Classical Ciphers Flashcards
Assume all algorithms are ____
public
Security is determined ____ by key size
only
Making your own security is _______
insecure
Simplest and earliest known use of substitution cipher
Caesar Cipher
Replace each letter in alphabet with another letter $n mod |A|$ places down
Caesar Cipher
Vulnerable to word / letter frequency analysis
Substitution Ciphers
Different monoalphabetic substitutions as one proceeds through plaintext message
Polyalphabetic cipher
Key determines which particular rule is chosen for given transformation
Polyalphabetic cipher
Key is required that is as long as the word
Vigenere Cipher
Usually a repeating keyword
Vigenere Cipher
Key: dogdogdog
Plaintext: abcdefghi
Ciphetext: dpigsljvo
Key and plaintext share same frequency distribution of letters
Vigenere Cipher
Plaintext written down as sequence of diagonals, read off as sequence of rows
Rail fence cipher
Plaintext (depth = 3):
whatsgoingonpal
Ciphertext: whtogpasioagnnl
Write messages in rectangle, row by row, and read message off column by column, permuting order of columns
Row transposition cipher
Order of the columns becomes key to algorithm
Row transposition cipher
Processes one bit or byte at a time
Stream cipher
Substitution and permutation techniques, one block at a time
Block cipher
Make the relationship between plaintext, key input and ciphertext output as complex as possible
Confusion
Spread the influence of each input bit across many output bits
Diffusion
Achieved mainly via substitutions
Confusion
Achieved via permutations
Diffusion
Each character of ciphertext depends on several parts of key
Confusion
Changing plaintext character should result in several changed ciphertext characters
Diffusion
