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ECE 419 Topic 2 Part 1 - Symmetric-key encryption
ECE 419 Topic 2 Part 1 - Symmetric-key encryption
81
Computer Science
Undergraduate 4
05/09/2012

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Term
what is the usage of linear feedback shift register in security?
Definition
stream cipher and block cipher will need it
Term
what is primitive polynomial?
Definition
a function that will give m-sequence
Term
what is a run?
Definition
binary sequence with period N, k consecutive 0s or 1s preceded by 1 and followed by 1 is a run with length k
Term
what is auto-correlation?
Definition
cross-correlation with itself
Term
how to compute auto-correlation?
Definition
a exor a shifted right by T bits. if exored bit is 1 then assign -1, if exored bit is 0 then assign 1. Then add them together
Term
what is auto-correlation useful at?
Definition
figuring out sequence of a sequence
Term
what is cross-correlation?
Definition
measures similarities of two sequences
Term
how to find period of sequence a and b from cross-correlation?
Definition
the resulting function is periodic
Term
in cross-correlation, what if two sequences have different length?
Definition
????
Term
Describe Golumb's three random properties for sequences
Definition
1. number of 1 is 1 more than number of 0
2. (1/2)^k should have run (of 0 and 1) with length k
3. auto-correlation when not same is always -1
Term
why is LFSR with m-sequence not good?
Definition
hacker can get 2n consecutive sequence and regover LFSR algorithm
Term
what is linear span?
Definition
???
Term
give an example of how stop-and-go NL LFSR work
Definition
???
Term
what is stream cipher?
Definition
Uses pseudorandom generator to produce cipher text
Term
is stream cipher symmetric key or public key crypto?
Definition
symmetric key
Term
describe one-time-pad principle and why it's important
Definition
key is used once -> key is unbreakable
Term
what are design principles of stream ciphers?
Definition
1. stream cipher algoirhm is public(i.e everybody show know it)
2. only secrete is pre-shared key
3. hacker can intercept communication so they have ciphertext
4. hackers can also create ciphertexts
Term
what are two phases in stream cipher?
Definition
1. KIA
2. PSG
Term
what does v mean in bit wise operation?
Definition
means or
Term
what does /\ mean in bit wise operation?
Definition
means and
Term
what does + mean in bit wise operation?
Definition
means exclusive or
Term
What does KIA do?
Definition
takes in IV and key, generate a random stream
Term
PSG usually use what to implement?
Definition
LFSR
Term
is IV public?
Definition
yes
Term
is key public?
Definition
no, it's secrete
Term
how many times does KIA execute for each encryption session?
Definition
1 time
Term
usually what are used to implement PSG?
Definition
LSFR, NL LSFR
Term
why should low power be used for KIA and PSG?
Definition
to protect side-channel attacks
Term
describe how linear cryptanalysis works?
Definition
???
Term
describe how differential cryptanalysis works
Definition
difference between itself and time shifted version, or difference between two keys or messages
Term
describe how correlation attack works
Definition
???
Term
what's the requirement for time-space attack?
Definition
for N bits, sqrt(s^N) should exceed computatiblity of current computors
Term
What is the requirement for internal state of stream cipher
Definition
If internal state has n-bits, sqrt(2^n) should resist exhausive attack
Term
how to attack using birthday attack
Definition
???
Term
why should stream cipher algorithm be public?
Definition
give hacker a lot of power, if still the hacker can't crack it, then it's good
Term
A5/1 uses what LSFR model?
Definition
stop-and-go LFSR model, 64 stages
Term
how many cycles does A5/1 take to output?
Definition
200 cycles
Term
Grain 2 uses what model?
Definition
1 LSFR and 1 NL LSFR
Term
how many cycles does Grain 2 take to output?
Definition
160 cycles
Term
WG-29 uses what model?
Definition
LSFR using vector, each holds 29 bits, 11 stages, linear span is 2^45
Term
how many cycles does WG-29 take to output?
Definition
22 cycles
Term
why is A5/1 bad?
Definition
output probability is unbalanced
Term
How did hackers hacked A5/1?
Definition
People reverse engineered
Term
draw a block cipher diagram
Definition
lecture notes
Term
what are differences between steam cipher and block cipher?
Definition
stream is bit by bit, block is block by block. So performance wise block cipher might be faster generally.
Generally steam cipher needs LFSR to compute output stream bit by bit, block cipher needs some state to mix a block up(using thing such as a LUT)
Term
how come requirements for stream cipher and block cipher are so different?
Definition
nature is different, stream cipher depends on function while block cipher depends on number of different operations
Term
what are two things block cipher require?
Definition
1. diffusion and confusion
Term
what is diffusion?
Definition
if message is changed by 1 bit, then cipher should change totally
Term
what is confusion
Definition
if key is changed by 1 bit, then cipher should change totally
Term
what is a substitution box?
Definition
a small box that takes in m bits of data and transform it to m bits of data
Term
how to improve security of block cipher which uses s-box in block cipher?
Definition
perform algorithm in s-box many times(rounds)
Term
what are two structures in block cipher?
Definition
1. Feistel structure(NLFSR) with input
2. substitute permutation network(a finite state machine)
Term
what is done in each round of block cipher?
Definition
1. add keys
2. NL permutation S-box layer(diffusion)
3. linear permutation layer(confusion)
Term
how is cipher text produced in DES?
Definition
run 16 rounds, then internal state is cipher text
Term
how can DES be hacked?
Definition
since the key is 56 bits, using birthday attack can hack
Term
how many rounds are there in AES?
Definition
10 rounds
Term
DES belongs to where in block cipher?
Definition
Feistel structure(NLLFSR)
Term
how many bits is message for DES?
Definition
64 bits
Term
how many bits is key for DES?
Definition
56 bits
Term
how many bits is message for AES?
Definition
128 bits
Term
how many bits is key for AES?
Definition
128 bits
Term
are 8 bit cipher text in AES same?
Definition
yes
Term
how is 128 bits represented in AES?
Definition
each 8 bit is a vector, there are 16 vectors in a matrix
Term
what's the usage of finite field?
Definition
finite field is a way to allow add and mult for binary numbers.
usually finite field is implemented in LUT, so computation of binary numbers are faster
Term
Describes the steps in AES
Definition
1. inverse
2. affine transform
3. shift row transform
4. mix column transform
Term
total of how many bits of keys are encrypted in AES?
Definition
block length * (number of rounds + 1)
Term
what are two types of encryption modes for block cipher?
Definition
1. block cipher mode
2. block cipher implemented as stream cipher
Term
what are two types of block cipher mode?
Definition
1. electronic codebook mode
2. cipher block chaining mode
Term
encryption mode???
Definition
1. Electronic code mode
2. CBC mode
3. Cipher Feedback Mode
4. Counter Mode
Term
what are three requirements in Secure Hash Functions?
Definition
1. collision resistance
2. second pre-image resistance
2. pre-image resistance
Term
what is second pre-image resistance and why it's important?
Definition
find y st h(y) = h(x)

