A few months ago, I attended a conference. A security
related session had just ended, when a fellow attendee that was sitting next to
me asked me if I understood what the speaker was talking about. When I answered
yes, the man asked me to explain it to him in laymen’s terms. After talking to
him for a few minutes, I realized that although he had worked with computers
for a few years, he did not really understand public key infrastructure (PKI).
He told me that he had tried to read about it on a few occasions, but that all
of the books or Web sites that he found on the subject either assume that you
already know all about PKI or they use so many big words that they are hard for
a beginner to understand. I thought about his statement and realized that he
was right. I have never seen anything on PKI that was geared toward a novice. I
decided right then that I wanted to be the one to write a beginner’s guide to
PKI.

Before I Begin

Before I get started, I just want to point out that this is
article is intended to be an introduction to PKI. There is no way that I could
possibly cover the topic thoroughly without writing a good sized book.

What is PKI?

As you’ve probably already figured out, PKI stands for
Public Key Infrastructure. PKI has lots of different uses, but it is used
primarily for encrypting and / or signing data. Encrypting data refers to
scrambling it in a way that makes it unreadable except to authorized persons.

Signing data basically refers to authenticating it. A good
example of this is signing an E-mail message. If an E-mail message contains a
valid digital signature, it proves two things. First, it proves that the
message has not been tampered with in transit. Second, it proves that the
message is from the person that it claims to be from. E-mail messages are not
the only thing that can be signed though.

You’ve probably seen device driver’s or applications that
contain a digital signature from the manufacturer. Such digital signatures
prove that the code was really developed by the company that it claims to be
from. I will talk a lot more about digital signatures later on.

The Anatomy of PKI

PKI is based on a mechanism called a digital certificate.
Digital certificates are sometimes also referred to as X.509 certificates or
simply as certificates. Think of a certificate as a virtual ID card.

In the real world, people use ID cards such as a driver’s
license, passport, or an employee ID badge to prove their identity. A
certificate does the same basic thing in the electronic world, but with one big
difference. Certificates are not just issued to people (users, administrators,
etc.). Certificates can also be issued to computers, software packages, or to
just about anything else that you may need to prove the identity of.

Certificates are very useful in high security situations.
For example, suppose that you needed to securely transmit data from a Windows
XP workstation to a Windows 2003 Server in your company. How do you really know
that you are transmitting the data to the actual server and not to an imposter?
One way of insuring the integrity of the transaction is to use digital
certificates to prove the identities of both machines. In fact, digital
certificates are one of the underlying requirements of the IPSec protocol that
is designed to securely transmit data over Windows networks.

OK, so a secure session can be established because two
machines are able to present each other with certificates. You might be
wondering though, what makes the certificates so trustworthy. These days, if
you want to leave the country, you pretty much have to have a passport
regardless of where you are going.

A few years ago though, you could travel to Canada, Mexico,
or to some of the Caribbean islands and use a birth certificate and a driver’s
license instead of a passport. That being the case, let’s imagine that these
forms of ID are still accepted and that you wanted to travel to the Bahamas.
When you arrive at the airport in Nassau, you give the immigration officer your
driver’s license. The officer has absolutely no idea who you are, but he knows
that your driver’s license has your name on it, along with your picture. The
fact that you hand the officer a plastic card with a name and picture on it
means nothing to him. However, your license was issued by government agency. Of
course the Bahamas is a foreign country and the officer may or may not have
ever heard of your state. What matters to him though is that the Federal
Government of the United States trusts the state to use due diligence in
proving your identity before they issue a license.

So what does this have to do with digital certificates? Let’s
go back to my earlier example in which a Windows XP machine needed to prove its
identity to a server. Think of the process of the Windows XP machine presenting
the server with its digital certificate as being similar to giving the
immigration officer a driver’s license. The certificate itself proves nothing
to the server. However, if the certificate was issued by a source that the
server knows and trusts then the server will accept the machine’s certificate
as proof of its identity.

This raises the question of where do certificates come from
and how do machines decide whether or not to trust them? Well, there are a
number of places that certificates can come from. One source is from third
party certificate authorities, such as VeriSign. Windows is configured by
default to trust anything with a valid VeriSign certificate. There are a few
other third party certificate authorities that are also trusted by default.

Windows 2000 Server and Windows Server 2003 also allow you
to create your own certificate authority. Sometimes it’s more practical and a
lot less expensive to just create your own certificate authority and allow it
to issue certificates on an as needed basis, according to your corporate
security policy.

Certificates and PKI

So far I have talked a lot about certificates, but you may
be wondering what certificates have to do with PKI. PKI works by assigning a
user a pair of keys. These keys are generated by running a mathematical process
against the user’s certificate. The keys themselves are nothing more than a
very long alpha-numeric string.

One of the keys is designated as the user’s private key,
while the other is designated as the user’s public key. The idea is that only
the user who owns the keys has the private key, but the user’s public key can
be freely given to anyone. Normally, a certificate authority or a key
management server passes out public keys whenever they are requested, but
public keys could really be distributed by any means.

