Transcript GIT 335 Lecture 6 - Arizona State University

GIT 335
Computer Systems Technology
Lecture 6
Communications
Dr. Thomas Schildgen, Professor
Arizona State University
Department of
Technology Management
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Lecture 6
Introduction to Information Technology
Content taken from the McGraw Hill Textbook Chapter 6
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Communications, Networks,
& Safeguards
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From the Analog to the Digital Age
• Analog signals use variation of a wave form to
send information
• In FM signals the Frequency is varied (Modulated)
• In AM signals the Amplitude is varied (Modulated)
• Radios send signals this way
• Light works this way
• Hearing and phones work this way
• Modems work this way
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From the Analog to the Digital Age
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From the Analog to the Digital Age
• Digital signals send data in terms of 1s and 0s
• The signal goes up and down abruptly because the only
values that matter are 0 and 1 and the timing of when they
occur
• Computers use digital signals
• Newspaper photographs are digital since they are made up
of little dots
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From the Analog to the Digital Age
• Since computers use digital signals but
phone lines use analog, modems must
translate from digital to analog, send the
signal along the phone line, then translate
back from analog to digital at the other end
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–
–
–
The process is called “modulation/demodulation”
Modulation means to translate from digital to analog
Demodulation means to translate from analog to digital
Modems have to do all this just to use standard analog
voice phone lines
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From the Analog to the Digital Age
• Tape recorders, voices, and musical
instruments are analog while CDs are
digital
• To burn a CD from a jam session, the
digital recording equipment must convert
from analog to digital
– The analog-to-digital converter samples the sound and
converts the height of the wave to a number
– Samples of the sound wave are taken at regular intervals
– about 44,100 times each second
– Because the digital samples are played back faster than
our ears can react, it sounds to us like a single
continuous sound wave
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From the Analog to the Digital Age
• Digital sampling is similar to showing
movies
– Movies show still pictures (frames)
– But they show them so fast that our eyes can’t react in
time
– So to us the series of still pictures look like continuous
motion
– Did you ever notice in movies when they show car tires
in motion they sometimes seem to move backwards?
• This is because the tires are moving at a rate that is
incompatible with the frame rate of the movie, so our
eyes think the tires are really moving backwards
when the car is actually moving forward!
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Networks
• A system of interconnected computers,
telephones, or other communications
devices that can communicate with one
another and share applications and data
• Before we had computer networks, people
used “sneakernet” to share data between
computers
– Person 1 saved their document to a floppy disk
– Then they walked over to person 2’s desk (wearing sneakers, of
course) and handed over the disk to person 2
– Person 2 loaded the disk into their computer to read and edit
the document
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Networks
• Since the days of “sneakernet”, networks
have become standard. They enable us to:
–
–
–
–
Share peripheral devices such as laser printers
Share programs and data
Use e-mail and other communication programs
Backup critical information because it is stored
centrally
– Access shared databases
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Networks
•
WAN – Wide Area Network
•
•
•
•
MAN – Metropolitan Area
Network
LAN – Local Area Network
•
HAN – Home Area Network
•
PAN – Personal Area Network
•
•
•
Covers a wide geographic area,
such as a country or the world
Covers a city or a suburb
Connects computers and
devices in a limited geographic
area such as an office, building,
or group of nearby buildings
Uses wired, cable, or wireless
connections to link a
household’s digital devices
Uses short-range wireless
technology to connect an
individual’s personal
electronics like cellphone, PDA,
MP3 player, notebook PC, and
printer
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Networks
• Client/Server
– Consists of clients, which are computers that request
data, and servers, which are computers that supply data
– File servers act like a network-based shared disk drive
– Database servers store data but don’t store programs
– Print servers connect one or more printers and schedule
and control print jobs
– Mail servers manage email
• Peer-to-Peer
– All computers on the network communicate directly with
each other without relying on a server
– For fewer than 25 PCs
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Networks
•
•
Peer-to-Peer (continued)
– Cheaper than client/server since servers are typically more
expensive than PCs
– There are often problems with knowing who has the current
version of documents and files
– Too slow for use in larger offices
Legal considerations
– Downloading copyrighted material without paying violates U.S.