Because attacker, if able to find y st h(y) = h(x), can use y to pretend to be person who generated h(x).
Term
what is pre-image resistance and why it's important?
Definition
given z, find x such that h(x) = z


Because attacker, if able to find y st h(y) = h(x), can use y to pretend to be person who generated h(x).
Term
what's the different reason why second pre-image resistance and pre-image resistance exist?
Definition

1. pre-image: to impersonate Alice

2. second pre-image: to change Alice's data

Term
how many bits are input to SHA1?
Definition
512 bits
Term
how many bits are output to SHA1?
Definition
160 bits
Term
how does SHA1 work??
Definition
????
Term
what are three ways MAC can be implemented?
Definition
1. normal stream cipher mode
2. CBC mode
3. hash mode
Term
What does it mean by KIA and PSG should be public but protected?
Definition
it means design should be public, but they should resist attack
Term
Symmetric key and public key, which one causes lower overhead to computation?
Definition
symmetric key, recall AES in 128 bits is over 4000 bits in RSA. So as soon as symmetric key is computed, should switch to symmetric key
Term
What are two advantages of stream cipher compared to block cipher?
Definition
1. one bit of error will result in output has one bit of error
2. stream cipher does not need to pad plain text, but block cipher need to
Term
what are two disadvantages of stream cipher compared to block cipher?
Definition
1. design principles of steam ciphers are hardly realized
2.if message is short, initialization if relatively long
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