Suppose for a moment that a user needed to encrypt a file.
The user would use their private key to encrypt the file. Once the file is
encrypted, only the public key can decrypt it. At first, this probably doesn’t
sound very secure since anyone in the world can have the user’s public key just
by asking for it. However, there is one detail that you need to consider. The
user’s public key can only decrypt files, it can not be used to encrypt
anything. Furthermore, it can only decrypt items that have been encrypted using
the corresponding private key. Therefore, if a public key is used to decrypt a
file, it absolutely guarantees that the person who encrypted it was the owner
of the corresponding private key (assuming that the private key hasn’t been
stolen). For example, if I encrypted a file with my private key, and you used
my public key to open it, then you can be sure that I was the person who
encrypted the file.

This concept is very important when it comes to producing
and authenticating digital signatures. To see how this concept works, let’s
pretend that I need to send you an E-mail message and that because of the
sensitivity of the message, you need to be absolutely sure that the message
came from me and not from someone pretending to be me. For this example, we
will also pretend that the message does not need to be encrypted. You just need
to be able to verify the identity of the person who sent the message (me) and
verify that the message was not intercepted and altered in transit. This is
what a digital signature does for E-mail in the real world. The example below
is simplified, but accurately illustrates how a digital signature works.

I would start out by typing the message. For simplicity’s
sake, let’s assume that the message says: The
check is in the mail
. The first thing that the E-mail program that I am
using would have to do is to produce a non reversible hash of the message. A
hash is nothing more than a mathematical computation based on the contents of the
message. In the real world, hashes are very complex, but for the sake of
demonstration, I will pretend that we are creating a hash by adding together
the ASCII values of each character in the message (including spaces and
punctuation). The hash would look something like this:

The check is in the
mail.

84 + 104 + 101 + 32 +
99 + 104 + 101 + 99 + 107 + 32 + 105 + 115 + 32 + 105 + 110 + 32 + 116 + 104 +
101 + 32 + 109 + 97 + 105 + 108 + 46 =Â
2180

Assuming that I have added the numbers correctly, the sum of
the ASCII values used to construct the message is 2180. As you may recall from
the previous paragraph, I said that we needed to use a non-reversible hash. The
reason why the hash is non-reversible is because there is no way that you can
look at the number 2180 and see that the message was “The check is in the mail”. You could theoretically run a brute
force attack against the number 2180 and look at all of the different
combinations of ASCII values that add up to this number in an effort to get the
original message, but there are some problems with that.

First, there would be a huge number of ASCII strings that
add together to produce the correct value. I won’t claim to be enough of a
mathematician to tell you how many possible strings there are, but it’s a
bunch. The second problem is knowing the algorithm that was used to produce the
hash. In this case, I used an extremely simple algorithm, but in real life the
algorithms are much more complex and are not highly publicized. The third
problem is picking out which of the results is the original message.

You could use a dictionary program to filter out junk
strings and leave only those messages that contain real words, but then
anagrams become a problem. Anagrams refer to being able to change the order of
the letters in a phrase to spell out something different. For example, if you
were to change the order of the letters in the word “dog” you could
produce the word “god”. Both words would both produce the same hash
value using the algorithm above. My point is that for all practical purposes,
the hash value is non-reversible.

Once my E-mail client has produced the hash, it appends the
hash to the end of the message. The idea is that when the recipient receives
the message, the recipient’s computer calculates the message’s hash by using
the same algorithm that my machine used to produce the hash in the first place.
If the recipient calculates the same value as the hash value that is appended
to the end of the message, then the recipient can be sure that the message has
not been altered in transit, and all is right with the world, right?

Not so fast� There is absolutely nothing stopping a hacker
from intercepting the message, changing the message’s contents, and then
changing the hash to reflect the message’s new contents. Besides, I haven’t
done anything to prove my identity to the message’s recipient.

This is where PKI comes into play. We have calculated a hash
value that the recipient should be able to use to verify that the message hasn’t
been modified in route, but presently there is nothing stopping a hacker from
modifying the hash value along with the message. Suppose however that I used my
private key to encrypt the hash value before I transmitted the message.

If I did this, then anybody can read the hash value because
it can be decrypted with my public key and my public key is freely available to
anyone who asks for it. Keep in mind though that the only things that my public
key can only decrypt things that were encrypted by my private key. Therefore, when
the recipient receives the E-mail message, they use my public key to decrypt
the hash value. If they are able to do this successfully, then they know beyond
a shadow of a doubt that I am the one who encrypted the hash value. They can
then compute a hash of the message for themselves. If their computed hash value
matches the now decrypted hash value that is appended to the message, then the
recipient knows for certain that I am the one who sent the message and that the
message has not been tampered with.

To put it simply, I have digitally signed the message. In
the real world, the algorithms used to create digital signatures are more
complex, but the process is very similar to what I have just shown you.
Furthermore, this process isn’t just used to sign E-mail messages. The process
can be used to sign software packages or just about anything else.

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