copyright laws
– Server-based online file sharing sites such as Napster have
been shut down
– Peer-to-Peer file-sharing sites have been more difficult to
control since there is no central server to shut down
– So publishers are suing individual downloaders instead
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Networks
Intranets, Extranets, VPNs
• Intranets
– An organization’s private network that uses the
infrastructure and standards of the internet and
the web
• Extranets
– Private internets that connect not only internal
personnel but also selected suppliers and other
strategic parties
• Virtual Private Networks
– Private networks that use a public network,
usually the internet, to connect remote sites
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Components of Networks
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Network
Components
• Connections
– Wired – twisted-pair, coaxial cable, or fiber-optic
– Wireless – infrared, microwave (Bluetooth), broadcast
(Wi-Fi) or satellite
• Hosts & Nodes
– Host: the central computer that controls the network
– Node: a device that is attached to the network
• Packets
– The format for sending electronic messages
– A fixed-length block of data for transmission
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Network
Components
• Protocols
– The set of conventions governing the exchange of
data between hardware and/or software components
in a communications network
– Built into the hardware or software you are using
– Govern the packet design and transmission standards
– Examples are:
•
•
•
•
•
TCP/IP for LANs and internet
AppleTalk for older Mac networks
SIP for Voice over IP (VoIP)
CDMA for cellphones
IPX for older Novell networks
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Network
Packets
• TCP/IP Packets carry four types of information
– Sender’s address (source IP number)
– Address of intended recipient (destination IP number)
– Number of packets the original data was broken into
• This happens because the amount of data the PC is sending
can be much larger than the space in a single packet
• So the data has to get broken up in one or more packets
• Then the packets have to be assigned a number like 1 of 6,
2 of 6, 3 of 6, 4 of 6, 5 of 6, and 6 of 6
– Packet number and sequence info for each packet
• Packets may arrive out of order (1, 6, 3, 2, 5, 4 for example)
• This information is used to resequence the packets and put
them back in the correct order (1, 2, 3, 4, 5, 6) so they can
be read
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Network
Devices
•
Daisy
Chain
• Hub
•
•
Switch
•
•
•
•
Bridge
Gateway
Router
•
•
•
•
Backbone
•
•
Used in peer-to-peer networks – direct
connections from one PC to the next
Used in small LANs to connect PCs and LAN
segments to each other. Forwards to all ports
Used in larger, busy LANs – faster than hubs
because it forwards only to correct destination
Used to connect two networks of the same type
Connects two networks of different types
Connects multiple LANs together. Routers are
the internet backbone
The main route that connects all the gateways,
routers, and other computers in an organization
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Network
Topologies
• Bus – all nodes are connected to a single wire or cable
• Ring – all nodes are connected in a continuous loop
• Star – all nodes are connected through a central host
Ring
Star
Bus
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Network
Packet Collision Schemes
• Collisions happen when two data packets are
going opposite directions on shared media
• Ethernet – deals with collisions
– All devices send data at once
– Collisions happen regularly
– Data is simply resent until it arrives
• Token ring – avoids collisions
– Devices take turns sending data
– Token is sent around the ring
– Wait to get the token, then send data
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Wired Communications Media
• Communications media carry signals over a
communications path
– Twisted-Pair Wire
• 2 strands of insulated copper wire twisted around each
other
• Twisting reduces interference (crosstalk) from electrical
signals
• Data rates are 1 – 128 Megabits per second
– Coaxial Cable
• Insulated copper wire wrapped in a metal shield and then
in an external plastic cover
• Used for cable TV and cable internet electric signals
• Carries voice and data up to 200 megabits per second
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Wired Communications Media
• Communications media continued
– Fiber-optic cable
• Dozens or hundreds of thin strands of glass
or plastic that transmit beams of light, not
electricity
• Can transmit up to 2 gigabits per second
• More expensive than twisted-pair or coax
• Lighter and more durable than twisted-pair
