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Cryptography > Modes of operation and random numbers > Flashcards

Modes of operation and random numbers Flashcards

1
Q

What is the purpose of having multiple modes of operation for block ciphers?

A

The different modes can provide different things. Some modes can be designed to provide confidentiality for data, authentication (and integrity) or both.

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2
Q

What is one thing that confidentiality modes includes?

A

Randomisation

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3
Q

What does randomised encryption schemes want to prevent?

A

The same plaintext block being encrypted to the same ciphertext block every time

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4
Q

How can randomisation be achieved in block ciphers?

A

Using an initialisation vector. This needs to be unique or random.

Can be achieved by including a variable state which is updated with each block.

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5
Q

What are some other features of modes?

A

Can allow parallel processing, enc and/or dec in parallel.

Error propagation: error in C result in multiple bit-errors in P after decryption

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6
Q

Why is padding used in some modes?

A

Some modes require plaintext to consist of complete blocks

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7
Q

What is ciphertext stealing?

A

An alternative to padding.

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8
Q

What is the ECB mode?

A

Enc:
Ct = E(Pt, K), plaintext block Pt

Dec:
Pt = D(Ct, K)

Blocks are appended to each other to make the message.

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9
Q

Why is ECB mode normally not used for bulk encryption?

A

Because it is deterministic

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10
Q

What are the properties of ECB?

A

Randomised: No
Padding: Required
Error propagation: Within block
IV: Not used
Parallel encryption: Yes
Parallel decryption: Yes

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11
Q

Describe the CBC mode

A

Random IV, sent together with the ciphertext

Enc:
Ct = E(Pt XOR Ct-1, K)
C0 = IV

Dec:
Pf = D(Ct, K) XOR Ct-1
C0 = IV

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12
Q

How does error propagate through CBC?

A

An bit error in block n result in a plaintext error for the block n, and a flipped bit in block n + 1,

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13
Q

What are the properties of CBC?

A

Randomised: Yes

Padding: Required

Error propagation: Within block, and into specific bits of next block

IV: Must be random

Parallel Enc: No

Parallel Dec: Yes

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14
Q

Describe CTR mode

A

Synchronous stream cipher.

Keystream generated by enc successive values of a ‘counter’ initialised using a nonce N.

Ot = E(Tt, K)
Tt: Concatination of nonce and block number t

Enc:
Ct = Ot XOR Pt

Dec:
Pt = Ot XOR Ct

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15
Q

What is a nonce in CTR mode?

A

A randomly chosen value

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16
Q

How is error propagated in CTR mode?

A

A one-bit change in C block n, produces a one-bit error in the plaintext at the same location

17
Q

What are the properties of CTR mode?

A

Randomised: Yes

Padding: Not required

Error propagation: Error occur in specific bits of current block

IV: Nonce must be unique

Parallel Enc: Yes

Parallel Dec: Yes

18
Q

When can using CTR mode be useful?

A

For access to specific P blocks without decrypting the whole stream

19
Q

Where is CTR mode used today?

A

Basis for authenticated encryption in TLS 1.3 and 1.3

20
Q

What is a TRNG?

A

True random number generator.

A physical process which outputs each valid string independently, with equal probability

21
Q

What is a PRNG?

A

Pseudo random number generator

Deterministic algorithm which approximates TRNG

22
Q

How can PRNG and TRNG be combined?

A

May use TRNG to provide a seed for a PRNG

23
Q

What is DRBG?

A

Deterministic Random Bit Generators. These are PRNG algorithms

Based on:
Hash functions
A specific MAC known as HMAC
Block ciphers in counter mode

24
Q

How does PRNGs work?

A

Each generator takes a seed as input and outputs a bit string. Then its state is updated.

Seed should be updated after a number of calls

Seed can be obtained from TRNG

25
Q

What are entropy sources?

A

Framework for design and validation of TRNGs

Includes physical noise source, digitalization process, and post-processing stages

Outputs any number of bits

26
Q

What are the functions of DRBGs?

A

Instantiate: Sets initial state of DRBG using seed

Generate: Provides output bit string

Reseed: Input new seed and update DRBG state

Test: Checks correct operation of the other functions

Uninstantiate: Deletes the state of the DRGB

27
Q

How is security defined for DRBGs?

A

Ability to destinguish reliably between its output and a truly random string.

2 properties: Backtracking resistance and forward prediction resistanse

28
Q

What is backtracking resistance?

A

If we have the current state, should not be able to distinguish between the output of earlier states, and random strings

29
Q

What is forward prediction resistance?

A

If we have the current state, should not be able to distinguish between the later outputs and random strings

30
Q

What is CTR_DRBG?

A

Uses block with CTR (AES-128 recommended)

Seed length is block-length + key length

Key and state (counter) is defived from high entropy seed. No separate nonce is used

Counter mode enc is run iteratively with no P added and output blocks form the output

31
Q

Define the update function in CTR_DRBG

A

Used in initialise, generate and reseed functions to generate new key and state.

Input: K and V (state/counter) and optional data input D

Output: K’ and V’

Computation for block size = key size:
- Generate new block O1 = E(V,K)
- Increment V
- O2 = E(V, K)
- K’ || V’ = (O1 || O2) XOR D

32
Q

How does the instantiate function work in CTR_DRBG?

A

Calls update with D equial to high entroy seed, K and V are zero strings

33
Q

How does the generate function work in CTR_DRBG?

A

Computes up to 2^19 bits by running CTR mode output from current state.
Update is then called with D empty

34
Q

How does the instantiate function work in CTR_DRBG?

A

Update with D as high entropy input, K and V in current state

35
Q

How many calls can be made to Generate before Reseed must be called, according to the standard for CTR_DRBG?

A

2^48

36
Q

In CTR_DRBG what provides backtracking resistance?

A

Update and Reseed

37
Q

In CTR_DRBG what provides forward prediction resistance?

A

Reseed

38
Q

What is Dual_EC_DRBG?

A

Older standard

Based on elliptic curve discrete logarithm problem

Slower than other DRBGs

No security proof exist,

39
Q

What are Cryptographically-secure pseudorandom number
generators (CSPRNGs)?

A

Algorithms, that given an unpredictable input, a much larger stream of unpredictable outputs are generated.

Cryptography (15 decks)

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