View
222
Download
1
Embed Size (px)
Citation preview
2
Revision
Before the break, we covered databases and their applications
Binary Search Tree Network DBMS Relational DBMS E-commerce Customer concerns
3
Overview
Today we look at data encryption for secret message transmission over public network
We also look at computer’s use in jobs involving multimedia components
We discuss images, video and audio data formats, compression and transmission
4
Encryption and Data Security
For successful e-commerce, we must ensure that the sensitive data has been encrypted and secured
Encryption transforms the data using a “key” into a value that is meaningless in its normal form
This encrypted value can only be decrypted by authorized agency or person
5
Encryption Basics
For encryption, XOR logical operation is widely used The data to be encrypted is XORed with the key XOR has the property that it can transform a binary
value into another totally different value. This encrypted value is sent instead of the original value
The recipient performs XOR of the received encrypted value with the key, restoring original value
6
Class Exercise
The data to be sent is: 0111 1011 The 4-bit key is 1010 SENDER: Encrypt the data and send it to
the recipient RECIPIENT: Decrypt the data
7
Encryption Keys
There are two types of encryption schemes in use
In symmetric or secret key ciphers, the encryption and decryption keys are symmetric and identical
As an example, consider this diagram then encrypt following message with your own encryption scheme
“Please transfer six million dollars into my checking account”
8
Symmetric Cipher
Adapted for academic use from “Exploring The Digital Domain” by Abernethy Allen, ITP 1999
9
Public Key Encryption
In symmetric secret key cipher, the key must be sent to recipient, otherwise the message cannot be decrypted
Sending the key on public network is like putting your house key in locksmith’s shop and allowing everyone to duplicate it
Alternatives are to restrict access to secret key to a few individuals
10
DES
DES (Data Encryption Standard) is one example of symmetric key encryption
DES uses a 56-bit key which can be doubled in length to 112 bits
Breaking messages encrypted with DES would require a considerable amount of computing time and effort
11
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
12
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?
13
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
14
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
15
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
16
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
17
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
18
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?
20
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.
21
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
22
Sampling Sound
Adapted for academic use from “Exploring The Digital Domain” by Abernethy Allen, ITP 1999
23
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
24
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
25
Compressing the Video
In its raw form, digital video delivery will push the computer to its limits
With 640X480 Frame, 24-bits color info per pixel, and 30 frames per second, we are talking about 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
26
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
27
Class Work Wednesday Nov29
For the class on Nov29th, please follow the link number 5 (“Ethics in Computing”) under useful links on the course homepage (http://www.cs.fredonia.edu/~zubairi/f2k/cs120.html).
Select one of the topics and go to “References” Read the references and then solve the
worksheet alone or in a group of 2 to 3 students