or coax
• More difficult to tap into than twisted-pair or
coax
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Wired Communications Media
for the home
• Ethernet
– Pull Cat5 cables through the house (yourself or
contractor)
– Connect to PC’s Ethernet network interface card (NIC)
– For several PCs, get a hub or switch to connect them all
– 10 or 100 megabits per second
• HomePNA
– Uses existing telephone wiring and jacks
– Requires HomePNA NIC in your PC
– Speeds of about 320 megabits per second
• Homeplug
– Uses existing home electrical lines
– Speeds of 200 megabits per second
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Wireless Communications Media
•
Electromagnetic spectrum of radiation is the basis of all
telecommunications signals
– Includes the longest radio waves (9 kHz) and audio waves
(sound), up through gamma rays that come from nuclear decay
(thousands of gigahertz)
– Radio-frequency spectrum is the part of the electromagnetic
spectrum that we use for radio communication
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Wireless Communications Media
Bandwidth
• Narrowband (or Voiceband)
– Used for regular telephone communications
– Transmission rate 1.5 megabits per second or less
• Medium Band
– Used for long-distance data transmission or to connect
mainframe and midrange computers
– Transmission rate 100 kb to 1 megabit per second
• Broadband
– For high-speed data and high-quality audio and video
– Transmission rate 1.5 megabits per second to 1 gigabit per
second or more
• US households get 4 – 5 MB while Japanese get 63 – 64 MB
per second
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Wireless Communications Media
• Infrared Transmission
– Sends signals using infrared light
– Frequencies are too low to see (1-16 Mbits per
second)
• Broadcast Radio
– AM/FM, CB, ham, cellphones, police radio
– Sends data over long distances using a
transmitter and a receiver
• Cellular Radio
– Form of broadcast radio
– Widely used in cell phones and wireless
modems
– Does not interfere with other cellular phone
calls
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Wireless Communications
Media
• Microwave Radio
– Superhigh-frequency radio waves (2.4 gigahertz
or higher)
– Requires line-of-sight transmitters and
receivers
• Communications Satellites
– “sky stations”
– Microwave relay stations in orbit around the
earth
– Launch costs between $50 and $400 million
– Basis for Global Positioning Systems (GPS)
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Wireless Communications Media
• Communications Satellites
– These are microwave relay stations in orbit around the
earth
•
Uplinking: transmitting a signal from ground station to a satellite
– Cover broad service area
– Cost $300 million to $700 million each + launch costs
– Can be placed at different heights: GEO, MEO, LEO
• GEO – geostationary earth orbit
– 22,300 miles up above earth
– Always above equator
• MEO – medium-earth orbit
– 5,000 – 10,000 miles up
• LEO – low-earth orbit
– 200 – 1,000 miles up
– Has less signal delay than GEO, MEO satellites
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Wireless Communications Media
GPS
• Global Positioning System
– 24 to 32 earth-orbiting satellites continuously
transmitting timed radio signals
– Each satellite circles earth twice each day at 11,000
miles up
– GPS receivers pick up transmissions from up to 4
satellites and pinpoint the receiver’s location
– Accurate within 3 – 50 feet, with a norm of 10 feet
accuracy
– GPS receivers contain map files that are displayed
based on the GPS position to guide users
– Many GPS receivers have speech chips too
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Wireless Communications Media
One-way Pagers
• One-way pagers are radio receivers that
receive data sent from a special radio
transmitter
– Radio transmitter sends out signals over the
special frequency
– Pagers are tuned to that frequency
– When a particular pager hears its own code, it
receives and displays the message
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Wireless Communications Media
Long-Distance Wireless
• Two-way pagers: Blackberry and Treo
• 1G: First Generation Cellular
– Analog cellphones
– Designed for voice communication using a system of
hexagonal ground-area cells around transmitter-receiver
cell towers
– Good for voice – less effective for data due to handing
off
• 2G: Second Generation Cellular
– Use same network of cell towers to send voice and data
in digital form over the airwaves
– Required digital receivers on original analog cell towers
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Wireless Communications Media
Long-Distance Wireless
• 3G: Third Generation Cellular
– Broadband technology
– Based in the U.S.
