3. Cryptography: introduction Flashcards
What is the shared secret for rail fence cipher?
k (natural number)
How do you encrypt with rail fence cipher?
Write plain text in columns of size k, ciphertext is the concatenation of resulting rows
How do you decrypt with rail fence cipher?
Write ciphertexy in rows if size |c| / k
Why is rail fence cipher weak?
Very small key space (k < |c|)
What is the shared secret for substitution cipher?
Mapping of characters to new characters
How do you encrypt with substitution cipher?
Apply substitution function to each character of plaintext
How do you decrypt with substitution cipher?
Apply the inverse substitution function to each character of the ciphertext
Why is substitution cipher weak?
Vunerable to frequency analysis (e, th, the)
What is the shared secret for vigenere cipher?
Word w over the English alphabet
How do you encrypt with vigenere cipher?
break plaintext into |w| sized blocks, add w to each block (mod 26), concatenate resulting blocks
How do you decrypt with vigenere cipher?
break ciphertext into |w| sized blocks, subtract w from each block (mod 26), concatenate resulting blocks
Why is vigenere cipher weak?
Weak to frequency analysis (every |w| character has the same offset)
