by Spencer Tiberi


  • Our data is under constant threat, but how can we defend ourselves?


  • Keeping people away from things you don’t want them to see
  • Computers are among the lease secure devices you own
    • Data or files are stored on them as 0s and 1s
      • Can be financial info, photos, etc.

Deleting Files

  • What does it mean to delete a file off of a hard drive?
    • Visually, it disappears from a desktop or folder
  • Files are stored on a computer as 0s and 1s
  • Some space needs to be allocated for the file
  • The operating system has a file that keeps track of files an their location on disk
  • Graphically, when a file is deleted, it moves to the trash (or recycle bin)
    • It can still be easily revived from here, until you empty the trash
  • However, an operating system doesn’t actually delete it from the hard drive
    • It simply forgets the location and existence of the file!
    • One can theoretically recover data by looking for familiar patterns of bits
  • So how do we delete more securely?
    • Re-saving a file with overridden information actually could not override the old bits but rather create more 0s and 1s stored on a hard drive!
    • Special software can wipe data off of a hard drive
  • Who do computers have this obvious flaw with deleting?
    • What if we accidentally delete a file?
      • This structure allows for recovery
    • Wiping data also takes a lot of time, so it’s much faster to just forget locations of data


  • A feature supported by HTTP
  • Little values a web server puts on a user’s browser
  • Used to remember if a user has visited a website before
    • Allows you to not have to log in every time you visit or refresh a page
      • When you log into a web server, a cookie is planted on your browser
    • Stored in a database
    • Browser will send value to web server to remind of previous login
  • When we make a request we send:
    GET / HTTP/1.1
  • We receive:
    HTTP/1.1 200 OK
    Set-Cookie: session=29823bf3-075a-433a-8754-707d05c418ab
    • The server gives us a cookie.
  • A cookie is like an ink-based hand stamp for an amusement park or club
  • Wireless information can be intercepted
    • What if a hacker could obtain the cookie
      • Session hijacking attack
      • If you have already logged in, hacker can pretend to be you
  • Encryption scrambles this value so hackers cannot easily use it
  • Browser history remembers everywhere you’ve been and everything you’ve done there
    • Convenient if you want to recall a website you’ve visited
      • But, so can anyone else with access to your browser
  • Can clear browser history and cookies
    • History likely not securely scrubbed
    • Will protect you from nosey friends
    • Websites will forget you visited as the cookies will be deleted as well!

Incognito Mode

  • Can open up a typically different colored browser window
  • Use if you want history automatically removed
  • Useful when building a website as sometimes you want a browser to forget old iterations of your website build


  • All of this assumes you log in
  • If you don’t use a passcode to protect your device, anyone can pretend to be you
    • What if you lose your phone or device?


  • On a phone could only be a few digits
    • Not super secure
    • __ __ __ __
      • With numbers, each space has 10 options
      • 10 x 10 x 10 x 10 = 10,000 possibilities
      • 0000-9999
  • On many smartphones, you will have to wait for an amount of time if you have entered a bad passcode
    • Slows down the process of someone guessing
  • Add more digits or letters of the alphabet
  • Using a-z, A-Z, 0-9
  • __ __ __ __
  • Each space now has 62 options (26 + 26 + 10)
  • 62 x 62 x 62 x 62 = 14,776,336 possibilities
  • Maybe you’re super secure and you have a 20-char password
    • You could forget it
    • Annoying to type in repetitively
  • No one fits all
    • Short = bad, longer = good
    • Don’t use popular words and phrases
      • Hackers will look for words or common phrases
  • Most common Passwords
    1. 123456
    2. 123456789
    3. qwerty
    4. 12345678
    5. 111111
    6. 1234567890
    7. 1234567
    8. password
    9. 123123
    10. 987654321
  • Hackers have dictionaries of bad passwords that they can search through and try
  • Random passwords
    • Usually have to confirm so it can be hard to replicate
  • Using numbers to represent letter is common
    • 1 for l
    • 4 for A
  • It’s suggested you mix uppercase, lowercase, and and throw in numbers
    • Good to use misspellengs
  • Don’t put your post-it with your password on your monitor!
  • Constant password changes can be a net negative
    • Can encourage easier passwords to help with memorization

Password Resetting

  • What if you forget your password?
    • Often can click on a link to reset your password
      • Asks you to type email address or username
    • Typically, you get an email with a link
      • Hopefully this goes back to the same website!
      • It likely has a random value in the URL
    • Once back at the website, you update your password
  • The website has a database
    • It generated a random number and stored it with a note indicating password recovery
    • The website assumes that anyone who has access to this value and to the user’s email is you
  • Typically, tech staff can’t tell you what your password is
    • Odds are your password is encrypted (scrambled) or, more technically, hashed in their database
  • Getting a password in email means that the password are not hashed or encrypted!
    • Also, sending a password over email opens that email to interception
    • This is a red flag if a website does this

Using The Same Password

  • You may have a favorite password that you reuse
    • Upside is that it’s convenient
  • However, what if one of the websites are hacked?
    • A hacker may try to use the password on other websites to see what she or he can get into!

