Cryptography Flashcards
When is a message, M, vulnerable in a communication?
M is vulnerable as soon as it leaves the sender’s system.
What is encryption?
The process of encoding a message into ciphertext so that its meaning is hidden
C = E(M)
What is decryption?
The process of decoding a message from ciphertext into its normal plaintext
D(C) == D(E(M)) == M
What is a cryptosystem?
A system in which rules are applied for the encryption and decryption of data.
Often keys, k, are used as a mechanism to adapt the encryption function’s output.
What is a cryptanalyst?
Break encryptions, their aim is to find both the plaintext and the decryption algorithm
Why do cryptanalysts want to know the decryption algorithm?
So that later messages can be decoded.
How are encrypted messages decoded?
Using the known decryption function
Noting common frequencies of letters and letter pairs
Exploiting vulnerabilities in encryption algorithms
Using compute power to brute force the encryption.
What cyphers use character manipulation to encode data?
Caesar Cipher One Time Pad (OTP) Vernam Cipher Book Cipher Vigenerè Tableau
What do substitution ciphers do?
Replace characters with other characters in the alphabet.
How does a one time pad work?
Uses non-repeating keys written on paper formed into a pad.
EG: Message has 500 chars, the sender requires 50 chars on a page so 10 pages would be used.
For the English alphabet the encryption is mod 26.
What is a Vernam Cipher and how does it work?
One time pad that uses a random sequence of numbers instead of characters
Take letter index in the alphabet and add the number, mod 26.
What is a book cipher?
A book cipher is used by the sender a receiver to encode texts. Both parties must know the book and a page apriori
Message encoded/decoded using the page in the text.
What is a Vigenerè Tableau and how does it work?
Matrix of letters used to help the encryption/decryption of messages.
Take row and column letter to give a resultant letter.
It provides resistance to frequency analysis as it uses two different characters for each letter, from different texts.
If one key is known it is easier to break.
What is columnar transposition?
Columnar transposition takes n columns and rearranges a message to diffuse the message.
What is columnar transposition susceptible to?
Frequency analysis can be used letters stay in the word, so common digrams can be found to find the column offset.
Can use moving window analysis to find these digrams.
What is symmetric encryption?
Key used is the same for both the encryption and decryption of data.
The key must remain secret at all times?
P = D(K, E(K, P))
What is asymmetric encryption?
Keys are complementary pairs in which there is a decryption key that inverts the encryption of the encryption key.
P = D(K_d, E(K_e, P))
When does symmetric encryption provide authentication?
When the key is kept secret and is only known by the two communicating parties in the conversation.
How many keys are needed for an N-user system using symmetric encryption?
n(n-1)/2 keys are needed.
What is a drawback of symmetric key encrypting?
Key exchange is an issue as A needs to get the key to B while keeping the key secret which can’t be guaranteed via transmissions if using symmetric encryption.
How do public key encryption work?
Public key encryption works as each user has a public and private key. Public keys are published freely and private keys are kept secret.
Information is encrypted using the recipient’s public key, which is then decrypted by the recipient upon receipt using the private key.
Does public key encryption scale?
Yes as each user is authenticated by their private key and users can send information to one-another using public keys.
What were the three conditions of Diffie-Helman’s paper with regards to public key encryption?
Computationally easy to encode/decode a message with a key
Computationally infeasible to derive the private key from a plaintext attack
Computationally infeasible to derive the private key from the public key.
What is taken advantage of to allow for public/private key systems to work?
The prime factorization problem of large prime numbers is used in the public/private key system. This problem is one such that it is computationally infeasible to factorize large primes in a timeframe that makes the data useful.
This property is used to create encryption keys as it means that it is computationally infeasible to find the primes used to create the key.
