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Think of an idea for an on-demand company you might launch.

It is hard for a traditional company with employees to compete with the on-demand model.

The on-demand economy just helps the rich get richer and the poor get poorer.

On-demand workers know what they're signing up for, so they shouldn't be suing these companies for not getting benefits.

An example of how, even though drivers aren't employees, Uber monitors them like employees:

A longer article, but with lots of information

Examples of

Research the on-demand economy. Here are some resources to get you started.

existed before the Internet, but technology has helped grow an ''on-demand economy.'' Many on-demand companies do not have regular employees, but hire independent contractors. This gives workers great flexibility about when they work, but costs them benefits like sick pay, health insurance, collective bargaining, job stability, or unemployment insurance if they are fired.

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Do some research about modern encryption systems such as the RSA cryptosystem, which is used to do secure transactions on the Internet

Read "The Key Agreement Protocol" and "Public Keys for Private Messages" (Blown to Bits pages 181-183) for more details on Public Key Encryption.

But this just pushes the problem back a layer. How does the Certificate Authorities know that you are who you say you are? The problem is a little bit like how your bank knows that you who you say you are when you call them on the phone. Namely, they ask you questions for which they hope only you know the answer.

Certificate authorities issue digital certificates that verify who owns the encryption keys used for secured communications.

Public key cryptography doesn't solve all the problems, because an eavesdropper (say, Eve) might publish a fake public key pretending to be Alice. Then Bob might accidentally encrypt their message for Alice using the Eve's fake key, and then the Eve can read the message. In practice, this is partly fixed by relying on trusted third parties, called Certificate Authorities, to certify public keys. In your browser's security options you can see all of the Certificate Authorities that it trusts.

Certificate Authorities

Open standards help security... In order to work properly, a cryptographic function has to be easy for the private key holder to invert, but hard for anyone else to invert. How do we know what "hard" means? For example, current cryptographic methods rely on the difficulty of finding prime factors of very large numbers. There's no proof that someone won't come up with a fast way to do that, but people are pretty confident about it because the problem has been well studied by many mathematicians. (On the other hand, when quantum computers become practical, factorization will be easy, and new cryptographic methods will be needed.) What makes it possible for mathematicians to study the difficulty of breaking Internet cryptography is that the method used—the cryptographic function—is openly published. This may seem strange; if you want to keep secrets, shouldn't you keep the technique secret, too? But secret algorithms can have weaknesses that go undiscovered until some bad guy exploits them. Open standards allow an algorithm to be studied before it is used in practice.

If your connection blocks YouTube, watch the video here, but start it at 4:40.

SSL/TLS (secure sockets layer/transport layer security) is the standard used for cryptographically secured information transfer on the Internet.

Secure HTTP connections (those that use https:// instead of http://) use a protocol called Transport Layer Security (TLS) or maybe an older version called Secure Sockets Layer (SSL). Both are based on public key cryptography. With SSL/TLS, the site you are visiting sends its public key, and your browser uses it to encrypt the information you send.

Secure HTTP

It's also possible to use the private key for encryption and the public key for decryption... That's no good for secret messages (why not?), but it's useful for digital signatures. I use my private key to encrypt a message; you use my public key to decrypt it. If you get a meaningful message as the result, that proves that the message was encrypted with my private key. (If I want both secrecy and digital signing, I encrypt the message with my private key to sign it, then encrypt the encrypted result again with your public key to keep it secret. You decrypt it twice, first with your private key and then with my public key.) This is a nice example of composition of functions: the output from the first encryption is the input to the second encryption.

With a partner, discuss how this method is different from symmetric cryptography described on previous pages. Would you trust this method to work to send a credit card number?

Here is a model of public key encryption (from wikimedia.org)

It may seem incredible that Alice can make her encryption key public and still no one except her can decrypt her message. The public key method relies on some mathematics and on some limitations on the speed of current computers. Read "Secrecy Changes Forever" (Blown to Bits pages 178-181) to understand some of how this works.

If your connection blocks YouTube, watch the video here, but start it at 2:25.

The public key idea was invented and first published by Whitfield Diffie and Martin Hellman in 1976. It turns out that it had been invented earlier but kept secret by governments.

Public key cryptography is a mathematical technique to avoid the need to communicate a secret key from one person to another. Instead, each person has two keys: a private key known only to that person and a public key that everyone in the world is allowed to know. If Bob wants to send Alice a secret message, he encrypts it with Alice's public key. Then no one but Alice can decrypt it. Only her private key can undo the encryption, and no one can figure out the private key from the public key.

