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Information Security > L6 - Introduction to Cryptographic Protocols > Flashcards

L6 - Introduction to Cryptographic Protocols Flashcards

1
Q

What is a protocol?

A

a series of steps,
involving two or more
parties,
designed to accomplish a task

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2
Q

What is an arbitrator?

A

A disinterested third party,

trusted to complete the protocol.

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3
Q

What is the purpose of an arbitrator?

A

Arbitrators can help complete protocols between parties

that don’t trust each other

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4
Q

Name three examples of real-world arbitrators.

A

Banks
Public Notaries
Lawyers

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5
Q

What are problems with arbitrators in the virtual world?

A
  • It’s more difficult for people to trust a faceless entity
  • An arbitrator can become a bottleneck, as he has to deal with every transaction (-> delays)
  • Lots of damage can be caused if arbitrator is subverted
  • Someone has to pay for running an arbitration service
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6
Q

What is an adjudicator?

A

An arbitrator that only gets activated in case of disputes.

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7
Q

What is the reason for using adjudicated protocols?

A

Arbitrators have high costs, so arbitrated protocols can be split into two sub-protocols:

  • A non-arbitrated part
  • An arbitrated part that is executed only if there is a dispute = adjudicated protocol
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8
Q

What is a real-world example of an adjudicator?

A

Judges:

Example: Alice and Bob can enter a contract without a judge; a judge only sees the contract if it is brought before a court

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9
Q

How do adjudicated protocols in the virtual world deter someone from cheating?

A
  • if someone SUSPECTS cheating, the protocol
    provides enough evidence to be able to detect this
  • In a good adjudicated protocol, this EVIDENCE also identifies the cheating party
  • Instead of PREVENTING cheating, adjudicated protocols DETECT cheating
  • The inevitability of detection acts as a deterrent
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10
Q

What is a self-enforcing protocol?

A
  • a self-enforcing protocol does not need an arbitrator or adjudicator
  • the protocol itself guarantees fairness
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11
Q

How do self-enforcing protocols work?

A
  • If one party tries to cheat, the other party is able to detect this immediately
  • the protocol then stops and/or punishes the cheating party
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12
Q

What is an example of a self-enforcing protocol?

A

dividing up things, e.g. a cake
1 Alice divides up the piece of cake
2 Bob chooses which piece to take

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13
Q

Name three applications of protocols involving cryptography.

A
  • secure communication
  • user authentication
  • key exchange
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14
Q

What could a SYMMETRIC cryptographic protocol look like?

A

1) Alice and Bob agree on a cryptosystem
2) Alice and Bob agree on a key
3 Alice encrypts her plaintext message using the
cryptosystem and key
4) Alice sends ciphertext to Bob
5) Bob decrypts ciphertext using the cryptosystem and
key

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15
Q

Name two potential problems (for world-spanning communication) in a SYMMETRIC cryptographic protocol which arise from the fact that Alice on Bob’s agreement on a key has to take place in secret?

A

Key management

Compromised Secrecy

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16
Q

How does one generally calculate the number of keys required in a SYMMETRIC cryptographic protocol?

A

(n/2)(n − 1)

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17
Q

Explain how COMPROMISED SECRECY would be problematic in a SYMMETRIC cryptographic protocol used for world-spanning communication.

A

If the key is compromised, then
– Eve can decrypt all messages encrypted with that key
– Mallory could intercept messages and send his own,
pretending to be Alice or Bob
– This protocol assumes that Alice and Bob trust each other: Either one could claim that the key has been compromised and publish the communication anonymously

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18
Q

Explain how KEY MANAGEMENT would be problematic in a SYMMETRIC cryptographic protocol used for world-spanning communication.

A

– If each pair of users in a network have their own key
then the total number of keys increases rapidly with the
number of users
– Sharing keys among users is not a solution:
If someone leaves the network or is not trusted anymore, all users sharing keys with that person have to change their
keys

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19
Q

Conventionally, which are the Dramatis Personae in cryptographic protocols?

A 
Alice: 1st participant
Bob: 2nd participant
Carol, Dave: more participants for multi-party protocols
Eve: Eavesdropper
Mallory: malicious active attacker
Trent: trusted arbitrator
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20
Q

What could communication with an ASYMMETRIC cryptographic protocol (public-key cryptosystem) look like?

