Cryptographic Tunneling and the OSI Model Write a paper consisting of 500-1,000 words (double-spaced) on the security effects of cryptographic tunneling based on an understanding of the OSI (Open Systems Interconnect) model (Review the OSI Simulation in the Week 3 Lecture). Provide input on the type of cryptographic tunneling protocols (e.g., L2TP, IPSEC, SSL, etc.) that may be used, the layer(s) of the OSI at which each operates, and also recommend how they may be implemented. Cryptographic tunneling is inherent in building any common virtual private network (VPN). This is the lecture material that was provided. Cryptography and Telecommunications Cryptography | Telecommunications and Firewalls Cryptography Back to Top The History of Cryptography It must be that as soon as a culture has reached a certain level, probably measured largely by its literacy, cryptography appears spontaneously--as its parents, language, and writing probably also did. The multiple human needs and desires that demand privacy among two or more people in the midst of social life must inevitably lead to cryptology wherever men thrive and wherever they write (Kahn, 1996, p. 84). Kahn, D. The codebreakers: The comprehensive history of secret communication from ancient times to the internet . New York, NY: Scribner. Cryptography: Cryptography is a discipline that embodies principles and methods for the transformation of data to hide its meaning, establish its authenticity; and prevent its undetected modification, repudiation, and unauthorized use. Cryptography is an old art. We know that the ancient Egyptians were using cryptographic techniques in 1900 B.C. The oldest known example is on the tomb of the Egyptian nobleman Khnumhotep II. For more of this history, you might find a tour of the National Security Agency's National Cryptologic Museum interesting, but this is not required for this class. The National Cryptologic Museum tour can be accessed at the following link: National Cryptologic Museum Forms of cryptography: There are two basic forms of cryptography: secret-key cryptography and public-key cryptography . These forms of cryptography are different because they use cryptographic keys in different ways. Secret-key cryptography: Secret key cryptography is the original form of cryptography. Let us deal with text at this point, and think of text as a string of characters. An understandable string of characters is called a plaintext . If we have a plaintext and we want to hide its meaning, there are basically two things we can do: We can rearrange the letters in the plaintext, or we can substitute the letters in the plaintext with other letters. These are the two forms of secret writing . They are called transposition and substitution methods, respectively. The text that results from encryption, which is the application of cryptography, is called the ciphertext . Type of Cryptography Plaintext Method Ciphertext Transposition cryptography rea.

1. Cryptographic Tunneling and the OSI Model
Write a paper consisting of 500-1,000 words (double-spaced) on
the security effects of cryptographic tunneling based on an
understanding of the OSI (Open Systems Interconnect) model
(Review the OSI Simulation in the Week 3 Lecture).
Provide input on the type of cryptographic tunneling protocols
(e.g., L2TP, IPSEC, SSL, etc.) that may be used, the layer(s) of
the OSI at which each operates, and also recommend how they
may be implemented. Cryptographic tunneling is inherent in
building any common virtual private network (VPN).
This is the lecture material that was provided.
Cryptography and Telecommunications
Cryptography
|
Telecommunications and Firewalls
Cryptography
Back to Top
The History of Cryptography
It must be that as soon as a culture has reached a certain level,
probably measured largely by its literacy, cryptography appears
spontaneously--as its parents, language, and writing probably
also did. The multiple human needs and desires that demand
privacy among two or more people in the midst of social life
must inevitably lead to cryptology wherever men thrive and
wherever they write (Kahn, 1996, p. 84).
Kahn, D.
The codebreakers: The comprehensive history of secret
communication from ancient times to the internet
. New York, NY: Scribner.
Cryptography:
Cryptography is a discipline that embodies principles and

