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  • Hi, I'm Carrie Anne, and welcome to CrashCourse Computer Science!

  • Over the last three episodes, we've talked about how computers have become interconnected,

  • allowing us to communicate near-instantly across the globe.

  • But, not everyone who uses these networks is going to play by the rules, or have our

  • best interests at heart.

  • Just as how we have physical security like locks, fences and police officers to minimize

  • crime in the real world, we need cybersecurity to minimize crime and harm in the virtual

  • world.

  • Computers don't have ethics.

  • Give them a formally specified problem and they'll happily pump out an answer at lightning

  • speed.

  • Running code that takes down a hospital's computer systems until a ransom is paid is

  • no different to a computer than code that keeps a patient's heart beating.

  • Like the Force, computers can be pulled to the light side or the dark side.

  • Cybersecurity is like the Jedi Order, trying to bring peace and justice to the cyber-verse.

  • INTRO

  • The scope of cybersecurity evolves as fast as the capabilities of computing, but we can

  • think of it as a set of techniques to protect the secrecy, integrity and availability of

  • computer systems and data against threats.

  • Let's unpack those three goals:

  • Secrecy, or confidentiality, means that only authorized people should be able to access

  • or read specific computer systems and data.

  • Data breaches, where hackers reveal people's credit card information, is an attack on secrecy.

  • Integrity means that only authorized people should have the ability to use or modify systems

  • and data.

  • Hackers who learn your password and send e-mails masquerading as you, is an integrity attack.

  • And availability means that authorized people should always have access to their systems

  • and data.

  • Think of Denial of Service Attacks, where hackers overload a website with fake requests

  • to make it slow or unreachable for others.

  • That's attacking the service's availability.

  • To achieve these three general goals, security experts start with a specification of who

  • yourenemyis, at an abstract level, called a threat model.

  • This profiles attackers: their capabilities, goals, and probable means of attackwhat's

  • called, awesomely enough, an attack vector.

  • Threat models let you prepare against specific threats, rather than being overwhelmed by

  • all the ways hackers could get to your systems and data.

  • And there are many, many ways.

  • Let's say you want tosecurephysical access to your laptop.

  • Your threat model is a nosy roommate.

  • To preserve the secrecy, integrity and availability of your laptop, you could keep it hidden in

  • your dirty laundry hamper.

  • But, if your threat model is a mischievous younger sibling who knows your hiding spots,

  • then you'll need to do more: maybe lock it in a safe.

  • In other words, how a system is secured depends heavily on who it's being secured against.

  • Of course, threat models are typically a bit more formally defined than justnosy roommate”.

  • Often you'll see threat models specified in terms of technical capabilities.

  • For example, “someone who has physical access to your laptop along with unlimited time”.

  • With a given threat model, security architects need to come up with a solution that keeps

  • a system secureas long as certain assumptions are met, like no one reveals their password

  • to the attacker.

  • There are many methods for protecting computer systems, networks and data.

  • A lot of security boils down to two questions: who are you, and what should you have access to?

  • Clearly, access should be given to the right people, but refused to the wrong people.

  • Like, bank employees should be able to open ATMs to restock them, but not mebecause

  • I'd take it all... all of it!

  • That ceramic cat collection doesn't buy itself!

  • So, to differentiate between right and wrong people, we use authentication - the process

  • by which a computer understands who it's interacting with.

  • Generally, there are three types, each with their own pros and cons:

  • What you know.

  • What you have.

  • And what you are.

  • What you know authentication is based on knowledge of a secret that should be known only by the

  • real user and the computer, for example, a username and password.

  • This is the most widely used today because it's the easiest to implement.

  • But, it can be compromised if hackers guess or otherwise come to know your secret.

  • Some passwords are easy for humans to figure out, like 12356 or q-w-e-r-t-y.

  • But, there are also ones that are easy for computers.

  • Consider the PIN: 2580.

  • This seems pretty difficult to guessand it isfor a human.

  • But there are only ten thousand possible combinations of 4-digit PINs.

  • A computer can try entering 0000, then try 0001, and then 0002, all the way up to 9999...

