IFSM 310 – Stage 4 Assignment – Due Sunday

Chapter 8

Secure Transport Layer

In the early days of the Internet, networks were small and all of

the routers were in secure locations. As long as each computer

connected to the Internet protected itself from unwanted incom-

ing connections, it was felt that there was no need to protect data

from prying eyes while it was crossing the network.

So the Link, Internetwork, and Transport layers were focused on

the efficient movement of data and solving the problems of a

large-scale shared distributed network without worrying about

the privacy of that data.

But as the use of the Internet grew rapidly in the late 1980s and

literally exploded when the Web became mainstream in 1994, se-

curity and privacy of network traffic became very important prob-

lems to solve. When we began using the Internet to conduct com-

merce and credit cards and bank account numbers were being

routinely sent across the network, securing data became essen-

tial. And when we started using wireless technologies like WiFi,

security became necessary for even the simplest uses of the In-

ternet.

There are two general approaches to securing network activity.

The first makes sure that all of the network hardware (routers

and links) is in physically secure locations so it is not possible for

someone to sneak in and monitor traffic while it is crossing the

Internet. This approach is not practical for hundreds of thousands

of network routers owned and operated by many different orga-

nizations. While you might be able to ensure that some of the

router operators adhered to strict security procedures and poli-

cies, sooner or later a mistake will be made. And once WiFi was

added to the mix and your packets went over radio waves, a net-

work attacker could just sit in a coffee shop and intercept packets

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90 CHAPTER 8. SECURE TRANSPORT LAYER

as they passed through the air.

Under these conditions, the only reasonable solution is to encrypt

data in your computer before it is sent across its first physical

link, and then decrypt the data in the destination computer after

it arrives. Using this approach, we assume that an attacker can

see all of the packets that you send, but they cannot decrypt the

data that they have captured. The encryption also guarantees

that there is no way to alter your data while it is crossing the

Internet.

8.1 Encrypting and Decrypting Data

The concept of protecting information so it cannot be read while

it is being transported over an insecure medium is thousands of

years old. The leaders in Roman armies sent coded messages to

each other using a code called the “Caesar Cipher”. The simplest

version of this approach is to take each of the characters of the

actual message (we call this “plain text”) and shift each charac-

ter a fixed distance down the alphabet to produce the scrambled

message or “ciphertext”.

Then we send the ciphertext via the courier or other insecure

transport to the other person. The courier cannot read the mes-

sage because it appears to be random characters unless you

know the technique used to encode the message.

As long as the person receiving the message knew the number

used to shift the message, they could unshift the characters in

the encoded message to reproduce the original message.

Here is a simple example of plain text and ciphertext using a shift

of one:

Plain text: Go to the river

Cipher text: Hp up uif sjwfs

We use the word “encrypt” to describe transforming the plain text

to the ciphertext and “decrypt” to describe the reverse process.

The Caesar Cipher is very simple to defeat, but it was used to

protect important messages until about 150 years ago. Modern

encryption techniques are far more sophisticated than a simple

character shift, but all encryption systems depend on some kind

of a secret key that both parties are aware of so they can decrypt

received data.

8.2. TWO KINDS OF SECRETS 91

8.2 Two Kinds of Secrets

The traditional way to encrypt transmissions is using a shared se-

cret (a password, a sentence, a number) that only the sending

and receiving parties know. With the secret, it is easy to decrypt

the received data, but if you received the data without possess-

ing the secret, it would be effectively impossible to decrypt the

message.

Figure 8.1: Shared Versus Asymmetric Keys

In the early days of the Internet, two people could send encrypted

email to each other by one person first calling the other person

on the phone and giving them the decryption secret. This worked

well when there were only a few users on the network, but could

not scale to situations where a company might have millions of

customers and could not afford to make a phone call to each cus-

tomer to establish a shared secret before they could make a pur-

chase.

It might seem like a good idea to distribute the shared secrets

over the Internet, but if we assume that the attackers are monitor-

ing and capturing all network traffic, they could also capture the

unencrypted message that contained the shared secret. At that

point it would be trivial for the attacker to use the shared secret

to decrypt a message. And even worse, the attacker could inter-

cept a message, delay it, then decrypt it, change and re-encrypt

it, and send the modified message back on its way. The receiving

92 CHAPTER 8. SECURE TRANSPORT LAYER

computer would decrypt the message and never know that it had

been modified by an attacker while in transit.

So shared secrets clearly would not work to solve the problem of

securing network traffic between trillions of pairs of networked

computers.

The solution to this problem came in the 1970s when the con-

cept of asymmetric key encryption was developed. The idea of

asymmetric key encryption is that one key is used to encrypt the

message and another key is used to decrypt it. The computer that

will be receiving the encrypted data chooses both the encryption

key and decryption key. Then the encryption key is sent to the

computer that will be sending the data. The sending computer

encrypts the data and sends it across the network. The receiving

computer uses the decryption key to decrypt the data.

We call the encryption key the “public” key because it can be

widely shared. We call the decryption key the “private” key be-

cause it never leaves the computer where it was created. Another

name for asymmetric keys is public/private keys.

