Cryptography

Question 1

Hash functions

Answer 1

Requirements for a one-way hash function h:
1.Ease of computation: given x, it is easy to compute h(x).
2.Compression: h maps inputs x of arbitrary bitlength to outputs h(x) of a fixed bitlength n.
3.One-way: given a value y, it is computationally infeasible to find an input x so that h(x)=y.
4.Collision resistance: it is computationally infeasible to find x and x’, where x ≠ x’, with
h(x)=h(x’) (note: two variants of this property).

Question 2

symmetric ciphers

Answer 2

Den hemmelige nøkkelen er brukt til både kryptering og dekryptering

Question 3

Status/usage of SHA-1, SHA-2 and SHA-3

Answer 3

Dette er noen kjente Hash funksjoner:
• SHA-1 (Secure Hash Algorithm):160 bit digest. Designed to operate with DSA (Digital Signature Standard). Attacks exist. Not recommended, but sometimes still in use.
• SHA-2 designed by NSA in 2001 provides 224, 256, 384, and 512 bit digest. Considered secure. Replacement for SHA-1.
• SHA-3: designed by Joan Daemen + others in 2010.
Standardized in 2015. Digest of: 224, 256, 384, and 512 bit.
SHA-3 has little use, because SHA-2 is considered strong.

Question 4

Parameters (block and key size) of AES

Answer 4

Advanced Encryption Standard. AES har en key sizes på 128, 192 or 256 bit
og en block size på 128 bit.

Question 5

Applications of Hash functions and symmetric ciphers

Answer 5

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Question 6

MAC (Message Authentication Code)

Answer 6

• En melding M med en simpel message hash h(M) kan bli endret av en attacker.
• In communications, trenger vi å verifisere dataens opprinnelse, i.e. trenger vi message authentication.
• MAC (message authentication code) kan bruke hash function som h(M, k) i.e. med message M og en secret key k som input.
• For å validate and authenticate en message, må mottakeren dele den samme secret key brukt to compute the MAC with the sender.
• A third party who does not know the key cannot validate
  the MAC.

Question 7

Basic principle: keyed hash function

Answer 7

Samme som MAC functions.

The algorithm used to compute a MAC. Eksempler. HMAC, CBC-MAC, CMAC

Question 8

(MAC) Security services

Answer 8

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Question 9

Asymmetric ciphers

Answer 9

Et par med private og public keys hvor det er beregningsmessig umulig å hente den private dekrypteringsnøkkelen fra den tilsvarende offentlige krypteringsnøkkelen.

Question 10

Key Exchange

Answer 10

Diffie-Hellman key agreement (provides no authentication)

Question 11

usage of keys in encryption and digital signature

Answer 11

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Question 12

Digital signature

Answer 12

I motsetning til MAC, så kan digital signatures bli verifisert av en 3.part.
– Used for non-repudiation,
– data origin authentication and
– data integrity

Question 13

(Asymmetric ciphers) security services

Answer 13

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Question 14

Threat to classical crypto from quantum computing

Answer 14

Dersom det blir tatt i bruk quantum computing (med minst 1 mill qubits) vil det være mulig å dekryptere alle krypterte meldinger kun ved å implementere Shor sin quantum algoritme.

Question 15

Cryptography

Answer 15

The science of secret writing

with the goal of hiding the meaning of a message.

Question 16

What can cryptography do?

Answer 16

Crypto can provide the following security services:
1. Confidentiality:
• Makes data unreadable to entities who do not have the
appropriate cryptographic keys, even if they have the data.
2. Data Integrity:
• Entities with the appropriate cryptographic keys can verify that data is correct and has not been altered, either deliberately or
accidentally.
3. Authentication:
• Entities who communicate can be assured that the other
user/entity or the sender of a message is what it claims to be.
4. Digital Signature and PKI (Public-Key Infrastructure):
• Strong proof of data origin which can be verified by 3rd parties.
• Scalable (to the whole Internet) distribution of cryptographic keys.

Question 17

Encryption (Kryptering)

Answer 17

Plaintext (vanlig tekst) blir konvertert til ciphertext under kontroll av en nøkkel

Question 18

Decryption (Dekryptering)

Answer 18

Med en nøkkel kan man konvertere tilbake fra ciphertext til plaintext.

Question 19

Factors for cryptographic strength

Answer 19

1. Key size: Full key-search time depends on the key size.
2. Algorithm strength: Key discovery by cryptanalysis can exploit statistical
   regularities in the ciphertext.

Question 20

Asymmetric chiphers

Answer 20

Public key brukt til å kryptere og private key brukt til å dekryptere.

Question 21

Shannon’s S-P Network

Answer 21

-Substitutions & Permutations
Du har plain text også har du noen “bokser” med bits som blir subsidert med andre bits. Når alle bitsene er subsidert vil det foregå en permutation (omrokering) av boksene.

Question 22

One-Time-Pad

Answer 22

A string of letters or a string of bits. Its a like a key, but much longer. If you want to encrypt a file of 1MG, the key needs to be 1MG. One-Time-Pad offers perfect security if the OTP key is perfectly random and only used once.

Question 23

Diffie-Hellman Key exchange (provides no authentication

Answer 23

Handler om at to personer setter opp en secret key gjennom en “offentlig” kanal. Alice sender g^a til Bob, og Bob sender g^b til Alice. Begge har dermed den hemmelige nøkkelen g^ab.