Year 11 · Computer Science · Spring 2 – Summer 1 · Final revision and the exams

Last full Paper 1

Exam practicePresent ▶

What this lesson covers

1.1 Number systems

Cambridge IGCSE Computer Science 0478 · Topic 1: Data representation · Paper 1

3.1 Computer architecture

Cambridge IGCSE Computer Science 0478 · Topic 3: Hardware · Paper 1

5.3 Cyber security

Cambridge IGCSE Computer Science 0478 · Topic 5: The internet and its uses · Paper 1

You will learn to

Complete a full Paper 1 in 1 hour 45 minutes, without a calculator.
Use the timing plan: about 1¼ minutes per mark, with 10 minutes to check.
Correct every lost mark using the mark scheme in the corrections lesson.

Success criteria

I can finish the paper and check it in the time given.
I can show my working on every calculation, with units.
I can write a full-mark correction for every question where I lost marks.

The syllabus points for these sections

Convert positive whole numbers between binary, denary and hexadecimal, both ways.
Explain why hexadecimal is used and give real uses.
Add two 8-bit binary numbers and spot overflow.
Carry out logical left and right shifts and explain their effect.
Use 8-bit two's complement for positive and negative numbers.
State the largest number that 8 bits and 16 bits can hold.
Describe the role of the CPU and what a microprocessor is.
Name the parts of the von Neumann architecture: ALU, CU, registers and buses.
Describe the fetch–decode–execute cycle, step by step.
Explain how cores, cache and clock speed affect performance.
Explain what an instruction set is.
Describe embedded systems and how they differ from general-purpose computers.
Describe each cyber security threat: brute-force attack, data interception, DDoS, hacking, malware, pharming, phishing and social engineering.
Describe the six types of malware: virus, worm, Trojan horse, spyware, adware and ransomware.
Explain how each security solution helps keep data safe.
Explain how proxy servers and SSL work.
Choose the best defence for a given threat and justify it.

Key words

Say each word, then read what it means. Every one is in the glossary too.

Bit: A single binary digit, 0 or 1.
Nibble: A group of 4 bits, which matches one hexadecimal digit.
Byte: A group of 8 bits.
Denary: The base 10 number system we use every day, with digits 0 to 9.
Binary: The base 2 number system computers use, with digits 0 and 1.
Hexadecimal: The base 16 number system, with digits 0 to 9 and A to F, used to write binary in a short form.
Overflow: An error when a result is too big to fit in the number of bits available.
Logical shift: Moving all the bits left or right, losing bits off one end and filling the other end with 0s.
Two's complement: A way to store negative numbers, where the leftmost bit has a negative place value (−128 in 8 bits).
Character set: A list of characters a computer can use, each with its own binary code.
Colour depth: The number of bits used to store the colour of each pixel.
CPU: The central processing unit, which processes instructions and data by fetching, decoding and executing them.
Microprocessor: A CPU made as an integrated circuit on a single chip.
Von Neumann architecture: A computer design where instructions and data are stored together in the same memory.
ALU: Arithmetic logic unit: does calculations and logical comparisons.
Control unit: Manages the fetch–decode–execute cycle and sends control signals.
Register: A tiny, very fast store inside the CPU that holds one item.
Bus: A set of parallel wires that carries addresses, data or control signals.
Cache: Small, very fast memory in or near the CPU that holds frequently used data and instructions.
Clock speed: The number of cycles the CPU does each second, measured in hertz.
Core: A complete processing unit; a multi-core CPU can process several instructions at once.
Instruction set: The list of all the instructions a particular CPU can carry out.
Embedded system: A computer system inside a larger device that does one dedicated job.
Sensor: An input device that measures a physical property and sends the reading to a computer.
NIC: Network interface card: the hardware that connects a device to a network.
MAC address: A unique 48-bit address given to a NIC at manufacture, written in hex.
IP address: A number that identifies a device on a network, such as 151.101.0.81.
Internet: The worldwide network of connected computer networks: the physical infrastructure.
URL: Uniform resource locator: the text address of a web page, made of protocol, domain name and file name.
HTTP / HTTPS: The rules for sending web pages; HTTPS adds encryption using SSL.
Web browser: Application software that fetches web pages and renders HTML so you can see them.
DNS: Domain Name System: servers that find the IP address that matches a domain name.
Digital currency: Money that exists only electronically.
Brute-force attack: Trying every possible password or key until the right one is found.
DDoS: Distributed denial of service: flooding a server with requests from many computers so real users cannot reach it.
Malware: Software designed to cause harm, such as a virus, worm, Trojan horse, spyware, adware or ransomware.
Phishing: A fake message that tricks the user into clicking a link and giving personal details on a fake site.
Pharming: Malicious code or a tampered DNS server redirects the user to a fake website, even if they type the correct URL.
Social engineering: Manipulating people into breaking normal security rules.
Firewall: Hardware or software that checks traffic in and out against rules and blocks anything that breaks them.
Proxy server: A server between the user and the internet that filters requests, hides the user's IP address and caches pages.
SSL: Secure Sockets Layer: a protocol that uses certificates and encryption to secure data between browser and server.

