01 - Cryptography

cryptography

the science of encryption, making information unreadable to unauthorized parties.

sender

person sending the message

receiver

person receiving (recipient)

attacker

eavesdropper/hacker trying to access the message

passive attack

only getting the message

active attack

intercepts the communication

authentication

verifies the sender is who they claim to be

confidentiality

information only intended receiver can understand

integrity

ensures message wasn’t modified

non-repudiation/non-repudiable

prevents sender/receiver from denying what they did

caesar cipher (shift cipher)

shifts letters in the alphabet by a fixed number

plaintext

original message

ciphertext

encoded message

key (symmetric)

shared parameter used for both decryption & encryption
— shift number (e.g., 3)

symmetric-key encryption

same key for both sender & receiver

brute force (caesar cipher)

trying all 26 possible shifts until plaintext (message) makes sense

public key

shared openly for encryption [mailbox address]

private key

secret key for decryption [mailbox address]

public-key cryptosystem — Diffie-Hellman, 1975

system using 2 keys: public key (encrypt) and private key (decrypt)

RSA

Rivest-Shamir-Adleman, public-key encryption system (1977)

modulus

product of two large prime numbers

Euler’s Totient (φ)

encryption/encoding exponent

number chosen such that

decryption/decoding exponent

number satisfying

coding (plaintext → ciphertext)

decoding (ciphertext → plaintext)

RSA public key

RSA private key

RSA security

based on difficulty of factoring large numbers