01 Ciphers Flashcards
What are 3 uses for encryption?
preserve privacy
check integrity
prove identity
Consider the use of a symmetric encryption algorithm and an asymmetric encryption algorithm. Which is used to authenticate identities? Which is used for the actual core encryption of data? Which is used to pass encryption keys?
- Asymmetric
- Symmetric
- Asymmetric
What is the main use of secret-key encryption?
Protecting the contents of a message
What is the main use of public-key encryption?
Proving the identity of a message’s sender or recipient
What is the main use of hashing methods?
Proving the validity of a message
What is the main use of message signatures?
Providing the identity of a message sender and the contents of the message
A code mapping in which pulses are sent along a medium, with silence in between groups of dots and dashes comprising individual charachters
morse code
What type of cipher is the Pigpen Cipher?
mono-alphabetic substitution
Describe the BIFID cipher
- grid-based, 5x5
- maps letters into numeric values
- 1901, Delastelle
Describe the Playfair cipher
- grid-based, 5x5
- encrypts pairs of digraphs
- frequency analysis does NOT work with it
- 1854, Charles Wheatstone / Lord Playfair
Describe the Homophonic Substitution Cipher
- aims to overcome frequency analysis
- varies the number of codes mapped to each letter
Describe the Caesar Cipher
- transposition cipher
- Julius Caesar
- originally, shifted the plaintext alphabet 3 chars
- modern implementations, both parties agree on the direction and degree of the shift
Describe the Scrambled Alphabet Cipher
- improved on Caesar cipher
- randomly maps plaintext to ciphertext
- 26! possible mappings (4.03x10^26)
- still susceptible to brute-force attacks (Eve needs to search through 50% of the possible solutions)
- can be cracked quickly by using frequency analysis
Describe the Vigenere Cipher
- an improvement on the scrambled alphabet cipher
- based on selected keyword & polyalphabetic mapping
- invented 1553, Giovan Battista Bellaso
- cracked by Kasiski, 1863
Describe One-time Pad
- used only once, then discarded
- generally considered unbreakable
Describe ASCII encoding
7-bit
8-bit
printable characters mapped to decimal, binary, & hex values
Describe Base-64 encoding
hexadecimal encoding
take values 6 bits at a time
Describe the Enigma machime
- multi-alphabet substitution cipher
- used machinery to accomplish encryption
- specifically, electromechanical rotors
Describe the Affine cipher
- mono-alphabetic substitution cipher
- each letter is mapped to a numeric equivalent, encrypted using a mathematical function, and converted back to a letter
What formula describes combinations?
nCk = n!/k!(n-k)!
What formula describes permutations?
nPk = n!/(n-k)!
Suppose you are considering the probability of two independent events A & B. What is the probability of both events occurring?
P(A and B) = P(A) * P(B)
Suppose you are considering the probability of two mutually exclusive events. How would you express this probability mathematically?
P(A and B) = 0
Suppose that you roll a dice. What is the probability of throwing a two or a three?
P = 1/6 (one in six chances of rolling a given number)
P(2) + P(3) = P(A and B)
1/6 + 1/6 = 1/3
Suppose that you throw two dice. What is the chance of two ones being thrown?
Each dice is independent (multiplication)
P(1) * P(1)
1/6 * 1/6 = 1/36
When performing a modulus operation, what value is returned?
The remainder of an integer division
Modulo-2 addition (or subtraction) is equivalent to this function.
X-OR
Modulo-2 multiplication is equivalent to this function
Logical AND
What is the GCD?
The largest possible integer that divides into two numbers without a remainder. Also called the greatest common factor.
What are characteristics of PRNGs?
- repeats the random numbers after some period of time
- fast
- deterministic
- return a repeatable set of random numbers (testing)
What are characteristics of TRNGs?
- generates a true random number
- takes entropic input
- slow (requires human interaction)
- non-deterministic
- aperiodic