The explanation

Read it here, or open the study page below to go one step at a time with things to try.

1.1 Number systems

Why computers use binary

A computer is made of billions of tiny switches called transistors. Each switch is either off or on.

We write off as 0 and on as 1. A number system with only two digits is called binary.

One binary digit is a bit. Eight bits together make a byte.

Every extra bit doubles the number of different patterns you can make.

e.g.Example

1 bit: 0, 1 → 2 patterns 2 bits: 00, 01, 10, 11 → 4 patterns 3 bits: 000, 001, 010, 011, 100, 101, 110, 111 → 8 patterns 8 bits: 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 = 256 patterns

?Why?

Two states are very reliable. A switch is clearly on or clearly off, so the computer rarely misreads a value.

Place value: denary and binary

Denary is our everyday number system. It has ten digits, 0 to 9, and each column is worth 10 times the column to its right.

Binary works the same way, but each column is worth 2 times the column to its right.

For 8 bits the column values, from left to right, are 128, 64, 32, 16, 8, 4, 2, 1.

To turn binary into denary, add up the column values that have a 1 under them.

Try the toy: switch on the 128 bit. What happens to the denary value? Now make 255.

128	64	32	16	8	4	2	1
0	1	0	1	1	0	1	0

e.g.Example

Binary 01011010 to denary. The 1s sit under 64, 16, 8 and 2. 64 + 16 + 8 + 2 = 90

!Watch out

Always write the column values above the bits first. Most mistakes come from adding the wrong column.

There is something to try for this step in the IGCSE library.

Denary to binary

Start at the left column, 128. Ask: does 128 fit into my number?

If it fits, write 1 and take 128 away. If it does not fit, write 0.

Move to the next column and repeat with what is left. Stop when you reach the 1 column.

Always write all 8 bits, including the 0s at the front.

e.g.Example

Denary 150 to binary. 128 fits → 1. Left: 150 − 128 = 22 64 does not fit → 0 32 does not fit → 0 16 fits → 1. Left: 22 − 16 = 6 8 does not fit → 0 4 fits → 1. Left: 6 − 4 = 2 2 fits → 1. Left: 0 1 does not fit → 0 Answer: 10010110 Check: 128 + 16 + 4 + 2 = 150

?Why?

Each column is used once at most, so taking away the biggest column first always gives the right pattern.

The biggest number n bits can hold

With n bits you get 2ⁿ patterns. One of those patterns is 0.

So the biggest number is one less than the number of patterns: 2ⁿ − 1.

8 bits hold 0 to 255. 16 bits hold 0 to 65,535.

A register is a small store inside the processor. An 8-bit register cannot hold anything bigger than 255.

e.g.Example

8 bits: 2⁸ = 256 patterns. Biggest = 256 − 1 = 255 (11111111) 16 bits: 2¹⁶ = 65,536 patterns. Biggest = 65,536 − 1 = 65,535 (1111111111111111, or FFFF in hex) Quick check for 8 bits: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

!Watch out

8 bits give 256 different values, but the largest value is 255, because counting starts at 0.

Hexadecimal: base 16

Hexadecimal (hex) has sixteen digits. It uses 0 to 9, then the letters A to F for 10 to 15.

Each hex column is worth 16 times the column to its right. Two-digit hex has columns 16 and 1. Three digits add a 256 column.

One hex digit stands for exactly four bits. Four bits are called a nibble.

Denary	Binary (nibble)	Hex
0	0000	0
1	0001	1
2	0010	2
3	0011	3
4	0100	4
5	0101	5
6	0110	6
7	0111	7
8	1000	8
9	1001	9
10	1010	A
11	1011	B
12	1100	C
13	1101	D
14	1110	E
15	1111	F

?Why?

16 is 2 × 2 × 2 × 2. That is why one hex digit matches four bits exactly, with nothing left over.

