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SCSC 455 Computer Security
Chapter 2 Secure Data
Dr. Frank Li
Index

Basics of cryptography

Kerckhoff’s Principle
The big picture of cryptography




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Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications
Cryptography
Cryptography


the science of encoding data so that it cannot be read
without special knowledge or tools;
Is a crucial part of network applications
Q: Why cryptography is a crucial part in network
applications?
Cryptography and Computer Security
Ans:

Network connections can be tapped, - sniffing the network


Sniffers are used by many system administrators to troubleshoot
networking problems.
Encryption can provide security in insecure communication channels
Q: What exactly does “security” mean?
History of cryptography: Caesar cipher
Julius Caesar (100–44 B.C.) developed a simple
encryption method -- shifted the alphabet by three
positions

Standard Alphabet:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cryptographic Alphabet:
DEFGHIJKLMNOPQRSTUVWXYZABC
e.g.1 Encypt “caesar”  ?
e.g.2 Decrpt “vhfxulwb”  ?

Caesar cipher is substitution cipher

a different letter of the alphabet is substituted for each letter in the
message
History of cryptography: Transposition Cipher
Transposition Cipher: rearrange letters in plaintext
to produce cipher text

Rail-Fence cipher


Plaintext is HELLO WORLD
Encryption:
HLOOL
ELWRD
 HLOOLELWRD
Q: How to decrypt the ciphertext?
History of cryptography:
Enigma machine in WWII

By the twentieth century it became possible
to carry out substitutions by using electrical
connections to mechanize the difficult work
of looking up tables in a handbook.

The basic Enigma was invented in 1918 by
Arthur Scherbius in Berlin.


It enciphers a message by performing a
number of substitutions one after the other.
Scherbius's idea was to achieve these
substitutions by electrical connections.
One-Time Pad

A one-time pad is a perfect encryption scheme
because it is considered unbreakable if implemented
properly

Is made up of random bits.

Is as simple as letter substitution

This encryption process uses a binary mathematic
function XOR.
Message stream
1001010111
Keystream
0011101010
Ciphertext stream
1010111101
OTP in action

One-time pads have been used throughout history to
protect different types of sensitive data.

Today, they are still in place for many types of militaries as
a backup encryption option if current encryption processes
are unavailable for reasons of war or attacks.
A Russian One-time pad, captured by MI5
The history of cryptography
(reading assignment -- article 1)
Modern cryptography: DES

The Data Encryption Standard (DES) was developed
in the 1970s

uses a 56-bit key to encrypt data using various
algorithms (substitution + Transposition Cipher)




56 bits provide for 256 possible key combinations.
was a widely implemented U.S. standard
However, it takes 20 hours to break a DES key in 1998
DES is being phased out, but it is still widely used


relatively few people have the equipment to break the
key
20 hours is still a relatively long time in the Internet age
Modern cryptography: Skipjack and Triple DES

There were several responses to the cracking of
DES:



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DES keys were increased to 1024 bits
Creation of a new and more secure algorithm called
Skipjack, which uses an 80 bit key
Triple DES relies on DES, but encodes each message
three times using three different keys
An new encryption algorithm Advanced Encryption
Standard (AES)
Modern cryptography:
Advanced Encryption Standard (AES)

Advanced Encryption Standard (AES)

NIST sponsored a competition in 1997 to create a
replacement for DES.



The winner is the Rijndael algorithm by two Belgians
Use three different key lengths: 128 bits, 192 bits, 256
bits
was approved for use by U.S. government agencies in
May 2002
Another way to hide data: Steganography

Steganography is a method of hiding data in another media
type so that the very existence of the data is concealed.