2. methods for the transformation of data to hide its meaning,
establish its authenticity; and prevent its undetected
modification, repudiation, and unauthorized use. Cryptography
is an old art. We know that the ancient Egyptians were using
cryptographic techniques in 1900 B.C. The oldest known
example is on the tomb of the Egyptian nobleman Khnumhotep
II. For more of this history, you might find a tour of the
National Security Agency's National Cryptologic Museum
interesting, but this is not required for this class. The National
Cryptologic Museum tour can be accessed at the following link:
National Cryptologic Museum
Forms of cryptography:
There are two basic forms of cryptography:
secret-key cryptography
and
public-key cryptography
. These forms of cryptography are different because they use
cryptographic keys
in different ways.
Secret-key cryptography:
Secret key cryptography is the original form of cryptography.
Let us deal with text at this point, and think of text as a string
of characters. An understandable string of characters is called a
plaintext
. If we have a plaintext and we want to hide its meaning, there
are basically two things we can do: We can rearrange the letters
in the plaintext, or we can substitute the letters in the plaintext
with other letters. These are the two forms of
secret writing
. They are called
transposition
and
substitution
methods, respectively. The text that results from encryption,
which is the application of cryptography, is called the
ciphertext

3. .
Type of Cryptography
Plaintext
Method
Ciphertext
Transposition
cryptography
rearrange the letters
ypcrtophygra
Substitution
cryptography
substitute letters
dszqujphsbqiz
Note of Interest - Codes and Ciphers
The discussion here is about ciphers. Another form of
cryptography uses codes which substitute words for words or
words for phrases. Codes use some sort of codebook that has the
translation between plaintext and code. Encoding is a very
powerful form of cryptography and is used with information
technology, but is not the primary form of cryptography that we
find in commercial products.
Elements of cryptography:
You can probably see that the transposition produces a
ciphertext that has the same letters as the plaintext, while the
substitution produces a ciphertext that has the same number of
letters, but the letters are changed (in this case, to the next
letter in the alphabet).
Cryptographic key:
It might seem from this simple example that substitution
produces a better ciphertext than transposition, but that is not
the case at all. Regardless, the bottom line is that any good
encryption method will use both substitution and transposition
several times. The methods used in encryption are called
cryptographic
algorithms
. Essentially, a cryptographic algorithm is a series of

4. substitutions and transpositions performed in a particular
sequence according to a
key
. A key is a bit of secret text that is used to drive the
substitutions and the transpositions within the algorithm. That
is, for a single algorithm, if the key is changed, the resulting
ciphertext produced from the same starting plaintext will be
different.
Shared secret:
So if two people want to communicate with one another and do
not want anyone else to be able to understand their
communication, they need to use the same algorithm and the
same key; the real problem is to ensure that they are the only
people who know that key. That is, cryptography is based on the
idea of a shared secret. A very important factor in cryptography,
then, is to keep that shared secret a secret by only sharing it
with those who are part of the group.
Decryption and cryptanalysis:
By the way, cryptography consists of encryption and
decryption. We just learned about encryption, which produces a
ciphertext from a given plaintext. Decryption reverses
encryption to produce the original plaintext from a given
ciphertext. You can think of it as a reverse process. As an aside,
decryption
without knowing the key
, that is through analytic processes, is called
cryptanalysis
.
Symmetric cryptography:
As you have probably already realized, the case of two people
wanting to communicate privately is not the normal situation,
although that is sometimes the case, especially when the
communication is through magnetic, optical, or solid-state
media rather than through network communications. It is more
likely, and more common, that groups of people want to
communicate privately with other groups of people. So while