– Carry data at high speeds
• 4G: Fourth Generation Cellular
– A nationwide 4G network in development
– Uses the WiMax (Worldwide Interoperability for
Microwave Access) standard
– Hope to provide broadband to rural areas
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Wireless Communications Media
2G Wireless
• There are two competing, incompatible
standards
– CDMA – Code Division Multiple Access
• Transmission rates 14.4 kilobits per second
• Used by Verizon and Sprint
– GSM – Global System for Mobile Communications
• Transmission rates of 9.6 kilobits per second
• Used by Cingular and T-Mobile, as well as Western
Europe, Middle East and Asia
• US GSM and European GSM use different frequencies
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Wireless Communications Media
2.5G Wireless
• Data speeds of 300–100 kilobits per
second
– GPRS – General Packet Radio Service
• An upgrade to 2.5G
• Speeds of 30 – 50 kilobits per second
– EDGE is Enhanced Data for Global Evolution
• A different 2.5G upgrade
• Speeds of up to 236 kilobits per second
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Wireless Communications Media
3G Wireless
• Third generation wireless
– High speed data: 144 kilobits per second up to 2
megabits per second
– Accept e-mail with attachments
– Display color video and still pictures
– Play music
– Two important upgrades:
• EV-DO – Evolution Data Only
– Average speeds of 400 – 700 kilobits per second, peaks of 2
megabits per second
• UMTS – Universal Mobile Telecommunications System
– Average speed of 220 – 320 kilobits per second
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Wireless Communications Media
4G Wireless
• Fourth generation wireless
– The latest standard
– Uses WiMax (Worldwide Interoperability for
Microwave Access) standard
– Transmission at a range of 6-10 miles
– Transmission speed up to 20 mbits/sec
– Transmits farther than Wi-Fi
– Two varieties: one fixed/one mobile
– Hoped to provide broadband in rural areas
– Several communications firms developing a
nationwide
4G network
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Wireless Communications Media
Short-range Wireless
• Local Area Networks
– Range 100 – 228 feet
– Include Wi-Fi (802.11) type networks
• Personal Area Networks
– Range 30 – 32 feet
– Use Bluetooth, ultra wideband, and wireless
USB
• Home Automation networks
– Range 100 – 150 feet
– Use Instead, ZigBee, and Z-Wave standards
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Wireless Communications Media
Short-range Wireless
• Wi-Fi (802.11) networks
– Wi-Fi b, a, and g correspond to 802.11b, 802.11a, and
802.11g
– 802.11 is an IEEE wireless technical specification
– 802.11b is older, transmits 11 megabits per second
– 802.11a is faster than b but with weaker security than g
– 802.11g is 54 megabits per second and transmits 50 ft
– Wi-Fi n with MIMO extends range of Wi-Fi using multiple
transmitting and receiving antennas – 200 megabits per
second for up to 150 ft
• Warning! Security is disabled by default on Wi-Fi
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Wireless Communications Media
Short-range Wireless
• Wi-Fi Security
– Why is it disabled by default?
• So non-technical users can get Wi-Fi working more
easily
– Why should this bother me?
• A person with a $50 antenna can eavesdrop on
everything your computer sends over wireless from a
block or two away
• This is called “wardriving”
– To read more about this problem, follow this link
• http://www.webopedia.com/TERM/W/wardriving.html
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Wireless Communications Media
Personal Area Wireless
• Bluetooth
– Short-range wireless standard to link cellphones, PDAs,
computers, and peripherals at distances up to 30 ft
– Named after King Harald Bluetooth, the Viking who
unified Denmark and Norway
– Transmits up to 3 megabits per second
– When Bluetooth devices come into range of each other,
they negotiate. If they have information to exchange, they
form a temporary wireless network
– Bluetooth can also be used to eavesdrop on networks
– Turn it off on your cellphone unless you need it at that
time
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Wireless Communications Media
Personal Area Wireless
• Ultra Wideband (UWB)
– Developed for military radar systems
– Operates in 480 megabit per second range up to 30 ft
– Uses a low power source to send out millions of bursts
of radio waves each second
– 100 times as fast as Bluetooth
• Wireless USB
– USB is the most used interface on PCs
– The wireless version could be a hit
– Range of 32 ft and maximum data rate of > 480 megabits
per second
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Wireless Communications Media
Short-Range Wireless for Home
• Insteon
– Combines electronic power line and wireless technology
– Can send data at 13.1 kilobits per second with 150 ft range
– Replaces X10
• ZigBee
– Entirely wireless very power-efficient technology
– Can send data at 128 kilobits per second with 250 ft range
• Z-Wave
– Entirely wireless power-efficient technology
– Can send data at 127 kilobits per second to range of 100 ft
– Allows you to remotely program your house!
