21: Security

What are the 3 issues considered in chapter? (note)

* is the encryption algorithm sufficiently robust to prevent the encrypted data ebing decrypted by some unauthorised third-party?
* how possible to ensure secret key remains secret?
* how can receiver of a communication be sure who sent communication?

Give the diagram overview of encryption and decryption

Plaintext→ Encryption→Ciphertext → Decryption → Plaintext

^ ^

| |

Key Key

Plaintext

Data before encryption

Ciphertext

The result of applying an encryption algorithm to data

What are the security concerns relating to a transmission? 5

* Confidentiality: Only intended recipient should be able to decrypt ciphertext
* Authenticity: Receiver must be certain who sent the ciphertext
* Integrity: ciphertext must not be modified during transmission
* Non-repudiation: Neither sendor nor receiver should be able to deny involvement in tranmission
* Availability: Nothing should happen to prevent the receiver from receiving the transmission

(chapter only consider confidentiallity, authenticity and integrity)

How could the concerns arise (confidentiality, integrity)?

Confidentialy concern arises because a message could be intercepted during transmission and the contents ready by an unauthorised person.

Concern about integrity reflects fact that the transmission may be interfered with deliberately but also there may be accidental corrpution of data during transmission

What is the fundamental principle of encryption?

the encryption algorithm must not be a secret: must be in teh public demain

In contrast, an encryption key must be secret.

What are the 2 alternative approaches?

Symmetric key encryption

Asymmetric encryption (public key encryption)

Symmetric Key Encryption

One private key is held by both sender and receiver and is used for both encryption and decryption.

Asymmetric Key Encryption

There is a public key and a private key one of which is used for encryption and the other for decryption.

Describe smmetric key encryption

just one key. This key is a secret shared by sender and receiver of a message. Sender uses encryption algorithm together with the key to encrypt some plaintext. Receiver dcyrpts the ciphertext using same key

What is the issue with symmetric key encryption?

Delivery of the secret key. Sender needs the key to encrypt but how can key be securely delivered to receiver to allow decryption?

Describe asymmetric encryption

2 dif keys used, one for encryption and other for decrpytion. only one of these is a secret. Process is initiated by someone in possession of 2 keys. One of theses is a public key which is sent to anyone who is going to partake in an encrypted communication. The other is a secret private key which is never sent to anyone. Having a means of secure transmission of a secret key is no longer an issues.

Most likely scenario is that the holder of the 2 keys wishes to receive a transmission. - describe

A sender uses the public key to encrypt some plaintext and sends the ciphertext to th ereceiver. Receiver now only person who can decrypt the message because the private and public keys are a matched pair. Public key can be provided to any number of differnt people allowing the receiver to receive a private message from any of the diffferent people.

note:

* if 2 people require 2 way communication, both communicators need a private key and must send the matching public key to the other person
* 2 requrements to ensure confidentiality should hte transmission be intercepted and message extracted: encryption algorithm must be complex and # of bits used to define the key must be large

(what missing factor is: organisation to provide keys and to ensure their safe delivery to individuals using them)

Using asymmetric encryption, decryption-encryption works if keys are used other way round. An individual can encrypt a message with a private key and send this to a recipient who has corresponding public key and who can use this to decrypt the received ciphertext. Approach would not be used if content of a message was confidential because Anyone may be in possession of the public key. However could be used if

important to verify who sender was. only sender has the private key and public key only works with that one specific private key. - digital signature identifying the sender

show a diagram of a sender using a one-way hash function to send a digital signature

Message → Cryptographic hash function → Digest

|

v

Sender’s private key → Encryption→ digital signature

What is hte disadvantage of using this method of applying a digital signature?

it is associated with an encryption of the whole of a message.