The fundamental problem that cryptography is trying to solve is how to get a message to your friend that can't be intercepted by your enemies. Symmetric encryption has a fundamental weakness: the encryption key is itself a message that needs to be send to your friend but not intercepted by your enemy.

you will learn about a commonly used method of cryptography that is more secure.

Create your own encryption/decryption scheme and implement it in Snap!. Terug

Implement a version of the Caesar Cipher that not only shifts the characters but also wraps them round the alphabet when the end of the alphabet is reached. You may wish to restrict your alphabet to the set of printable characters given above in the Unicode table.

If you came across a long message encrypted in the Caesar Cipher but did not know the shift value, what are some ways you might be able to break the system and decrypt the message? Discuss the weaknesses of a Caesar Cipher and how it is prone to breaking.

On paper, use a shift cipher to encrypt and decrypt a short message to get a feel for how this cipher works.

In this project you will develop a program that uses a shift cipher that does not wrap around, but instead uses other characters like [ and { to follow Z and z.

A Caesar cipher (or shift cipher) is a simple encryption method. Each letter in what's called plaintext (the un-encrypted text) shifts some fixed number of positions along the alphabet. After Z, the shifting "wraps around" and goes back to A. For example, “ABCZ123abcz” shifted by 4 would become “EFGD567efgd”. This technique is named after Gaius Julius Caesar, who ruled Rome 49-44 BC and used encryption in his correspondence.

you will work with the Caesar Cipher.

Do some research on other types of ciphers used historically. Especially read about the Vigenere Cipher which was used extensively in communicating sensitive information during World War 2.

If you copy your encrypted message with a method other than by using copy and paste (for example, by writing it by hand or typing it into a phone), some characters may disappear from your message. This is because some of the Unicode characters after 126 are printing characters that symbolize things like "delete." These characters won't get displayed in Snap!, so you can't copy them by hand, but if you use copy and paste, Snap! knows they are there. In Take It Further exercise A, you can develop a method of encryption that avoids this problem.

I'm missing some letters. Where did they go?

Now test your work. Agree with your partner on a shift value for the encryption. Then use your program to encrypt a secret message and e-mail it to your partner. Then let your partner decrypt your message by using the program to reverse the shift.

You can extract the encrypted messages from the Snap! interface by right-clicking on the variable that holds the encrypted message and selecting the “Export” option which will download a text file to your computer which then you can copy/paste.

Try to code the shift cipher on your own in Snap! using the algorithm you have developed. If you get stuck, look at this page for hints on how to proceed.

Develop an algorithm for this procedure that works for any input text and any shift value.

Why do we see characters like

The unicode as letter block reports the character that a given Unicode number represents:

The unicode of block reports the number that is used for a particular character:

Internally, computers store keyboard characters (capital and small letters, punctuation marks, space, digits, symbols, and so on) and others (like Enter, or Command-Z, or Shift-Ctrl A) as numbers—binary sequences. The computer industry standard numbering is called Unicode. For most purposes, even programmers and web developers don't need to know what number represents what character, but sometimes we do need to specify a character by its number. This table shows the Unicode for some of the keyboard characters

computers voor nodig.

You might have used a substitution cipher to encode your message, substituting each letter of the alphabet with some other letter. You could substitute letters in any order, like this

The 192.168 domain (the block of IP addresses that all start with 192.168) is reserved for local networks, but no computer on the Internet has an address in that range. Another such domain is 10.0.

Look up your current local IP address in your system preferences or settings. It's usually under network or Internet settings and may be listed with the computer device supporting that connection (wifi, Ethernet, wifi, Bluetooth, etc.).

Most likely, the router in your home or school uses a protocol that allows all the computers on the local network (such as in one building) to share a single IP address on the Internet, which can be cheaper. The router that creates the local network gives each computer a local address. For example, although the outside world may think someone's computer has IP address 108.26.181.226, that computer itself might think its address is 192.168.1.11.

Read

Is your IP address in IPv4 or IPv6?

Will this always be enough, or will we need to upgrade the addressing system again? There are an estimated 1029 stars in the observable universe. So even if the Internet is extended to include other planets or space aliens, we'll still have enough addresses with IPv6.

The long-term solution is to increase the length of an IP address. The new IP addresses are 128 bits wide, which is enough to support 2128 (about 1038) computers.

Why does IPv4 support 232 computers? There are 32 bits in an IPv4 address (see right), and each bit can be one of two possible values (0 or 1). So, there are 232 possibilities with thirty-two bits.

