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crypto:

Cryptography

and You

attend.osucyber.club

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Announcements

Engineering meeting this week

Thursdays @ 7:30 PM Eastern
See Jackson/Discord for more info�

CSAW Quals!!!!!!!

2022-09-09 11:00am est to 2022-09-11 11:00am est 🤢
One of the oldest and biggest CTFs
We’ll meet Friday night (free food)�

Bootcamp CTF

https://bootcamp.osucyber.club/
Free and up 24/7
Get that practice in for CSAW

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Background

Problem: sensitive information needs to be moved from one place to another through a public medium

How do we make sure no bad guys are yoinking our secrets?�

Solution: make our data unreadable

Just mess it up
Spies can’t intercept your information if it’s nonsense

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Background

Problem: sensitive information needs to be moved from one place to another through a public medium

How do we make sure no bad guys are yoinking our secrets?�

Solution: make our data unreadable

Just mess it up
Spies can’t intercept your information if it’s nonsense �

...ok, but now the good guys can’t read it either.

Don’t just change the data randomly
Process needs to be controlled and reversible
But also only by us (trusted party)

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Enter Cryptography

What is cryptography/crypto?

“constructing and analyzing protocols that prevent third parties or the public from reading private messages” - Wikipedia :D�

When you use a cryptographic protocol to secure a message, that is called encryption

The opposite is decryption�

In computing, this protocol is an algorithm

Tons of cryptographic algorithms
The algorithm being used may or may not be a secret
A good algorithm will keep an attacker out even if it’s identified

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Encoding vs. Encryption

Encoding is translating data to a different form so it can be transferred effectively

Not meant to hide data, easily reversible
Specific domain
Ex: ASCII encodes letters as bytes�

Encryption is similar to encoding (transforms the data), but is built for security

Meant to hide data, hard to reverse
Needs a key
General domain
Ex: ChaCha20 encrypts any kind of data

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Important Concepts

Key: used by algorithm to encrypt or decrypt input

Symmetry: whether an algorithm uses one or two keys

Symmetric: algorithm uses one key for both encryption and decryption
Asymmetric: algorithm uses one key for encryption and a different key for decryption

Hash/digest: irreversible result of an algorithm, typically used for integrity verification

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Basic Crypto Algorithm: XOR Function

Symmetric algorithm with one byte as the key
For each byte of content: f(x, key) = x XOR key

f(f(x, key), key) = x # XOR operation is symmetric

h e l l o w o r l d

104 101 108 108 111 119 111 114 108 100

8 5 12 12 15 23 15 18 12 4

Weakness: small key space (one byte = 0-255)
Operates on each byte in sequence -> “stream cipher”

f(x,96)

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Popular Crypto Algorithm: RSA

Asymmetric algorithm with a private key and a public key

Uses exponents and big numbers -> large key space�

For each block of content:

f(x) = x^e mod n = X # e and n are part of the exchanged public key
g(X) = X^d = x # d is part of the private key�

Weakness: key generation phase

n = pq where p and q are prime numbers that are NOT shared
If either of p or q is found, the private key can be reversed

This needs some fancy math; see sources slide�

Operates on blocks of data -> “block cipher”�

Yes, this is a lot. Crypto gets easier with practice