Integrity and Non-Repudiation
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35 hours 25 minutes
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>> Now if we continue on,
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let's talk about integrity and non-repudiation,
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>> our next stages.
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>> We've already said that,
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in an asymmetric environment,
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if you want to get privacy,
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you're going to be using the receiver's
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public key to encrypt.
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That way, only the receiver's private key can decrypt,
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which only that receivers should have.
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Then we looked at authenticity,
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and we said, hey, if you want authenticity,
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then the sender needs
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>> to encrypt something off the message,
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>> but some add on,
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if you will, with the sender's private key.
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That way when the receiver gets the message,
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they can try to decrypt that add on,
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with the receivers public key.
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If that works, proves it was encrypted
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with the sender's private key,
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>> which only they have.
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>> It's getting a little tricky here.
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But now we're going to move on
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to talking about integrity.
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Now, integrity really is neither here nor there.
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By that I mean,
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it's neither symmetric nor asymmetric.
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Integrity is where we're going to get a guarantee that
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a message has not been changed or hasn't been modified.
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But then, we're going to use asymmetric cryptography,
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to add authentication to integrity
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>> and give us true non-repudiation,
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>> we'll see that in just a moment.
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But let's start out by looking at integrity.
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We get integrity through a process called hashing.
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Hashing produces checksums,
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which can also be referred to as message digests.
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Can also produce hashes.
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So, hashes, checksums,
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message digests, all the same.
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What we're trying to do,
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is we're trying to guarantee
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that a message has not been modified.
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Let's say you and I are communicating
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across a really unreliable link,
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there's lot of interference, packets get dropped.
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I want you to have an assurance that what I sent,
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is what you received.
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I have this message, I'm going to send you, hello.
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What we've agreed to do ahead of time,
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is so that you'll be able to make sure
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>> the message hasn't changed.
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>> What I'm going to do before I send you my message,
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is I'm going to take the numeric equivalent
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for each letter in my message.
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So H is the 8th letter of the alphabet,
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E is the 5th, L is the 12th.
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L is the 12th, O is the 15th.
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I'm going to figure out with a numeric value
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for each letter of the alphabet is,
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not each letter of the alphabet,
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that each letter of my message is going to be.
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Then I'm going to add all those numbers together.
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I came up with the number 52.
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8 plus 5 is 13, plus 12 is 25, 37, 52.
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Before I send you this message,
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I'm going to tuck the number 52
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>> onto the bottom of the message.
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>> I'm going to send the message to you.
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When you receive it,
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your application's going to figure out
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>> the numeric value for each of the letters.
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>> Add them all together.
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If you come up with the value 52,
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and that's what I came up with,
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then we have a reasonable assumption
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there's been no modification.
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Now let me pause here and say,
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that's a really simplistic method
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and there are problems with it.
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One letter could change one direction,
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another letter could change the other.
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There are ways that we could have
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different messages produce the same hash value.
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That's because this is just
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>> a little basic hashing algorithm
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>> we just made up for this example.
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Real hashing algorithms are
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much more sophisticated and complex,
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but you get the idea here.
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If the message changes, the hash changes.
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When you get the message,
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if your hash values 53,
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you say, wait, Kelly thought the hash was 52.
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I came up with 53,
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that means the message has been
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corrupted, let's discard it.
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If you've ever downloaded a file
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from the Internet and you go to open it up
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>> and you get a message that says
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>> this file has been damaged or corrupted,
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that's what's going on,
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>> the hash values aren't working
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>> or ain't matching, so to speak.
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>> With real hashing algorithms,
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every message should produce its own unique hash.
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There should be no two messages
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that produce the same hash value.
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That's really important.
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As matter of fact, the hashes sometimes referred
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to as a fingerprint or a thumbprint,
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because every message should have its own unique value.
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If the hash changes,
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then I know the message has changed.
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Now that being said,
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it is technically possible
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to have two different messages,
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create the same hash value,
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and that's called a collision.
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I can't say it's impossible for two hashes.
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There are two different documents
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>> to produce the same hash.
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>> It's possible,
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>> but it should be mathematically infeasible.
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>> By that it means,
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it should take so many resources,
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so much time, so much processing,
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that it is incredibly mathematically infeasible,
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the two different messages would produce the same hash.
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Now, if you're using a hashing algorithm
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>> where collisions happen,
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>> that's a really lousy hash algorithm,
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because the strength of hashing
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is to be able to compare the file hash,
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with the received file hash,
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and say, hey, these two match.
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Therefore, I know that I know,
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that I know it hasn't been modified.
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If you do get collisions
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with the hashing algorithm, that's bad news.
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You got to get new hashing algorithm.
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Now, we've used some hashing algorithms
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throughout the years, MD-5,
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which used for a long time,
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and it produced 128-bit hash.
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>> That was broken.
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>> SHA-1 came around
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>> and that produced 160-bit hash,
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>> that was broken.
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>> So we went to SHA-2,
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which can produce 256-bit hash
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>> or 384, 512, 1024, can have much longer size.
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>> So most of the time for this exam,
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you do not need to memorize bit lengths.
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When we were talking about asymmetric algorithms,
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you don't need to know that
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>> there has an effective key length of 56-bits.