Password Managers

  • Difficult to remember all these passwords
  • Software called password managers exist that store on your phone or hard drive all usernames and passwords in an encrypted way
    • You have a master password that logs you into everywhere!
      • Store it physically in somewhere like a safety deposit box
  • Password managers create long random passwords and will log in for you
    • All websites have different passwords!
  • However, if you lose the master password, you cannot get the accounts back!

Two Factor Authentication

  • First factor is a password
    • Historically, something “only” the user knows
    • Can be guessed
  • Second factor should be fundamentally different
    • Should be something you have
      • An RSA device displays a unique value that is synced with a server RSA
        • This number needs to be typed in too!
        • As long as this device isn’t stolen by someone with your password, they can’t get in as easily
      • Phones now run software that allows you to get a code and type them in
  • Should think about what websites you care about the most and enable two factor authentication
    • Some companies can use sms (text messages)

Network security

  • So many of our current networks are wireless
    • You probably been conditioned to look for free wifi
      • Sometimes still might not connect for various reasons
  • If the wireless connection has not padlock (no password to log in) the connection is not secure
    • You may still visit https or secure websites
    • However, everything you do on http sites can be seen
  • What to do?
    • Don’t use that network
    • Use a VPN (Virtual Private Network)
      • Connection to internet is encrypted VPN1
        • With an unsecured connection, anyone can access your data


  • First establish encrypted connection to a server and let this server communicate for you
    • The connection between the VPN server and website can still be insecure!
  • Because we are encrypting data through an algorithm, using a VPN can slow down speed


  • A physical firewall is a wall between connected buildings that prevents the spread of fire
  • In the world of computer science, a firewall is software that looks at IP addresses and helps keep bad guys out and user data inside
  • Helps prevent people from accessing your computer


  • Suppose I want to send a secret message for “HI”
    • HI ➟ IJ
    • Change each letter by 1
    • The recipient needs to know how it changed to revert
  • Plaintext ➟ Cyphertext ➟ Plaintext
    • HI ➟ IJ ➟ HI
  • This is called a caesar cypher
    • Rotational cyphers are not that secure
      • Can be guessed easily
      • Not used for internet encryption
    • For this to work, recipient needs the key
      • To know the key, we need to agree in advance
        • Can’t send it encrypted as well as they need the key!

Public Key Cryptography

  • The last example with a caesar cypher is secret-key cryptography
    • Only one key
  • In public key cryptography there are two keys, one public and one private
    • Mathematical relationship between them
    • Use public key to encrypt, private key to decrypt Alice and Bob
    • Bob’s private key can undo the effects of his public key
    • When Bob responds… Bob Response
    • Bob sends a message using Alice’s public key
  • Your browser has its own public and private keys
    • So does websites like Google and Amazon
      • This allows them to communicate securely with you
  • Often this processes is used to exchange a secret key


  • These kind of attacks have become so prevent that the following has been posted around Harvard’s campus phishing
  • Phishing attacks are when an adversary sends a somewhat official-looking email
    • May contain a link asking for a password or account info
    • The email may contain an elaborate backstory “justifying” the request
    • The malicious email is trying to obtain information from you
  • Odds are that the link provided doesn’t go to the website being claimed
    • Can go to a website that looks legit
      • People can just copy HTML
  • Results in giving up private information
  • It’s healthy to distrust most email you get
    • Don’t follow links, type in the address for the company yourself
    • Sketchy emails may have typographical errors


  • Malicious software can also be sent via email
  • Windows is particularly vulnerable
  • Software can be injected into your browser and your computer to erase your hard drive, make your computer send spam, or hold your data hostage
  • Some malware encrypts your data and asks for large sums of money to get the key to decrypt it
    • Key could not even work!
    • This is called ransomware
  • Malware can ultimately do anything on your computer


  • At the end of the day, all of security and privacy boils down to trust
    • People around you
    • Algorithms/software
    • Manufacturers
  • We’ve downloaded software with trust that it will only do what it claims
    • Word could log your key strokes
    • Chrome could monitor you even when not on Google’s website
    • Snapchat could not delete posts after being seen
  • There have been cases where software was written to cover tracks of being monitored!
  • Who’s to say the software we’re using is actually doing what we say?
  • It’s east to curl up into a ball and worry, but we need to decide who to trust
  • Security measures make it more difficult for someone to be malicious, but ultimately they can’t guarantee privacy
  • You have to decide what data you’re comfortable with storing, what you view on the internet, who to trust, and how much to trust them