Hex to denary and denary to hex

Hex to denary: multiply each digit by its column value, then add. Change letters to numbers first (B = 11, F = 15).

Denary to hex: divide by 16. The whole-number answer is the left digit. The remainder is the right digit.

Change any digit from 10 to 15 into its letter.

e.g.Example

Hex B7 to denary: B = 11, so 11 × 16 = 176 7 × 1 = 7 176 + 7 = 183 Denary 200 to hex: 200 ÷ 16 = 12 remainder 8 12 = C, so the answer is C8 Check: 12 × 16 + 8 = 192 + 8 = 200 Three digits, hex 1A3 to denary: 1 × 256 = 256 A = 10, so 10 × 16 = 160 3 × 1 = 3 256 + 160 + 3 = 419

!Watch out

A remainder of 10 to 15 must be written as a letter. Writing "128" for C8 is a common slip.

Binary and hex using nibbles

Binary to hex: split the binary into groups of 4 bits, starting from the right. Change each group into one hex digit.

Hex to binary: change each hex digit into its 4-bit nibble. Write them side by side.

Each nibble uses the small column values 8, 4, 2, 1. This keeps the sums easy.

The fastest route between denary and hex is often through binary.

e.g.Example

Binary 10110110 to hex: Split: 1011 | 0110 1011 = 8 + 2 + 1 = 11 = B 0110 = 4 + 2 = 6 Answer: B6 Hex 4E to binary: 4 = 0100 E = 14 = 8 + 4 + 2 = 1110 Answer: 01001110 12 bits, 000110100011 to hex: 0001 | 1010 | 0011 → 1, A, 3 → 1A3

!Watch out

Always group from the right. If the left group is short, add 0s at its front. Keep the leading 0s when you change hex to binary, so each digit gives four bits.

Why hexadecimal is used

Computers do not use hex. Inside, everything is still binary. Hex is for people.

Hex is shorter: 8 bits become 2 hex digits. It is easier to read, remember and type.

People make fewer mistakes copying hex than long strings of 0s and 1s.

It converts to binary very easily, because each digit is one nibble.

Use	Real example	What the hex means
MAC address	00:1A:2B:3C:4D:5E	The unique number of a network card. 12 hex digits instead of 48 bits.
HTML colour code	#FF8800	Red FF = 255, green 88 = 136, blue 00 = 0. This is orange.
Error codes	0x80070005	A Windows error code (access denied). The 0x means "hex follows".
URLs	my%20file.html	%20 is hex 20 = denary 32, the code for a space.
Memory dumps	3F A7 00 B6	The contents of memory shown for a programmer finding a bug.
IPv6 addresses	2001:0db8::7334	Long network addresses written in hex groups.

!Watch out

Do not write "hex takes up less memory". It does not. The computer stores the same binary. Hex is only a shorter way for humans to write it.

Adding binary: the four rules

You add binary in columns, from right to left, like denary addition.

In binary, 1 + 1 makes 2, and 2 is written 10. So you write 0 and carry 1 to the next column.

If a column has 1 + 1 plus a carry of 1, the total is 3. Three is 11: write 1 and carry 1.

Try the toy: it starts with 91 + 54. Press Step to add one column at a time and watch where each carry goes. Then press Overflow example.

Column adds up to	Denary total	Write	Carry
0 + 0	0	0	0
0 + 1 or 1 + 0	1	1	0
1 + 1	2	0	1
1 + 1 + carry 1	3	1	1

There is something to try for this step in the IGCSE library.

A full 8-bit addition

Write the two numbers one under the other, with the column values on top.

Write each carry above the next column on the left. Then add that column, including the carry.

Check your answer by changing everything to denary.

	128	64	32	16	8	4	2	1
Carry	1	1	1	1	1
109	0	1	1	0	1	1	0	1
+ 54	0	0	1	1	0	1	1	0
= 163	1	0	1	0	0	0	1	1

e.g.Example

Column 1: 1 + 0 = 1 Column 2: 0 + 1 = 1 Column 4: 1 + 1 = 10 → write 0, carry 1 Column 8: 1 + 0 + carry 1 = 10 → write 0, carry 1 Column 16: 0 + 1 + carry 1 = 10 → write 0, carry 1 Column 32: 1 + 1 + carry 1 = 11 → write 1, carry 1 Column 64: 1 + 0 + carry 1 = 10 → write 0, carry 1 Column 128: 0 + 0 + carry 1 = 1 Answer: 10100011 Check: 128 + 32 + 2 + 1 = 163, and 109 + 54 = 163

!Watch out

Show your carries in the exam. A wrong answer with clear carries can still earn a working mark.

