Where certainty stops

This one topic asks you to learn logic gates, then networks, then security. Three subjects in a row, and most students revise them as three unrelated lists. ⭐⭐ THEY ARE JOINED BY A QUESTION ABOUT CERTAINTY, AND ONCE YOU SEE IT THE ORDER STOPS BEING ARBITRARY. Start with the logic, because it is the one place in this whole qualification where you can be completely certain of something. ⭐ A LOGIC CIRCUIT WITH TWO INPUTS HAS EXACTLY FOUR POSSIBLE SITUATIONS. WITH THREE INPUTS IT HAS EIGHT. YOU CAN WRITE EVERY ONE OF THEM DOWN, WORK OUT WHAT COMES OUT, AND WHEN YOU REACH THE LAST LINE THE BEHAVIOUR OF THAT CIRCUIT IS SETTLED FOREVER. Not tested. Not probably fine. Settled. There is no situation left that you have not already looked at. ⚠️ NOTHING ELSE IN COMPUTING OFFERS YOU THAT, AND IT IS WORTH NOTICING HOW UNUSUAL IT IS. Now send something out of the machine. ⚠️⚠️ YOU CANNOT LIST EVERY PERSON WHO MIGHT BE LISTENING. YOU CANNOT LIST EVERY ROUTE THE MESSAGE MIGHT TAKE, OR EVERY MACHINE IT PASSES THROUGH ON THE WAY, OR EVERY PERSON WHO MIGHT LATER TRY A PASSWORD. The set of cases is not large. It is open. There is no last line to reach. ⭐⭐ AND THAT IS EXACTLY WHY SECURITY IS THE THIRD THING IN THIS ROW. SECURITY IS WHAT YOU DO WHEN YOU CAN NO LONGER CHECK EVERY CASE. YOU STOP TRYING TO PROVE THINGS AND YOU START MAKING THE BAD CASES EXPENSIVE. Once you hold that, a lot of confusing things line up. ⭐ WHY DOES A SYSTEM NEED A LOGIN AT ALL? Because a network delivers bits and nothing else. It has no way of knowing who was sitting at the keyboard. Identity is not something that arrives with the message, so it has to be established by something the bits can carry. ⚠️ AND EVERY WAY OF DOING THAT IS A TRADE RATHER THAN A PROOF. A password accepted tells you somebody knew the password. It does not tell you who they were. ⚠️⚠️ WHICH MEANS THE COMMONEST SENTENCE IN GCSE SECURITY ANSWERS IS WRONG. NOTHING STOPS HACKERS. MEASURES CHANGE WHAT AN ATTEMPT COSTS, WHAT IT REQUIRES, AND HOW LIKELY IT IS TO BE NOTICED. ⭐ CARRY ONE QUESTION THROUGH EVERYTHING THAT FOLLOWS: COULD I CHECK EVERY CASE HERE, OR ONLY MAKE THE BAD ONES HARDER?

Three places, three kinds of check

The same question asked of each part of this topic: what can actually be established here?

Tap the two you could check completely

Tap the TWO situations where every possible case can actually be listed and checked.

The behaviour of a logic circuit with three inputs
Whether two logic circuits with the same inputs always produce the same output
Everybody who might be able to read a message crossing a network
Everybody who might attempt to log in to a system in the future

Four rows, and no more

A student writes out every possible input situation for a two-input logic circuit and works out the output for each. Why is that a stronger kind of result than testing a program with several sample inputs?

Because the list of possible situations is complete, so there is no untested case left, while sample testing can only ever cover the cases somebody thought of
Because logic circuits are simpler than programs
Because it uses mathematics rather than practical testing
Because it is quicker than running a program many times

Five terms for this unit

Five terms, each defined by what it is. The middle three are the ones a security answer has to use precisely.

Match each situation to what can be established

A logic circuit with three inputs, fully worked through
A message arriving across a network claiming to be from a named user
A password that a system has just accepted
Data held in a form that cannot be read without the right key
A system that has never yet had a break-in reported

Its behaviour completely, because every possible situation has been listed and none is left over
That some device sent these bits, and nothing whatever about who was at the keyboard
That whoever typed it knew it, which is a different thing from being the person it belongs to
That reading it needs something the reader must also hold, rather than that nobody could ever obtain that
That no successful attempt has been noticed so far, which is a good deal weaker than a guarantee

Two that follow from not being able to enumerate

Select the TWO statements that follow from the set of cases being open once data leaves the machine.

A security measure changes what an attempt costs or requires, rather than making one impossible
Identity has to be established by something the message itself carries, because the network cannot supply it
A well configured system can be made completely secure against unauthorised access
Testing a network against every possible threat is difficult but achievable

