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In the previous lesson,

we covered one of two major categories that

encryption ciphers fall into, symmetric key ciphers.

In this next lesson, we'll cover the second class of

ciphers called asymmetric or public key ciphers.

Remember why symmetric ciphers are referred to as symmetric?

It's because the same key is used to encrypt as to decrypt.

This is in contrast to asymmetric encryption systems because as the name implies,

different keys are used to encrypt and decrypt.

So how exactly does that work?

Well, let's imagine here that there are

two people who would like to communicate securely,

we'll call them Suzanne and Daryll.

Since they're using asymmetric encryption in this example,

the first thing they each must do is generate a private key,

then using this private key,

a public key is derived.

The strength of the asymmetric encryption system comes from

the computational difficulty of figuring out

the corresponding private key given a public key.

Once Suzanne and Daryll have generated private and public key pairs,

they exchange public keys.

You might have guessed from the names that

the public key is public and can be shared with anyone,

while the private key must be kept secret.

When Suzanne and Daryll have exchanged public keys,

they're ready to begin exchanging secure messages.

When Suzanne wants to send Daryll an encrypted message,

she uses Daryll's public key to encrypt the message and then send the ciphertext.

Daryll can then use his private key to decrypt the message and read it,

because of the relationship between private and public keys,

only Daryll's private key can decrypt messages encrypted using Daryll's public key.

The same is true of Susanne's key pairs.

So when Daryll is ready to reply to Suzanne's message,

he'll use Suzanne's public key to encode his message

and Suzanne will use her private key to decrypt the message.

Can you see why it's called asymmetric or public key cryptography?

We've just described encryption and decryption

operations using an asymmetric cryptosystem,

but there's one other very useful function the system can perform, public key signatures.

Let's go back to our friends Suzanne and Daryll.

Let's say, Suzanne wants to send a message to Darryll and she wants

to make sure that Daryll knows the message came from her and no one else,

and that the message was not modified or tampered with.

She could do this by composing the message and combining it

with her private key to generate a digital signature.

She then sends this message along with the associated digital signature to Daryll.

We're assuming Suzanne and Daryll have already

exchanged public keys previously in this scenario.

Daryll can now verify the message's origin and authenticity by combining the message,

the digital signature, and Suzanne's public key.

If the message was actually signed using Susanne's private key

and not someone else's and the message wasn't modified at all,

then the digital signature should validate.

If the message was modified,

even by one whitespace character,

the validation will fail and Daryll shouldn't trust the message.

This is an important component of the asymmetric cryptosystem.

Without message verification, anyone could use

Daryll's public key and send him an encrypted message claiming to be from Suzanne.

The three concepts that an asymmetric cryptosystem grants us are

confidentiality, authenticity, and non-repudiation.

Confidentiality is granted through the encryption-decryption mechanism.

Since our encrypted data is kept confidential and secret from unauthorized third parties.

Authenticity is granted by the digital signature mechanism,

as the message can be authenticated or verified that it wasn't tampered with.

Non-repudiation means that the author of

the message isn't able to dispute the origin of the message.

In other words, this allows us to ensure that

the message came from the person claiming to be the author.

Can you see the benefit of using

an asymmetric encryption algorithm versus a symmetric one?

Asymmetric encryption allows secure communication over an untrusted channel,

but with symmetric encryption,

we need some way to securely communicate the shared secret or key with the other party.

If that's the case, it seems like asymmetric encryption is better,

right? Well, sort of.

While asymmetric encryption works really well in untrusted environments,

it's also computationally more expensive and complex.

On the other hand, symmetric encryption algorithms are faster,

and more efficient, and encrypting large amounts of data.

In fact, what many secure communications schemes do is take advantage of

the relative benefits of both encryption types by using both, for different purposes.

An asymmetric encryption algorithm is chosen as a key exchange mechanism or cipher.

What this means, is that the symmetric encryption key

or shared secret is transmitted securely

to the other party using

asymmetric encryption to keep the shared secret secure in transit.

Once the shared secret is received,

data can be sent quickly,

and efficiently, and securely using an asymmetric encryption cipher.

Clever?

One last topic to mention is somewhat related to

asymmetric encryption and that's MACs or Message Authentication Codes,

not to be confused with media access control or MAC addresses.

A MAC is a bit of information that allows authentication of a received message,

ensuring that the message came from

the alleged sender and not a third party masquerading as them.

It also ensures that the message wasn't modified

in some way in order to provide data integrity.

This sounds super similar to digital signatures

using public key cryptography, doesn't it?

While very similar, it differs slightly since the secret key that's

used to generate the MAC is the same one that's used to verify it.

In this sense, it's similar to symmetric encryption system and the secret key must be

agreed upon by all communicating parties beforehand or shared in some secure way.

This describes one popular and secure type of MAC called

HMAC or a Keyed-Hash Message Authentication Code.

HMAC uses a cryptographic hash function along with a secret key to generate a MAC.

Any cryptographic hash functions can be used like Shahwan or MD5 and the strength or

security of the MAC is dependent upon

the underlying security of the cryptographic hash function used.

The MAC is sent alongside the message that's being checked.

The Mac is verified by the receiver by

performing the same operation on the received message,

then comparing the computed MAC with the one received with the message.

If the MACs are the same,

then the message is authenticated.

There are also MACs based on symmetric encryption ciphers,

either block or stream like DES or AES,

which are called CMACs or Cipher-Based Message Authentication Codes.

The process is similar to HMAC,

but instead of using a hashing function to produce a digest,

a symmetric cipher with a shared keys used to encrypt

the message and the resulting output is used as the MAC.

A specific and popular example of a CMAC though slightly

different is CBC-MAC or Cipher Block Chaining Message Authentication Codes.

CBC-MAC is a mechanism for building MACs using block ciphers.

This works by taking a message and encrypting it

using a block cipher operating in CBC mode.

CBC mode is an operating mode for block ciphers that

incorporates a previously encrypted block cipher text into the next block's plain text.

So, it builds a chain of encrypted blocks that require the full,

unmodified chain to decrypt.

This chain of interdependently encrypted blocks means that any modification to

the plain text will result in

a different final output at the end of the chain, ensuring message integrity.

In the next section, we'll check out some common examples of

asymmetric encryption algorithms and systems. I'll see you there.