CISSP D3 Preview | Digital signatures

In this lecture, we are going to talk about digital signatures and PKI– Public Key Infrastructure. In PKI, we can use both symmetric and asymmetric encryption as well as hashing, and we use that to provide and manage digital certificates, just like we do with asymmetric encryption, we keep our private key secret.
But for PKI, we also store a copy of the key pair somewhere secure.
And that is a key repository, because what if I just kept my private key on my computer and the computer was stolen?
Even if we have full disk encryption, they can’t get you the key, that is nice, but my key is still gone.
Which means that any of the messages that were sent using my public key, I can no longer open because my private key is gone.
So to make sure we can still access the data that we need, regardless if the key is destroyed or lost, we have a key repository.
I have worked in a couple of places where for one reason or another, I needed a new desktop.
After I got the desktop, I would contact the department that handled our digital signatures.
We had the right policies in place on how and when you can retrieve them.
I think in our organization at the time, the request had to come from someone that was a senior director or higher and someone that was in your reporting structure.
They say, yes, Thor is getting a new PC and he needs his private key.
Then we would have two security administrators access the key repository and find my key.
And we had two because it is a dual control.
That means that no single security administrator can access the key repository and retrieve keys that they should not.
Having that dual control significantly reduces the chance of someone doing something malicious.
And I say reduce because it does not remove.
It is just much less likely that two security administrators are both in agreement on doing something wrong. It’s the same dual control you see in movies, when on the spaceship, they have to turn the key at the same time to stop the self-destruct or to start it.
To make sure that Bob, who just lost all his space money in a poker game, doesn’t get angry
and destroy the space station, we have that dual control.
Even if you use digital signatures just for private use, make sure you have a copy of your secret key stored somewhere secure.
And that last part is important, secure.
The attackers will always look for the weak implementation.
So you need to make sure it is not there.
A Key Escrow is similar to our own key repository.
It is a backup of our key pairs that is kept somewhere, but it is kept by our third party and most often it is kept at the request of law enforcement.
Let’s say for whatever reason, our organization is under investigation by some branch of law enforcement.
They can then demand we put our keys in key escrow.
So in six months we can’t say, oh no, we can’t open all those emails because we lost the private key.
And I’m sure that key escrow is a real thing because many companies did that.
They chose to lose the private key so they would not incriminate themselves with the proof of their wrongdoings in the emails or on the servers.
Now, let’s take a look at how a digital signature works and the actual flow of the data.
Over here on the right, you can kind of see a flow.
I am sending an email to Bob and remember, digital signatures give us integrity and non repudiation.
We can add confidentiality to that.
But the way it is most commonly used, it is only integrity and non repudiation.
I send my email to Bob, that is the data packets you see here.
Then I have a hashing algorithm that provides a hash of the data.
I then encrypt that with my private key and that gives us the digital signature along with the data.
I send that over the Internet, Bob receives it, he then uses my public key to decrypt it, then he uses the same hashing algorithm and those two hashes from the algorithm and the one I sent, they have to match.
If they do, we have message integrity.
And since Bob decrypted this with my public key, then that proves that message came from me.
That provides us now repudiation.
We have talked about we can technically add confidentiality to this as well, although it is not very common.
The way we would do this is after I have encrypted the message with my private key, I would then encrypt it again using Bob’s public key.
Since he should be the only person that has his private key, he is the only one that can decrypt my email.
So as you can see on the flow here, we used to hashing for integrity.
Then we use the private and public key for non repudiation and in some cases confidentiality.
Then we send it over the Internet.
Here we mainly use symmetric encryption.
And then Bob again uses the hashing and the asymmetric.
And I have had students that ask me, “Thor do I really need to understand this flow?”
And my default answer is, every time, if I teach it to you, then yes, it is possible to see it on the exam.”
Now, that doesn’t mean you will see it on the exam.
I have no clue.
The question databank for the actual exam is huge.
You may or may not see something on the flow until digital signatures, but why take that chance?
This flow, I think is pretty simple and I have said it before.
The exam is not going to give you definition questions.
You may get something where they might describe the flow of a digital signature and knowing that you can then pick the right answer.
Or they might say we want integrity, non repudiation and confidentiality.
And then the four answer options will give you a flow saying encrypt with Thor’s private key, then encrypt with Bob’s public key.
Then they might say private key, public key, then hash it, then send it.
And if you don’t completely understand the flow, then how can you answer what we should do to get integrity, non repudiation and confidentiality?
Or they might just want integrity and non repudiation.
Well then it is a regular digital signature.
Most of the questions on your exam, you’re going to have to use logic, figure out what are they really asking, and with that knowledge, find the most right answer or the least wrong.
Now, let’s finish this lecture by looking at digital certificates.
Digital certificates are public keys signed with a digital signature.
They can either be client based or server based.
If they are server based, that can be SSL or TLS.
It is assigned to a specific server and stored on the server.
If it is client based, well, then it is your digital signature.
It is assigned to you and stored on your PC.
Since we already covered your digital signature, let’s look at the server based ones, for SSL and certificates, we will most likely use a public CA– Certification Authority.
And that could be someone like GoDaddy or VeriSign, regardless if it is a public or something we have internally in our company, the CA’s job is to issue and revoke certificates.
Then we have an ORA– Organizational Registration Authorities, and that is something we have within our organization.
It authenticates a user or a system and then it issues a certificate.
On top of issuing the certificates, it also looks at which ones have expired and would have been compromised.
And for that it uses a certification revocation list.
Let’s say one of our server certificate has been compromised.
Well, then the ORA revokes that certificate.
Or if an employee leaves our organization, we keep the certificate to make sure that we can decrypt messages.
But we retired from active use.
I have had some students that suggested that we should delete the certificate when they leave the organization.
And no, first off, we need to be able to read their emails.
And what if in six months we get a court order saying we suspect the employee that left you has been part of insider trading, you have ten days to provide us with all the emails from this time period.
So, yeah, we keep the certificates after they leave.
Up till some years ago for SSL and TLS certificates, we did the same thing.
Now we have moved to an online certification status protocol, which is a client server hybrid, and that is OCSP– Online Certification Status Protocol.
Before we will check every single certificate to see if it was expired with the new version we just checked is this specific certificate expired.
And that, of course, is much, much faster.
And the certification and revocation is an ongoing process.
The list is never static, which is also why it makes so much more sense to use that server client hybrid instead of having to go back every time and check.
And while we’re on this topic, let’s take a real quick look at the Clipper chip.
The Clipper chip was a chipset that was developed and promoted by the US NSA.
And what they said was the intention behind it was an encryption deviceto secure data and voice messages, but somehow they were also smart enough to leave built in back doors so they can listen in on all our conversations.
So the intent was that this little chip would be embedded in every device to secure us.
And while it might have secured some things, obviously the main purpose was for them to listen in.
So luckily when they published this, there was a huge public outcry.
It was seen as a huge invasion of privacy, which we now know it was, and that made them pull the plug on it, which was very, very lucky, because after the fact, we discovered a bunch of security holes in the Clipper chip that would have made many more people able to listen in and see everything that you did, not just the NSA.
Many of the security flaws that the Clipper chip had came from the Skipjack cipher, a cipher that was never secure.
And with that, we are done with this lecture.
I will see you in the next one.