# Symmetric Cryptography

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>> Now I mentioned this a little bit earlier when we were

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talking about historic uses of cryptography.

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But let's go ahead and define

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this process a little bit more clearly,

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and then we're going to talk about

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some of the difficulties that

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come with symmetric cryptography

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as well as some of the benefits.

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Then we're going to talk about

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the two types of symmetric algorithms.

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We have stream algorithms and we have block algorithms.

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Don't forget algorithm in cipher mean the same thing.

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Sometimes I'll say stream cipher,

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I might say stream algorithm

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just to mix it up a little bit,

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but no difference between the terms.

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Symmetric cryptography.

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This is what all of

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our historical types of

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cryptography or the Caesar

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cipher and the Enigma machine,

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they were all symmetric.

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We really didn't have an asymmetric algorithm

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until the late '70s.

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Two gentlemen, Whitfield Diffie and

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Martin Hellman came out

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with the Diffie-Hellman algorithm,

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which was our first asymmetric,

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so everything historical is

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going to fall in the category of being symmetric.

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Now, symmetric cryptography,

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remember we have one key shared between two parties.

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I'm going to use that key to encrypt,

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you're going to use the key to decrypt.

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Now the tricky part is we have

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to share that key between us.

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Now, remember we referred to that as being

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out-of-band key exchange and

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our encryption is only as strong as our key exchanges.

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If we have weak key exchange,

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we have weak encryption

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because anybody could intercept that key.

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I have to find a good secure way for me

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to get the secret to you.

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That's problem number 1.

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Now the second problem is

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that symmetric cryptography is

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not great for large environment.

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In a large environment,

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I need a key with

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every individual I'm going to be communicating with.

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Every individual needs a key

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for everyone they'll be communicating with.

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We wind up having a lot of keys in

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symmetric environments if we were going to

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have it implement just purely symmetrically.

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If you think about this, let's say that I want to start

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a dog walking club and I get

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50 of my closest friends and

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neighbors to participate in this dog walking club,

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and we've decided that we're going to want

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anybody in our club to be able

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to walk anybody else's dog.

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I've got 50 people.

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I'm going to need a house key for the 49 other people and

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each of them are going to need a house key for

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the 49 other people in our group.

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Even though 50 isn't

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a tremendously large number of folks,

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the number of keys we're going to have in

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that type of environment is going to be very large.

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As a matter of fact, there's actually

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a formula that you can use.

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This is going to be referenced later,

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but I'll just mention it now.

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The formula is n times n minus 1 divided by

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2 is the number

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of keys that you would need in a symmetric environment.

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If we just think about that, it would be 50,

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which is n times n minus 1,

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which is 49, divided by 2.

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That means in our little dog walking club,

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there would be 1,225

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keys distributed between the parties.

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That's a lot of keys to have to keep up with.

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Symmetric cryptography does not

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grow well, it just doesn't.

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Now the last problem with

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symmetric cryptography, if you'll remember,

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we talked earlier that

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our desired security services are privacy,

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authenticity, integrity, and non-repudiation.

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The only one of those security services we can

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get with symmetric cryptography is privacy.

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We cannot get integrity,

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can't get non-repudiation,

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or authenticity only privacy.

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Now we do get good privacy with symmetric cryptography,

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but we don't get those other elements.

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If you think about that, those are some big problems.

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We have out-of-band key exchange that makes it difficult.

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You can't use symmetric cryptography

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in a large environment,

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and we don't get authenticity or integrity,

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so of course, we don't get non-repudiation.

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Why in the world do we even want to use

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symmetric cryptography then with all those problems?

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Well, the reason that we want to is because it's fast.

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Very beneficial to have a means to exchange data that

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has very quick performance

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because we've already said

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there's always a trade-off for security,

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and we want to minimize the costs

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associated with security as much as possible.

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We've got the pros and cons.

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To be honest with you,

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the most difficult thing about

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symmetric cryptography

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is all the different names you can call it.

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As a matter of fact, you can

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call it symmetric cryptography,

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of course, but you can also call it secret key.

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You can call it private-key cryptography.

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You can call it shared key

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because the two parties are sharing the same key,

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and session keys are also symmetric in nature.

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You need to know all of those names

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because they may use them interchangeably.

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Symmetric, secret, private,

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shared, session keys,

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they're all symmetric cryptography.

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The heart and soul of it,

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same key is used to encrypt that is used to decrypt.

