[MUSIC] Welcome back. In the last lesson we looked
at the ideas that we need in order to be able to understand
symmetric key encryption. Now I'm going to introduce you to both
symmetric key encryption and asymmetric key encryption, which is what's used in
order to then perform key negotiation. After this lesson,
you will be able to compare and contrast these two
cryptography basic concepts. So let's first start with
symmetric key encryption. Now we just looked at
the ideas that we need in order to be able to understand
symmetric key encryption. And these ideas are highlighted
here in our concept tree. Now we're going to move on to talking
about symmetric key encryption. Now, we know that when you encrypt
you have to have a private key, but the receiver needs to have
a private key as well. In symmetric key encryption,
both the sender and the receiver have to have the same key. Now you're probably wanting to ask how
is it that they have the same key? For now, I would like to ask
you to accept this as true, that they both have the same key,
and we'll go over how they negotiate what private key that
they're going to use in the next section. But at any rate,
that's what symmetric key encryption is. You have the sender and the receiver both
using the same private key to either encrypt a message or decrypt a message. An example of this symmetric
key algorithm is AES, or also known as advanced
encryption standard. Now, the way symmetric key encryption
works is on the sender's side it has the plaintext message and
a private key, and it feeds those two pieces of data
into an encryption algorithm. And the resulting operation
outputs is ciphertext message. And this ciphertext message is what
gets sent over to the receiver. So when the receiver is
ready to read the message, they take the ciphertext message and use
this as input to a decryption algorithm. The receiver also gives it as input, the same private key that the center
has used to encrypt this message. And when the receiver runs this
decryption algorithm process, the output is a plaintext message that
the sender intended to send the receiver. So now we just looked at
symmetric key encryption. Now we'll move on to introducing
asymmetric cryptography and mention public and private keys. Asymmetric cryptography is that concept that leads us to having the ability
to agree on a shared secret key. And that shared secret key is then used
to perform symmetric key encryption. Now, in asymmetric cryptography,
what the sender needs is a public key of the receiver, which is used to
encrypt their message of the receiver. The receiver needs the matching
private key to that public key in order to decrypt the sender's message. Effectively, during
asymmetric key encryption. The message is the agreed upon symmetric
private key that the sender would like the receiver to use in future
symmetric key encryption processes. An example of this key exchange
protocol is Diffie-Hellman. So now let's move on to
talking about public and private keys in a little bit more detail. Public and private keys are generated
using a special algorithm, a key generation algorithm. And the relationship between the public
and the private key is that there is only one private key to one public key,
it's a one to one relationship. The public key is what the perceiver
announces to the world basically. The private key is what
the receiver keeps as secret, because this private key is used to
decrypt messages intended for them. So now, how we go on and
use this is the following. First, the receiver generates a public and
matching private key, and using a special key generation algorithm,
the receiver goes on and sends that public key
into a public key repo. And this can happen once, you don't
need to do this many times, just once. Unless you want to change your
private key, but that's an aside. So let's say they publish their first
public key into this repository and then along comes this sender who
wants to send them a message. So what the sender does is they
retrieve this receiver's public key from the public key repository. And they use that public key
as the encryption key for their encryption algorithm. So they take the plain text message
that they want to send, and they use that as input to
the encryption algorithm. And they use the receiver's public
key as the encryption key for this encryption operation. The resulting ciphertext is
then sent over to the receiver. And when the receiver is ready to read
the message they take the ciphertext that they had just received and put that
as input to the decryption algorithm. And they use the private
to decrypt the message, and outcome's the plain text message that the
sender intended to send to the receiver.