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![Page 1: 2. Key Technical Concepts. Topics Basic Computer Operation Bits & Bytes File Extensions & File Signatures How Computers Store Data RAM: Random Access.](https://reader035.fdocuments.us/reader035/viewer/2022062303/5517a73c5503463e368b5cd6/html5/thumbnails/1.jpg)
2. Key Technical Concepts
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Topics
• Basic Computer Operation• Bits & Bytes• File Extensions & File Signatures• How Computers Store Data• RAM: Random Access Memory• Volatility of Data
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Topics
• The Difference Between Computer Environments
• Active, Latent, and Archival Data• Allocated and Unallocated Space• Computer File Systems
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Bits & Bytes
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Bits & Bytes
• A Bit is 0 or 1• 8 bits is a byte
o 00000000 to 11111111o 256 possible bytesoCan be written as a number 0 to 255o In Hexadecimal, 00 to FF
• Binary Games
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ASCII Text
• One byte per character• 7 bits encode character, one parity bit• 94 printable characters• Originally used for English• Adapted to other languages
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ASCII file in Hexadecimal
• 20 hex = 32 decimal = SPACE• 0D 0A = 13 10 = CR LF
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ASCII
• From Wikipedia (Link Ch 2a)
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Unicode
• Encodes all "commercially significant" languages
• Two bytes per character
• FF FE at the start is a Byte Order Mark o Link Ch 2c
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File Headers & File Carving
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GIF Image (13x16 pixels)
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GIF File Header
• GIF89a – Version of GIF• 0D 00 0A 00 – 13 pixels x 16
pixels
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GIF Specification
• Link Ch 2d
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GIF Specification
• Link Ch 2d
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File Carving
• Rebuilding files by assembling blobs of data found on a disk
• Relies on file headers and footers• Done automatically by all-purpose
forensic suites like FTK and EnCase• Many other tools exist to carve files
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Project X1: Identifying File Types
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File Extensions & File Signatures
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File Extensions
• Usually three letters long• Appear at the end of a file name,
after a dot• Hidden in Windows by default• Used to specify the file type,
icon, and default application
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Hide File Extensions
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Incorrect File Extension
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Wrong Default Application
• Any stream of bytes can be interpreted as ASCII
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Open With…
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How Computers Store Data
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Storage Methods
• ElectromagnetismoHard disks and floppy disks
• Microscopic Electrical Transistorso SSDs, USB flash drives, SD cards, etc.
• Reflecting LightoCDs, DVDs, Blu-ray
• They are all nonvolatile – they retain data without power
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Magnetic Disks
• Platter spins at 7,000 rpm to 15,000 rpm
• Spindle is the axis• Read/write head is
an electromagnet mounted to an actuator armo Image from textbook
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Disk Controller Card
• Stores and retrieves data from the platters• Controlled by firmware stored in the Host
Protected Areao Image from http://static.ddmcdn.com/gif/ide-
controller2.jpg
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Flash Memory
• Made of transistors• Solid State Devices (SSDs)
o Faster than hard disksoUse less poweroMore expensive
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Optical Storage
• Microscopic pits encode bits
• Area between pits are called lands
• There is one long spiral track for the whole disk
• Data is read with laser lighto See Link Ch 2eo Image from
http://www.backgroundsy.com/file/large/blu-ray-disc-isolated.jpg
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Volatile v. Nonvolatile Memory
• Memory is short-term storage• Storage devices (hard disks,
SSDs, and optical disks) are nonvolatile—data is retained without power
• RAM is main system memoryoRAM is volatile—data is lost when
power goes off
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Volatility of RAM
• From Princeton (Link Ch 2f)
5 sec 30 sec 60 sec 5 min
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RAM Forensics
• RAM contains important evidence that is not normally written to the hard disko Instant messageso Network connectionso Running processes
• BUT there are no time-stamps on RAM contentso It can be misleading
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Computing Environments
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Four Categories
• Stand-alone• Networked• Mainframe• Cloud
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Stand-Alone
• A computer not connected to any other computero Such as a laptop not connected to Wi-Fi
or cellular dataoBUT networks are everywhere now, even
in BART or on airplanes
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Networked
• A computer connected to at least one other computer
• Evidence might be on servers and network devices as well as the local computer
• Almost every computer is networked now
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Mainframe
• A powerful computer used at a business, or shared by many users
• Located in a data center or colocation centero Image from
http://danialsharifudin.blogspot.com/2012/08/classification-of-computer.html
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Cloud Computing
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Examples of Cloud Computing
• Gmail• Facebook• Twitter• Amazon Web Services• CloudFlare
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Cloud Services
• Infrastructure as a Service (IaaS)• Platform as a Service (PaaS)• Software as a Service (SaaS)
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• From Wikipedia (Link Ch 2m)
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IaaS
• The most basic cloud service• Outsources hardware needs
o Servers, storage, routers, switches…
• ExamplesoAmazon EC2oWindows Azure Virtual MachinesoGoogle Compute EngineoRackspace Cloud
• Link Ch 2m
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PaaS
• Provides a computing platformoOS, programming language execution,
database, and Web server
• ExamplesoAWS Elastic BeanstalkoHerokuoGoogle App EngineoWindows Azure Compute
• Link Ch 2m
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SaaS
• Providers install and operate application software in the cloud
• Users access the software from cloud clients
• ExamplesoGoogle AppsoMicrosoft Office 365
• Link Ch 2m
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IaaS
• Outsource hardware needso Servers, storage, routers, switches…
• ExamplesoAmazon EC2oWindows AzureoGoogle Compute Engine
• Link Ch 2m
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• From link Ch 2g
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• From link Ch 2g
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Instagram• Online photo-sharing site• In Dec. 2012, Instagram changed its
terms of serviceo Perpetual rights to all photosoRight to sell photos to advertisers
without payment or notice to the user
• Instagram lost half its daily users in a montho Links Ch 2h, Ch 2i
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AWS Outage
• Dec. 24, 2012• Netflix was down, because they rely
on AWS (Link Ch 2j)• Amazon has had several other major
outages (Link Ch 2k)
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• From 2011 (Link Ch 2l)
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Cloudflare Growth