A

1) Alice and Bob agree on a public-key cryptosystem
2) Bob sends Alice his public key
3) Alice encrypts her plaintext message using Bob’s public key
4) Alice sends ciphertext to Bob
5) Bob decrypts ciphertext using his private key

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21
Q

What are the advantages of ASYMMETRIC crypto communication?

A

The key exchange does not have to take place in secret
anymore!

!Compromised Secrecy:
Even if Eve listens in on step 2 and 4, she only has the public key and the ciphertext; This will not help her in recovering the private key or the plaintext

Key Management
Everyone who wants to communicate with Bob uses Bob’s
public key- There’s no need to have a separate key for each pair of users

22
Q

How can an ASYMMETRIC crypto communication protocol be attacked (other than chosen plaintext)?

A

a man-in-the-middle attack!
- In step 2, Mallory intercepts Bob’s public key and sends his own to Alice
- Alice encrypts her message using Mallory’s public key
(thinking it’s Bob’s) and sends it to Bob
- On the way back Mallory intercepts Alice’s ciphertext,
decrypts it, encrypts it with Bob’s public key, then sends it to Bob
- Mallory could even change the message
- Alice and Bob think they have a secure connection

23
Q

Why are public-key algorithms not used all the time instead of symmetric algorithms?

A

Public-key algorithms are much slower than most
symmetric algorithms
Public-key algorithms are vulnerable to chosen-plaintext
attacks:

24
Q

What is a chosen-plaintext attack?

A
  • If there are only a few possible messages, then it’s possible to recover the plaintext (although not the private key)
  • Encrypt all possible messages with the public key
  • If one of the generated ciphertexts matches the message ciphertext, then you know what was sent
25
Q

What could communication with HYBRID CRYPTOSYSTEM look like?

A
  • protocol combining symmetric algorithm with a public-key algorithm
    1) Alice and Bob agree on a public-key cryptosystem and a symmetric cryptosystem
    2) Bob sends Alice his public key
    3) Alice generates a random session key for the symmetric algorithm
    4) She encrypts the session key using Bob’s public key
    5) Alice sends ciphertext to Bob
    6) Bob decrypts ciphertext using his private key to recover the
    session key
    7) Alice and Bob can now continue with the symmetric
    algorithm
26
Q

Summarise the advantages of a HYBRID CRYPTOSYSTEM.

A
  • Using public-key cryptography for distributing a key
    improves key management
  • The key exchange can take place in public, even though a symmetric algorithm is used for the bulk of communication
  • The session key is only used for a limited time and then destroyed
  • The longer a key sits around, the higher the chances that it is vulnerable to compromise
  • The public-key cryptosystem is only used very sporadically, generating a very small number of ciphertexts
  • The less data, the harder it is to break a code
27
Q

What are some different names for a one-way hash function?

A
  • message digest
  • fingerprint
  • (cryptographic) checksum
  • message integrity check
  • manipulation detection code
28
Q

What is a hash function?

A

A hash function is a function that takes a variable-length
input string (called pre-image) and converts it to a
fixed-length (generally smaller) output string (called hash
value)

29
Q

What is an example use case of a hash function?

A

Can be used as a quick check if two pre-images are the

same: same pre-images have the same hash value

30
Q

What could be a simple example hash function?

A

A simple example is a function that takes the pre-image and returns a byte consisting of the XOR (exclusive-or) of all input bytes

31
Q

What is a ONE-WAY hash function?

A

A one-way hash functions is a hash function that works in one direction
It’s easy to compute a hash value from a pre-image, but it is hard to generate a pre-image that hashes to a particular value

32
Q

What does it mean that, a good one-way has function is collision-free?

A

it’s hard to generate two pre-images with the same hash value

33
Q

How to one-way functions achieve security?

A

The hash function is public, the security lies in its
one-wayness: it is computationally infeasible to find a
pre-image hashing to a particular hash value

34
Q

How is authentication performed using hashed values?

A

Instead of storing passwords, a host stores one-way hash
values of the passwords
1 Alice sends the host her password
2 The host performs a one-way hash function on the
password
3 Host compares the result to the stored value

35
Q

What is the benefit of storing one-way hashed passwords rather than cleartext passwords?

