Tugasan 3 : Computer Security

Computer Security??
When we use the Internet, we're not always just clicking around and passively taking in information, such as reading news articles or blog posts -- a great deal of our time online involves sending others our own information. Ordering something over the Internet, whether it's a book, a CD or anything else from an online vendor, or signing up for an online account, requires entering in a good deal of sensitive personal information. A typical transaction might include not only our names, e-mail addresses and physical address and phone number, but also passwords and personal identification numbers (PINs).
The incredible growth of the Internet has excited businesses and consumers alike with its promise of changing the way we live and work. It's extremely easy to buy and sell goods all over the world while sitting in front of a laptop. But security is a major concern on the Internet, especially when you're using it to send sensitive information between parties.
Let's face it, there's a whole lot of information that we don't want other people to see, such as:
  • Credit-card information
  • Social Security numbers
  • Private correspondence
  • Personal details
  • Sensitive company information
  • Bank-account information ­
­ Information security is provided on computers and over the Internet by a variety of methods. A simple but straightforward security method is to only keep sensitive information on removable storage media like portable flash memory drives or external hard drives. But the most popular forms of security all rely on encryption, the process of encoding information in such a way that only the person (or computer) with the key can decode it.
Encryption
Computer encryption is based on the science of cryptography, which has been used as long as humans have wanted to keep information secret. Before the digital age, the biggest users of cryptography were governments, particularly for military purposes.
Computer encryption systems generally belong in one of two categories:
  • Symmetric-key encryption
  • Public-key encryption

Authentication

As stated earlier, encryption is the process of taking all of the data that one computer is sending to another and encoding it into a form that only the other computer will be able to decode. Another process, authentication, is used to verify that the information comes from a trusted source. Basically, if information is "authentic," you know who created it and you know that it has not been altered in any way since that person created it. These two processes, encryption and authentication, work hand-in-hand to create a secure environment.
There are several ways to authenticate a person or information on a computer:
  • Password - The use of a user name and password provides the most common form of authentication. You enter your name and password when prompted by the computer. It checks the pair against a secure file to confirm. If either the name or the password does not match, then you are not allowed further access.
  • Pass cards - These cards can range from a simple card with a magnetic strip, similar to a credit card, to sophisticated smart cards that have an embedded computer chip.
  • Digital signatures - A digital signature is basically a way to ensure that an electronic document (e-mail, spreadsheet, text file) is authentic. The Digital Signature Standard (DSS) is based on a type of public-key encryption method that uses the Digital Signature Algorithm (DSA). DSS is the format for digital signatures that has been endorsed by the U.S. government. The DSA algorithm consists of a private key, known only by the originator of the document (the signer), and a public key. The public key has four parts, which you can learn more about at this page. If anything at all is changed in the document after the digital signature is attached to it, it changes the value that the digital signature compares to, rendering the signature invalid.
Recently, more sophisticated forms of authentication have begun to show up on home and office computer systems. Most of these new systems use some form of biometrics for authentication. Biometrics uses biological information to verify identity. Biometric authentication methods include:
This article is retrieved from: http://computer.howstuffworks.com/encryption8.htm

How to Encrypt a File in Window XP
You can encrypt files only on volumes that are formatted with the NTFS file system. To encrypt a file:
  1. Click Start, point to All Programs, point to Accessories, and then click Windows Explorer.
  2. Locate the file that you want, right-click the file, and then click Properties.
  3. On the General tab, click Advanced.
  4. Under Compress or Encrypt attributes, select the Encrypt contents to secure data check box, and then click OK.
  5. Click OK. If the file is located in an unencrypted folder, you receive an Encryption Warning dialog box. Use one of the following steps:- 
    • If you want to encrypt only the file, click encrypt the file only, and then click OK.
    • If you want to encrypt the file and the folder in which it is located, click Encrypt the file and the parent folder, and then click OK.
Soalan 3:

Pemampatan Data

·         Pemampatan (compression) bermakna mengurangkan saiz fail data bagi storan dan penghantaran
·         Nyahpemampatan (decompression) bermakna mengembangkan data yang telah dipadatkan dan menghasilkan semula data asal
·         Pasangan proses ini dikenali sebagai codec (compression/decompression)
·         Secara amnya, codec mengurangkan redundancy in data
·         Maka betapa banyak saiz fail dapat dikurangkan bergantung kepada betapa banyak maklumat redundant yang wujud dalam data
-        Satu fail yang simbol-simbolnya wujud dengan satu kekerapan yang rawak adalah susah untuk dikurangkan saiznya
-        Bagi perwakilan simbolik yang berulang, senang untuk menggantinya dengan satu corak tertentu yang lebih pendek
·         Codec tidak boleh menjamin bahawa data dapat dijana semula seperti asal. Terdapat dua kaedah pemampatan utama : loosy  dan losless
-        Dengan losless , codec dapat hasilkan data seperti asal dari format termampat
·         Kelajuan codec  adalah penting bagi aplikasi-aplikasi tertentu
-        Codec  boleh berfungsi secara simetrik dan asimetrik
·         Bagi pemampatan secara simetrik, compression dan decompression perlu berlaku pada kelajuan yang sama seperti bagi aplikasi video conferencing  yang berlaku dalam masa nyata bagi komunikasi dua hala
·         Bagi pemampatan asimetrik, kelajuan memampat boleh berlaku secara perlahan seperti bila pemampatan dilaku sekali sahaja semasa peyimpanan atas CD-ROM tetapi pencapaian data boleh berlaku kerap-kali dan decompression perlu berlaku pada kadar cepat
·         Sintatik
-        Juga dikenali sebagai pengkodan entropy
-        Tidak menganalisa kandungan data
-        Data dilihat sebagai satu jujukan simbol
·         Semantik
-        Mengambilkira ciri-ciri khas sesuatu set data dan menyingkirkan maklumat yang tidak perlu dikod
-        Contoh : bagi data audio, nilai amplitud bagi satu frekuensi disimpan dan diguna untuk menjana semula bunyi. Bagi nilai frekuensi yang tidak dapat didengar oleh manusia, adalah tidak perlu untuk merekod nilai amplitudnya kerana jika ia dimainkan semula, manusia tidak dapat mendengarnya dan maka membazir storan menyimpan maklumat tak berguna
·         Hybrid
-        Mungkin gabungkan kedua-dua teknik
-        Guna sintatik dahulu dan kemudian semantik