Overflow

An 8-bit register can hold 0 to 255. Sometimes an addition gives more than 255.

Then the last column makes a carry that has nowhere to go. It would need a 9th bit.

This is called an overflow error. The 9th bit is lost, so the stored answer is wrong.

The processor sets an overflow flag so the program can deal with it.

	Lost	128	64	32	16	8	4	2	1
Carry	1	1
200		1	1	0	0	1	0	0	0
+ 100		0	1	1	0	0	1	0	0
Stored	1	0	0	1	0	1	1	0	0

e.g.Example

200 + 100 = 300. But 300 is more than 255. The full answer needs 9 bits: 1 00101100. The register keeps only the right 8 bits: 00101100 = 32 + 8 + 4 = 44. 300 − 256 = 44, because the lost 9th bit was worth 256.

?Why?

A register has a fixed number of bits. Anything that does not fit is lost, so the computer cannot store a value above 2ⁿ − 1.

Logical left shift

A logical shift moves every bit left or right by a number of places.

In a left shift, bits fall off the left end and are lost. 0s fill the gaps on the right.

Each place shifted left multiplies the number by 2. Two places is × 4. Three places is × 8.

If a 1 falls off the left end, the answer is wrong. This is a kind of overflow.

Try the toy: load 22 (00010110), choose 2 places and press Shift left. Then load 53 (00110101) and shift it left 3 places. Which answer is wrong?

	128	64	32	16	8	4	2	1	Denary
Before	0	0	0	1	0	1	1	0	22
Left 2	0	1	0	1	1	0	0	0	88

e.g.Example

22 shifted left 2 places: 00010110 → 01011000 0 and 0 fall off the left. Two 0s come in on the right. 01011000 = 64 + 16 + 8 = 88. And 22 × 4 = 88. Correct. 53 shifted left 3 places: 00110101 → 10101000 0, 0 and 1 fall off the left. The lost 1 was important. 10101000 = 128 + 32 + 8 = 168. But 53 × 8 = 424. Wrong answer, because 424 is too big for 8 bits.

?Why?

Moving a bit one column left puts it in a column worth twice as much. So the whole number doubles.

There is something to try for this step in the IGCSE library.

Logical right shift

In a right shift, bits fall off the right end and are lost. 0s fill the gaps on the left.

Each place shifted right divides the number by 2. Two places is ÷ 4. Three places is ÷ 8.

If a 1 falls off the right end, the remainder is lost. The answer is rounded down, so accuracy is lost.

	128	64	32	16	8	4	2	1	Denary
Before	0	0	0	1	0	1	1	0	22
Right 2	0	0	0	0	0	1	0	1	5

e.g.Example

180 shifted right 1 place: 10110100 → 01011010 = 64 + 16 + 8 + 2 = 90. And 180 ÷ 2 = 90. Exact. 22 shifted right 2 places: 00010110 → 00000101 = 4 + 1 = 5 But 22 ÷ 4 = 5.5. The bits 1 and 0 fell off the right end. The lost 1 was worth 0.5 after dividing, so the answer is not exact.

!Watch out

In a logical shift the new bits are always 0. Do not copy the old end bit back in, and do not wrap bits round to the other end.

Two's complement: negative numbers

So far 8 bits only held positive numbers, 0 to 255. Two's complement is a way to store negative numbers too.

The only change: the leftmost column is worth −128 instead of 128. All other columns stay the same.

If the leftmost bit is 0, the number is positive. If it is 1, the number is negative.

8-bit two's complement holds −128 to +127.

Try the toy: press Smallest (−128) so only the −128 bit is on. Then tap the 64 bit. Watch the value climb towards 0.

−128	64	32	16	8	4	2	1	Denary
0	1	0	0	1	1	0	1	64 + 8 + 4 + 1 = 77
1	0	1	1	0	1	1	0	−128 + 32 + 16 + 4 + 2 = −74
1	1	1	1	1	1	1	1	−128 + 127 = −1

e.g.Example

Two's complement 10110110 to denary. The 1s are under −128, 32, 16, 4 and 2. −128 + 32 + 16 + 4 + 2 = −128 + 54 = −74

?Why?

With a −128 column, normal binary addition still works for negative numbers. The processor needs only one adding circuit.

There is something to try for this step in the IGCSE library.

Denary to two's complement

Positive number: write it in normal 8-bit binary. The leftmost bit will be 0.