5. secret-key cryptography can certainly be used for one-to-one
communication, it is more commonly used to protect
information between secure sites or secure computers that share
information, rather than between individuals. The U.S.
government standard for shared secret cryptography, more
commonly called
symmetric cryptography
, is the Advanced Encryption Standard (AES).
Blocks and streams:
One final point is that symmetric cryptography can operate on
streams of data or on blocks of data.
Stream ciphers
, as they are called, use algorithms that encrypt each character
as it passes through the cryptographic algorithm and decrypt the
resulting ciphertext in the same way.
Block ciphers
, as they are called, use algorithms that encrypt blocks of
characters all at the same time as those blocks pass through the
algorithm, and decrypt blocks as well. You might consider
where symmetric cryptography fits and where it does not fit in
the OSI model that you have been studying, and particularly
what the overall effect of that cryptography would be.
Public-key cryptography:
In the late 1960s, a new form of cryptography became public
knowledge through the work of Diffie and Hellman. Their idea
was to split the shared secret into two parts: a shared part and a
secret part. In this way, the shared part could be shared widely,
while the secret part could be kept secret. The shared part was
(and is) called a
public key
, while the secret part was (and is) called a
private key
.
Every subject who is an authorized user would be assigned a
unique public key, and all of those public keys would all be
stored in a repository to which all authorized users have access.

6. Every user would generate a private key from his or her
assigned public key (together with a specially chosen password
known only to that user). The private key is formed in such a
manner that it will convert ciphertext produced with the public
key to plaintext (and vice versa). Note that each public key and
private key form a pair, and are not related to any other public
key or private key.
Digital signing:
Now, it is also the case for many public-key cryptosystems that
a text string encrypted with a private key can also be decrypted
with the corresponding public key. This is the idea behind
digital signing. Let us say that Jack wants to make sure that Jill
can tell that the message is from Jack. So Jack uses his own
private key to encrypt a special text string. When Jill receives
Jack's message, she still decrypts the message using her private
key, but now she also can ask the cryptographic system to find
Jack's public key in the key repository and use it to decrypt that
text string. In most systems, Jill does not have to ask to have
this done; rather, the signature is checked automatically. This
text string is specially constructed as will be discussed below,
but the fact that Jill can decrypt this text string using Jack's
public key means that it could only have been encrypted using
Jack's private key, which only Jack has, and therefore the
message must be from Jack. Notice that digital signing uses the
sender's private key and the sender's public key.
The hash value:
The text string that Jack encrypts with his private key to form
his digital signature is a special text string. It contains a value
that is calculated from the message that Jack is sending to Jill.
That value is called the
hash
of the message,
or digest
for the message. The hash value is calculated as a function of
the message, and that value is encrypted with Jack's private key.
This means that if Jack's public key can correctly decrypt the

7. digital signature, and the resulting hash value matches a new
hash value calculated for the same message, then the message is
from Jack
and
it has not been changed since Jack signed it. This hash value
therefore validates the integrity of the message, and the
authenticity of the message.
Time:
The time that Jack signed the message is sometimes (usually)
very important, so messages can also be time-stamped with the
time of creation of the digital signature so that Jill can tell that
the message from Jack is a current message, and not an old one
that has been resent for some reason.
Messages and Data
We have been looking at the OSI model throughout this class. If
you go back to the OSI simulation from Week 3, you will notice
that messages pass between protocol layers and between the
corresponding layers on the sending and receiving side of the
protocol stack (corresponding layers on sending and receiving
sides of the protocol stack are called
apposite
). These messages are contained in headers, and those headers
contain information or messages for the apposite layer, as well
as for the next higher layer in the protocol stack. Within the
whole protocol package, though, there is data that was sent into
the protocol stream from an application--that might be the
actual text message sent by Jack to Jill.
Symmetric cryptography again:
You should now understand that asymmetric cryptography is
used to encrypt and decrypt messages and symmetric
cryptography is used to encrypt and decrypt data. In fact, one of
the functions of a public-key system is to distribute symmetric
keys to recipients so that they can use those symmetric keys to
privately encrypt and decrypt data, but that is beyond the scope
of this class. In the normal case, for reasons of efficiency, data
is normally encrypted using symmetric cryptography and

