1 - Classical Symmetric Algorithms Flashcards
Caesar/Shift cipher
key K 1 to 25
Replace each plaintext character with letter K positions further
Inverse = backwards
Components of a cryptosystem
Set of possible plaintexts
… ciphertexts
… keys (keyspace)
the encryption function E(K,m) (K key is parameter)
the decryption function D(K,c)
The encryption (E) and decryption function (D) much be such that..
w/r to a fixed K (key)
- For fixed K, different messages are encrypted to different cipher texts. injective/one to one
- For fixed K, E/D are inverses
Exhaustive Keysearch Attack
Try decrypting c using all possible keys in turn, until result makes sense.
Combatted by large keyspace
Key is string of n bits. How many keys?
2 to the power of n
How many keys in a 128bit key
2 to the power of 128
Absolute minimum number of bits
80bits. Most common 128
Substitution Cipher
Plaintext: alphabet
ciphertext: 26 characters
Key: A substitution table of two rows (top - OG, bottom - cipher)
Encryption/Decryption: Substitute the letters
How can context be used to crack a cipher?
The environment or known patterns of the plaintext can be used.
How many possible keys in a substitution cipher
26!
26 factorial = 403291461126605635584000000
26 possibilities on first char, 25 on second, 24 on third
How can letter frequencies be used to crack a cipher?
Some letters are more frequent in languages and so, patterns can be recognised using this fact.
Vigenere cipher
Key: sequence of n numbers from 0 to 26.
Shifts applied corresponding to each number in the key
Vigenere Cipher: Number of keys
26 to the power of n
Permutation cipher: Concept
anagrams
Split message into fixed length blocks and apply same permutation in each block
Permutation Example Key
1 2 3 4 5
3 1 5 2 4
Plaintext: nobod y here
ciphertext: oondb hryee
Inverse:
1 2 3 4 5
2 4 1 5 3
Permutation Number of keys
n!
n factorial (reasonably large)
Does the hacker know the algorithm?
Assume yes
Ciphertext only attack
Eve only has ciphertexts. Obtan plain or key
Known plaintext attack
Eve has plaintext(s) and corresponding ciphertext(s)
Chosen plaintext attack
Eve obtained temporary access to encryption and encrypted a custom plaintext
Chosen ciphertext attack
Eve has temporary access to decryption and decrypts a chosen ciphertext