Negative number, method 1: write the positive version in binary. Flip every bit (0 becomes 1, 1 becomes 0). Then add 1.

Negative number, method 2: start with −128. Work out what you must add to reach your number. Put that in the other 7 columns.

Use one method and check with the other, or check by adding the column values.

e.g.Example

Change −45 into 8-bit two's complement. Method 1 (flip and add 1): +45 = 00101101 Flip every bit: 11010010 Add 1: 11010011 Method 2 (start at −128): −45 = −128 + 83 83 = 64 + 16 + 2 + 1 So: 1 in the −128 column, then 1010011 Answer: 11010011 Check: −128 + 64 + 16 + 2 + 1 = −45

!Watch out

+45 and −45 are not the same pattern with a different first bit. 10101101 is −83, not −45. Always flip and add 1, or use the −128 method.

3.1 Computer architecture

The CPU and the microprocessor

The CPU (central processing unit) is the part of the computer that processes instructions and data. It carries out the instructions of every program.

It does this by fetching an instruction from memory, decoding it, then executing it. Then it repeats, billions of times a second.

A microprocessor is a CPU built as an integrated circuit on a single chip. An integrated circuit is a whole electronic circuit made on one small piece of silicon.

e.g.Example

The chip inside a laptop, a phone or a games console is a microprocessor. A microprocessor also sits inside a microwave oven, running its timer.

Von Neumann architecture

Most computers follow the von Neumann architecture. Its key idea is the stored program: instructions and data are both stored in the same main memory.

The CPU has a control unit (CU). The CU manages the fetch–decode–execute cycle. It decodes instructions and sends control signals to the other parts.

The CPU has an arithmetic logic unit (ALU). The ALU does calculations (such as add and subtract) and logic operations (such as comparing two numbers).

The CPU also has registers: very small, very fast stores inside the CPU. The CPU is joined to memory by three buses (sets of wires).

?Why?

Keeping instructions and data in one memory means the computer can run any program you load. Change the program in memory and the same hardware does a new job.

There is a diagram for this step in the IGCSE library.

The five registers

A register holds one item at a time: an address, an instruction or a piece of data.

An address is the number of a location in memory, like a locker number in a school corridor.

Register	Full name	What it holds
PC	Program counter	The address of the next instruction to be fetched
MAR	Memory address register	The address of the memory location about to be read from or written to
MDR	Memory data register	The data or instruction that has been fetched from memory, or about to be written to memory
CIR	Current instruction register	The instruction that is being decoded and executed right now
ACC	Accumulator	The result of calculations done by the ALU

!Watch out

PC and MAR hold addresses. MDR and CIR hold instructions or data. Mixing these up is the most common error in FDE answers.

The three buses

The address bus carries addresses from the CPU (the MAR) to memory. It is one-way (unidirectional), because only the CPU chooses addresses.

The data bus carries data and instructions between the CPU (the MDR) and memory. It is two-way (bidirectional), because data is both read and written.

The control bus carries control signals from the control unit, such as 'read', 'write' and the clock signal. It is two-way, because devices also send signals back.

!Watch out

Exam answers often say 'the address bus carries data'. It carries addresses only. Data and instructions travel on the data bus.

The fetch–decode–execute cycle

Every instruction goes through the same three stages. Learn the fetch stage in order: it is worth most marks.

Fetch: the address in the PC is copied to the MAR, using the address bus. The PC is incremented (1 is added), so it points to the next instruction. The instruction at that address is sent along the data bus to the MDR. It is then copied from the MDR to the CIR.

Decode: the control unit decodes the instruction in the CIR. It works out what the instruction means and what needs to happen.

Execute: the instruction is carried out. The control unit sends signals on the control bus. If there is a calculation, the ALU does it and the result goes in the ACC. Then the cycle starts again.

e.g.Example

PC = 100. The instruction at address 100 is LOAD 105 (put the value at address 105 into the ACC). Fetch: 100 is copied from PC to MAR → PC becomes 101 → LOAD 105 travels from address 100 to the MDR → copied to the CIR. Decode: the CU sees 'load the value at address 105'. Execute: 105 goes into the MAR → the value at 105 (say 7) comes back on the data bus to the MDR → it is copied to the ACC. ACC = 7. Try it in the toy: its tiny program starts at address 0 (LOAD 5, ADD 6, STORE 7, HALT). Press Step and watch each register change. Notice when the PC goes up by 1.

?Why?

The PC is incremented during fetch, so the CPU always knows where the next instruction is, even while it is busy with the current one.

There is something to try for this step in the IGCSE library.