8. messages with asymmetric cryptography.
A Note of Interest - Secure Socket Layer (SSL)
Secure Socket Layer:
The question seems to arise as to whether a particular security
solution is a public-key solution or a private-key solution. The
answer is usually that either it is a shared secret-key solution
alone, or it is a combination public-key and secret-key solution.
You see, symmetric cryptography is the better choice for
encryption and decryption of data because it is efficient.
Asymmetric cryptography is the better choice for encryption of
messages because it is very flexible. In fact, a standard use of
public-key cryptography is to distribute secret keys to entities
that have been authenticated through public-key encrypted
messages. Let us look at a particular solution in a little more
detail.
Secure Socket Layer (SSL) is actually two protocols that reside
between TCP and HTTP (or FTP or SMTP). We will assume
HTTP for simplicity.
The first protocol is called the SSL
handshake protocol
. The handshake protocol helps apposite applications agree upon
a shared secret key to be used for encryption (i.e., a symmetric
key) and another shared secret key (i.e., also a symmetric key)
used for hashing the message content for message integrity.
Please note that there are other elements of the handshake
protocol (e.g., change cipher, alert) that are not discussed here.
The second protocol, called the
record protocol
, goes through a five-step sequence, which takes the data to be
sent to the recipient, and
fragments the data into equal-sized pieces of 16348 bytes
(characters);
compresses each piece;
calculates the hash value for each piece and attaches that hash
value to each compressed piece of the data (this hash value is
called the Message Authentication Code, or MAC) and it is

9. attached as a trailer;
encrypts each piece of data including the hash value trailer; and
then
appends the SSL record header and sends each resulting piece to
TCP for transportation through the TCP/IP stack. You may be
interested in knowing that the SSL header contains four fields:
the type of content (HTTP in this case), the version of SSL that
is used, the minor version of SSL that is used, and the
compressed length of the pieces or fragments.
The SSL handshake protocol expects to accept an X.509
certificate from each user of the SSL session (that contains a
public key for that user). These public keys are used to
calculate a
48-bit master secret key
, and that master secret key is sent to both correspondents
encrypted under their public keys. If no certificates are
available, then the protocol uses a standard algorithm to create
the master secret key. In either case, the actual encryption of
data is symmetric, while the exchange of messages establishing
the master secret key is asymmetric. A similar process is used
to create the hash key to produce the Message Authentication
Code.
Public-Key Infrastructure (PKI):
Public-key cryptography is usually called asymmetric
cryptography. As you can probably tell, the use of such a
cryptographic system for a large organization requires that keys
be managed carefully--both public and private keys. But public
keys must be made available to every authorized user, while
private keys are made available only to individual users. An
infrastructure
is needed to manage such a capability, and that infrastructure is
quite naturally called a Public-Key Infrastructure (PKI). The
following Flash presentation provides an introduction to PKI
and asymmetric cryptography.
Tutorial

10. Click below to open the Flash presentation (required viewing).
This tutorial instructs on the use of a Public-Key Infrastructure
in order to allow parties to communicate via the Internet in a
more secure fashion. Both encrypted and unencrypted messages
are addressed, along with a discussion of the OSI model and
link-level encryption.
Public-Key Infrastructure (PKI)
Time: approx 5 min
A Note of Interest - PKI and Symmetric Keys
You may have been mildly surprised that SSL uses symmetric
keys that are generated from public keys (or through a Diffie-
Hellman exchange) to encrypt the actual data sent through an
SSL session. The same is true for most asymmetric exchanges
using Public-Key Infrastructures. The infrastructure guarantees,
using asymmetric cryptography, that the public keys found in
certificates belong to the user to whom the certificate is
assigned. However, when data of more than a few characters are
to be exchanged across a PKI-supported session, it is common
practice for a
symmetric secret master key
to be generated by the system and delivered to the participants
in a session encrypted under their public keys. The user's
system decrypts the secret master key using the individual's
private key. This symmetric secret master key is then actually
used to encrypt the data content of the packages that are
exchanged, while the public key is used to encrypt the
messages.
The secret master key is active for the duration of a session,
and when the session is terminated, the key is destroyed. This
means that it is technically feasible to have several
simultaneous sessions on a single connection with assurance
that they will not leak information to each other. This capability
is rarely (if ever) used, however, due to concerns about
interference, reliability, and availability.
Telecommunications and Firewalls
Back to Top