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>> You don't need to memorize block sizes.
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The only time I would make an exception for that is,
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I would know MD5,
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SHA-1 and SHA-2,
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and the bit lengths associated with those algorithms.
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Algorithms, the hashing algorithms
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produced a fixed length hash.
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Sometimes you'll hear the phrase,
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variable length message, fixed length hash.
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So whether my message is one character, or a 1,000,
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the hash will always be the same size.
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With SHA-1, whether I have one character or 1,000,
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the hash will always be 160-bits. That's important.
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You don't want the size of the hash to change,
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based on the size of the message.
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You want that hash to be consistent.
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Every file should produce a unique hash.
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No two different files should produce the same hash.
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If that happens, it's collision.
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There's also an attack,
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that's job it is to cause collisions.
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It's basically saying look,
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you might just dumb luck into creating a collision
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>> if you try enough variations.
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>> That's called the birthday attack.
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But the idea is,
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collision should be mathematically infeasible.
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The longer my hashes,
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the more mathematically in feasible it is
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>> that a hash would be created.
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>> Or that a collision would be forced.
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Now, one other thing I want to stress is that
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>> a hash doesn't use a key,
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>> it's not symmetric, it's not asymmetric.
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The magic of the hash,
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is that it uses what's referred to as one-way math.
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You might hear that associated with
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the idea of a trapdoor,
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because it's very easy to fall through a trapdoor.
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It's hard to get back through one door.
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It always makes me think, I don't know if any of
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you guys out there watch, The Simpsons.
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But if you do,
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Mr. Burns has an office and his desk,
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and in front of his desk.
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If somebody comes in,
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stands in front of his desks and asks him for a raise,
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he has this button under his desk that he pushes
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>> and it drops the trapdoor
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>> and the employees are dropped down to the basement
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>> where the dogs are apparently.
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That idea, it's easy to fall through a trapdoor,
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it's hard to get back.
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With one-way math, for instance,
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even my little basic hashing algorithm
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>> uses one-way math.
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>> For instance, I told you with the math was.
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Figure out the numeric value
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for each letter of the alphabet,
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and then add those values together.
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Well, came up with a value 52,
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and that was easy to do one direction.
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But if you'd look at the number 52,
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let's say the message was encrypted,
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and you didn't know what the message
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was that was encrypted
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and all you saw was the hash 52.
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You wouldn't be able to figure out what the message is.
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That's a really important rule of hashing,
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is you should never be able to look at a hash,
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and reverse it and say,
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oh, I know what your message was.
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Even with my ridiculously basic hashing algorithm,
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if you saw the number 52,
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>> you would know the message is hello.
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>> When you take these real hashing algorithms
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like SHA-1 and SHA-256,
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was a much more sophisticated process,
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it just makes it that much harder.
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Those are some important rules, with hashing.
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You shouldn't get the two different documents
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that produce the same hash.
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You shouldn't be able to look at a hash
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>> and reverse it and figure out what the text is.
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>> If you take that into consideration,
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I shouldn't have to protect
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the confidentiality of a hash,
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because an attacker can see the hash,
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but they can't do anything with it.
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If you see the hash,
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1B3F269438F2, you should be able to say,
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oh, that message said, "Hi Kelly, how are you today?"
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The one way nature of the math is what makes that work.
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Let's build on this.
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What if I take a hash,
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and that hash gives me integrity?
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The hash proves a message hasn't been changed.
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What if I were to encrypt the hash?
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I'm the sender, with my private key.
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The hash gives integrity
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by encrypt it with my private key,
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yes, send it to you.
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Your email application says,
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oh this looks like it comes from Kelly.
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Let's try Kelly's public key to decrypt the hash.
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If Kelly's public key can decrypt the hash,
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you know the message came from Kelly.
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Then your application hashes the document.
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The two hashes match,
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you know it hasn't been modified.
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So I've given you authenticity and integrity.
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The two together are non-repudiation.
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That's what a digital signature does.
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Takes the hash of the document,
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encrypts the hash with the sender's private key.
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The hash is integrity.
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Using the sender's private key gives authenticity.
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That is non-repudiation,
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through a digital signature.
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On your end,
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your system uses the sender's public key
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to decrypt the hash.
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Then they hash the document,
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compare the two hash values.
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If they're the same, then we know
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the document has not been modified.
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We take pieces for integrity,
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we combine them with the pieces for authenticity,
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and now we have digital signature
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which gives us non-repudiation.
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Ultimately, throughout these last couple of videos,
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we've described privacy,
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authenticity, integrity, and non-repudiation.
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Just a good cheat sheet for you right here.
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The receiver's public key is used for privacy.
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Authenticity is achieved through
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the sender's private key.
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With integrity, we use a hash
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>> which could also be called the checksum.
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>> CRC's are old school,
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so a hash, a checksum, message digest.
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Then for non-repudiation,
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>> we use digital signatures
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>> which are the hash encrypted
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>> by the sender's private key.
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>> So that wraps up this section
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>> on asymmetric cryptography.
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>> Make sure before you move to the next set of videos,
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that you can get privacy,
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authenticity, integrity, non-repudiation.
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You can describe that
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>> by using the asymmetric tools
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>> that we've talked about.
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