1 Applications of Computers Lecture-4 2 Asymmetric or Public Keys 4 Asymmetric keys give a more robust performance 4 The RSA algorithm works as follows:

1

Applications of Computers Lecture-4

2

Asymmetric or Public Keys

Asymmetric keys give a more robust performance

The RSA algorithm works as follows:– Sender gets the public key of recipient

(available to everyone)and uses it to encrypt the message

– Receiver uses private key (only known to receiver) to decrypt the message

3

Digital Signatures

Authentication is a core issue in e-commerce Authentication is to verify the source of a

document Authentication of paper documents is done with

watermarks, stamps, signatures and seals How to authenticate the electronic documents?

4

Authentication

With little effort, the public-private keys can be applied in reverse to perform verification of e-documents

For example, consider this e-conversation between two persons on the internet

5

Authentication

Bob--> Chuck Hi, Bob here Chuck-->Bob Prove you are Bob Bob-->Chuck Send me a random

message, I will return a digital signature (message digest encrypted using my private key)

Chuck-->Bob Random message Bob-->Chuck digital signature Chuck decrypts the message using Bob’s public key

and matches it to message digest

6

Multimedia Applications

Computers have become multimedia devices with an increasing number of applications with audio, video and graphics images

We look at basics of digitizing images and other multimedia stuff

7

Images

Computer Screen consists of thousands of pixels. Each pixel is intersection of a row and a column

Images can be digitized using bit-mapping Bit-mapped images contain pixel color and

intensity information for all the pixels on screen

8

Images

Let us assume we want to store a bit-mapped black-and-white image to memory

Each pixel can be represented by one bit that can be zero (black) or one (white)

Determine the number of bytes needed to store the image if the screen resolution is 1024 X 768

SOLUTION: 1024X768 equals 786,432 bits 786,432 divided by 8 equals 98,304 bytes

9

Images

Normally, the images stored in the computer contain much more information

TRUE-COLOR images contains 24-bit information for each pixel.

This translates into 8 bits for each of the RGB colors that are emitted in the CRT

Each color’s effect can be enhanced by increasing its 8-bit value

How many colors are possible?

10

How Many Colors?

11

How Many Colors?

With 2 bits per pixel, we can store 2 power 2 colors (4 colors)



With 24 bits per pixel, we can store 2 power 24 colors (16,777,216 or 16 Million colors)

12

Animation

Animation is a visual effect that can show things moving by rapidly changing displayed frames on screen

Animated GIF’s are easy to construct and shareware GIFConstruction Set can be used to build a series of frames that are displayed in rapid succession

Morphing is a visual technique that transforms one image into another.

13

Audio

In order to digitize analog signals such as sound, we need an ADC (analog to digital converter)

Normal conversation can be recorded in 8-bit values but for hi-fi sound, we will need 16-bit samples

Audio CD’s are recorded at the rate of 44.1KHz, the higher the sampling rate, the better is the reproduction

14

Sampling Sound

Adapted for academic use from “Exploring The Digital Domain” by Abernethy Allen, ITP 1999

15

Editing Sound

Once sound is digitized, we can edit it using digital signal processing techniques

Adding an echo effect involves inserting amplitudes equal to a percentage of some selected values at a slightly later time

Changing amplitudes involves clipping, or re-adjusting values outside the range to the max value

16

Digital Video

The analog video already consists of frames that are in a time sequence

The digitization involves capturing each frame and sampling its pixels

Raw digital video has a lot of digital still images

Digital video can be edited, it is randomly accessible and scalable

17

Compressing the Video

In its raw form, digital video delivery will push the computer to its limits

Suppose 640X480 Frame, 24-bits color info per pixel, and 30 frames per second

It translates to 216,000 Kbits per second Mix with it audio rate of 1378 Kbits per second

(stereo 16 bits per channel 44,100 samples per second) and we get total 26.5 MB per second

18

Compressing the Video

Therefore, it is necessary to compress the video at source and decompress it on displaying

This will also be good for our networks as we mostly have 10Mbps LAN connections

MPEG is the compression standard for video and it is based partly on JPEG

MPEG-1 has 150KB/s and MPEG-2 has 1.2 MB/s data rate

19

Applications of Computers (R)

We will go over the key concepts in this module

Databases: How do they work? How are they indexed?

Web opportunities E-commerce concerns Encryption, key delivery problem Multimedia

20

Databases

Databases have files. Files contain records. Records have information in fields

For example, “Employee Information” file will have one record for every employee. Each record will have fields like last name, first name, SS#, address, division etc.

In Ms-Access, tables are the database files, each field is defined as a column. A row is a record

21

Databases

To perform search in a database, we must define a field as a primary key

This primary key indexes into the database It accelerates searching by value of a field

in all records. For example, search by last name, search by

zip code

22

Binary Search Tree

In order to accelerate searching through the database, we need some efficient data structure

We introduced BST (Binary search tree) as the data structure that holds the values of a primary key field

We can build a BST of SS# or a BST of last names

23

BST Exercise

Build a BST whose root node has a value 15. Next values arrive in this order. 17, followed by 34, then 12 and finally 18

Given a BST, determine the steps needed to complete the searching

24

DBMS

Database Management Systems are of two types

Network and Relational What is the difference? Which one is faster? Which one is more flexible? Web + Database leads to ===>????

25

E-Commerce

Buying and selling online Main concern? For encryption, the fundamental operation is

the XOR with a key Original Message XOR key ==> Ciphertext Ciphertext XOR key ==> Original Message Key delivery problem

26

Asymmetric Keys

In some apartment complexes, the laundry room is locked

Every tenant gets a key to the laundry room. This is like a “public” key

If the management wants to enforce operating hours (for example, 8am to 10pm), they would install an additional lock

The key of this lock is not duplicated. It stays with the management

27

Asymmetric Keys

In a similar way, a site creates its own public and private key pair, related to each other in “strange” ways

It lets everyone download the public key but it will keep the private key secret

Suppose that John wants to send a secret message to Citibank

John will go to the Citibank website and obtain their public key

28

Asymmetric Keys

John will encrypt the message using the public key of Citibank

The message will be transmitted over the Internet

Anyone who gets this message and tries to open it using the public key of Citibank will fail (why? Remember laundry room after 10pm?)

29

Multimedia

Images are stored either as bitmapped, grayscale or color images

Storage requirements increase when we use a lot of colors

Digital video is even more demanding

1 Applications of Computers Lecture-4 2 Asymmetric or Public Keys 4 Asymmetric keys give a more robust performance 4 The RSA algorithm works as follows:

Documents