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A Brief Introduction to Cybersecurity

David J. Malan

malan@harvard.edu

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OpenCourseWare

CS50 AI Introduction to Artificial Intelligence with Python
CS50B Computer Science for Business Professionals
CS50 Cybersecurity Introduction to Cybersecurity
CS50G Introduction to Game Development
CS50L Computer Science for Lawyers
CS50P Introduction to Programming with Python
CS50R Introduction to Programming with R
CS50S Introduction to Programming with Scratch
CS50 SQL Introduction to Databases with SQL
CS50T Understanding Technology
CS50W Web Programming with Python and JavaScript
CS50x Introduction to Computer Science

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https://edx.org/cs50

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passwords

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123456
admin
12345678
123456789
1234
12345
password
123
Aa123456
1234567890

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iloveyou
qwertyuiop
P@ssw0rd
********

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brute-force attacks

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4-digit passcode

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10 × 10 × 10 × 10

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10,000

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4-letter passcode

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52 × 52 × 52 × 52

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7,311,616

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4-character passcode

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94 × 94 × 94 × 94

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78,074,896

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8-character passcode

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94 × 94 × 94 × 94 × 94 × 94 × 94 × 94

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6,095,689,385,410,816

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password managers

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1Password
LastPass
…

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Apple iCloud Keychain
Google Password Manager
Microsoft Credential Manager
...

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two-factor authentication

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one-time passwords

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hashing

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alice:apple�bob:banana�…

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hash function

input →

→ output

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f

x →

→ f (x)

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password →

→ hash

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apple →

→ 1

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apple →

→ ..ekWXa83dhiA

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banana →

→ 2

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banana →

→ ..ZS4zkCo/P7E

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alice:apple�bob:banana�…

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alice:..ekWXa83dhiA�bob:..ZS4zkCo/P7E�…

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alice:$y$j9T$DIXfskUxbz6we5RbfdPCz0$zFd93JMuTyEJHrdIf.6bZ8Rbw4otHvybdOuLn.eD.s3

bob:$y$j9T$ty3B9GwDhm4f6zQIgm9uL.$SbFq.iSFt48A5iQlue8DtUd.57KaBN1tIEdLPmtEjwC�…

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rainbow table

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password	hash

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alice:apple�bob:banana�carol:cherry�charlie:cherry�…

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alice:..ekWXa83dhiA�bob:..ZS4zkCo/P7E�carol:..rj98gxDTYfM�charlie:..rj98gxDTYfM�…

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alice:..ekWXa83dhiA�bob:..ZS4zkCo/P7E�carol:..rj98gxDTYfM�charlie:..rj98gxDTYfM�…

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cherry →

→ ..rj98gxDTYfM

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salting

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password →

→ hash

salt →

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cherry →

50 →

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cherry →

→ 50kbbTM.xAinU

50 →

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cherry →

→ 50kbbTM.xAinU

50 →

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cherry →

→ 50kbbTM.xAinU

50 →

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cherry →

→ 493vg4DKt4MKg

49 →

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cherry →

→ 493vg4DKt4MKg

49 →

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cryptography

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cipher

plaintext →

→ ciphertext

key →

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cipher

ciphertext →

→ plaintext

key →

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secret-key cryptography

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symmetric cryptography

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A →

→ B

1 →

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asymmetric cryptography

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public-key cryptography

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plaintext →

→ ciphertext

public key →

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ciphertext →

→ plaintext

private key →

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HTTPS

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passkeys

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challenge →

→ signature

private key →

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signature →

→ challenge

public key →

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end-to-end encryption

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deletion

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0000000000000000000000000000000000000001111111111111111111111111111111110000011111111111110000000000000

0000000000000000000000000000000000000001111111111111111111111111110111000000000011111111100000000000000

0000000000000000000000000000000000000001111111111111111111111111111111100000000000111111100000000000000

0000000000000000000000000000000000000011111111111111111111111111111111000000000000111111100000000000000

0000000000000000000000000000000000000011111111111111111111111111111100000000000000111111100000000000000

0000000000000000000000000000000000000011111111111111111111111111111111000000000000111111100000000000000

0000000000000000000000000000000000000011111111111111111111111111111111111111000000111111000000000000000

0000000000000000000000000000000000000011111111111111111111111111011111111111111000011110000000000000000

0000000000000000000000000000000000000001111111111111111111111111000111111111111100011110000000000000000

0000000000000000000000000000000000000001111111111111111111111111000011111111110000011100000000000000000

0000000000000000000000000000000000001111100001111111111111111110000000000000000000011100000000000000000

0000000000000000000000000000000000001111100011111111111111111110000000000000000000000010000000000000000

0000000000000000000000000000000000001111111111111111111111111111110000000000000000000001000000000000000

0000000000000000000000000000000000000111111111111111111111111111111000000000000000011000000000000000000

0000000000000000000000000000000000000011111111111111111111111110000000000000000000001000000000000000000

0000000000000000000000000000000000000001111111111111111111111110000000000000000000000000000000000000000

0000000000000000000000000000000000000000000011111111111111111111110000000000000000000000000000000000000

0000000000000000000000000000000000000000000011111111111111111111111111000000000000000000000000000000000

0000000000000000000000000000000000000000000001111111111111111111111000100000000000000000000000000000000

0000000000000000000000000000000000000000000001111111111111111111110000000000000000000000000000000000000

0000000000000000000000000000000000000000000000111111111111111111110000000000000000000000000000000000000

0000000000000000000000000000000000000000000000111111111111111110000000000000000000000000000000000000000

0000000000000000000000000000000000000000000000111111111111111111000000000000000011110000000000000000000

0000000000000000000000000000000000000000000000111111111111111111111111000000000001111111000000000000000

0000000000000000000000000000000000000000000011111111111111111111111110000000000001111111111111111100000

0000000000000000000000000000000000000000000111111111111111111111110000000000010000111111111111111111111

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