Hello. Now,
we'll look closer at asymmetric cryptography.
As we talked about symmetric cryptography,
we said there are three main faults with it.
Out-of-band key-exchange.
You've got to figure out some way to exchange
the keys and doesn't scale well,
and it doesn't give us authenticity or integrity,
and if it doesn't give us either of those,
you know it doesn't give us
non-repudiation because that is
a combination of authenticity and integrity.
We need to solve those problems and
still be able to have secure cryptography.
We have asymmetric cryptography
>> to solve those problems.
>> Now, with asymmetric cryptography,
every user has a key pair.
Every user gets two keys and only two keys.
Right off the bat, you solve the scalability problem.
Everybody doesn't have to have a unique key
for everyone the way they want to communicate with.
Each person just has two keys that they need to worry
about and those two keys make up a key pair.
The key pairs each contain
one public key and one private-key.
Captain obvious is going to
tell you that the public key is
available to anybody who wants it and shared freely.
There's nothing sensitive on a public key.
Now, captain obvious will also
tell you that a private key is private.
It only belongs to its owner and is
tightly bound to that person's identity.
If your private keys compromise and someone can read
your encrypted messages and
signed documents as if they were you.
You must keep your private key private.
Now, the relationship between the two keys
is what makes asymmetric cryptography work.
Anything encrypted with one key
can only be decrypted by the other.
For example, if something
is encrypted with your public key,
it can only be decrypted with your private key.
Similarly, if something is
encrypted with your private key,
it can only be decrypted by your public key,
so it's not mathematical relationship between
the two keys that is in
the magic in asymmetric cryptography.
We already said this solves the problem of scalability,
but this also solves the problem of key exchange.
Now, I will use your public key
to encrypt a message for you.
The only thing that will decrypt a message
encrypted with your public key is your private key,
which only you have.
You've just gotten privacy by using
the receiver's public key as
the sender encrypt the message
for you with your public key,
and only your private key can decrypt it.
Now, I've been talking about this like I
actually ask you for your public key,
but actually, my application
asks your application for the public key.
For example, let's say that
I'm sending an e-mail message to you.
Well, usually what happens is your network
administrator is going to incorporate
>> public keys into the mail server.
>> When I open a message for you and I click the checkbox,
that's just encrypts it.
What is actually happening is
my email application is pulling
your name out of the global address list
along with your public key.
When you receive it on your end,
your e-mail application uses
your private key to decrypt it.
Or it could be a web client and a web browser.
A web client wants to make a secure connection to
a web server and
that web browser requests the public key,
and then the web server provides it.
Again, we're not discriminating
about who gets our public key.
We shared that widely and freely,
because that's how people get
information to us privately.
Privacy will always come through
>> a receiver's public key.