Week 3 Flashcards
(20 cards)
What is cryptography?
A way of turning plaintext (secret message) into ciphertext (unreadable form) that can be reverted back to plaintext.
What four elements does encryption link together?
Plaintext m, ciphertext c, key k, algorithm E (so c = Eₖ(m))
State one principle of modern cryptography.
Security depends only on secrecy of the key, not the algorithm (Kerckhoff’s principle)
What does “small change in plaintext results in large change in ciphertext” help resist?
Differential and statistical attacks by ensuring avalanche effect.
Symmetric vs Asymmetric Encryption: which uses the same key for both encrypting and decrypting?
Symmetric encryption uses the same secret for both operations.
What is the main drawback of symmetric key cryptography?
Key distribution problem: n parties need n(n-1)/2 unique keys.
How does asymmetric (public-key) encryption work?
Uses a key pair: public key to encrypt, private key to decrypt: enables digital signatures
What hybrid approach combines symmetric and asymmetric cryptography?
Uses asymmetric to exchange a symmetric session key, then use symmetric for bulk data encryption.
Describe the Caeser cipher encryption and decryption process.
Encrypt by shifting each letter forward by k (mod 26); decrypt by shifting back by k.
What’s the difference between substitution and transposition ciphers?
Substitution replaces symbols; transposition rearranges symbol positions.
Give an example of a monoalphabetic and polyalphabetic cipher.
Monoalphabetic: Simple substitution
Polyalphabetic: Vigenère Cipher
What are the block size and key size of DES?
64-bit block size; 56-bit key
How does the Triple DES (3DES) extend DES?
Applies DES three times with multiple keys to lengthen effective key size
Which algorithm replaced DES and 3DES as the NIST standard in 2001?
AES (Rijndael)
What problem does Diffie-Hellman key exchange solve?
Securely establishing a shared symmetric session key over an insecure channel.
Name two standard protocols that use DH for key establishment.
TLS/SSL (Transport Layer), IPSec (Network Layer)
Who developed RSA and in what year?
Rivest, Shamir and Adleman in 1978
What area the core steps of “textbook” RSA key setup?
Choose primes p, q; compute n = p·q; derive e and d; publish (n,e), keep d secret
State the RSA encryption and decryption formulas.
Encryption: c = mᵉ mod n; Decryption: m = cᵈ mod n
How can RSA be used for digital signatures?
Sign by “encrypting” a hash with the signer’s private key; verify with their public key.