The instruction set

An instruction set is the list of all the instructions (commands) that a particular CPU can understand and carry out.

Each instruction is a binary code, called machine code. Examples are load, add, store, compare and jump.

The CU decodes instructions using this set. Different CPU families have different instruction sets.

e.g.Example

Most laptops use x86 processors. Most phones use ARM processors. A program compiled for one will not run directly on the other, because their instruction sets are different.

Cores, cache and clock speed

Clock speed is the number of fetch–decode–execute cycles the CPU can do per second, measured in hertz (Hz). A 3.5 GHz CPU has 3,500,000,000 clock ticks per second. Higher clock speed means more instructions per second.

Cache is a small amount of very fast memory inside or next to the CPU. It stores instructions and data that are used often, so the CPU does not have to wait for slower RAM. More cache means fewer slow trips to RAM.

A core is a complete processing unit (with its own ALU, CU and registers). A quad-core CPU has four cores, so it can process four instructions at the same time.

?Why?

All three let the CPU get through more instructions in a given time. That is what 'better performance' means.

!Watch out

Doubling the cores does not double the speed. Cores must communicate with each other, and some programs cannot be split into parts. Raising clock speed also makes the CPU hotter, and cache is expensive, so there are limits.

Embedded systems

An embedded system is a small computer system built inside a larger device, to do one dedicated job.

It uses a microprocessor (or a microcontroller) and usually runs one fixed program stored in ROM.

A general-purpose computer, like a laptop, can run many different programs and do many jobs.

Embedded system examples	What it controls
Washing machine	Water level, temperature, drum speed for each wash programme
Microwave oven	Timer and power level
Central heating	Turns the boiler on and off using a temperature sensor
Car engine management	Fuel and air mix, using many sensors
Traffic lights	The light sequence and timing

?Why?

Embedded systems are small, cheap to mass-produce and use very little power, because they only do one job. The drawbacks: they are hard to update or change, and they can only do the task they were built for.

5.3 Cyber security

Brute-force attack

A brute-force attack is when a criminal tries every possible password (or key) until one works. Software tries millions of guesses each second.

The aim is to get into an account or system, then steal data or cause harm.

Longer passwords with a mix of character types have far more possible combinations, so they take much longer to crack.

e.g.Example

A 4-digit PIN: 10 choices per digit. 10 × 10 × 10 × 10 = 10,000 possible PINs. An 8-character password using a–z, A–Z and 0–9: 62 choices per character. 62^8 = 218,340,105,584,896 possible passwords (about 218 million million).

?Why?

Each extra character multiplies the number of guesses needed. That is why length matters more than anything else.

!Watch out

Defences: long, complex passwords; locking the account after a few failed attempts; two-step verification.

Data interception

Data interception is when data is stolen while it is being sent, for example over Wi-Fi or across the internet.

A criminal can use a packet sniffer, a program that captures and examines data packets as they pass. On an unsecured public Wi-Fi network this is easy.

The aim is to read passwords, card numbers or private messages.

e.g.Example

In a café, someone on the same free Wi-Fi captures the packets from your laptop. If a website uses plain HTTP, they can read the password you typed.

!Watch out

Defences: encryption (HTTPS/SSL so captured data cannot be read); avoid public Wi-Fi for banking, or use a VPN; use strong Wi-Fi encryption at home.

Distributed denial of service (DDoS)

A denial of service (DoS) attack floods a web server with so many requests that it cannot cope. Real users cannot reach the website.

In a distributed DoS (DDoS), the requests come from thousands of computers at once. These are often ordinary devices infected with malware and controlled by the criminal. This group is called a botnet.

The aim is to take a website offline, often to cause loss of sales or to demand money.

e.g.Example

On the morning concert tickets go on sale, a botnet of 50,000 infected computers and cameras sends requests to the ticket website. The server is overloaded and real fans see an error page.

?Why?

Requests from so many different places are hard to block, because they look like normal visitors.

!Watch out

Defences: a proxy server or firewall that filters out the flood of traffic; anti-malware on your own devices so they do not join a botnet.

Hacking

Hacking is gaining unauthorised access to a computer system. Unauthorised means without permission.

A hacker may steal, change or delete data, or install malware. This is illegal in most countries.

Ethical hacking is different: a company pays an expert to try to break in, so weaknesses can be found and fixed.

!Watch out

Defences: firewalls, strong passwords and authentication, access levels, and keeping software updated so known weaknesses are fixed.

