Brute Force Cryptanalysis: Quiz and Flashcards on Cryptography Attacks

Study Notes

Cryptanalysis

Relies on the nature of the algorithm and knowledge of properties of plain text or even knowledge of sample plaintext-ciphertext pairs
Types of cryptanalytic attacks:
- Ciphertext-only attack: only know algorithm and ciphertext, statistical knowledge about plaintext type
- Known plaintext attack: know/suspect plaintext and ciphertext to attack cipher
- Chosen plaintext attack: select plaintext and obtain ciphertext to attack cipher
- Chosen ciphertext attack: select ciphertext and obtain plaintext to attack cipher
- Chosen text attack: select either plaintext or ciphertext to en/decrypt to attack cipher

Brute Force Search

Always possible to try every key
Most basic attack, proportional to key size
Assume either know or recognize plaintext
Examples: DES, AES, 3 DES

Classical Substitution Ciphers

Replace letters of plaintext with other letters, numbers, or symbols
Example: Caesar Cipher
- Substitutes each letter by 3rd letter on
- Can define transformation mathematically
- Only have 26 possible ciphers, can try each in turn (brute force search)
- Recognize when have plaintext

Monoalphabetic Cipher

Shuffle the alphabet arbitrarily
Single cipher alphabet is used per message
Key is 26 letters long
Example: Plain: abcdefghijklmnopqrstuvwxyz, Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Security: total of 26! possibilities
Use single, double, and triple letter frequencies to cryptanalyze

English Letter Frequencies

Discovered by Arabian scientists in 9th century
Calculate letter frequencies for ciphertext
Compare counts/plots against known values

Example Cryptanalysis

Given ciphertext, count relative letter frequencies
Guess P & Z are e and t
Guess ZW is th and hence ZWP is the
Proceed with trial and error to get plaintext

Transposition Ciphers

Hide the message by rearranging the letter order
Recognize since have the same frequency distribution as the original text
Example: Rail Fence cipher
- Write message letters out diagonally over a number of rows
- Read off cipher row by row
Example: Row Transposition cipher
- Write letters of message out in rows over a specified number of columns
- Reorder the columns according to some key before reading off the rows

Product Ciphers

Using several ciphers in succession to make harder
Two substitutions make a more complex substitution
Two transpositions make a more complex transposition
A substitution followed by a transposition makes a new much harder cipher

Steganography

Hides existence of message
Examples:
- Use only a subset of letters/words in a longer message marked in some way
- Invisible ink
- Hiding in LSB in a graphic image or sound file
Drawbacks: high overhead to hide relatively few info bits

Symmetric Encryption

Sender and recipient share a common key
All classical encryption algorithms are private-key
Basic terminology:
- Plaintext: original message
- Ciphertext: coded message
- Cipher: algorithm for transforming plaintext to ciphertext
- Key: info used in cipher known only to sender/receiver
- Encipher (encrypt): converting plaintext to ciphertext
- Decipher (decrypt): recovering plaintext from ciphertext
- Cryptography: study of encryption principles/methods
- Cryptanalysis (codebreaking): study of principles/methods of deciphering ciphertext without knowing key
- Cryptology: field of both cryptography and cryptanalysis

Symmetric Cipher Model

Two requirements for secure use:
- Strong encryption algorithm
- Secret key known only to sender/receiver
Cryptography can be characterized by:
- Type of encryption operations used (substitution, transposition, product)
- Number of keys used (single-key or private-key, symmetric; two-key or public, asymmetric)
- Way in which plaintext is processed (block, stream)

How to Attack Secret-Key Encryption

Brute force search
Cryptanalysis
Other methods

Description

Explore the concepts of cryptanalysis, which relies on the nature of algorithms and knowledge of plaintext properties, as well as brute-force attacks where every possible key is attempted on ciphertext. Learn about different types of cryptanalytic attacks such as ciphertext-only and known-plaintext attacks.