The whole process is designed so that if an attacker has the public

key (which was sent unencrypted) and the encrypted text, it is

virtually impossible to decrypt the encrypted data. There is a lot

of math with large prime numbers that makes it hard to guess the

private key from the public key and encrypted data.

So with the advent of public/private key technology, the only

question left was how to apply it in our network model.

8.3 Secure Sockets Layer (SSL)

Since network engineers decided to add security nearly 20 years

after the Internet protocols were developed, it was important not

to break any existing Internet protocols or architecture. Their so-

lution was to add an optional partial layer between the Transport

layer and the Application layer. They called this partial layer the

Secure Sockets Layer (SSL) or Transport Layer Security (TLS).

When an application requested that the Transport layer make a

connection to a remote host, it could request that the connec-

tion either be encrypted or unencrypted. If an encrypted connec-

tion was requested, the Transport layer encrypted the data before

breaking the stream into packets. This meant that the Transport

layer, Internetwork layer, and physical (link) layers could still per-

form exactly the same way whether the packets were encrypted

8.4. ENCRYPTING WEB BROWSER TRAFFIC 93

Figure 8.2: Where Encryption and Decryption Happens

or non-encrypted. The applications making the connections were

also spared the details of how encryption and decryption worked.

Since encryption was a simple and transparent addition to the

Transport layer, there was no need to change the routers that

operate at the Internetwork and Link layers. There was no need

to change any Link layer hardware to make encryption work. And

applications did not need to be modified except to request that a

connection be encrypted when appropriate.

8.4 Encrypting Web Browser Traffic

Since web browsers and web servers operate at the application

layer, we barely notice whether we are using encrypted or un-

encrypted connections. Web browsers use the URL convention

of replacing “http:” with “https:” to indicate that the browser is

to communicate with the web server using the Secure Transport

Layer instead of the unencrypted Transport layer. Your browser

will usually show a “lock” icon in the address bar to let you know

that you are communicating with a secure web site.

94 CHAPTER 8. SECURE TRANSPORT LAYER

There is a small overhead in setting up the https connections and

a small cost to encrypt and decrypt the data that is being sent.

Since https was slightly more costly, for a while it was used only

for pages that contained passwords, bank account numbers, or

other sensitive data.

But over time as networks have become faster and the https im-

plementations have gotten much more efficient, there is a trend

toward encrypting all web server interactions whenever you are

interacting with a web server where you have an account. The

current trend is towards using https for all web traffic.

8.5 Certificates and Certificate Authorities

While public/private key encryption works to allow the distribution

of encryption keys across insecure networks and the use of those

keys to encrypt transmissions, there is still a problem of knowing

if the public key that you have received when you connected to a

server is really from the organization it claims to be from.

Figure 8.3: Certificate Authorities and Public Keys

Perhaps you think you are connecting to www.amazon.com

but a rogue computer intercepts your traffic, claiming to be

8.6. SUMMARY 95

www.amazon.com and giving you a public key to use for encryp-

tion. If your web browser trusts the key, it will use the rogue

computer’s public key to encrypt your banking information and

send it to the rogue computer. Since the rogue computer gave

you the public key, it also has the corresponding private key and

is able to decrypt and abscond with your banking information.

So your computer needs to know who the key is actually coming

from. This is achieved by sending you a public key that is digi-

tally signed by a Certificate Authority (CA). When your computer

or browser is initially installed, it knows about a number of well-

known certificate authorities. If your browser is given a public

key that is signed by one of the well-known certificate authorities,

it trusts the key and uses it to encrypt and send your data. If

your computer receives a public key that is not signed by one of

its trusted certificate authorities, it will warn you before sending

your data using the key.

If you see a warning message about an untrusted certificate, you

should probably say “no” and figure out why your network traffic

is not being routed to the server that you think it is going to before

sending any sensitive data.

8.6 Summary

Since the Internet was nearly 20 years old before we needed

broadly deployed security, we had to find a way to add security to

the already existing four-layer model. The perfect place to add se-

curity was as an option in the Transport layer. This is why we call

secure connections on the Internet “Secure Sockets Layer” (SSL)

or “Transport Layer Security” (TLS). There are subtle differences

between SSL and TLS but they both encrypt data at the Transport

layer.

The invention of public/private key encryption was well timed in

that it solved the key distribution problem of shared-secret en-

cryption approaches. With public/private keys, the public encryp-

tion key can be routinely shared across insecure media. This

means we can use an unencrypted connection to exchange data

and upgrade the connection to a secure connection.

By inserting the secure layer at the top of the Transport layer, we

were able to avoid changing the Application, Internetwork, and

Link layers while still easily securing any Transport layer connec-

tion. This approach ensures that all data being sent across a con-

nection is encrypted before it leaves your computer. Given that

96 CHAPTER 8. SECURE TRANSPORT LAYER

many of us use wireless connections like WiFi, which are easily

monitored by attackers, it is a good idea to encrypt data before it

is sent across WiFi.

Browsers support secure connections by changing the prefix on

the URL from “http:” to “https:”. By keeping an eye on the URL,

end users can make sure they never send sensitive data across

insecure connections. A series of trusted Certificate Authorities

will sign public keys to give you an assurance that the key you

received is indeed from the organization you expect it to be.