What is an alternative method of applying a digital signature?

for the sender to use a public cryptographic one-way hash function which creates a number that is uniquely defined for the particular message called a digest. Process at senders end of transmission is same as diagram before. Private key is used to encrypt the digest. The encrypted digest is the digital signature. The mesage can be transmitted as plaintext together with teh encrypted digest as a separate file. Because the digest is much smaller than if the whole message were encrypted.

Give the diagram of the receiver checking that the received transmission is authentic and unchanged from the method before.

Message → Cryptographic hash function → Digest

\
Digital signature → Decryption → Digest

|

Sender’s public key

Describe the receiver end

same public one-way hash funciton is used to create a digest from the received message. Then the encrypted version of the original digest is decrypted using the public key. If hte 2 digests are identical the receiver can be confident that the message is authentic and has been transmitted unaltered.

The digital signatrure is different each time this process is used. This is becuase the digest is uniquely defined by teh hash funciton being applied to that particular message.

What is problem with this type of method for digital signature

the authenticity only confirms to the receiver that the message was sent from the person who had sent them the public key. Doesn’t consider if someone may create a public key and pretend to be someone else.

Therefore a more strict way of ensuring authentication is needed. Who can provide this?

Provided by a Certification Authority (CA as part of a Public Key Infrastructure (PKI)

Show a diagram of processes involved in obtaining a digital certificate (go over in textbook)

Person A (receiver) → (person A’s public Key) → CA

| (person A’s public key) | |

v (CA’s private key) V V

Person A’s website Digital certificate creation process

\
(Digital certificate creation procress goes back to Person A with Person A’s digital certifiace)

Process of receving a CA and obtain digital certifiace to allow safe public key delivery - shown in diagram above. Summarise/ explain in 7 steps

1. An individual (Person A) who is a would-be receiver and has a public-private key pair contacts a local CA
2. CA confirms identify of person A
3. Person A’s public key is given to CA
4. CA creates a public key certificate (digital certificate) and writes person A’s public key into this document
5. CA uses encryption with the CA’s private key to add a digital signature to this document
6. Digital certificate is given to person A
7. Person A posts the digital certificate on a website

note

Anyone who wishes to extract hte public key from the certificate has to use the CA’s public key.

For overall process to work there is a need for standards to eb defined regarding hte public key infrastructure and the produciton of the digital certificate.

What are the 2 similar processes of showing authentication to a process?

One case someone with a private key sends a public key to someone else.

Other case: the CA sends a digital certificate containing a public key

notes to summarise options available - summary 8 points

* starting position is someone who has a public-private key pari which are associated with a specific asymmetric key encryption algorithm
* this person could jsut make the public key available to anyone who is going to be either a sender or a receiver
* for optimum security the person instead sends the public key to a CA
* The CA creates a digital certificate which contains the public key with proof of the ownership of the public key
* anyone wishing to use the public key obtains it form this digital certificate
* a message encyrpted wiht the public key could be sent to the owner of the private key
* a message encrypted with the private key could be sent to anyone having the public key
* owner of the private key could use it to create a digital signature that oculd be used to authenticate an email ie

What does DES stand for? What type of encryption is it?

Data Encryption Standard (symmetric key encryption)

S-DES - simplified DES for this topic

S-DES is an example of a block cipher. What is a block cipher?

means encrypting blocks of bits

In S-DES how many bit blocks are encyrpted and what key is chosed?

8-bit blocks encrypted

10-bit key chosen

What is the first stage in S-DES?

create 2 8-bit keys from the 10-bit key. Permutation (reordering of digits) done. Next step is to apply a circular left shift to the first 5 bits and to the last 5 bits. Finally, the first of the 2 8-bit keys to be used in teh encryption is creating using hte permutation 6 3 7 4 8 5 10 9. Slightly modifed version is used to create a second 8-bit key.

What is the second stage? 5 steps.

Encryption:


1. initial permutaiton
2. application of a funciton using the permuted code and the first encryption key
3. a switch of the first and last 4-bit parts
4. a repeat application of the function but this time with teh second encryption key
5. final permutation using the reverse of the initial permutation sequence

The decryption by the receiver of a transmission uses the same generated 8-bit keys and follows the revers of the above process.

