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