scsc 455 computer security chapter 2 secure data dr. frank li

SCSC 455 Computer Security

Chapter 2 Secure Data

Dr. Frank Li

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

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 SecurityAns: 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 simpleencryption method -- shifted the alphabet by threepositions

Standard Alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZCryptographic 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: 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. 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.

E.g.1 the least significant bit of each byte of the image can be replaced with bits of the secret message. 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 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 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.


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?


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 non-repudiation. Why?


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 : 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.


In different scenarios, we can choose to use either public key or

private key to encrypt and decrypt. Scenario 1: C = E_a(M) M = D_a(C) Scenario 2: C = D_a(M) M = E_a(C)

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?


Common asymmetric algorithms 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 symmetricalgorithm 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


Authentication the public key

A potential weakness of public-key cryptographyQ: 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 variable-length string, and produces a fixed-length hash value.

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:CertificateUsing 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


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) 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. Client accesses a URL with https2. Server sends back its certificate (public key encrypted by CA)3. Client validates certificate, generates a symmetric session key,

encrypts by server’s public key and sends back4. Server gets session key, communication begins…

Demo of Https

Tools: Ethereal Http vs. Https (www.Netbank.com)

What will we look for? Three-way TCP/IP handshake Clear text packets of Http SSL setup procedure Encrypted packets Https

Index


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 yourselfgpg –gen-key Choose which algorithm GPG will use

DSA: Digital signature algorithm El Gamal: an asymmetric key encryption algorithm or both (the default)

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

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 keygpg –o frank.key –export [email protected]

create a binary file of public keygpg –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 keygpg –import thomas.keykeyring – 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 Assistanthttp://www.gnupg.org/related_software/gpa/

The Seahorse GUI to GPG

The Gnu Privacy Assistant

Index


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 public-key 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

scsc 455 computer security chapter 2 secure data dr. frank li

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