Ringkasan

·         3 kaedah bagi klasifikasikan kaedah-kaedah pemampatan adalah berdasarkan samada ia adalah
-        loosy atau losless
-        simetrik atau asimetrik
-        sintatik, semantik atau hybrid


Pemampatan data teks dan numerik

·         Perlu menggunakan kaedah losless
·         Beberapa kaedah
-        Run-length encoding
·         Bila satu simbol berlang, ia diwakili dengan satu simbol lain bagi jujukan tersebut
·         Contoh : ABBCCDDDDDDDDDEE
-        Boleh diganti dengan ABBCCD#9E
-        Hauffman codes
·         Tidak menetapkan bilangan bit bagi perwakilan sesuatu data
·         Contoh : bagi mewakili aksara ‘e’ yang kerap berlaku dalam teks, mungkin kod 4 bit diguna tetapi bagi aksara ‘q’ yang jarang diguna, 8 bit diguna
-        LZW compression
·         Bagi perwakilan string  atau jujukan aksara
·         Gunakan satu kod bagi satu string sebalik menggunakan satu kod bagi satu aksara
·         Contoh : Bagi perkataan this yang kerap diguna, satu kod 4 bit boleh diguna sebalik mengguna 8 bit bagi ‘t’ , 8 bit bagi ‘I’ dan seterusnya.


Pemampatan Imej

·         Beberapa piawaian
-        GIF compression
-        TIFF compression
-        JPEG compression

Pemampatan video

·         Beberapa piawaian
-        Quicktime and AVI
-        Digital Video Interactive (DVI)
-        Motion-JPEG
-        MPEG
-        Piawaian px74
·         2 kaedah utama – spatial compression  dan temporal compression
·         Pemampatan berdasar ruang (spatial compression)
-        Mengurangkan maklumat yang diperlu bagi mewakili satu bingkai dalam satu bingkai jujukan video
-        Juga dikenali sebagai intraframe compression  dan mengurangkan kaedah pemampatan imej
·         Pemampatan berdasarkan masa (temporal compression)
-        Menyingkirkan data maklumat antara bingkai-bingkai
-        Juga dikenali sebagai interframe compression
-        Beberapa pendekatan
·         Pendekatan 1
·         Pendekatan 2

Pendekatan 1


·         Katakanlah untuk memadatkan satu jujukan 4 bingkai, bingkai pertama dirujuk sebagai key frame
·         Key frame menjadi asas untuk menentukan jumlah perbezaan pergerakkan yang berlaku di antara bingkai-bingkai
-        Maklumat latarbelakang seperti langit, bangunan dan sebagainya biasanya tidak berubah antara 4 bingkai. Hanya kereta bergerak
-        Maka 3 bingkai  selepas key frame  hanya mengkod maklumat imej dengan kereta bingkai-bingkai ini dipanggil diffrence frames

Pendekatan 2


·         Pendekatan 1 merekod perbezaan mengikut satu arah, iaitu bingkai-bingkai dihadapan key frame
·         Pendekatan 2 yang diguna oleh MPEG
·         Menggunakan kaedah ramalan
·         Keyframe  dirujuk sebagai intrapicture ( I picture)
·         Dari satu I picture, satu bingkai diramal dan difference frame  ini dirujuk sebagai predicted picture (P picture)
·         Dari P picture ini beberapa difference frames  lagi  diperolehi sebagai beberapa bingkai sebelumnya dan selepasnya. Bingkai-bingkai ramalan berdasar P frame dikenali sebagai bidirectional pictures (B pictures)

Pemampatan audio


·         Kaedah pemampatan biasa yang diguna bagi audio ialah ADPCM (adaptive differential code modulation)
·         Kaedah quantising maklumat audio yang telah kita bincang dalam topik audio dikenali sebagai kaedah PCM (pulse code modulation)
·         Dengan ADPCM, amplitud pertama yang disampel disimpan
-        Bagi amplitud-amplitud seterusnya, hanya nilai perbezaan amplitud semasa dan sebelumnya disimpan kerana nilai perbezaan adalah lebih kecil dari nilai amplitud sebenarnya
-        Untuk mengurangkan saiz lagi, nilai perbezaan ini boleh dibahagi dengan satu angkatap (coefficient)





Contoh perisian pemampatan.











No Response to "Tugasan 3 : Computer Security"

Post a Comment