Cryptosystems

Question 1

Name the formal definition of a one-way function

Answer 1

f(x) can be efficiently computed while f^-1(y) cannot be efficiently computed

Question 2

Name a real world one-way function

Answer 2

Find a telephone number in a telephone book that is alphabetically sorted

Question 3

Name some one-way mathematical functions

Answer 3

exponential: y = g^x / RSA: y = x^e / Square: y = x^2

Question 4

what is a trapdoor function?

Answer 4

its a one-way function if there exists some additional information that may help to efficiently invert f

Question 5

what is a hash-function?

Answer 5

an efficiently computable function that takes as input an arbitrary long input string and generates an output string of a fixed length

Question 6

Name specific properties of a hash function

Answer 6

preimage resistent (one way), second-preimage resistent (weakly collision resistent), collision resistent (strongly collision resistent)

Question 7

What does collision resistence mean?

Answer 7

it implies second-preimage resistence (there shouldnt be a lot of messages that result in the same hash value)

Question 8

name some exemplary cryptographic hash functions

Answer 8

MD5, SHA-1, SHA-2, SHA-3

Question 9

What and Why random bit generators?

Answer 9

a device that generates an output randomly without any input - it is necessary for the security of a crypto-system. The output is a sequence of independent and equally distributed bits

Question 10

Salt vs. Seed?

Answer 10

Salt is random data added to the hash function (e.g. when hashing passwords) in order to make sure that two same passwords do not get the same hash value.
A seed is the input for a pseudorandom number generator in order to make sure that not everyone get the first random value from the generator