E.g.1 the least significant bit of each byte of the image can be
replaced with bits of the secret message.


does not use algorithms or keys to encrypt information.
A message can be hidden in a WAV file, in a graphic, or in unused
spaces on a hard drive or sectors that are marked as unusable.
This practice does not affect the graphic enough to be detected.
E.g. 2 can also be used to insert a digital watermark on digital
images so that illegal copies of the images can be detected.
(This technique will be covered in lab.)
Index

Basics of cryptography

Kerckhoff’s Principle
The big picture of cryptography





Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications
The anatomy of cryptography
M: plaintext, the message to transmit
C: ciphertext, encrypted message
E: encryption
D: decryption

Cipher, a.k.a. rules or algorithms of encryption and decryption

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E.g. allow letter-substitution to convert plaintext to ciphertext
The level of complexity of an algorithm can be increased by using
a key
 Key is a code necessary to encrypt or decrypt a message
correctly using the algorithm
Kerckhoff’s Principle

Kerckhoff’s Principle (1883)

the only secrecy involved with a cryptography system
should be the key; the algorithm should be publicly
known;

Good security assumes an eavesdropper knows the
cipher, but the key must be kept secret
Index

Basics of cryptography

The big picture of cryptography

Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications



The big picture of cryptography

Symmetric encryption algorithms

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
Use the same key and algorithm to encrypt and decrypt a message
The key used is called a private key, because it must be kept
secret for the message to be secure
A.a.k. classical cryptography, classical cipher
(Every encryption algorithm covered so far belongs to symmetric
encryption algorithm.)

Asymmetric encryption algorithms


Use one key to encrypt and another key to decrypt
The key you can reveal to everyone is called a public key
Symmetric Algorithms

The sender and receiver use two instances of the same key
for encryption and decryption

If an intruder were to get this key, the intruder could decrypt any
intercepted message encrypted with this key.
Symmetric Algorithms


The security of the symmetric encryption method is
completely dependent on how well users protect the key
Each pair of users who want to exchange data using
symmetric key encryption must have two instances of the
same key.

Q 1: How many keys are required if 50 people want to
communicate using symmetric algorithm?

Q 2: How symmetric keys are actually shared and updated?
Symmetric Algorithms

It is not safe to just send it in an e-mail message



the key is not protected and can be easily intercepted
and used by attackers
The out-of-band method
Other implications of symmetric algorithms


Both users have the same key to encrypt and decrypt
messages
Symmetric cryptosystems can provide confidentiality
but they cannot directly provide authentication or nonrepudiation.
Why?
Symmetric Algorithms

Advantages of symmetric algorithms



Symmetric algorithms are very fast, comparing to asymmetric
algorithms
Symmetric algorithms are hard to break if a large key size is used.
Examples of symmetric algorithms :

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Data Encryption Standard (DES)
Triple-DES (3DES)
Blowfish
IDEA
RC4, RC5, and RC6
Advanced Encryption Standard (AES)
Asymmetric Cryptography

In public key systems, each entity has a pair of
different keys, or asymmetric keys.

The two different asymmetric keys are mathematically
related.



The public key can be known to everyone
The private key must be known and used only by the
owner.
It must be computationally infeasible to derive the
private key from the public key.
Asymmetric Cryptography
In different scenarios, we can choose to use either public key or
private key to encrypt and decrypt.
Scenario 1
Q: Is there any problem in scenario 1?
Scenario 2
Q: Is there any problem in scenario 2?
Scenario 3
Q: Is this any problem in scenario 3?
Asymmetric Cryptography

Common asymmetric algorithms

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Diffie-Hellman
RSA
Elliptic curve cryptosystem (ECC)
El Gamal
Digital Signature Algorithm (DSA)
Knapsack
RSA is the most popular public-key encryption


Developed by Ronald Rivest, Adi Shamir, and Len Adleman
RSA is used in a number of products from many vendors
 Web browsers, Virtual private networks (VPN)
Asymmetric vs. Symmetric Cryptography

Pro: Asymmetric algorithms
 Can provide authentication and non-repudiation.
 also provide for easier and more manageable key
distribution

