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Diffie Helman comes in different forms.

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Diffie Hellman one is 768 bits in length the film and two it's 1024 bits in length DIFI home and five

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is 1536 bits in length.

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Once again the longer the key length the more secure.

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But the downside is more processing power would be required.

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Now just to reiterate asymmetric key algorithms are used in VPN today not for bulking corruption of

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data but they help with the establishment of a shared secret.

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They're also used for other things like a syndication which I get to talk about in a moment symmetric

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key algorithms such as Alias are used for Bolt encryption of data.

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So we've covered confidentiality encryption.

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Let's look at the second goal which is integrity.

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We want to ensure that data has not been tampered with.

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In other words we want to know that that data has traversed the internet or other network unchanged

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between the two parties.

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Data integrity uses algorithms known as hashing algorithms also known as Treptow or message digests

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these a one way algorithms unlike encryption algorithms which can be reversed hashing algorithms convert

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arbitrary data into a fixed length hash.

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An example would be in the five or message digest algorithm 5 which has a fixed length of 128.

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That's now to demonstrate hashing.

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Notice I can take a piece of arbitrary information.

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Let's say my name and I can hash it in this case using schaw Shaugh or secure hash algorithm is more

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secure than M.D 5.

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This is the hexadecimal very Forshaw and the binary value Forshaw.

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Now it is if I change one value for instance making that David 1 and hash it again notice the entire

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hash changes.

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But notice it's of a fixed length.

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I could put a bunch of people's names in there

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and hash it again.

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Notice the entire hash changes.

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But he's of a fixed length.

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I could go and copy some text from let's say USA Today.

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Arbitrary length.

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I could take the Encyclopedia Britannica put it through an empty five hash and come up with 128.

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That's.

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So for example I could take that USA Today article put it into an empty five hash generator and notice

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it will come up with a 128 bit hash value.

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Or I could replace that with let's just say my name and it'll come up with a 128 bit hash value ashing

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is nonreversible because data is lost.

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You cannot take 128 but M.D five hash reverse it and come up with the Encyclopedia Britannica.

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But you can take the Encyclopedia Britannica hash it and come up with 128 but value.

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Please note that the hash will change.

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As I've demonstrated if any part of the input value changes so with hashing we can take data of arbitrary

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length put it through an empty five or char hash.

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In this case it's M.D five and come up with a fixed 128 but ash Valley you cannot take the 128 bit hash

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value and reverse the process and come up with the original data.

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It is a one way function or trapdoor function.

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There are various hashing algorithms that can be used M.D 5 once again is 128.

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That's M-B 5 is not recommended today.

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In networking environments Shaw one is 160 Betson length shot to these 256 or Sabran 12 bits in length

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and Shaugh 3 is scheduled for release in 2012.

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Just be aware that there are various hashing algorithms once again shot to is what's recommended in

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today's networking environments.

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So as an example if Peter wanted to send data to sirra in sharing confidentiality and integrity the

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following would happen.

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Peter's private information that no one else except Sarah should read is encrypted firstly with an encryption

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algorithm like a yes.

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Now in this case we're assuming that a shared secret or shaid key has been derived.

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So assuming that that's happened Peter can encrypt the data using a symmetric key algorithm like a s.

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So the kid text information is encrypted into ciphertext.

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This provides confidentiality Petah then takes being corrupted text will ciphertext and hashes it with

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a hashing algorithm like Shawa M.D. 5 which comes up with a fixed length hash.

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This will ensure data integrity.

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Because if any part of the date is changed remember the hash will also change.

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So Peter takes the clear text encrypted with an algorithm like s to come up with ciphertext he hashes

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that encrypted text and comes up with a hash.

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Even a pin is the hash to the encrypted ciphertext and sends it to Sarah.

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Sarah upon receipt of the data in this case the encrypted ciphertext wants to make sure that the data

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hasn't been tampered with before going through all the effort of decrypting the text.

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So Sarah will take the encrypted text and hash it herself to come up with a M.D five Wilshaw hash.

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She will then compare the hash that she derived with the hash appended to the encrypted data.

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Only if the hashes are the same as she bother decrypting the text now if the hashes are the same.

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It means that the data hasn't changed in transit.

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If the hashes are the same Saraa can decrypt the data by reversing the ace encryption knowing that the

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data hasn't been tampered with.

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However that being said what stops Joe hacker receiving the data changing it so manipulating the data

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before it reaches Cerra encrypting it with a yes hashing that fake data with let's say Shaw and appending

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a new hash to the data and then transmitting it to Sarah.