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Now with our symmetric ciphers,

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we said the algorithm itself

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is the type of math that's used.

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Symmetric ciphers can specifically either

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use stream functions or block functions.

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Symmetric ciphers are either stream or block.

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Let me talk about that just a little bit more.

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When we look at stream encryption,

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what we're doing is we're encrypting one bit at a time,

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or possibly one bite at a time

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if we're doing one character at a time.

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The idea is bit by bit by bit, we encrypt data.

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Now the alternative to that is using

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a block cipher and

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a block cipher chunks data into blocks,

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and each chunk goes through a series of

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math functions called S-boxes, substitution boxes.

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That's what I demonstrated several

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videos ago when we talked

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about the algorithms and how they work,

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because block ciphers are the most common by far.

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All this data we chunk it may be in 128 bit blocks.

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Each block goes through a series of

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math functions where substitution happens,

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and that's how the magic of block ciphers work.

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Just to look at this a little bit more depth

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with stream ciphers.

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Stream ciphers frequently use [NOISE]

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a process called XORing or eXclusive OR.

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I've got a little example of how XOR works down below.

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If you take a look,

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what you can see is I have

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some characters and I've got two bytes worth of data.

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Up at the top the 1101001

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and its corresponding second byte of data,

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we'll assume that that's plain text.

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Then we have the XOR function,

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which is what our key is going to do,

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and then underneath we

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have the ciphertext that's produced.

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Now I know this looks complex,

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it's actually really easy.

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What happens is each bit

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of the plain text is matched with a bit of the key,

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and if the values are the same, ciphertext becomes zero.

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If the values are different,

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the ciphertext becomes one.

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If you look at this, one and zero are different,

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so the ciphertext is just one.

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Same thing, one and zero are different, ciphertext one.

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Zero and one, different ciphertext one.

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Different, different, all the way to the last two bits.

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The second bit from the end here,

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zero and zero are the

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same so the ciphertext becomes zero.

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One and one are the same.

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Ciphertext becomes zero.

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Ultimately, this XOR process just requires a bit of

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the plain text being XORed against the bit of the key.

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If the values are alike,

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the ciphertext becomes zero,

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if the values are different, ciphertext becomes one.

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Now this is actually very

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quick to produce encrypted text using XORing.

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That's the thing about stream ciphers.

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They are fast.

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Boom, boom, boom.

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As matter of fact, a lot of

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times they're going to be used with

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hardware encryption devices because you need

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a hardware encryptor to keep up with

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the capabilities of how fast stream ciphers can be.

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Now the downside.

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If it's super quick to encrypt,

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it may also be super-quick to decrypt.

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That usually goes hand in hand.

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The idea is stream ciphers are very fast,

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but they don't provide

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the same sophistication of

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encryption that a block cipher would.

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Long story short, stream ciphers

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are considered to be less secure.

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Of note, I want you to remember the algorithm

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RC4 is the only stream cipher

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that I want you to know for this course.

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It's the only one that's going to come up,

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it's the only one we're going to ask you about.

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Is AES a stream or a block? It's a block.

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Why? Because it's not RC4.

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The only time I want you to answer

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stream is when you see RC4.

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But Kelly, what about our C2?

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Is it RC4?

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Nope. Then it's a block.

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Only RC4 is the stream we care about.

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Then remember, this is very comparable to

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the illustration we had earlier

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when we were looking at algorithms and keys.

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At each one of these S-boxes,

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there is a math function that's performed.

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Like we said, with your block ciphers

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static it's chunked into blocks,

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in this case maybe 64 bits.

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Each block goes through series of math functions.

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Which math function and

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in what order and how many math functions,

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that's what the key dictates.

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Now we have a list here of symmetric algorithms.

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I think you might see a question or two where you have to

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know whether an algorithm is symmetric or asymmetric.

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You want to take a look at these may be

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screenshot them and make

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sure that you can associate these

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with being symmetric in nature.

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In a little bit I'll show you the list

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of our asymmetric algorithms also.

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Just to wrap things up,

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we gave an overview of

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symmetric cryptography and talked

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about some of its pros and cons.

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Then we looked at stream ciphers versus block ciphers,

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and we also just gave a list of

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some of the common symmetric algorithms.

Up Next

Asymmetric Cryptography

Authenticity

Integrity and Non-Repudiation

Common Asymmetric Algorithms

Symmetric vs. Asymmetric Review

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