Block vs Stream Ciphers

Type of encryption operations used

substitution / transposition / product

Number of keys used

single-key or private / two-key or public

Way in which plaintext is processed

block / stream

cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key

objective to recover key not just message

general approaches:

cryptanalytic attack / brute-force attack

cryptology - field of both cryptography and cryptanalysis

Cryptanalytic Attacks

Ø ciphertext only

Ø known plaintext

Ø chosen plaintext

Ø chosen ciphertext

Ø chosen text

Unconditional security no matter how much computer power or time is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext

Computational security given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken

Classical Substitution Ciphers

where letters of plaintext are replaced by other letters or by numbers or symbols

or if plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns

Caesar Cipher earliest known substitution cipher, by Julius Caesar , first attested use in military affairs, replaces each letter by 3rd l

tter on, example:

meet me after the toga party

PHHW PH DIWHU WKH WRJD SDUWB

Monoalphabetic Cipher rather than just shifting the alphabet , could shuffle (jumble) the letters arbitrarily , each plaintext letter maps to a different random ciphertext letter, hence key is 26 letters long

Plain: abcdefghijklmnopqrstuvwxyz

Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN

Plaintext: ifwewishtoreplaceletters

Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

Polyalphabetic Ciphers:polyalphabetic substitution ciphers ; improve security using multiple cipher alphabets ; make cryptanalysis harder with more alphabets to guess and flatter frequency distribution ; use a key to select which alphabet is used for each letter of the message ;use each alphabet in tur n; repeat from start after end of key is reached

Security of Vigenère Ciphershave multiple ciphertext letters for each plaintext letter;hence letter frequencies are obscured;but not totally lost;start with letter frequencies:see if look monoalphabetic or not;if not, then need to determine number of alphabets, since then can attach each

Transposition Ciphersnow consider classical transposition or permutation ciphers ; these hide the message by rearranging the letter order ; without altering the actual letters used; can recognise these since have the same frequency distribution as the original text

Product Ciphersciphers using substitutions or transpositions are not secure because of language characteristics; hence consider using several ciphers in succession to make harder, but: 1.two substitutions make a more complex substitution 2. two transpositions make more complex transposition 3.but a substitution followed by a transposition makes a new much harder cipher ; this is bridge from classical to modern ciphers

Rotor Machines before modern ciphers, rotor machines were most common complex ciphers in use; widely used in WW2: German Enigma, Allied Hagelin, Japanese Purple; implemented a very complex, varying substitution cipher; used a series of cylinders, each giving one substitution, which rotated and changed after each letter was encrypted; with 3 cylinders have 26³=17576 alphabets

Steganographyan alternative to encryption

hides existence of message

1.using only a subset of letters/words in a longer message marked in some way

2.using invisible ink 3. hiding in LSB in graphic image or sound file 4. has drawbacks

5.high overhead to hide relatively few info bits

Block vs Stream Ciphers

· block ciphers process messages in blocks, each of which is then en/decrypted

· like a substitution on very big characters

· 64-bits or more

· stream ciphers process messages a bit or byte at a time when en/decrypting

· many current ciphers are block ciphers

· broader range of applications

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