Each of the four numbers in a typical IP address today is an eight-bit byte with a value between 0 and 255 (see right). A 32-bit IPv4 (the "v" stands for "version") address is big enough to support 232 computers. That's about four billion (4 · 109), but there are more than seven billion people on Earth, so there aren't enough IP addresses to go around.

Some of the information might have slight inaccuracies. IP addresses often give the location of an Internet service provider, possibly at a different location.

The amount of detailed information available from an IP address is pretty amazing (and a little scary), especially when you think about the ways that information can be used.

You can add any IP address to the end of that URL like this:

Visit http://ipinfo.io/. What information does that page give you? (You don't need to enter your email address. Just click "See more details.")

Visit http://bot.whatismyipaddress.com/ for your current IP address.

Discuss how this shape is related to how people connect to the Internet (though an Internet Service Provider, etc.). Write out a brief description and/or explain it to someone else.

Why does the graph of the Internet look like a tangle in the middle with fireworks on the outsides?

An IP address is a unique number assigned to each device on a computer network. Packet switching means that the Internet sends short bursts of information, not long continuous strings.

Usually, there are many possible paths from one endpoint to another. This redundancy allows IP to find an alternate pathway if some system in the middle doesn't respond to requests by finding an alternate pathway. This is the principle of fault tolerance. When you send a data over the Internet, the IP program in your computer divides it into packets that it sends individually. (Each may take a different path.) This process is what makes the Internet a packet switching network.

Image from

Every device on the Internet has a unique Internet Protocol (IP) address (or more than one, if it's a router), like a postal or email address. The Internet Protocol specifies how a router handles a request for a different IP address. Each router knows the layout of its specific neighborhood of the Internet and knows which way to send each message to get it a little bit closer to where it's going. The fact that each router doesn't have to know the complete Internet improves scalability.

How Do Routers Know Where to Find the Computer You Want?

The end to end architecture of the Internet means that routers only know how to find an IP address; they don't do anything with the content of the message. Making sense of content is the job of the endpoint computers: the sender and the receiver.

Depending on where you live, you might pronounce "route" differently, but "router" is always pronounced the same: like "outer."

A router is a computer that passes information from one network to another.

The Internet isn't just a network of computers. It's a network of networks. The connection points between networks are called routers, networking devices that route traffic between subnetworks on the Internet. The routers only know how to pass information on to the next router or to the final destination; the routers do not analyze what's inside each packet of data (as long as you live in a country without Internet censorship). Making sense of the information happens at the destination computer. This is called the end to end principle.

you will learn how machine-readable computer addresses work.

you will learn about some of the abstractions that make the Internet work.

Read

Explain how each of these protocols is an abstraction. What details does each one hide? HTTP: HyperText Transfer Protocol—the protocol that your browser uses to access an HTML web page DNS: Domain Name System—the hierarchical addressing protocol that is human-readable TCP: Transmission Control Protocol—the protocol that assures reliable transmission of data IP: Internet Protocol—the hierarchical addressing protocol that manages routing of data between computers; we are upgrading from IPv4 to IPv6 for more addresses

ust think... Your T-Mobile cell phone can talk to your friend's Verizon phone. You can send email to someone in a country that's considered an enemy of your country (from the US to Iran, for example). An engineer at Microsoft can read a web page at Apple even though their companies are competitors. Before the Internet, there were several different network protocols that were secrets belonging to particular manufacturers. So if you had a particular brand of computer or router, it could talk only to other computers of the same brand.

These are all open standards: anyone can look up a protocol and code with it to make new hardware or software without permission. The Internet is probably the largest and most complicated artifact in human history, and it relies on cooperation. Despite some governments' attempts to censor the net, the big picture is one of strong cooperative spirit.

Open Protocols

You may connect to the Internet with an Ethernet cable or perhaps a WiFi radio antenna inside the case of your computer. Either connects computers to a local network router which then connects to an Internet provider. Cell phones use a longer-range cellular connection to a phone carrier. All four of these levels include more protocols than listed here.

Network Interface Hardware (Link Layer): All Internet devices connect through a physical interface that uses a protocol to manage the connection to the local network. These local protocols are the least abstract because they deal directly with your physical hardware.

Every device on the Internet needs an IP address so other devices can find it. IP (Internet Protocol) addresses are upgrading from IPv4 to IPv6. Routers use Internet layer protocols to detect and work around network congestion.

Internet Layer Protocols manage the pathways that the data packets travel across networks. These protocols treat the Internet like one large network even though the physical reality on the lower level is one of many tiny subnetworks.

