Cryptography I Flashcards
What parts of CIA are ensured by cryptography?
Confidentiality and Integrity
Kerchoff’s principle?
security of an encryption system should not depend on secrecy of system, but on secrecy of the key.
Describe symmetric encryption
The encryption and decryption keys are the same
What does Symmetric Key Distribution mean?
- each pair of communicators needs a distinct key
=> you need a quadratic number of keys for pairwise communication
Describe the Julius Caesar’s Chiper
You shift each letter by 3
Describe the Alphabet Shift Cipher
You shift each letter by k (generalized Caesar’s)
Issues with the Alphabet Shift Cipher
- insecure ( you can guess k by trying all values and seeing which makes sense)
Describe the Permutation Cipher
- each letter is replaces by another one (permutation of letters)
- nb of possible keys: 4 x 10^26
Issues with Substitution Cipher
- can be cracked by frequency analysis
Describe One Time Pad
- key = sequence of random bits, same length as plaintext
- to encrypt: C = K bitwiseXOR P
- to decrypt: P = K bitwiseXOR C
What are the advantages and disadvantages of One Time Pad?
+ each bit of C is random
+ fully secure if k only used once
- key needs to be as large as plaintext => hard to generate/share
- key CANNOT be reused
What are some One Time Pad pitfalls?
- if you reuse key - you can XOR the Cs and get the XOR of the Ps
- imperfect randomness: the randomly generated key might end up being 0000 or smth => useless
What are the Characteristics of Cryptographic Hash Functions?
- One-way:
Given x, hard to find P such that h(P) = x - Weak Collision Resistance:
Given P, hard to find Q such that h(Q) = h(P)
-String Collision Resistance:
Hard to find pair of Q and P such that h(Q) = h(P)
How does CHF solve file integrity? (how to see that files were not tampered with)
- A computes CH of each file, stores locally
- A checks CH of retrieved files against what he has stored
WEAK CR => difficult for B to change file such that hashes are equal
How does CHF solve password authentication? (I dont want to store password directly in database)
- store CH of password but not the password
One-way => diff to recover password if hacked
WEAK CR => hard to guess other password with same hash
How does CHF solve coin tossing online? (Fair protocol to toss a coin)
- A and B agree on a CHF h
- A picks random int R, sends d = h(R) to B
- B guesses whether R is odd or even
- A reveals R to B
- if B correct, tails, if not, heads
What is entropy?
= Formal measure of uncertainty in the outcome of a process
= suma de la i=0 la n-1 din ( p(ei) log2( 1/p(ei) ) )
