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Have you ever downloaded
a file from the internet.

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Maybe it was a file
for a router update,

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or a file for your
computer or an application.

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And wondered is this file
really from the vendor

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or is this a file that an
attacker or a hacker got,

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manipulated, included
some malware,

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and now I have it
on my computer?

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That's actually a very
good question to ask.

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That's why we should download
from reputable sources,

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but beyond just hoping
the files are OK.

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Another method we can use
for verifying data integrity

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in the world of cryptography
is the concept of using a hash.
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in the world of cryptography
is the concept of using a hash.

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And here's how a hash works.

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So we're sitting
here at our computer

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and we download the file.

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And what we can do is
we can have that file.

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It's on our hard disk.

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And instead of just
trusting, Oh, yeah.

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That's the official file.

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We can go ahead and
verify the hash.

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And then we can go up to-- if
it's a Cisco file, for example,

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we Cisco.com.

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And on their website, they
would publish the hash.

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So a hash is like a little
string of data source.

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If the file is 200
megabytes, or 50 megabytes,
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If the file is 200
megabytes, or 50 megabytes,

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or 1 megabyte based on the
hash that they're using,

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it'll create this little,
teeny string of text

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that is created as a
result of creating a hash.

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And so what we can do is
we can download the file.

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We can take a look on
Cisco's official site

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and see what the hash is.

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We can run that same
hashing algorithm again.

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And if the hash we
generate matches the hash

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they published, guess what?

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We have data integrity.

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And that way we can
validate that it is indeed

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the file that we think it is.
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the file that we think it is.

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Or if it's on a Cisco
iOS image, you can--

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let's go on the top here.

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You can write this command
right here, verify slash MD5.

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That's verifying the MD5 hash.

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It'll generate the
hash, and then we'd

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compare that hash
generated to the hash

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that Cisco publishes for.

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So this is an example
of using a hash

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to verify the data
integrity of a file,

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but the same concept
can be used to verify

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the data integrity
of packets as they're

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being sent over a network.

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So in the event of
sending a packet,

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if R1 is sending a
packet over to R2,
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if R1 is sending a
packet over to R2,

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if we're using some kind
of a hashing function.

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R1 and R2 can validate, based on
the hash, each and every packet

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that's being sent to make
sure the hashes match up,

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which is going to
help verify the data

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integrity of those
packets-- that they haven't

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been manipulated or changed
in transit by some third party

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device.

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So let me give you a very
vivid example of how a hash can

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help us verify data integrity.

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This is a Passwordgenerator.net.

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I just found it online.

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And I'm going to
paste in-- right click

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and paste the Declaration
of Independence.
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and paste the Declaration
of Independence.

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And I'm also including
the unanimous declaration,

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the first few words there.

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So if we have this document
with all these words,

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all the syntax.

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If we want to generate
a hash based on that,

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we'd paste that text
in, click on Generate.

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This little string
right here is the hash

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that's now being generated
based on all that text.

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Now check this out.

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If we change one character or
anything else in that document,

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it's going to modify the hash.

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So you can take a
look at the first,

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like 1C5F, or at the end, 5850.
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like 1C5F, or at the end, 5850.

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Either way is fine
because the entire hash

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is going to change if
we change one character.

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So going to change
instead of a capital T,

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I'm going to go ahead
and put a lowercase t.

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And you'll notice the
entire hash changes.

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And so if I generate a hash
on the original document,

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you're on the same
hashing algorithm.

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When you receive
the document, you

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can know if the hashes match,
that the data has integrity.

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It hasn't been manipulated
or modified since I sent it.

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So hashing is pretty cool.

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Now one of the challenges
with hashing though,
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Now one of the challenges
with hashing though,

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is that what if an
attacker takes packets

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and they modify the data, and
then they generate a new hash?

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Because the hashing algorithms
like SHA512, SHA256, SHA1,

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those are not secret algorithms.

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So a hacker could
theoretically take a packet,

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modify the contents, create a
new hash, send it on it's way.

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And the receivers say, Oh, yeah.

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The hash matches.

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That's the problem.

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So another aspect
regarding hashing

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is instead of just using the
hashing algorithm by itself,

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why not also include in the
formula, in the equation,

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include some secret key.
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include some secret key.

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Some secret that's only known
by the sender and the receiver.

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So if we're on a
hashing algorithm

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and we include a secret as
part of the calculation,

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that way only the
sender and receiver

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can correctly create new
hashes and verify the hashes

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on that data.

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So when we do that
in IP networking

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to protect the traffic,
that's referred

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to as hashed message
authentication

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code, or HMAC for short.

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And let me give you
an example of how

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adding that additional
component of some secret

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that both party knows, how
that changes the equation.
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that both party knows, how
that changes the equation.

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So here is yet another site.

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It's called Freeformatter.com.

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And I will go ahead and
paste the Declaration

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of Independence in there again.

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And we can choose our algorithm.

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So bigger is better.

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MD5, we're not using too much
anymore because it's not too

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secure, but let's do SHA512.

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That's pretty hefty.

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Now it's also asking
us for a secret key.

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So the secret is
the element that's

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going to make this hash only
calculatable by the devices who

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both have that key.

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So let's use our secret here.
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So let's use our secret here.

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And we'll click on Compute HMAC,
which is the hashed message

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authentication code.

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And there it is.

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There is the SHA512 HMAC
for the entire document.

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If we change one
character of the text,

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or we change any aspect of our
key, which I will go and just

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call it our SEC and
click on Compute HMAC

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is going to modify
that entire string.

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Again, a different
secret is going

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to affect the outcome
of the calculation.

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So in this video,
we've taken a look

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at the concept of hashing
to validate data integrity,
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at the concept of hashing
to validate data integrity,

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whether it's on a file or
whether it's on a packet that's

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being sent over a network.

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Hashing and its
bigger brother, HMAC,

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are important tools to have
in our cryptography tool belt.

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Now hashing and data integrity
are pretty darn important,

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but also in the world
of cryptography,

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we have tools to help
keep the data private

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and hidden from
unauthorized eyes.

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And we'll take a look at
that in the next video.

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So I'll see you there
in just a moment.

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Meanwhile, I hope this has
been informative for you.

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And I'd like to thank
you for viewing.
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And I'd like to thank
you for viewing.