Week 3 Flashcards
What is the difference between Cryptography and Stenography?
Cryptography = Secret Writing
Stenography = Concealed Writing
What is Cryptanalysis?
Secret Analysis
What is difference between Symmetic and Asymmetric Encryption?
Symmetric = Single key to encrypt and decrypt, key1 = key2
Asymmetric = 2 Keys, one for encryption, one for decryption
Private key for decryption
Public key for encryption
What is a Mono-Alphabetic Cipher?
Each letter in the plaintext is replaced with a different one
What does Key Space mean?
the set of all valid, possible, distinct keys of a given cryptosystem
What is Frequency Analysis?
Frequency analysis is used to break classical ciphers by studying the frequency of letters in a ciphertext
For example, in English, the most common letters are E, T, A, and O, while Z, Q, X, and J are rare
What is a Polyalphabetic Cipher?
Uses multiple substitution alphabets
Each letter could be encryped differently by position
What is an example of a Polyalphabetic Cipher?
Vigenère cipher
Which shifts letters by varying amounts using a key word
eg. using word “CAT” as key
GOOD MORNING
IOHF MHTNBPG
What is a One-Time Pad (OTP)?
Thereotically 100% secure
Uses random key same length as message
Each bit encrypted with corresponding pad using XOR
Key must remain secret and be destroyed after use
Must ensure randomness in key generation
What is the Security like for Substitution Ciphers?
Huge key space of 26!
Brute forcing a key is difficult
However it is very easy to crack using Frequency Analysis
How does the Vigenere Cipher work?
The Vigenère Cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword.
It is often referred to as a polyalphabetic cipher because it uses multiple cipher alphabets to encrypt the message, making it more secure than a simple Caesar cipher, which uses just one shift.
What is a Transposition (Permutation) Cipher?
Rearranges character positions
Wrap the ciphertext (encrypted) around a cylinder to spell out its decrypted message on the face of the cylinder
The shared secret is the diameter of the cylinder
What is a Composite Cipher?
Combines substitution and transposition ciphers
S-Boxes - Confuse input bits
P-Boxes - Diffuse bits across S-box inputs
What was the First Cryptographic Standard?
16 round Feistal Cipher and a key-scheduler
Block cipher, encrypting 64-bit blocks
First Cryptographic standard:
US federal standard in 1977
Heavily used in banking
Extended to triple-DES to overcome the key-length problem
How was the Security of DES?
Main attack: exhaustive search
7 hours with 1 million dollar computer (in 1993)
7 days with $10,000 FPGA-based machine (in 2006)
