Cryptography Flashcards
How do we call the input and output of the encryption process
The input to an encryp- tion process is commonly called the plaintext, and the output the ciphertext.
What are some of the cryptography primitives
There are a number of cryptographic primi- tives—basic building blocks, such as block ciphers, stream ciphers, and hash functions.
What are the possible classifications for a block cypher
Block ciphers may either have one key for both encryption and decryption, in which case they’re called shared key (also secret key or symmetric), or have separate keys for encryption and decryption, in which case they’re called public key or asymmetric.
How can we make a stronger cypher
There are basically two ways to make a stronger cipher: the stream cipher and the block cipher.
How does the encryption works in a stream cipher?
you make the encryption rule depend on a plaintext symbol’s position in the stream of plaintext symbols
Stream cipher, commonly ascribed to the Frenchman Blaise de Vigenère, works by adding a key repeatedly into the plaintext. also known as a running key.
How does the encryption works in a block cipher
while in the latter you encrypt several plaintext symbols at once in a block. Let’s look at early examples.
How can we make a stream cipher proof against attack? What does it implies and what is its price?
One way to make a stream cipher of this type proof against attacks is for the key sequence to be as long as the plaintext, and to never repeat (one time pad)
Its effect is that given any ciphertext, and any plaintext of the same length, there is a key that decrypts the ciphertext to the plaintext.
the price of the perfect secrecy of the one-time pad is that it fails completely to protect message integrity. it also consumes as much key material as there is traffic, hence is too expensive for most ap- plications.
What is a one way function?
The third classical type of cipher is the one-way function. This evolved to protect the integrity and authenticity of messages
What are assymetric primitives
Finally, some modern cryptosystems are asymmetric, in that different keys are used for encryption and decryption.
What is a random oracle model?
The random oracle model seeks to formalize the idea that a cipher is “good” if, when viewed in a suitable way, it is indistinguishable from a random function of a certain type.
When a cryptography primitive can be consider pseudorandom?
cryptographic primitive pseudorandom if it passes all the statistical and other tests that a random function of the appropriate type would pass, in whichever model of computation we are using.
What are random functions and how do they work?
The first type of random oracle is the random function. A random function accepts an input string of any length, and outputs a random string of fixed length, say n bits long.
Random functions are our model for one-way functions or cryptographic hash functions, which have many practical uses.
What are the inputs and outputs of hash functions
The output of the hash function is known as the hash value or message digest; an in- put corresponding to a given hash value is its preimage; the verb to hash is used to refer to computation of the hash value.
What are some properties of random functions?
The first main property of a random function is one-wayness. Given knowledge of an input x, we can easily compute the hash value h(x); but it is very difficult given the hash value h(x) to find a corresponding preimage x
A second property of pseudorandom functions is that the output will not give any in- formation at all about even part of the input.
A third property of pseudorandom functions with sufficiently long outputs is that it is hard to find collisions
