Cryptography

Please free me

Confidentiality in data is?

the assurance that data cannot be viewed by an
unauthorised user

Integrity in data is?

the assurance that data has not been altered in an unauthorised
(which includes accidental) manner.

Data Origin Authentication is?

the assurance that a given entity was the original source of received data

Non-repudiation is?

the assurance that an entity cannot deny a previous
commitment or action.

Entity authentication is?

the assurance that a given entity is involved and
currently active in a communication session

the encryption key and the decryption key are
essentially the same in which arch?

symmetric cryptosystems

the encryption key and the decryption key are
fundamentally different in which arch?

asymmetric/public-key cryptosytems

Caesar cipher an encryption system where what happens?

a single symbol (letter) in the alphabet is shifted
by a constant amount

how do you decrypt a ceasar cipher?

un-shift each symbol (letter) in the message by that
constant amount. The ‘constant’ becomes the key and is shared amongst the sender and receiver

A Vigenere cipher goes farther by?

having a separate (arbitrary) key to determine the
’encrypted’ character

Using a one time pad (OTP) requires the use of what?

a single-use pre-shared key that is not
smaller than the message being sent. OTP
based encryption offers perfect secrecy.

Stream ciphers process how many bits of the plaintext
at a time.

one

Symmetric Stream Ciphers:
The BLANK (K) is typically shorter than the BLANK (P), so the keystream

key, plaintext

In order to decrypt, the receiver must have both BLANK and the BLANK for the keystream generator

the key and the algorithm

Advantages of symm stream cipher?

1. No error propagation

2. Speed

3. ‘On-the-fly’ operation: no need to wait and fill a buffer

Disadvantages of symm stream cipher?

Requires synchronization (the ‘steps’ of the keystream generator need to be synchronized)

A block cipher takes as input a BLANK
and a key, and outputs a BLANK.

plaintext block, 

for symm block cipher, If the blocksize is too small then?

then the number of different plaintext blocks that
can ever be encrypted may be small enough to ‘leak’ information using other techniques

for symm block cipher, If the blocksize is too large, then?

symm block cipher advantages? (3)

1. Versatile - used for encryption, hashing, signatures

2. Compatible - widely implemented/supported

3. Adaptable - different modes of operation

Disadvantages of symm block cipher? (2)

1. Error propagation - if P changes by 1 bit, a ‘good’ algorithm will have 50% of C changed

List some examples of block cipher algorithms (3):

1. DES - old and broken

2. TripleDES - still good, but slow

3. AES - the current star

main problems with symm key crypto?

1. Trust (if both parties have the sole/all-powerful key, the other party can effectively masquerade as the first party)

2. Key management (includes establishment, transfer, revocation, etc.)

we can identify which 6 requirements for
public-key cryptography?

1. encryption and decryption ought to use different keys

2. anyone who wishes to receive messages needs to have a private decryption key

3. anyone who wishes to receive messages needs to share encryption key

4. anyone who wishes to send a secure message will want assurance that the encryption key they have belongs to the person they wish to send the message to

5. decryption and encryption keys must not be obviously ’related’ (there should be no way to ’build’ a padlock with just the key or ’build’ the key from just the padlock)

6. The encryption standard (padlock) must be sufficiently strong to resist unauthorized access

2 limitations with public key encryption:

1. Computations in encryption and decryption of public key algorithms are ‘expensive’, meaning they are slow. As a result, the number of encryptions/decryptions are kept to a minimum.

2. A message must be shorter than the digits of the modulus of the public. One can use the strategies discussed in symmetric algorithms for chaining together short messages into a single long message, but computational cost discourages that strategy.

explain diffie hellman key exchange?

Alice and Bob each choose a private color and agree on a public color g. They each mix the public color with their private color and share the result (also public). To create the key, each takes the two-part mixed color and add their own private color. This ‘three-color’ key is now the keys for this session.

what is a block chain?

a distributed, immutable “ledger” wherein each block contains a has of the previous block in the “ledger”, thus making it a chain.

Two types of block chains are?

1. permission-less: typically open to all

2. permissioned: private (all members known and implicitly trusted)

what is block chain proof of work?

Each agent always listening for new transactions and
solutions. While listening they are solving the current block challenge.

The cryptographic challenge is dynamic such that?

every solution takes about the same amount of time

The winner of block chain challenge gets which 2 things?

1. to add the new block to the chain

2. broadcast solution to all nodes

what happens in block chains in terms of ties, duplicate blocks, etc?

The longest chain (or agent with most work invested) is selected as being correct.

Proof of Stake is defined as?

in order to be qualified to be a verifier, you must put up a
stake (money). A verifier is then chosen (picking algorithm: time, amount staked, random selection, etc) to do the verification

True or False, PoS does not require additional overhead in the
’staking’ process.

False

Issues with Blockchains? (3)

1. privacy concerns as all data on chain is visible,

2. identity and trust issues,

3. life-cycle issues

smart contracts dictates what?

code runs automatically once all conditions are met,
allowing workflows to be automated based on transactions being written to the blockchain