Computer science free courses pdf download

Cambridge International AS and A Level Computer Science Coursebook offers students detailed descriptions of the concepts, reinforced with examples that outline complex subject matter in a clear way. Alongside fundamental definitions, higher level programming skills are developed through the explanation of processes and consolidated by practical exam-type questions for students to attempt.

This title provides students with fundamental definitions, detailed descriptions of the concepts and numerous examples for consolidation of knowledge. It also features practice and end-of-unit exam-style questions.

As the digitalization rises the expectation from college and school pass outs aspirants are also high to meet the upcoming demand of twenty first century. In this era the use of computers is everywhere, so there is a huge career opportunity to shape their future and make a bright career. There are a variety of career options available to transform their career and to fulfill their goal, some of the popular jobs are software engineer or software developer, programmer etc.

It has many competitors, the most famous being the open source Mozilla Thunderbird and the free Windows Live Mail. It can be useful for various reasons: it does not require the installation of a mail reader program; old received emails are always available on the website and can thus be accessed from home, office and while traveling, even without a personal laptop; the mailserver takes care of emails backup.

But on the other hand it requires a continuous fast connection even to write a single long email, which can be costly and, in some situations, impossible and usually the email space is limited. The most famous website interfaces are the Microsoft Outlook Web App, where the web interface looks exactly like Microsoft Outlook, and the Webmail interface, used and personalized by most Internet providers.

Posta Elettronica Certificata PEC When sending an email, the sender has no proof that it has been sent, for example to be used in a court of justice, and no guarantee that the email has been dispatched. Some mail readers use a receipt system, but the receiver is not obliged to send back the receipt.

In order to overcome these problems, many solutions have been proposed. When an email is sent from a PEC address to another PEC address, the sender receives two receipt: the first one is a proof that the email has been sent with date and time, while the second one is a proof that the email has been dispatched to the mailbox of the receiver or, equivalently, received by the sender.

However, it is important to note that PEC alone does not guarantee that the sender is really the person who claims to be and that content remains unread until it reaches destination. In order to overcome these last two problems, encryption and digital signature see section 4.

The major difference with respect to most Internet communication systems is that WhatsApp uses a fully centralized architecture, i. Search engines A search engine is a special program running on a website which offers to the user the possibility of searching other websites for specific web pages. The user needs to connect to the search engine website Page 16 of 33 Edition 8. Paolo Coletti and digit the keywords, or sometimes even a complete question, and the website returns the list of relevant web pages.

Search engines use a crawler technique: they continuously go through the known web pages memorizing their content and trying to discover other web pages through the contained links. The most popular search engines are Google, the current market leader, Yahoo! In order to choose the order in which web pages are displayed to the user, search engines use scoring system.

Internet connections There are many different ways to connect to the Internet. This phenomenon is called digital divide: there are people e. In order to overcome this social problem, WiMAX is spreading, a sort of very long range Wireless which arrives up to 10 Km but works only if the transmitting and receiving antennas are in line of sight and whose theoretical speed of 70 Mbps decreases with distances to about 40 Mbps.

Paolo Coletti Basic Computer course book 4. Computer security Being connected to the Internet means giving anybody access to the computer. This law requirements apply clearly to all companies and professionals which handle data, but they also apply to personal users who communicate data.

To personal users who do not communicate data still the security requirements to avoid data theft apply. Encryption Encryption is a text masking technique, derived from military use, which transforms information in such a way that it may be correctly read only with a special password called key.

It uses two keys, a public key for encrypting, usually known only to one computer or person, and a private key for decrypting, usually known by all the computers or people which legitimately may read the information. The size of these keys, and thus the difficulty to be guessed, is expressed in bits, with bits being the typically most secure size used.

The sent messages are encrypted and later decrypted by A with his private key. In case somebody Page 20 of 33 Edition 8. Even when somebody uses the public key to decrypt, it does not work. The same process happens whenever a browser tries to send a password or secret information to a website using a secure connection see section 4. In this case private and public keys coincides and are kept secret.

The encryption and decryption process is done automatically by a program PDF creation programs or compression programs can do it, see page 11 for instructions how to do it or even by the operative system if the entire disk is encrypted , which asks the password to the user every time. The two following schemas illustrate instead the usage of encryption for digital signature. A wants to publish a publicly available document with its signature.

Paolo Coletti authority to distribute private and public keys. Even though theoretically a simple password is enough, to be sure that the user does not give the private password around, the certification authority gives him, after having identified him through a governmental identity card, a password usually together with another identification tool a smart card, a telephone number for an OTP to be send , which, when used together, correspond to his private key.

An automatic signature program takes care of automatically encrypting documents. The service to digitally sign documents is offered by private certification companies, with prices currently affordable also by private users and with alternative devices such as smartcards or OTP devices see section 4. Documents for which it is important to determine the exact date of the signature have moreover a temporal mark signed directly by the certification authority. Passwords On the Intranet the user is identified only by his username, known to everybody, and his password, known only to him.

Therefore it is absolutely necessary to keep passwords secret. Unfortunately many people use very trivial passwords. This is the list of the most common passwords in password, , , , qwerty, dragon, pussy, baseball, football, letmein, monkey, , abc, There exist automatic programs which are able to try 4 billion passwords each second, and they usually start trying combinations of words and numbers the complete set of all Italian, German and English words can be tried in less than 1 second.

Unfortunately every website asks the user to register with a password and users who use always the same password are giving it away to every website they register, even untrustworthy ones. It is a good procedure to have at least three passwords: one for important use bank account , a second one for everyday use and a last one for unimportant use registering to unknown websites or to services that will not be used anymore. They seem to be unreadable, but computer experts can reveal them instantly.

Store passwords in programs only if that computer has a single user i. Page 24 of 33 Edition 8. Paolo Coletti 4. Usually biometric identification is considered to be very secure and thus it is used to replace completely the password system.

For very important activities, such as digital signature or bank operations, usually a personal device is instead added to the standard login and password system. The usual password is remembered personally by the user and a personal device provides the second part of the password. This device can be a smartcard, such as the national health card, which is inserted into a card reader or a USB token and they provide to the program or web site the second half of the password which is stored inside the object.

This big advantage of this second system is that, even if both parts of the password are intercepted or guessed, the second part can be used only that time and will expire after a few seconds. Viruses From the Internet many unauthorized connection attempts arrive. Some of these are mistakenly authorized and manage to reach the Intranet or at least to come in contact with programs which are behind the firewall.

If these connections carry malign intentions, usually their aim is to explore and use the Intranet computers, to destroy Intranet data or to stop some Intranet services which is a dangerous attack if these services are managing stock trades or telephone calls.

Defense against these kinds of attacks is in charge system administrators. The virus is a little program which has this name because its life cycle is the same of a biological organism: survive and duplicate. It arrives on the computer through email attachments, downloaded files, CDs and floppy disks or directly from the Intranet.

It is often hidden inside other good files or programs, which are called infected. As soon as the user mistakenly runs it often trying to run the good program or to open the good file , the virus orders the computer to run itself every time the computer is turned on, thus assuring its survival.

It starts duplicating itself, infecting other files, CDs and floppy disks, and trying to send itself around by email or on the Intranet. No viruses are known to be able to damage hardware. These types are not exclusive: for example a Trojan horse which is at the same time a spyware and an adware. An infected computer can be recognized by some symptoms. To check the computer for viruses and to try to remove viruses from the computer, the user can run a special program called antivirus.

If a virus is found, it tries to remove it and to repair damaged files. Some files can be unrecoverable. If there is an infected file, it tries to delete the virus and repair it.

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