11. Providing reliable and secure access to computing resources is a
serious charge for security professionals.
Note of Interest - A Very Large Domain, Indeed!
The study of telecommunications security fills many volumes,
but here we will concentrate on the nature and use of firewalls
for network security (see Objective 10). You have also learned
about cryptography, the OSI model, and virtual private networks
in this class. It is highly recommended that you continue your
study of telecommunications security with introductions to e-
mail security, Internet Protocol security, web security, and
network security management for a complete understanding of
the Telecommunications, Networks, and Internet Security
Domain.
A firewall is a set of security controls with four distinct goals.
All network traffic that enters or exits an entity, such as a
network, a host, a device, or an application, must pass through
the firewall. This means that all other possible access to the
entity is blocked.
Only authorized traffic, as defined by security policy, will be
allowed to pass into or out of the firewall.
The firewall is simple enough to be thoroughly evaluated.
The firewall cannot be tampered with.
You might note that the security goals of a firewall are exactly
the goals of the Reference Monitor discussed in Week 3, though
the language is slightly different.
Every firewall also uses some specific techniques to meet its
goals. They are as follows:
Object control:
A firewall allows certain objects to pass through it once it is
shown to meet the security policy, and blocks others even
though they may have been able to meet the security policy.
Direction control:
A firewall determines the direction in which a particular object
request will be honored. This can allow, for example, e-mail to
be sent but not received.
Subject control:

12. Controls access to the entity it is protecting, based on
authenticated identification of the subject.
Behavior control:
Controls how particular objects are used. For example, the
firewall may filter spam from an e-mail message, or allow
access to only a portion of a protected object.
A firewall is essentially a choke point that controls access
between subjects and objects that it protects. There are several
types, some of which are listed below.
A packet-filtering bridge is an OSI Layer 2 mechanism. A
firewall can apply data-link controls to permit or deny an
association between data-link layer devices.
A packet-filtering router is an OSI Layer 3 mechanism. The
firewall can apply a security policy to each IP packet that enters
the router and either allow that packet to pass, or discard it
based on information contained in the packet, including the
source address, the destination address, port numbers, and other
information.
Application-level gateway is an OSI Layer 4 mechanism. Also
commonly called a
proxy server
, a gateway can relay application-level traffic (such as File
Transfer or Telnet), or block it, based on a security policy. An
application-level gateway is also a very nice place to audit
incoming and outgoing application traffic.
A circuit-level gateway is a specialized OSI Layer 4
mechanism. It interposes itself into a transport-layer
connection, splitting it into two separate connections, and
manages the flow of information through the
Transport
protocol (nominally TCP in real networks) as two separate
connections. A circuit level gateway is said to "break the
circuit."
A bastion host is an OSI seven-layer mechanism that can
support either an application-level gateway (proxy), a circuit-
level gateway (circuit breaker), or both. A bastion host is a

13. minimal host, stripped of applications and utilities that are
normally delivered with a computer system. It may even
resemble a security kernel, as mentioned in Week 3.
A proxy is a software server that substitutes itself for a subject
and can be used in a variety of different ways. A proxy is often
referred to as an application-level firewall. It is a mechanism
that protects a specific application or service from unauthorized
use. It is built in to, or for, the application interface, protocol,
or service to control incoming and outgoing access requests. A
proxy server is generally an application-level firewall that will
only support the connections and services allowed by the
security policy, and will block all others. The proxy is also
minimized to the smallest bit of software needed to carry out its
functions, and is able to be thoroughly evaluated. For example,
a firewall might well support an HTTP proxy and no others. In
that case, only the HTTP protocol would be able to pass through
the firewall, and it would pass through the proxy server for
validation of content.
It is standard practice to combine these mechanisms in various
ways to achieve the intended policy for the firewall. That is, it
should not be the case that a company installs a firewall; a
firewall is put into place to enforce a policy. Let us look at a
couple of configurations that seem to be common.
A
screened host firewall
consists of a packet-filtering router that sits on the network
side (the external side) of a bastion host. The bastion host
controls access to an information server, which contains
information to be shared through the screened host firewall. The
bastion host may also permit certain requests to pass to the
internal infrastructure of the organization.
A
screened-subnet firewall, or demilitarized zone
, consists of two routers with a bastion host between them. The
bastion host has access to an information server. The bastion
host allows an external subject access to the information server,

