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- [Instructor] Information
security professionals

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must also apply controls to protect

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the integrity of information.

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As the second leg of the CIA triad,

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integrity controls ensure that information

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is not altered without authorization.

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Integrity controls protect
an organization's information

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from accidental or intentional tampering

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that may come as the result
of many different issues.

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Integrity failures may result from

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the intentional alteration of information

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such as an employee
altering his or her salary

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or a student altering grades.

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They may come from user error

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such as the data entry
clerk accidentally entering

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the wrong information into a field.

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Or they may come from
software or hardware error

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such as an application or
hard drive malfunctioning

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and writing erroneous data.

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Finally, they may come from acts of nature

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such as a lightning strike
that alters information

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stored on a disc.

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Hashing is one of the core controls

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used to protect integrity.

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A hash function is a
mathematical algorithm

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that computes a unique digest
from a file of any length.

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This digest is like a
fingerprint for the file.

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It's a short piece of data
that can uniquely identify

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the contents of a file and
tell if it's been modified.

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Let's take a quick look
at hashing in action.

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Here, I'm going to use
a common hash function

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called md5, short for
message digest version 5,

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along with a very simple text file,

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the Gettysburg Address.

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So, this webpage that we're looking at

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and md5 hash generator will take any text

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that you type into the string field

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and compute the md5 hash
value for that string.

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I'm going to go ahead and paste

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the text of the Gettysburg
Address into this,

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and as you can see, the webpage

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already computed for me the
md5 hash value of this text.

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This long string beginning with 78e35

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and ending with fd7 is the hash value

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of the text of the Gettysburg Address.

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Now, if I go back up
here and make a change,

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let's say, I change this from

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"Four score and seven years ago"

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to "Four score and eight years ago,"

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notice that the hash value has changed

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to a completely different value.

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If I go ahead and change this back

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and type "seven" again, I
have that original hash value

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that began with 78e35 and ended with fd7.

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That's a major change to the file,

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but hash values are extremely sensitive.

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Notice, for example, if I take this comma

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after "new nation" and delete it,

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the hash value, again, changes completely.

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Hash values are very good at telling us

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if any change has been made to a file,

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but they can't tell us
what changes were made

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or how significant those changes were.

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Hashes can easily detect
changes in a file.

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You can compute the hash
value of a file today

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and then compare it to the hash value

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that you compute tomorrow.

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If the hash value hasn't
changed, the file hasn't changed.

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If the two hashes are different, you know

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that some modification to
the file has taken place.

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Hashes are the foundation for many

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different integrity controls.

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One of those is digital signatures,

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a control that helps us achieve the goal

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of non-repudiation.

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Non-repudiation means
that the creator of text

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can not later dispute that he or she

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was the real originator of that text

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by claiming that the
message was a forgery.

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Just as you'd sign a paper document

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with your physical signature,

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you can apply a digital signature

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to a digital file to
achieve non-repudiation.

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To create a digital signature,

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the person signing a document
first uses a hash function

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to create a digest of that document

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and then encrypts the hash value

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using his or her private encryption key.

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The recipient of a
digitally signed message

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can then use the sender's public key

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to decrypt the signature and
then compute the hash value

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of the message themselves
and compare the values

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from step one and step two.

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If they match, the message is authentic.

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If they don't match, the message

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may have been tampered with.

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There's a bit more technology behind this

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that I cover in the CISP
security engineering course.

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Digital signatures can also be used

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to create digital certificates.

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Digital certificates are files used

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to provide a system or
individual's public encryption key

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to previously unknown third parties.

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Organizations known as
certificate authorities

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create these certificates
and then digitally sign them

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to show that they are authentic.

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Again, I cover this in more detail

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in the CISP security engineering course.