>> Hello. We talked about
the basic elements of cryptography,
and in the last section we talked about algorithms.
We said that algorithms are
the math functions that are used to provide
the substitution from plaintext to ciphertext,
and then we said an algorithm
>> has to be used with the key
>> because a key provides
the instructions on how to
use the mouth of the algorithm.
Now we're going to talk about two different types of
cryptography: symmetric and asymmetric.,
and these terms refer to the key that is used.
It has to do with what key is,
how you get the key, and how you
use the key, and so forth.
The first type we're going to talk
about is symmetric cryptography.
With symmetric cryptography,
the same key is used by both parties.
Encrypt data with that key and when you get it,
you have to decrypt it with the same key.
It's like your house key you, use it to
lock the door and unlock the door.
Now let's take a look at some of
the drawbacks to symmetric cryptography.
It's really important with symmetric cryptography that
we find a way to securely distribute the keys.
Let's say I need a person to watch my dogs
this weekend and you offer to do it.
I have two puppies and a 14-year-old dog.
As you can imagine the 14-year-old dog
is not amused by the puppies at all,
but anyway the problem is that I
need to find a way to give my house key to you.
I could hide it under my little garden or
my front porch, but that's not secure.
How can I get it to you in a secure fashion?
The most secure way is to walk
over and give it to you in-person,
but that's not practical.
Since I will be giving you the key in
a different environment that are normal communications,
we call this an out-of-band key distribution.
That's one of the downsides of symmetric cryptography.
Symmetric cryptography is usually
pretty easy to understand
because most of the logs we work with in
our lives use symmetric cryptography.
The hardest thing about symmetric cryptography
is to remember all the different names it can go by;
secret key, private key,
shared key, or in some cases session key.
Now going back to my need for a dog walker, Let's see,
I've chosen one person to walk my dogs,
and let's see that I do find a way to
get that person the key security.
As long as we're both trustworthy, it can work.
Next, let's say we decide
we're going to join in dogs and in club,
and at any given moment you could ask one of
us to come over that evening and walk your dogs.
They would be able to do it because we are
all going to share our house keys.
Everyone would have everyone's house key.
Can you imagine how many
keys we would have to keep up with?
That's just not possible, that's crazy.
Well, that's the second problem
is symmetric cryptography.
As your numbers increase you need a key for
each communication channel but
also they need a key for each communication channel,
so this is not a solution that scales well.
Then the other problem with
symmetric cryptography is that
it does not provide non repudiation.
Remember when we talked about pain and
the security services you want with
cryptography: privacy,
authenticity, integrity, and non-repudiation,
we said that those are security systems
that we want with cryptography.
Well, the only one of those services that you get
with symmetric cryptography is privacy.
There is nothing that guarantees
non-repudiation because the key is shared.
If you and I share a key to
some data and that data gets leaked to media,
I can only say that you leaked
it because we both share the same key.
Or as another example,
if you and I share a locker
>> in the back of the classroom
>> and one of us leaves a [inaudible] damage
in the locker over the weekend,
even though we all know it was me I could
see it was you because we share the same logger.
With all the issues in symmetric cryptography,
why do we even talk about it?
Because it's fast.
We want speed, so just put
it in the back of your mind for future use
that we want to send bulk data with
symmetric cryptography due to its speed.