Malware: virus, worm and Trojan horse

Malware (malicious software) is any software written to cause harm. Cambridge lists six types. The first three spread or get in; the last three do specific damage.

Type	How it works	Example of harm
Virus	Attaches itself to a program or file. Needs a user to run that file. Then copies itself to other files.	Deletes or corrupts files; makes the computer crash
Worm	A standalone program that copies itself across a network on its own. No user action needed.	Uses up bandwidth and slows networks; can carry other malware
Trojan horse	Disguised as useful, legitimate software. The user installs it willingly.	Once installed, it lets in other malware such as spyware, or gives the criminal remote access

e.g.Example

A student downloads a free "game cheat" from an unknown site. It works, but it also installs spyware. That download was a Trojan horse.

!Watch out

Virus versus worm: a virus needs a host file and a user to run it. A worm spreads by itself across a network.

Malware: spyware, adware and ransomware

These three types are named after what they do once they are on a device.

Type	How it works	Example of harm
Spyware	Secretly records what the user does and sends it to the criminal. A keylogger records every key pressed.	Steals passwords and bank details
Adware	Shows unwanted adverts, often as pop-ups. May redirect the browser to advertising sites.	Annoying, slows the device; some adware also contains spyware
Ransomware	Encrypts the user's files so they cannot be opened. Demands payment for the key to unlock them.	A hospital cannot open patient records until it pays or restores from backup

?Why?

Regular backups are the best answer to ransomware. If the files are backed up elsewhere, the victim can restore them without paying.

!Watch out

Defences for all malware: anti-malware software, automatic updates, not opening unknown attachments, downloading only from trusted sources.

Phishing and pharming

Phishing is sending a fake email or message that looks like it comes from a trusted organisation, such as a bank. It tries to make the user click a link to a fake website and type in their personal details.

Pharming is when malicious code is installed on the user's computer, or a DNS server is tampered with. It redirects the user to a fake website, even when they type the correct URL.

Both aim to steal login details, card numbers or other personal data.

	Phishing	Pharming
How the user reaches the fake site	Clicks a link in an email or text	Is redirected automatically, even after typing the correct URL
Needs the user to click a link?	Yes	No
Main defence	Check spelling, tone and URLs; do not click links in unexpected messages	Anti-malware; check the URL and padlock before entering details

e.g.Example

Phishing: a text says "Your Correos parcel is held. Pay 1.99 euros here" with a link to a fake site. Pharming: you type your bank's correct address, but malware on your PC sends you to a copy of the bank's site.

!Watch out

Learn the difference: phishing tricks you into clicking; pharming redirects you without you knowing.

Social engineering

Social engineering is manipulating people, rather than computers, into breaking security. It uses trust, fear, curiosity or urgency.

Phishing is one kind. Another is a phone call from someone pretending to be the IT department.

Baiting leaves a USB stick labelled "Salaries" in the car park, hoping someone plugs it in. Scareware is a fake pop-up saying "Your PC is infected, call now".

The aim is to get passwords, access to a building or system, or to get malware installed.

e.g.Example

A caller says: "Hi, it's Sam from IT. Your account is locked. Tell me your password and I'll fix it now." The caller is not from IT.

?Why?

People are often the weakest link. The best technical security fails if someone gives away a password.

!Watch out

Defence: training staff to be suspicious, never sharing passwords, checking who is really asking.

Solutions: access levels, anti-malware and authentication

Access levels give each user only the rights they need. A student can read the school timetable but cannot change it. This limits the damage if an account is hacked.

Anti-malware software includes anti-virus and anti-spyware. It scans files and compares them with a database of known malware. It quarantines or deletes anything harmful and checks downloads as they arrive.

Authentication proves a user is who they say they are.

Authentication method	How it works	Strength / weakness
Username and password	User types a secret word or phrase	Easy to use; can be guessed, stolen or brute-forced
Biometrics	Uses a body feature: fingerprint, face, iris, voice	Hard to fake and cannot be forgotten; costly; cannot be changed if copied
Two-step verification	After the password, a second step: a code sent to your phone or an app	A stolen password alone is not enough; needs the phone with you

?Why?

Anti-malware needs to update its database often, because new malware appears every day.

Solutions: updates, checking messages and links

Automating software updates means the OS and apps download and install fixes by themselves. Updates close security holes that hackers and malware use.

Checking the spelling and tone of communications helps spot phishing. Warning signs are spelling mistakes, a general greeting such as "Dear customer", and an urgent or threatening tone.

