Cryptography Final

(2^3)^2 =

(2^2)^3

(2^3)^2 mod 3 =

1

For a truely random encryption scheme, to trasmit a message of size 10GB, the Key lenght must be at least

10 GB

A 4 bit shift register's current values are 0110 (from left to right). The output of the right most unit is fed back as the input of the left most unit. What will be the values of the given shift register after three shifts to the right?

1100

If the key of a stream cipher =1111111111, then for a plaintext = 0011111000, the ciphertext will be

1100000111

In DES, in each round the number of bits actually encrypted is

32

In DES, the "round key" size is

48 bits

SHA-1 algorithm has ____ rounds

80

To send an integrity verifiable message to Bob, Alice should encrypt the hash of her message with ______, so that an adversary cannot replace it easily

Alice's private key

If A=0, B=1, ....Z=25, then (C+Z)%26=

B

DES uses permutation at the

Bit level

In a game, Stefany randomly picks a letter from "A", "B" and "C" and Brandon randomly picks a number from "1","2","3" and "4". To win, Stefany needs to guess Brandon's number and Brandon needs to guess Stefany's letter. If played many times who is expected to win the game?

Brandon

The purpose of Byte Substituition Layer in AES is to inroduce

Confusion

Example of diffusion methods include

Expansion

A perfect stream cipher that uses a true random key, can re-use the key safely

False

All collision resistant hash functions are irreversible

False

For enhanced security, an organization should use secret encryption algorithms that nobody knows

False

Functions E() and F() in Diffie-Hellman key exchange algorithm must be kept secret

False

Given E() and the value of E(alpha), "alpha" is easily computable

False

In English language, the frequency of A is less than the "frequency of AB+ frequency of AC"

False

In the MixColumn Sublayer of AES, each row of the input matrix is replaced by another same size row generated by multiplying the input row with a fixed 4x4 matrix

False

Keys used in an encryption, and the corresponding decryption functions must be the same

False

Knowing that entropy is a measure of randomness (more randomness =less predictability = more entropy), we can tell that ABCDEFGHHHHHHH has higher entropy than ABCDEFGH

False

PRNG can generate random keys that do not repeat

False

Prime p and generator g in Diffie-Hellman key exchange algorithm, ideally, should be kept secret

False

RSA is computationally expensive since computing public and private keys require expensive math operations

False

The hash of a 100000 GB message will be significantly larger than the hash of an eight-character password

False

To break an LFSR based PRNG, an attacker just needs to know the K bits

False

Use of the same cryptographic key for multiple purposes is a recommended key management practice

False

For decryption, AES uses

Inversion of every entcryption setp in reverse order

In a Feistal Network, such as DES, if we know L1 and R1, we can determine L0 and R0 provided we also know

K0

Cryptographic hash funtions should not be reversible is specified by the following property

Pre-image resistance

Diffusion layer in AES includes

ShiftRows Sublayer and MixColumn Sublayer

Example of confusion methods include

Substitution and Permutation

AES supports key lengths of 128, 192 and 256 bit

True

AES, chosen by NIST through a multi-year competitive selection process, is the most widely used symmetric cipher today

True

During Diffie-Hellman key exchange, the order of key exchange (i.e., which party sends the part of the "key" to the other party first) does not matter

True

E(alpha) is easily computable

True

For a Shannon cipher(E,D) defined over (k,m,c), D(k, E(k,m))=m

True

Hash of a plaintext is usually shorter than its ciphertext

True

If PB, and PR are respectively the public and private keys of an RSA cryptosystem, then C=PB(M) generates a ciphertext C for the plaintext M, and PR(C) recovers M.

True

If PB, and PR are respectively the public and private keys of an RSA cryptosystem, then C=PR(M) generates a ciphertext C for the plaintext M, and PB(C) recovers M.

True

If a Shannon cipher (E, D) defined over (k,m,c) is perfectly secure, then the length of k>=the length of m

True

If a XOR b = c, then c XOR b= a

True

If a, b, c are binary variables, a XOR b XOR c= a + b + c mod 2

True

In 128 bit AES, input data is structured into a 4x4 byte matrix

True

In DES although key size is 64, actually 56 of them are used for encryption

True

In RSA endcryption, E(D(M))= D(E(M))

True

In a Feistal Network, such as DES, if we know L0, R0 and K0, we can determine L1 and R1

True

In each round of DES, 32-bit R_i is XORed with a 48-bit K_i by expanding 32-bit R_i into 48 bits

True

In each round of DES, the 48-bit XORed output of K_i and expanded R_i is reduced to 32 bits, which is then XORed with 32-bit L_i

True

In the Byte Substituition Layer of 128 bit AES, each 8 input bit is replaced by another 8 output bit using a known look up table

True

Invention of quantum computer may cause the death of RSA cryptosystems.

True

It is possible for an m-bit LFSR based PRNGE to have 2^m-10 unique steps

True

It is possible that an adversary can make someone believe that he/she is using an authentic public key of a company, while the key is actually generated by the adversary.

True

Knowing that entropy is a measure of randomness (more randomness = less predictability = more entropy), we can tell that ABCDEFGHIJKLMN has higher entropy than ABCDEFGH

True

Large bandwidth requirement is another problem with RSA since the size of the associated ciphertext is usually large

True

Protocol ElGamal key agreement (Menezes, Page 517) is nothing but the Diffie-Hellman key exchange with part of the "key" computed by one party is made public

True

RSA is computationally expensive since associated encryption and decryption processes require expensive math operations

True

Recommended number of rounds in AES is determined by the length of its selected key

True

ShiftRows Sublayer in AES just rearranges data at the byte level

True

To make a hash funtion not reversible (i.e., one way), we must lose some information about the input during the hashing process

True

If a plaintext is encrypted using an RSA private key, the ciphertext can be broken by

anybody

The encrypted hash of a message that certifies message integrity is a form of

digital signature

If a function is collision resistant, then

finding two inputs with the same hash will be difficult and replacing my name "Karim" with some other name, both having the same hash is difficult

A Merkle tree is built for 8 different files (each file corresponds to one leaf node). If one file is corrupted, how many hash values in that tree will be affected

four

If a function is second pre-image resistant, then

replacing my name "Karim" with some other name, both having the same hash is difficult

Knowing RSA public key does not compromise a cryptosystem since the associated encryption function is

reversible, but not in a reasonable amount of time

If we can guess the bits (0's and 1's) of a digital key by the power consumption of a decryptor circuit, we can call our effort a

side channel attack

RSA is usually

used to transmit part of the communication involving short messages, such as AES Keys