Cons: Asymmetric algorithm works much more slowly than
a symmetric algorithm


Symmetric algorithms carry out relatively simplistic mathematical
functions – substitution and transposition
Asymmetric algorithm uses much more complex mathematics to
carry out their functions.
Hybrid cryptosystem
In a hybrid system, asymmetric algorithm and a a symmetric
algorithm are used in a complementary manner
 A hybrid cryptosystem = symmetrical key cipher + public-key
cipher


An asymmetric algorithm creates keys that are used for
automated key distribution.
A symmetric algorithm creates keys that are used for encrypting
bulk data
Index

Basics of cryptography

The big picture of cryptography

Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications



Authentication the public key

A potential weakness of public-key cryptography
Q: How do you know that the pubic key you have for
an individual is really for that individual?

The solution is authentication public key


Authentication is the process of proving that you are in
fact the person you say you are.
E.g., A phone ID is commonly used to authenticate a
person.
Q: How to authenticate a public key?
One way to authenticate public key:
Signatures

Signatures let you authenticate a public key

How the signature works?

You verify that another person’s key really belongs to that person.
And then sign that public key with your own private key.

Others get that public key can see your signature and know you trust
that key, so they may decide to trust it OR may decide to verify that
key themselves.

Form a web of trust -- a peer to peer trust relationship

Example …
Q: How to verify another person’s public key?
Verify public key

Verify the public key in person or call the owner of
the public key and check the key
 A key usually has hundred of digital


Check bit by bit is not very efficient
A fingerprint is a smaller number that is derived from
a very lengthy public key

Fingerprints are created by hashing the public key,

Hashing is a process by which a mathematical function
is used that converts larger numbers into smaller
numbers
Hash function
A hash function is a function that takes a variablelength string, and produces a fixed-length hash value.


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
The hash value is also called fingerprint, checksum, or message digest
A fingerprint is changed  the contents have been altered
Example …
Two commonly used hash functions


Message digest hash (MD5) provides 128 bits fingerprint
Secure hash algorithm (SHA-1) provides 160 bits fingerprint
The second way to authenticate public key:
Certificate
Using digital certificate -- with PKI

A certificate is a numeric code that is
used to identify an organization

Certificate authority (CA) verifies the
credential of an organization of
individual.
Then CA issues a client’s public key
and sign it with CA’s private key
E.g. VeriSign is an well-known CA

Index

Basics of cryptography

The big picture of cryptography

Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications



Using Cryptography in a Browser

A small lock or key in the lower left corner of the
browser window


When visiting a web page that has been transmitted to
your computer using encryption,
Most encrypted Web pages, such as shopping carts,
appear with a URL that starts with https

https indicates the web server used an encrypted
protocol Secure Socket Layer (SSL) to transmit web
page.
SSL and TLS

Secure Sockets Layer (SSL)

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
Developed by Netscape for transmitting private
documents via the Internet
Uses a public key to encrypt data that is transferred
over the SSL connection
URLs that require an SSL connection start with “https:”
instead of “http:”
Transport Layer Security (TLS)


Is the latest version of SSL
Not widely available in browsers yet.
SSL/TLS Protocol

Runs on top of the TCP and below higher-level protocols


Uses TCP/IP on behalf of higher-level protocols
Allows both machines to establish an encrypted connection


Allows SSL-enabled server to authenticate itself to SSL-enabled
client
Allows client to authenticate itself to server
Secure Hypertext Transfer Protocol (Https)
Https is communications protocol designed to transfer
encrypted information between computers over the World
Wide Web


Online purchasing or exchange of private information over
insecure networks
Https set up steps:

1.
2.
3.
4.
Client accesses a URL with https
Server sends back its certificate (public key encrypted by CA)
Client validates certificate, generates a symmetric session key,
encrypts by server’s public key and sends back
Server gets session key, communication begins…
Demo of Https

Tools: Ethereal


Http vs. Https
(www.Netbank.com)
What will we look for?