Checking URLs in links before clicking: hover over the link to see where it really goes. Look for misspellings such as `paypa1.com` or extra words such as `bbva-secure-login.net`.

e.g.Example

Email: "URGENT!! Dear costumer, your acount will be close in 24 hours. Click here." Red flags: spelling errors (costumer, acount), generic greeting, urgent threat. The link shows http://santander.verify-acc.ru when you hover. The domain is not the bank's real domain.

Solutions: firewalls and privacy settings

A firewall sits between a computer (or network) and the internet. It can be hardware, software or both.

It examines all traffic coming in and going out. It checks each packet against a set of rules, such as allowed IP addresses or ports. Traffic that breaks the rules is blocked.

It can also log traffic and warn the user when a program tries to connect to the internet.

Privacy settings control who can see your information, for example on social media. Setting a profile to "friends only" stops strangers seeing your location and photos, which social engineers use.

!Watch out

A firewall does not remove malware that is already on a computer. That is the job of anti-malware.

Solutions: proxy servers

A proxy server is a server that sits between the user's computer and the web server. Requests go to the proxy first, and the proxy fetches the page on the user's behalf.

It can filter requests, blocking access to certain websites. Schools use this to block unsuitable sites.

It hides the user's IP address, because the web server only sees the proxy's address. This protects the user's identity.

It can cache (store copies of) popular pages, so they load faster the next time.

In front of a web server, it can absorb and filter heavy traffic, helping to protect against DDoS attacks.

?Why?

Putting a middle step between the user and the internet means harmful traffic can be stopped before it reaches either side.

Solutions: SSL

SSL (Secure Sockets Layer) is a security protocol that encrypts data sent between a browser and a web server. Its modern version is called TLS, but the exam uses the name SSL.

It is what turns HTTP into HTTPS. It uses an SSL certificate: a digital document that proves the website is who it claims to be.

e.g.Example

How an SSL connection starts: 1. The browser asks the web server to identify itself. 2. The server sends a copy of its SSL certificate. 3. The browser checks the certificate is real and trusted. 4. If it is, the browser sends a message back to the server. 5. The server replies with a digitally signed acknowledgement to start the session. 6. All data sent from now on is encrypted.

?Why?

Even if the data is intercepted, it cannot be read. The certificate also helps the user know they are on the real website.

!Watch out

SSL protects data in transit (while it travels). It does not stop malware on your own computer or protect data stored on the server.

Matching threats to defences

Exam questions often give a scenario and ask you to name the threat and the best defence. Try the threats toy: read each scenario, name the threat, then pick the defence.

Always explain how the defence stops that threat. "Install a firewall" is a weak answer. "A firewall blocks incoming traffic that does not meet its rules" is stronger.

Threat	Good defences
Brute-force attack	Long, complex passwords; lock after failed attempts; two-step verification
Data interception	Encryption, SSL/HTTPS; avoid public Wi-Fi
DDoS	Proxy server; firewall; anti-malware so devices do not join a botnet
Hacking	Firewall; strong authentication; access levels; automatic updates
Malware	Anti-malware; automatic updates; do not open unknown attachments; backups for ransomware
Phishing	Check spelling, tone and URLs; do not click links in unexpected messages
Pharming	Anti-malware; check the URL and SSL certificate before entering details
Social engineering	Staff training; privacy settings; never share passwords

There is something to try for this step in the IGCSE library.

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Open the study page and go one step at a time.

Computer Science topic 1

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Success criteria

Why computers use binary

Place value: denary and binary

Denary to binary

The biggest number n bits can hold

Hexadecimal: base 16

Hex to denary and denary to hex

Binary and hex using nibbles

Why hexadecimal is used

Adding binary: the four rules

A full 8-bit addition

Overflow

Logical left shift

Logical right shift

Two's complement: negative numbers

Denary to two's complement

The CPU and the microprocessor

Von Neumann architecture

The five registers

The three buses

The fetch–decode–execute cycle

The instruction set

Cores, cache and clock speed

Embedded systems

Brute-force attack

Data interception

Distributed denial of service (DDoS)

Hacking

Malware: virus, worm and Trojan horse

Malware: spyware, adware and ransomware

Phishing and pharming

Social engineering

Solutions: access levels, anti-malware and authentication

Solutions: updates, checking messages and links

Solutions: firewalls and privacy settings

Solutions: proxy servers

Solutions: SSL

Matching threats to defences

Data representation

Data transmission

Hardware

Software

The internet and its uses

Automated and emerging technologies

Algorithm design and problem-solving

Programming

Databases

Boolean logic