Hill cipher, stream ciphers and the One Time Pad Flashcards

1
Q

What is the hill cipher?

A

Polygram cipher (polygraphic cipher)

Perform a linear transformation on d number of plaintext characters, to get d ciphertext characters.

Enc:
Multiply a d x d matrix K by the block of plaintext P
C = KP

Dec:
Multiply the matrix K⁻1 by the block of the ciphertext C
P = K⁻1C

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2
Q

What is a weakness of the Hill cipher?

A

It is linear, known plaintext attacks are easy

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3
Q

Do a cryptanalysis of the Hill cipher

A

Known plaintext are possible given d number of plaintext ciphertext matching blocks.

We are given d blocks of matching P and C,
Then we are able to C = KP for K

And use K to get rest of plaintext:
P = K⁻1*C

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4
Q

What are stream filters?

A

Characterised by the generation of a keystream of any required length.

Each element of the keystream encrypts one or more ciphertext characters

Usually symmetric ciphers

Keystream must have good randomness properties

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5
Q

What is a synchronous stream cipher?

A

Keystream generated independently of the P

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6
Q

How does enc and dec in synchronous stream ciphers work?

A

Enc: c(t) = p(t) XOR s(t)

Dec: p(t) = c(t) XOR s(t)

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7
Q

What is the one time pad?

A

Key: truly random sequence of characters, independently generated

Each character in key is used one time only

non-periodic binary synchronous stream cipher

Provides perfect secrecy

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8
Q

Define perfect secrecy

A

Message set: {M1, M2, …, Mk}
Ciphertext set: {C1, C2, …, CK}

P(Mi|Cj): Probability of Mi was encrypted given that Cj was observed

Perfect secrecy:
P(Mi|Cj) = P(Mi)

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9
Q

How does enc and dec work in the one time pad?

A

Enc:
ci = (pi + ki) mod 26

Dec:
pi = (ci -ki) mod 26

mod 26 when using roman alphabeth

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10
Q

With P = HELLO
K = EZABD

What is C?

A

LDLMR

Given C, note that P can be any 5-letter message

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11
Q

How does enc and dec work in binary one time pad?

A

Enc:
ci = pi XOR ki

Dec:
pi = ci XOR ki

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12
Q

What are some properties of perfect secrecy, in regards to the number of keys?

A

Any cipher with perfect secrecy, myst have as many keys as there are messages.

This makes one-time pad the only unbreakable cipher.

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13
Q

What is the main problem with one time pad?

A

Key distribution and random keys

Dealing with keys are difficult when these are so large

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14
Q

What is visual cryptography?

A

Application of one time pad

Split image into two shares

Dec: Overlaying the twoshares

Each pixel is shared in a random way

Each share reveals no information about the image

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15
Q

How does encryption work in visual cryptography?

A

Generate a one time pad P (random string of bits) with length equals to number of pixels in the image

Generate an image share by replacing each bit using these sub-pixels patterns:
0 bit: black-white
1 bit: white-black

Generate the other share as follows:
- The same as the first share for all the white pixels of the image
- The opposite of the first share for all black pixels of the image

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16
Q

How do we decrypt in visul cryptography?

A

Overlay the two shares