Week 2 Flashcards
What is cryptography?
Describes how to transfer messages between participants without anyone else being able to read/modify.
What is a code?
Any way to represent data.
E.g. Bit strings, morse code.
What is a cipher?
A code which is difficult to derive data from.
Plain text encodes to cipher text via encryption.
What does the UNIX command xxd do?
Allows us to see hex representation (on disk) of a file.
How does the Caesar Cipher work?
Replaces each letter of the alphabet with the character three letters to the right.
What is Kerckoffs’ Principle?
A cipher should be secure even if the attacker knows everything about it apart from the key.
What is Frequency Analysis?
Counts number of times each symbol, pair of symbols occurs and draws conclusion from this.
Explain Symmetric Cryptography?
When the same key is used for encryption and decryption.
Explain Symmetric Cryptography?
When the same key is used for encryption and decryption.
What are the advantages and disadvantages with One Time Pads?
+ Perfect encryption.
- They key needs to be as long as the message.
- Must use key only once.
Explain Block Ciphers?
Work on blocks of plain text, not just single symbols.
Made up of a series of permutations and substitutions repeated on each block.
Describe AES block cipher?
Advanced Encryption Standard.
Works on blocks of 128 bits (4x4 matrix) by generating ten round keys from a 128 bit key.
Uses one permutation (ShiftRows) and three substitutions (SubBytes, MixColumns, AddRoundKey).
Describe 3-DES?
Takes 3 keys:
Ek1,k2,k3(M) = Ek3(Dk2(Ek1(M)))
Explain a mechanism for Padding?
PKCS (5/7).
If one byte of space write 01, and so on up until filled block (16….16).
5 - 8 byte block, 7 - 16 byte block.
Explain the Electronic Codebook Mode of block ciphers?
Each block encrypted individually, and assembled in same order.
If blocks are repeated in plain text, this is revealed in cipher text.