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Cyberthreats, Hackers, &
Safeguards
• Problem: internet was begun to foster
collaboration among universities and scientists.
They trusted each other. No security was built into
the internet.
• Problem: the internet is open-access and is used
by some people who are not trustworthy, who take
advantage of the lack of built-in safeguards.
• Problem: Most people connect to the internet and
use their computers in LANs. All it takes is one
computer on a LAN that has been compromised
for all computers on it to be vulnerable.
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Cyberthreats, Hackers, &
Safeguards
• Denial of Service Attacks
– Consist of making repeated requests of a computer or
network device, thereby overloading it and causing
legitimate requests to be ignored
– Used to target particular companies or individuals
• Worms
– A program that copies itself repeatedly into a computer’s
memory or disk drive
– May copy itself so much it crashes the infected computer
– Famous worms include: Code Red, Nimda, Klez, Sasser,
Bagle
– Primarily target PCs running Microsoft Windows
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Cyberthreats, Hackers, &
Safeguards
• Viruses
– A deviant program that hides on a floppy, hard drive, CD,
or e-mail that causes unexpected side effects such as
destroying or corrupting data
– Viruses self-replicate and try to secretly distribute
themselves to other systems
– Famous viruses include the “I Love You” virus
– Viruses are published at the rate of about one per day
– To see what the latest ones are, go to
• http://www.symantec.com/avcenter/vinfodb.html
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Cyberthreats, Hackers, &
Safeguards
• Trojan Horses
– Programs that pretend to be a useful program
such as a free game or screensaver
– Carry viruses or malicious instructions that
damage your computer or install a backdoor or
spyware
– Backdoors and spyware allow others to access
your computer without your knowledge
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Cyberthreats, Hackers, &
Safeguards
• How they spread
–
–
–
–
–
–
Via e-mail attachments
By infected floppies or CDs
By clicking on infiltrated websites
By downloading from infected files from websites
Through infiltrated Wi-Fi hotspots
From one infected PC on a LAN to another
• What can you do about it?
– Install anti-virus software and subscribe to the automatic
anti-virus update service
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Cyberthreats, Hackers, &
Safeguards
•
•
Hackers are either
– Computer enthusiasts, people who enjoy learning about
programming and computers (good)
– People who gain unauthorized access to computers or
networks, often for fun or to see if they can (not good)
Crackers
– Malicious hackers who break into computers for malicious
purposes
• Script kiddies are technically unsophisticated teenagers who use
downloadable software for perform break-ins
• Hacktivists are hacker activists who break into systems for a
political purpose
• Black-hat hackers are those who break into computers to steal or
destroy information or to use it for illegal profit
• Cyberterrorists attack computer systems so as to bring physical or
financial harm to groups, companies, or nations
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Cyberthreats, Hackers, &
Safeguards
• Safeguards
– Use antivirus software, and keep it current
– Install a firewall to monitor network traffic and filter out
undesirable types of traffic and undesirable sites
– Use robust passwords:
• Minimum 8 characters with letters, numbers, characters
• 4cats is not a good password, but f0UrK@tTz is safer
– Install antispyware software
– Encrypt financial and personal records so only you can read
them
– Back up your data, so if your PC is attacked and must be
reformatted, you can restore your data
– Never download from a website you don’t trust
– Consider Biometric authentication
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Cyberthreats, Hackers, &
Safeguards
• Encryption
– The process of altering readable data into unreadable
form to prevent unauthorized access
• Uses powerful mathematical ciphers to create coded
messages that are difficult to break
– Two forms:
• Private Key encryption means the same secret key is
used by both the sender and receiver to encrypt and
decrypt a message
• Public Key encryption means that two keys are used
– The public key of the recipient is published and is
used by the sender to encrypt the message
– The private key of the recipient is secret and is the
only way to decrypt the message
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