  • in a fraction of a second.

  • This is called a brute force attack, because it just tries everything.

  • There's nothing clever to the algorithm.

  • Some computer systems lock you out, or have you wait a little, after say three wrong attempts.

  • That's a common and reasonable strategy, and it does make it harder for less sophisticated

  • attackers.

  • But think about what happens if hackers have already taken over tens of thousands of computers,

  • forming a botnet.

  • Using all these computers, the same pin – 2580 – can be tried on many tens of thousands

  • of bank accounts simultaneously.

  • Even with just a single attempt per account, they'll very likely get into one or more

  • that just happen to use that PIN.

  • In fact, we've probably guessed the pin of someone watching this video!

  • Increasing the length of PINs and passwords can help, but even 8 digit PINs are pretty

  • easily cracked.

  • This is why so many websites now require you to use a mix of upper and lowercase letters,

  • special symbols, and so onit explodes the number of possible password combinations.

  • An 8-digit numerical PIN only has a hundred million combinationscomputers eat that

  • for breakfast!

  • But an 8-character password with all those funky things mixed in has more than 600 trillion

  • combinations.

  • Of course, these passwords are hard for us mere humans to remember, so a better approach

  • is for websites to let us pick something more memorable, like three words joined together:

  • green brothers rockorpizza tasty yum”.

  • English has around 100,000 words in use, so putting three together would give you roughly

  • 1 quadrillion possible passwords. Good luck trying to guess that!

  • I should also note here that using non-dictionary words is even better against more sophisticated

  • kinds of attacks, but we don't have time to get into that here.

  • Computerphile has a great video on choosing a password - link in the dooblydoo.

  • What you have authentication, on the other hand, is based on possession of a secret token

  • that only the real user has.

  • An example is a physical key and lock.

  • You can only unlock the door if you have the key.

  • This escapes this problem of beingguessable”.

  • And they typically require physical presence, so it's much harder for remote attackers

  • to gain access.

  • Someone in another country can't gain access to your front door in Florida without getting

  • to Florida first.

  • But, what you have authentication can be compromised if an attacker is physically close.

  • Keys can be copied, smartphones stolen, and locks picked.

  • Finally, what you are authentication is based on... you!

  • You authenticate by presenting yourself to the computer.

  • Biometric authenticators, like fingerprint readers and iris scanners are classic examples.

  • These can be very secure, but the best technologies are still quite expensive.

  • Furthermore, data from sensors varies over time.

  • What you know and what you have authentication have the nice property of being deterministic

  • either correct or incorrect.

  • If you know the secret, or have the key, you're granted access 100% of the time.

  • If you don't, you get access zero percent of the time.

  • Biometric authentication, however, is probabilistic.There's some chance the system won't recognize you

  • maybe you're wearing a hat or the lighting is bad.

  • Worse, there's some chance the system will recognize the wrong person as youlike

  • your evil twin!

  • Of course, in production systems, these chances are low, but not zero.

  • Another issue with biometric authentication is it can't be reset.

  • You only have so many fingers, so what happens if an attacker compromises your fingerprint data?

  • This could be a big problem for life.

  • And, recently, researchers showed it's possible to forge your iris just by capturing a photo

  • of you, so that's not promising either.

  • Basically, all forms of authentication have strengths and weaknesses, and all can be compromised

  • in one way or another.

  • So, security experts suggest using two or more forms of authentication for important

  • accounts.

  • This is known as two-factor or multi-factor authentication.

  • An attacker may be able to guess your password or steal your phone: but it's much harder

  • to do both.

  • After authentication comes Access Control.

  • Once a system knows who you are, it needs to know what you should be able to access,

  • and for that there's a specification of who should be able to see, modify and use what.

  • This is done through Permissions or Access Control Lists (ACL), which describe what access

  • each user has for every file, folder and program on a computer.

  • Readpermission allows a user to see the contents of a file, “writepermission

  • allows a user to modify the contents, andexecutepermission allows a user to

  • run a file, like a program.