TCP (Transmission Control Protocol) simulates a reliable, long-term connection between two computers by only displaying data once all packets have arrived. When speed is more important than accuracy, people use UDP (User Datagram Protocol), such as for real-time video streaming, where one missed packet doesn't matter much.

Transport Layer Protocols manage the breakdown of a message into packets to be transmitted by lower level protocols and also the reconstruction of the message from the packets upon arrival.

Browsers use HTTP (HyperText Transfer Protocol) to interpret HTML instructions for page formatting. DNS (Domain Name System) converts user-friendly web addresses into IP addresses. Your email inbox may use SMTP (Simple Mail Transfer Protocol) to send and IMAP (Internet Message Access Protocol) to read email.

Application Layer Protocols are the highest level of abstraction because they manage how data is interpreted and displayed to users. These protocols give meaning to the bits sent by lower-level protocols; user and server computers must agree on what the bits mean, and application protocols (like HTTP) offer this.

This hierarchy of abstractions manages the complexity of the Internet by hiding the details of lower levels of the system. The highest level of abstraction includes the most general features of the Internet that have to work the same across all devices. At lowers levels of abstraction, things get more device-specific.

Internet Abstraction Hierarchy

There are a lot of protocols! The Internet was designed with several layers of abstraction that sort the protocols according to what part of the process they support.

There are billions of devices connected to the Internet, and hundreds of different kinds of devices: laptops, tablets, phones, refrigerators, handheld credit card readers, and so on. How do they all know how to find and talk to each other? Protocols (standards) ensure that the variety of devices interact with each other smoothly.

you will learn about the communication standards used on the Internet and how they work together.

A Hierarchy of Open Protocols

To solve this problem, you'll need a way to keep track of the order of the data and a way to re-request missing packets: First, solve the problem of packets arriving out of order. You can include extra header information in addition to the packet data in order to help the receiver reconstruct the message. This will require cooperation by both sender and receiver (that is, changes to both gray blocks). Then, solve the problem of packets not arriving at all. That is, make the transmission reliable even though IP is unreliable. This, too, will require changing both sender and receiver.

Do not change the definition of . That block simulates the unreliable network. You could "solve" the problem by rewriting this block to simulate a perfect network instead of an imperfect one, but that misses the point.

Build a simple TCP. Resolve the unreliability so that messages are received reliably despite the limitations of IP packets. You'll need to change the definitions of:

Read Blown to Bits pages 306-309.

TCP works by including additional information along with each packet so that the receiving computer can keep track of how many packets it has received, re-request any missing packets, and reorder the packets to reconstruct the original message. In this simulation, a packet either arrives correctly (even if it's out of order) or it doesn't arrive at all. But on the Internet, it's possible for a packet to arrive with erroneous data, so the real TCP has to check for errors and request re-transmission of packets with errors too.

This project provides a simulation of unreliable data transmission by Internet Protocol. Click the green flag to initialize the incoming transmission variables before each experiment. Click either character to enter a message for it to send to the other one. In this simulation, the complete message is a string of text that is divided into packets of one letter each. In reality, the packet length is not so strictly limited and messages are usually much longer. Compare the result with what you sent. What problems do you see?

The TCP/IP end-to-end design of the Internet is an abstraction: The computers (including servers) at the two endpoints of a communication run the Transmission Control Protocol (TCP), which guarantees reliable transmission. The routers at every connection point on the Internet run the Internet Protocol (IP), which transmits packets from one IP address to another. The routers don't know anything about the messages they carry; all they care about is transmitting them. The computers that send and receive the messages are the only ones concerned with what the messages mean.

Reliable Transmission on Unreliable Networks: TCP

If your connection blocks YouTube, watch the video here.

TCP/IP is a pair of protocols that provide an abstraction. IP lets your computer pretend it has a direct connection to another computer. TCP lets your computer pretend it has a reliable connection to the other computer.

Computers, servers, and routers are fairly reliable, but every once in a while a packet will be lost, and devices on the Internet need to tolerate these faults. One way to tolerate faults is not to care. (If you lose one frame of video, it doesn't matter.) Another way (called TCP) is to keep sending packets until they are acknowledged as having been received correctly. TCP (Transmission Control Protocol) guarantees reliable data transmission by keeping track of which packets have been received successfully, resending any that have been lost or damaged, and specifying the order for reassembling the data on the other end.

you will learn the system for ensuring that communication is reliable on the Internet.

Unit 4 Lab 2: Communication Protocols, Page 2

Unit 4 Lab 2: Communication Protocols, Page 1

you will learn about the history of the Internet.

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