# Symmetric Ciphers Stream and Block

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Time
7 hours 50 minutes
Difficulty
Beginner
CEU/CPE
8
Video Transcription
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>> Hello. Let's talk a little bit more in-depth on
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how symmetric algorithms work and talk about
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the two types, stream and block.
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Now, stream ciphers encrypt data one bit at a time.
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Sometimes it's one byte
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but just think of it as being bit by bit,
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and it's very fast.
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As a matter of fact, a lot of times you'll
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use a hardware encrypter for stream ciphers.
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Just because you've got to have
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something that you can keep up with
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the process, it's very fast.
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The way stream algorithms work is they
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either use a process called transposition,
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which is basically just shuffling
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>> the characters around,
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>> or substitution,
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which is replacing one character for another,
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or they use a process called XOR,
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which we'll talk about in a minute.
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These are some very fast algorithms,
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but they're not as secure.
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Like we said, sometimes we
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trade security for performance.
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The only algorithm that I want you to know for the test
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that is a stream cipher is RC-4.
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If I ask if AES is a stream or block, you say block.
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Why? Because it's not RC-4.
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What about RC-5?
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It's a block because it's not RC-4.
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That is the only stream cipher they
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will ask you about is RC-4.
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Now, remember that we already talked about
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some of the things that made WEP work.
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One of those reasons was that it actually use RC-4,
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is not as secure as when they needed it to be.
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RC-4 uses either a 64-bit or a 128-bit key.
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But that's not very secure bytes, today's standards.
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RC-4 was one of the problems with WEP.
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Even when we improved it with WPA,
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we were still using IC-4,
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so it could be backward compatible.
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But with WPA2,
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we got away from RC-4 and use the block cipher.
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Block ciphers chunked data into blocks.
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Each block goes through a series of
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math functions called S-box.
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Now, going through all these functions takes more time,
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but it is more secure.
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The block algorithm that came with WPA2 is called AES,
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and that stands for Advanced Encryption Standard.
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AES is an algorithm that can
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provide variable length encryption.
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You can use AES in a 128-bit mode,
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192 or 256, with 256 giving you the best performance.
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I'll also mention that sometimes we hear about
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these algorithms like the Advanced Encryption Standard,
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but that's actually not an algorithm, it's a standard.
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The government will release
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these standards and then vendors
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will produce algorithms that satisfy those standards.
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Currently, the algorithm that satisfies
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the AES is called Region del.
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I'm not sure if they will ask
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>> you about that on the test,
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>> but it's something to be aware of.
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This is an illustration of block ciphers.
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You can see the S-blocks.
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Within each conceptual S-block,
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a math function is performed.
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The key is going to dictate
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what order those math functions are performed in.
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Those algorithms and keys work together.
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Now, I mentioned earlier that
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stream ciphers either do transposition,
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substitution, or a process called XOR,
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and that stands for Exclusive OR.
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For XOR, remember we are
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encrypting one bit of data at a time.
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If you look at the bottom of this slide,
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we have plain text on the top line and the key is next,
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and what we produce is ciphertext.
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Let's look at the first bit of plain text,
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which is one, and the first bit of the key is a zero.
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If these two values are alike,
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the ciphertext becomes zero.
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If the values are different, it becomes one.
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Since the first bit for each
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>> one and zero are different,
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>> the ciphertexts bit becomes one,
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and that pattern follows through for each bit.
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Only where there are both the same
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does the ciphertexts bit come through as a zero.
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That's how the XOR process works.
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It's very quick and very easy to reverse,
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particularly when you have a key.
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We have to keep that in mind.
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Anything quick to encrypt is
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likely going to be quick to decrypt.
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These are just some common symmetric algorithms.
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Now, I already mentioned RC-4 and AES.
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I think I already mentioned DES,
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Data Encryption Standard.
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That happens a lot where there is
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a standard and the algorithm gets created for it.
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When you see that S on the end,
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it likely means it's a standard and
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an algorithm gets created to satisfy it.
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An AES is by far the most commonly used one,
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it's the de-facto standard.
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The government set apart this standard to encrypt
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sensitive but unclassified information.
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We know we're going to see AES pop up on the exam.
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IDEA is a proprietary algorithm that is used for PGP,
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which is pretty good privacy.
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That's an email encryption application
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created by Phil Zimmerman.
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He noted that for national security reasons,
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the government would like to decrypt
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anything that we encrypt.
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Zimmerman wondered why are
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we using algorithms that the government
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standardized if their goal is
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to decrypt anything that's been encrypted.
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Zimmerman came up with an algorithm called
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IDEA, which was proprietary.
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That's what he used to secure
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email and his application, PGP.
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That one will most likely come up on the test.
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PGP is an alternative that's
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proprietary to standards-based email.
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Now, I do think that on the test,
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you'll need to look at a list and pick out
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which algorithms are symmetric versus asymmetric.
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I'll show you how to do that later on.
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