  • For organizations with users at different levels of access privilegelike a spy

  • agencyit's especially important for Access Control Lists to be configured correctly

  • to ensure secrecy, integrity and availability.

  • Let's say we have three levels of access: public, secret and top secret.

  • The first general rule of thumb is that people shouldn't be able toread up”.

  • If a user is only cleared to read secret files, they shouldn't be able to read top secret

  • files, but should be able to access secret and public ones.

  • The second general rule of thumb is that people shouldn't be able towrite down”.

  • If a member has top secret clearance, then they should be able to write or modify top

  • secret files, but not secret or public files.

  • It may seem weird that even with the highest clearance, you can't modify less secret files.

  • But, it guarantees that there's no accidental leakage of top secret information into secret

  • or public files.

  • Thisno read up, no write downapproach is called the Bell-LaPadula model.

  • It was formulated for the U.S. Department of Defense's Multi-Level Security policy.

  • There are many other models for access controllike the Chinese Wall model and Biba model.

  • Which model is best depends on your use-case.

  • Authentication and access control help a computer determine who you are and what you should

  • access, but depend on being able to trust the hardware and software that run the authentication

  • and access control programs.

  • That's a big dependence.

  • If an attacker installs malicious softwarecalled malwarecompromising the host

  • computer's operating system, how can we be sure security programs don't have a backdoor

  • that let attackers in?

  • The short answer iswe can't.

  • We still have no way to guarantee the security of a program or computing system.

  • That's because even while security software might besecurein theory, implementation

  • bugs can still result in vulnerabilities.

  • But, we do have techniques to reduce the likelihood of bugs, quickly find and patch bugs when

  • they do occur, and mitigate damage when a program is compromised.

  • Most security errors come from implementation error.

  • To reduce implementation error, reduce implementation.

  • One of the holy grails of system level security is a “security kernelor a “trusted

  • computing base”: a minimal set of operating system software that's close to provably secure.

  • A challenge in constructing these security kernels is deciding what should go into it.

  • Remember, the less code, the better!

  • Even after minimizing code bloat, it would be great toguaranteethat code as written

  • is secure.

  • Formally verifying the security of code is an active area of research.

  • The best we have right now is a process called Independent Verification and Validation.

  • This works by having code audited by a crowd of security-minded developers.

  • This is why security code is almost always open-sourced.

  • It's often difficult for people who wrote the original code to find bugs, but external

  • developers, with fresh eyes and different expertise, can spot problems.

  • There are also conferences where like-minded hackers and security experts can mingle and

  • share ideas, the biggest of which is DEF CON, held annually in Las Vegas.

  • Finally, even after reducing code and auditing it, clever attackers are bound to find tricks

  • that let them in.

  • With this in mind, good developers should take the approach that, not if, but when their

  • programs are compromised, the damage should be limited and contained, and not let it compromise

  • other things running on the computer.

  • This principle is called isolation.

  • To achieve isolation, we cansandboxapplications.

  • This is like placing an angry kid in a sandbox; when the kid goes ballistic, they only destroy

  • the sandcastle in their own box, but other kids in the playground continue having fun.

  • Operating Systems attempt to sandbox applications by giving each their own block of memory that

  • others programs can't touch.

  • It's also possible for a single computer to run multiple Virtual Machines, essentially

  • simulated computers, that each live in their own sandbox.

  • If a program goes awry, worst case is that it crashes or compromises only the virtual

  • machine on which it's running.

  • All other Virtual Machines running on the computer are isolated and unaffected.

  • Ok, that's a broad overview of some key computer security topics.

  • And I didn't even get to network security, like firewalls.

  • Next episode, we'll discuss some specific example methods hackers use to get into computer

  • systems.

  • After that, we'll touch on encryption.

  • Until then, make your passwords stronger, turn on 2-factor authentication, and NEVER

  • click links in unsolicited emails!

  • I'll see you next week.

Hi, I'm Carrie Anne, and welcome to CrashCourse Computer Science!

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サイバーセキュリティ。クラッシュコース コンピュータサイエンス #31 (Cybersecurity: Crash Course Computer Science #31)

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