14. but does not allow internal subjects to exit the external router
and does not allow external subjects to enter the internal router.
A
dual homed, screened host firewall
is also a demilitarized zone, but in this case, the bastion host
does not allow internal subjects to access the information server
except for the process that updates the information in the
information server. The bastion host does allow external
subjects to access the information server.
You might consider the overall policy each of these
configurations implements, and think through how you might
implement other policies using Layer 2, Layer 3, Layer 4, and
application firewalls.
Virtual Private Networks (VPN):
Traditional virtual private networks are Layer 2 controls. They
can be used to establish multiple logical networks on a single
communication medium, using cryptographic isolation. This
allows, for example, an encrypted virtual private network to be
secured on a commercial, nonsecured communications medium.
These VPNs have been in existence for many years. Newer
forms of virtual private networking operate between Layers 3
and 4, and between Layers 4 and 7. VPNs can be established
from firewalls and are often part of the solution to specific
security policies.
Note of Interest - VPNs
L2TP:
Protecting a network at Layer 2 is possible if the owner has
installed or leased a dedicated communications facility between
sites. The security services that one can achieve at Layer 2 are
strong site-to-site authentication, confidentiality, a continuously
authenticated channel, antispoof protection, and traffic flow
security-. If a firewall or equivalent function- is used for traffic
flow security, it is inserted before the link cryptography. Many
refer to Layer 2 VPNs as
site-to-site
VPNs.

15. IPSEC:
At Layer 3, networks support dynamic rout-ing and switching
of information and may offer more affordable communica-tions
than Layer 2 solutions. With a layer-pro-tected system, one only
has to enable access control features to allow new connec-tivity.
Layer 3 VPNs allow superior cost control (volume control) and
continuous site-to-site authentication, and they provide
flexibility for adding new nodes. Many refer to Layer 3 VPNs as
client-to-client
VPNs.
SSL:
Layer 4, the Transport Layer, is the first layer in which
security services can be broken out per application. Layer 4/7
VPNs are used to control access by actual users to applications
such as portals and databases. Layer 4/7 solutions are very good
for control of remote user access. But Layer 4 secu-rity services
must be imple-mented in end systems and are subject to all the
invocation concerns of ap-plication-layer services. This means
that an SSL solution may need to be supported with
helper applications
, or other
add ons
. Note that SSL runs on top of the Hypertext Transport Protocol
(HTTP). Many refer to Layer 4/7 VPNs as
user-to-application
VPNs, or even
Portal Access VPNs
, though they support more than just portal access.
Firewalls and security operations: Regardless of the type of
firewall and the mechanisms used, a firewall is a particularly
good place from which to collect readiness information for the
operations center that we discussed in Week 4. We can look at a
firewall as a sensor, or a set of sensors, that can detect and
prevent possible intrusions. Reporting on rejection rates for
particular access requests should be made part of the firewall
mechanism. We will talk more about this next week when we

16. discuss intrusion detection and prevention systems. You also
might notice that the purpose of a firewall is to maintain the
known good state of the entities that it protects, and that a
firewall is an element of defense-in-depth.
That's it for this week. Next week, we will talk about some of
the more futuristic aspects of Information Security, though we
will find that the future is coming ever more quickly, and by the
time you read this, the future may be the present!