The design of the Secure Transport Layer provides a secure and

yet easy-to-use mechanism for secure communications across

the Internet at a scale of trillions of pairs of interacting comput-

ers.

8.7 Glossary

asymmetric key: An approach to encryption where one (public)

key is used to encrypt data prior to transmission and a different

(private) key is used to decrypt data once it is received.

certificate authority: An organization that digitally signs public

keys after verifying that the name listed in the public key is actu-

ally the person or organization in possession of the public key.

ciphertext: A scrambled version of a message that cannot be

read without knowing the decryption key and technique.

decrypt: The act of transforming a ciphertext message to a plain

text message using a secret or key.

encrypt: The act of transforming a plain text message to a ci-

phertext message using a secret or key.

plain text: A readable message that is about to be encrypted

before being sent.

private key: The portion of a key pair that is used to decrypt

transmissions.

public key: The portion of a key pair that is used to encrypt

transmissions.

shared secret: An approach to encryption that uses the same

key for encryption and decryption.

SSL: Secure Sockets Layer. An approach that allows an appli-

cation to request that a Transport layer connection is to be en-

8.8. QUESTIONS 97

crypted as it crosses the network. Similar to Transport Layer Se-

curity (TLS).

TLS: Transport Layer Security. An approach that allows an ap-

plication to request that a Transport layer connection is to be en-

crypted as it crosses the network. Similar to Secure Sockets Layer

(SSL).

8.8 Questions

You can take this quiz online at http://www.net-intro.com/quiz/

1. How do we indicate that we want a secure connection when

using a web browser?

a) Use https:// in the URL

b) Use a secure web browser

c) Open an incognito window

d) Manually encode the address of the server using SHA1

2. Why is a shared-secret approach not suitable for use on the

Internet?

a) Because people would lose or misplace the secret

b) It is difficult to distribute the secrets

c) Encryption and decryption with shared secrets are too easily

broken

d) Encryption and decryption with shared secrets take too

much compute power

3. What is the underlying mathematical concept that makes

public/private key encryption secure?

a) Continuous functions

b) Taylor series

c) Karnaugh Maps

d) Prime numbers

4. Which of the keys can be sent across the Internet in plain

text without compromising security?

98 CHAPTER 8. SECURE TRANSPORT LAYER

a) Encryption key

b) Decryption Key

c) Shared Secret

d) Univerally Safe Key (USK)

5. Where does the Secure Sockets Layer (SSL) fit in the four-

layer Internet architecture?

a) Below the Link layer

b) Between the Link and Internetworking layers

c) Between the Internetworking and Transport layers

d) Between the Transport and Application layers

6. If you were properly using https in a browser over WiFi in a

cafe, which of the following is the greatest risk to your losing

credit card information when making an online purchase?

a) Someone captured the packets that were sent across the

WiFi

b) Someone captured the packets in the gateway router

c) Someone captured the packets as they passed through a

core Intenet router

d) You have a virus on your computer that is capturing

keystrokes

7. With the Secure Sockets Layer, where are packets encrypted

and decrypted?

a) They are encrypted and decrypted as they pass through the

router

b) Each physical link has its own separate encryption

c) They are encrypted in your computer and decrypted in the

server

d) They are encrypted in the WiFi gateway and decrypted in the

last router before the destination computer

8. What changes to the IP layer were needed to make secure

socket layer (SSL) work?

a) No changes were needed

b) We had to add support for Secure IP (IPSEC)

c) We needed to support longer packets in IP

8.8. QUESTIONS 99

d) The Time-To-Live (TTL) value needed to be encrypted

9. If a rogue element was able to monitor all packets going

through an undersea cable and you were using pub-

lic/private key encryption properly, which of the following

would be the most difficult for them to obtain?

a) What servers you were communicating with

b) How often you used the servers

c) How much data you retrieved from the servers

d) Which documents you retrieved from the servers

10. What is the purpose of a Certificate Authority in pub-

lic/private key encryption?

a) To make sure people do not forge badges for learning activi-

ties

b) To make sure packets get routed to the correct destination

computer

c) To assure us that a public key comes from the organization

it claims to be from

d) To choose when a particular country must switch from IPv4

to IPv6

11. The ARPANET network was in operation starting in the 1960s.

Secure Sockets Layer (SSL) was not invented util the 1980s.

How did the ARPANET insure the security of the data on its

network?

a) By using public/private keys and encrypting all transmis-

sions

b) By using encryption at the Link layer

c) By making sure no one could access the physical links

d) By only using secure WiFi routers

12. Which of these answers is “Security is fun” encrypted with a

Caesar Cipher shift of 1.

a) Ptsjduao rt dii

b) Wentudhs di dju

c) Tfdvsjuz jt gvo

d) Asdfghjk qw zxc

100 CHAPTER 8. SECURE TRANSPORT LAYER

13. What Caesar Cipher shift was used to encrypt “V yvxr fr-

phevgl”?

a) 1

b) 6

c) 13

d) 24