Lec 3: Basic Cryptography I Flashcards
What is cryptography?
Greek for “secret writing”
What do you do with Cryptography?
- Confidentiality - obscure a message from eavesdroppers
- Integrity - assure recipient that a message was not altered
- Authentication - verify that a message was sent by a certain identity
- Non-repudiation - convince a 3rd party that what was said is accurate
What is a cryptographer?
invents cryptosystems
What is a cryptanalyst?
breaks cryptosystems
What is cryptology?
the study of crypto systems
What is a cipher?
mechanical way of encrypting text
What is code?
semantic translation - e.g. “eat breakfast tomorrow” = “attack on Thursday”
What do we mean when we say a cryptosystem is “secure”?
1- if enemy intercepts ciphertext, cannot recover plaintext
What is the issue in making cryptosystem’s secure precise?
1, What else might your enemy know?
-The kind of encryption function you are using?
— Kerckhoffs’ principle: assume everything except key is public knowledge
- Some plaintext-ciphertext pairs from last year?
- Ciphertext for plaintext the enemy selected?
- Some information about how you choose keys?
2. What do we mean by “cannot recover plaintext” ? - Ciphertext contains no information about plaintext, or
- No efficient computation could make a reasonable guess
What are the kinds of cryptographic analysis?
What is the caesar cipher? What are the advantages and disadvantages?
What is a monoalphabetic ciphers?
What is the cryptanalysis of monoalphabetic ciphers?
What is the order of frequency of single letters in English (the first 5)?
How to break monoalphabetic ciphers?
What are the most used digrams and trigrams?
What is the problem with monoalphabetic ciphers?
How does polyalphabetic substitutions?
What is the Vigenère Tableau?
- Multiple substitutions
- Can choose “complimentary” ciphers so that the frequency distribution flattens out
- More generally: more substitutions means flatter distribution - Vigenère Tableau
- Invented by Blaise de Vigenère for the court of Henry III of France (c. 1500’s)
- Collection of 26 permutations
- Usually thought of as a 26 x 26 grid
- Key is a word
What is the weakness of polyalphabetic substituion ciphers?
What is that Kasiski method?
What is variance?
How do you estimate variance from frequency?
What is the index of coincidence?