A

If someone breaks into the host, they cannot steal the

passwords. The one-way hash function cannot be reversed to obtain the passwords.

36
Q

Why is a file of passwords encrypted with a one-way hash function still vulnerable?

A

Dictionary Attacks!

37
Q

How can a Dictionary Attack be performed on a list of one-way hashed passwords?

A

Mallory can take a list of the most popular passwords and a dictionary and run it through the one-way hash function
After stealing the password file, he can then compare the password list with the generated hash values to find
matches

38
Q

What can be done to combat Dictionary Attacks when storing one-way hashed passwords?

A

Salting the passwords!

39
Q

What is Salt?

A

A Salt is an string that is combined with the password

40
Q

How does Salting work?

A

For example, assume the password selected by the user is ‘hello’, the computer will automatically create a salt, say‘e6f20fd420’
Another user also selects the same password, the
computer generates another salt,‘a30e26e83b2’
Simply appending the salt to the password, can generate
different representations for the same password in the
password file
SHA-256(“helloe6f20fd420”)=
50815056236f8b36a0c182de286f2786c87d8ec684ec85dd6ea0bce062b950b7
SHA-256(“helloa30e26e83b2”)=
c058814af38cee43c7deb31cd686a895d20b77cb7636a15c7b507a34a7e613aa
Note, we need to record the salt with each account

41
Q

What is considered best practice when salting passwords?

A

Don’t use the same salt, it should be unique per-user
per-password
Don’t make the salt too short, otherwise it is vulnerable to a brute force attack (going through every possible
combination of salt)
Salts should be selected as randomly as possible, so they are not predictable given prior knowledge of salts already used.

42
Q

What is Key Stretching?

A

using a Hash Function that deliberately

takes a long time to evaluate is called key stretching, the hash function is applied repeatedly.

43
Q

What is the purpose of Key Stretching?

A

The rationale is to reduce the rate at which hashes can be generated during brute force attacks.

44
Q

What is Password Eavesdropping?

A

Eve can intercept the password anywhere on the network or on the host before it is hashed
For example, early electronic locks sent serial number in clear

45
Q

What can we do to combat password eavesdropping?

A

Authentication with Public-Keys

46
Q

How could a sample protocol for Authentication with Public-Keys (rather than passwords) work?

A

The host keeps a file of every user’s public key, all users keep their private key
1) The host sends Alice a random string
2) Alice encrypts the string with her private key and sends it
back (along with her name)
3) The host looks up Alice’s public key and decrypts the
message with it
4) If the decrypted string matches what the host originally sent, then access is granted

47
Q

What are two potential weaknesses of public-key authentication (challenge-response-protocol)?

A

Cipher and plaintext are transmitted

Random numbers issued by host may not be truly random

48
Q

Explain how the transmission of cipher and plaintext within a challenge response protocol may present a security weakness?

A

Having access to (a large number of) plaintexts and
corresponding ciphertexts makes the job of a cryptanalyst easier
Eve could listen in on Alice’s authentication attempts to
obtain information to deduce private key

49
Q

Explain how a lack of true randomness among the numbers (challenges) issued by the host within a challenge response protocol may present a security weakness?

A
  • Mallory (knowing the sequence of future random number) could intercept host’s challenge
  • Mallory forwards one of the future random numbers, gets it encrypted by Alice; Alice’s login will fail
  • However, Mallory now has a correct response to a future challenge
50
Q

How can a simple challenge-response protocol public-key authentication protocol be improved in terms of the lack of truly random numbers??

A

Random number generator has to be implemented very

carefully

51
Q

How can a simple challenge-response protocol public-key authentication protocol be improved in terms of the the transmission of cipher and plaintext?

A

use the following protocol
(derived from SSHv2)
1) Alice sends her public key with a request for authentication to the host.
2) The host sends Alice a random number encrypted using Alice’s public key
3) Alice decrypts the message with her private key. She then uses a hashing algorithm to create a hashed version of the number and sends it to the host.
4) The host computes its own hashed version of the data in Step 2.
5) If the two outcomes match, access is granted.

Alice proves to the host, that she knows her private key
without giving away any information on her private key and without actually encrypting plaintext with her private key

Information Security (11 decks)

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