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Sarah has no way of knowing that the data has been manipulated because when she reverses the process

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by hashing this new data the hash will be the same as Joe Hakas hash that he appended to the new data.

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So to combat that what Pete needs to do is use something called hash message into question code or SCHMOCK

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and there are two variants of this.

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You have a schmuck M.D 5 and H Mac Shaw.

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And what PD needs to do is take the data of arbitrary lengths.

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In other words the data that he wants to send to Sarah Plus a secret key that only Sarah and he knows

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and now hash those two values with the five or Shaw to get the hash that will combat Joe hacker from

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manipulating the data.

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Because Joe hacker won't know what the secret key is that Peter and Sirah are using in combination with

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the hashing algorithm Joe hacker will not know what the secret key is.

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So when he hashes the data Sarah will know that the data has been manipulated because of the hash that

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she derives will not be the same hash seru will be taking the encrypted data.

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In combination with the secret key and hashing those two together to come up with her hash Joe hacker

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will not know what the secret key is.

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So when Joe hacker hashes the data he's hash will not be the same as the new hash that Sarah derives

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and she will therefore know that the data has been tampered with.

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Only Peter and Sarah know what that secret key is not.

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Joe hacker say he can not successfully manipulate the data and derive the same hash fairly.

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Thus data integrity is provided with a smack in combination with M.D 5 and Shaw.

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The third goal to accomplish is authentic cation.

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Now authentication is knowing that data received is the same data that was sent and that the claim sender

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is in fact the actual sender.

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Now we've already spoken about integrity.

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Now we are looking at a thing to Kading appear to make sure that there are actually who they say they

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are.

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This goes beyond validating the source attempting to access a service sharing initial logging.

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You should also validate that the source has not been replaced by an attacking host.

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In the course of the conversation which is known as session highjacking you want to make sure that the

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person that you are talking to is the person that they say they are and that they haven't been replaced

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by a hacker.

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They are two types of authentication so we could have sent Akkad wrote a one to rodded to using either

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appreciate key which is a secret key value entered into each peer manually and is used to indicate the

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pier.

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Or we could use RSA signatures which encrypt the hash with a private key.

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So firstly appreciate key in this example.

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Peter needs to be authenticated by Sarah.

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In this case Peter it takes a Diffie Helman shape key that they derived the pre-shared key that was

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agreed upon with Sarah which should have been done out of band and other information relating to OPSEC

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and he hashes that with either M.D Feibel Shaw and he attaches the hash to a packet with his identification

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information which may be the IP address or hostname that is used for the VPN.

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Sarah can then hash a local copy of the information which includes the agreed upon pre-shared key and

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derive and in the final shall Hesh she can then compay her locally derived hash with the hash that you

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received from Peter.

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If they the same she knows that Peter has the same pre-shared key as she does and she can any indicate

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Peter if the hashes are different she knows Peter does not have the correct pre-shared key and therefore

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the VPN is not set up.

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The second option is to use digital signatures digital signatures have multiple advantages including

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the automatic exchange of keys without the need of programming static authentication keys on multiple

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devices.

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This allows for scalability.

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The key L'anse are also a lot greater appreciate keys should be changed on a regular basis.

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And in reality that often doesn't happen.

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Another advantage of digital signatures is non repudiation.

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Which means you can not deny being involved in a conversation because you're the only person that has

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your private key.

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So the way it works is pita in this example takes a Diffie Hellman shaky and other information and hashes

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it in a very similar way to pre-shared keys.

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But notice the preset key is not in this list.

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That hash is now signed with Peter's private key and remember that Peter is the only person that has

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that private key that creates what's called a digital signature.

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So a digital signature is created when information is encrypted with a private key.

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Please remember that if something is encrypted with someone's private key only that person's public

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key can decrypted peed in our sins.

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That information to Sarah Sarah takes the received signature from Peter and decrypts it with Peter's

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public key which he had previously received from Peter.

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That will result in the original hash that Peter created.

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Sarah Not takes the same information that she has locally and hashes it herself to derive her own hash

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of the various parameters.

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She then compares the two hashes.

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If say the same she knows firstly that PETA has all the correct information.

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She also knows that this information could only have come from PETA because only PETA's public key can

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decrypt something encrypted with Peter's private key.

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So the digital signature proves that the information came from Peter and that all of this information

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is correct.

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She has thus been able to think Akkad Peter.

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Now the reverse will happen for both pre-shared keys and digital signatures.

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Peter Will authentic syndicate Sarah.

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So there's mutual two way into question either by using pre-shared keys or by using digital signatures.