Current Internet Cryptography (Public Key Systems)

• Most internet uses public key cryptography systems
• Key = algorithm converting readable text/data into predictable mess that can be undone
• Public key means everyone knows how to encrypt, only recipient knows how to decrypt
• Works through "easy to do, hard to undo" mathematical problems
• Analogy: treasure chest that locks when closed, only recipient has unlock method

RSA Algorithm

• Most well-known public key system
• Named after inventors: Rivest, Shamir, Adleman
• Based on factorization of large numbers into primes
• Easy to multiply two large prime numbers
• Very difficult to find prime factors from product
• Public key derived from product of primes, doesn't contain prime factors themselves
• Can be broken in principle but takes 100,000+ years on current computers
• Practically safe for current purposes

Quantum Computer Threat

• Quantum computers can vastly speed up computation
• Can break cryptographic protocols like RSA quickly
• Major security risk for credit cards, trade, national security
• Currently no quantum computers powerful enough exist
• Risk high enough for next decades that solutions being developed

Post-Quantum Cryptography

• Cryptographic protocols that cannot be broken by quantum computers
• Uses mathematical problems where quantum computers provide no advantage
• Also called "quantum resistant cryptography"
• Does NOT use quantum effects itself
• Name only indicates resistance to quantum computer attacks
• Situation could change if new quantum algorithms discovered
• Most nations planning switch to post-quantum cryptography within decade

Quantum Key Distribution (BB84 Protocol)

• Method for two parties to securely share encryption key
• Uses quantum physics to keep key safe
• BB84 protocol: Bennett and Brassard, 1984
• Standard naming: Alice (sender), Bob (receiver), Eve (eavesdropper)

Basic Process

• Alice creates random sequence of particles with spin up/down
• Both measure spin: up = 0, down = 1
• Creates shared random bit sequence for encoding
• Problem: Eve can intercept, measure, pass on to Bob

Quantum Enhancement

• Alice randomly chooses spin direction: up-down OR left-right
• Doesn't tell Bob which direction used
• Bob randomly picks measurement direction
• Correct direction = correlated result, wrong direction = uncorrelated (50% probability)
• Alice later sends direction information via unencrypted channel
• Bob discards wrong-setting measurements, keeps correlated ones as secret key

Security Mechanism

• Eve cannot intercept without detection
• Eve doesn't know Alice's chosen direction
• Wrong measurement direction changes particle spin
• Eve has no way to know if this happened
• When Bob uses correct setting, his result won't correlate with Alice's if Eve interfered
• Alice and Bob compare sequence portions to check correlation
• Lack of expected correlation reveals eavesdropping attempt
• Based on quantum no-cloning theorem: impossible to copy arbitrary quantum state without destroying it

Current Implementation Status

• Quantum key distribution exists commercially but not widespread
• Can be done through optical fiber or free space
• Encoded messages still sent through classical channels
• General quantum cryptography (beyond key distribution) exists only theoretically

Summary of Three Approaches

• Post-quantum cryptography: Non-quantum methods resistant to quantum computers, exists and being adopted
• Quantum key distribution: Uses quantum effects for secure key sharing, exists but not widely used
• General quantum cryptography: Uses quantum effects for message encoding, theoretical only

Sponsor Content - NordVPN

• Uses AES (Advanced Encryption Standard) not RSA
• AES based on symmetric key known to both parties
• 68% off two-year plan + one free month
• Code: sabine at nordvpn.org/sabine
• Works on all platforms: Android, Windows, iOS
• 5000+ servers worldwide for location flexibility