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Three-way TCP/IP handshake
Clear text packets of Http
SSL setup procedure
Encrypted packets Https
Index

Basics of cryptography

The big picture of cryptography

Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications



Using Encryption Utilities

Pretty Good Privacy (PGP) is the first utility to
provide public-key encryption to the public


although PGP software was formerly included in Linux, it has
been replaced with GPG
Gnu Privacy Guard (GPG) is a public-key encryption
utility and uses non-patented algorithms


is the GNU project's complete and free implementation of the
OpenPGP standard as defined by RFC2440 .
allows to encrypt and sign your data and communication, features
a versatile key management system
GPG operation
Create a key pair for yourself
gpg –gen-key

Choose which algorithm GPG will use

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Decide a key size for the DSA (1024-bit by default)
Select an expiration time or key does not expire
Define a user ID (real name, email address, organization,
brief comment) for the key pair
Assign a passphase to protect the key pair


DSA: Digital signature algorithm
El Gamal: an asymmetric key encryption algorithm
or both (the default)
A long password
Move mouse around or play with keyboard to generate the
random data in the OS that GPG needs to create a key pair
GPG operation

View your public key
gpg –list-keys

Export your public key
gpg –o frank.key –export [email protected]
create a binary file of public key
gpg –a –o frank.key –export [email protected]
create the text of public key, so that it can be pasted into
message

Import other’s public key
gpg –import thomas.key
keyring – a file on your system that contains all the public keys
Q: How to ensure the authenticity of public keys?
GPG operation

Bogus public key causes man-in-the-middle attack

Review: two ways to provide the authenticity of
public key


Using digital certificate -- with CA
Using singed public key -- web of trust


A peer to peer trust relationship
E.g. 1, Your friend Alice handed you a copy of her public
key on a disk. You believe this key is real and sign it
with your private key. Others can access Alice’s public
key from you (with your signature)
gpg –eidt-key [email protected]
GPG operation

E.g. 2, Your friend Bob emailed you a copy of his public key.
You are not sure whether this key is real, since it came in
email through insecure Internet. To verify the key, you print
out the fingerprint of Bob’s key
gpg –fingerprint [email protected]

Fingerprint – hash value, message digest

MD5, SHA-1
You call Bob. You know where he works, as well as his voice.
You recite the fingerprint over the phone and Bob verifies that
it is correct. You believe this public key indeed belongs to
Bob. Now you can sign Bob’s public key with your private key
GPG operation

The sender encrypts a file with GPG



Encrypt a file with a recipient’s public key
Sign the encrypted file with your private key
Store the cipher file in a named file
gpg –o report.gpg –encrypt [email protected]
report.doc

The recipient decrypts a encrypted file
gpg –o report.doc –decrypt report.gpg
Graphical utilities for GPG
Two graphical utilities for GPG:

Seahorse is a Gnome front end for GnuPG.
http://seahorse.sourceforge.net/



Data encryption and digital signature creation can
easily be performed through a GUI
Key Management operations can easily be carried out
through an intuitive interface.
The Gnu Privacy Assistant
http://www.gnupg.org/related_software/gpa/
The Seahorse GUI to GPG
The Gnu Privacy Assistant
Index

Basics of cryptography

The big picture of cryptography

Digital signature and digital certificate
Cryptography in a browser
Encryption utilities
Other encryption applications



SSH and VPN

Secure Shell (SSH) is an encrypted version of Telnet



provides secure remote access
SSH allows other protocols to ride on top of it
A Virtual Private Network (VPN) is a secure organizational
network that uses an insecure public network (Internet) for
communications

VPNs are often created with specially designed software that
integrates many networking functions with cryptographic
protocols and system management software
(SSH and VPN will be covered in details in later chapter.)
Virtual Private Network (VPN)
Other Security Applications

Red hat package manager (RPM) security can check a publickey signature on any package to verify that it came from its
stated creator

Cryptographic File System (CFS) enforces cryptographic
authentication on all users who want to share files across the
network

Transparent Cryptographic File System (TCFS) operates
transparently to users

IPSec and CIPE provide for IP packet encryption