Arguing about risk without exaggerating

Security questions are marked on precision, and the commonest answer is a list of measures with a sentence each saying that it keeps data safe. ⚠️ THAT SENTENCE IS DOING NO WORK, AND IT IS ALSO NOT TRUE. A shape that works on any measure you are asked about. ⭐⭐ SAY WHAT THE MEASURE ACTUALLY ESTABLISHES, IN ONE SENTENCE. A password accepted establishes that somebody knew the password. Encrypted data establishes that reading it requires the key. Be exact, because exactness is the mark. ⭐ THEN SAY WHAT IT LEAVES OPEN, AND THIS IS THE HALF MOST ANSWERS SKIP. Knowing the password is not being the person. Needing a key is not the same as nobody having one. THEN SAY WHAT AN ATTEMPT NOW COSTS. More time, more knowledge, more equipment, or a greater chance of being noticed. ⭐ THAT IS WHAT A MEASURE BUYS, AND SAYING SO IS MORE HONEST AND SCORES BETTER THAN CLAIMING PREVENTION. AND IF SEVERAL MEASURES ARE USED TOGETHER, SAY WHY. Each one closes something the others leave open, which is the actual argument for using more than one. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS THE WORD STOPS. Nothing stops anybody. Write what it requires of an attempt instead, and the sentence immediately becomes assessable. ⚠️ THE SECOND IS TREATING ABSENCE OF TROUBLE AS EVIDENCE OF SAFETY. ⭐ A SYSTEM WITH NO REPORTED BREAK-IN MAY BE WELL PROTECTED, OR MAY SIMPLY NOT HAVE NOTICED. THOSE ARE DIFFERENT AND AN ANSWER SHOULD NOT CONFUSE THEM. ⚠️ AND THE THIRD IS IMPORTING LOGIC'S CERTAINTY WHERE IT DOES NOT BELONG. You can finish checking a circuit. You cannot finish checking a network, and writing as though you could is the error this whole topic is arranged to prevent. One more thing, and it is a boundary rather than a technique. Your course asks what measures do and why they are needed. It does not ask how to defeat them, and an answer is not improved by describing how an attack would be carried out. Keep to what is established, what is left open, and what an attempt costs.

One message, four things it cannot prove

A message arrives at a server. It says it comes from a particular account, it carries a password that matches, and it asks for something to be changed. The server accepts it. Reasonable enough. Now look at what actually arrived. ⭐⭐ A SEQUENCE OF BITS. THAT IS ALL. EVERYTHING ELSE IS SOMETHING THE SERVER HAS INFERRED. ⚠️ IT CANNOT ESTABLISH WHO TYPED IT. The bits carry a password and no person. Somebody knew the password, and the machine has no way to distinguish the owner from anybody else who knew it. ⚠️ IT CANNOT ESTABLISH WHERE THE MESSAGE REALLY BEGAN. What it can see is where the message reached it from, which is not the same thing, and the difference is invisible from the receiving end. ⚠️ IT CANNOT ESTABLISH THAT NOBODY ELSE SAW IT ON THE WAY. A message crosses equipment belonging to people the sender has never heard of. Scrambling the contents so that a listener obtains something unusable changes what they GET; it does not change whether anybody was there. ⚠️ AND IT CANNOT ESTABLISH THAT THIS IS THE FIRST TIME THE MESSAGE HAS ARRIVED, unless something was built in to tell. ⭐ FOUR THINGS THE SERVER WOULD LIKE TO KNOW, AND NOT ONE OF THEM IS SETTLED BY THE FACT THAT THE MESSAGE TURNED UP. Now compare that with the logic half of this topic. A circuit with three inputs has eight possible situations, and after you have gone through all eight there is nothing left to wonder about. ⚠️⚠️ HERE THERE IS NO LIST TO FINISH. YOU CANNOT WRITE OUT EVERY PERSON WHO MIGHT HAVE OBTAINED THAT PASSWORD, OR EVERY MACHINE ON THE ROUTE, BECAUSE THOSE SETS ARE OPEN. ⭐⭐ SO WHAT DO YOU ACTUALLY DO? YOU ADD THINGS THAT MAKE A FALSE MESSAGE HARDER TO PRODUCE. Requiring something in addition to knowledge, so knowing alone is not enough. Limiting which connections are accepted in the first place, so fewer messages even reach the question. Keeping records, so an attempt that does succeed is more likely to be noticed afterwards. ⚠️ NONE OF THOSE MAKES A FALSE MESSAGE IMPOSSIBLE. EACH RAISES WHAT PRODUCING ONE COSTS, AND THAT IS THE HONEST DESCRIPTION OF WHAT SECURITY IS FOR.

Order what a login actually establishes

Put these six statements in order, from what is most certainly established to what is least.

A sequence of bits arrived at the server
Those bits contained a password matching the one held for that account
Whoever produced the message knew that password
That person had obtained access to the password somehow
That person is the owner of the account
Nobody else is able to produce the same message

Build the sentence that names the limit

This is the shape a security answer takes when it is precise. Assemble it.

The logic and networks run

Five questions on what can and cannot be established. Three lives.

Complete the networks and security facts

A table listing every possible combination of inputs to a logical operation, with the output for each, is a _____. Establishing that somebody is who they claim to be is _____. Turning data into a form that cannot be read without the right key is _____. A system that controls which network traffic is allowed to pass between networks is a _____.

truth table authentication encryption firewall exhaustive checking a protocol a topology malware

What a checked case cannot cover

A student has worked through every input situation for a circuit and found it behaves as intended. They conclude the finished device will work correctly. Where does that reasoning overreach?

The check covers the logic completely and says nothing about the physical device: a wire could be connected wrongly, or a component could fail, and no truth table would reveal it
They should have checked the input situations more than once
Working through every situation does not really settle the behaviour
There are more input situations than they realised

Three answers to make exact

Three student answers on security. Each contains something true, and each is one clause away from the marks.

A student writes that encrypting data stops anybody from reading it. What needs correcting?
A student argues that a company with no history of break-ins clearly has effective security. What is wrong with the inference?
A student explains that a system uses several security measures together, and lists them without saying why more than one is used. What would you add?

Explain where certainty ends

Explain why the behaviour of a logic circuit can be established completely, why the same is not possible for data crossing a network, and what security measures are actually for as a result.

Explain what makes the set of cases for a logical operation complete, and what that allows you to conclude
Explain why the set of cases stops being complete once data leaves the machine, giving at least two things that cannot be listed
Explain why a network cannot establish who sent a message, referring to what actually arrives
Take one security measure and say precisely what it establishes and what it leaves open
Explain what a measure buys, in terms of what an attempt now costs or requires, rather than what it prevents
Finish by explaining why several measures are used together, and avoid claiming that any of them makes a system completely secure

Logic, Networks and Security

Get the method right under pressure

What you'll cover