What does AES stand for?

Advanced Encryption Standard - defines block length as 128 bits. User can choose to use 128,192 or 256 bits for the key. Chances of the key being identified from the transmitted ciphertext are small. Main concern is the safety of the method used to provide the key to both sender and receiver

RSA, the usual method for public key encryption. What are the major features of the method? 2

the key generation algorithm

Encryption function

Key generation can be followed in 6 points:

1. 2 very large prime numbers p and q are chosen and their product n is calculated
2. the product (p-1)(q-1) is calculated
3. a prime number e less than (p-1)(q-1) and not a factor of it is chosen (65537 usual choice)
4. another number d is found which satisfies the condition that the produce of d times e when divided by (p-1)(q-1) gives a remainder of 1
5. public key becomes the pair (n,e)


1. private key becomes the pair (n,d)

What does the security of the algorithm depend on?

the fact that finding factors of a very large number is not feasible within any reasonable time scale.

Encryption works on numbers so how does encrypted text work?

must first have the characters replaced by numbers according to a sensible coding scheme.

Compare assymetric key encryption to symmetric

public key encryption is inherently more secure but the algorithms are not as fast. Quite common for public key encryption to be used to deliver securely a key that can then be used for symmetric key encryption

SSL was created to give assurance to a website user when a client-server application is underway. The intercae between an application and TCP uses a prot number. In the absense of a security protocol, TCP services an application using the port number. What is a socket

The combination of an IP address and a port number.

What happens when the SSL protocol is implemented?

When teh SSL protocol is implemented it funcitons as an additional layer between TCP in the transport layer and application layer. When SSL portocol is in place, the application protocol HTTP becomes HTTPS

Give some facts about SSL 10

* Although referred as a protocol is in fact a protocol suite
* There is a Record protocol taht deals with teh format for data transmission
* also a Handshake protocol responsible for security
* operation of SSL happens whithout any action from user
* starting point for SSL implementation is a connection between the client and server ebing established by TCP
* client browser then invokes handshake protocol from the SSP suite
* handshake protocol requests from the server its SSL certificate which is a digital certificate confriming its identity
* server sends this SSL certificate and its public key
* brower uses this public key to encrypt a key which is to be used as a one-off session key for symmetric key encryption to be used for the data transfer druing the session
* may also be a need at this time to agree which encryption algorithms are to be used

TLS is an upgraded version of SSL

note

alternatives for encyrption 2

symmetric: using one kye

asymmetric using 2 dif keys

Photons

particles that transmit light

demonstrate wave behavior so each photon appears to vibrate in a particular direction at right angles to its direciton of trave.

Polarisation

the direction each photon vibrates in and is represented in a diagram as a double-edged arrow.

A photon can be created with a specific polarisation to represent a value for a bit.

what are the 2 ways to represent a 1 and 2 ways to represent a 0 from state of polarisation

| = 1 -- =0 / = 1 \\ =0

just giving directions all should be double arrows

What do + and x represent on a shared secret code table

* represents using vertical and horizonal
* x representing diagonal pair

How does work?

sender chooses the bit pattern at random and also the polarisation basis for each value at random. receivers choice for polarisation basis afor each value also random choice. Following teh transmission the sender informs the receiver about the polarisation basis used for each value. Receiver responds by saying which ones were chosen to match. FOr these matches there is now a stored value for a bit.

above scheme been incorporated with QKD systems what are QKD’s?

Quantum Key Distribution - offers alternative for key tranfer

What is the advantage of QKD

transfer doesn’t invole defined values just photons. Anyone tyring to intercept the flow of photons in an attempt to disover their polarisation will destryo the photons. A photon cannot be detected and measured hten sent on again.

What is the main drawback of quantum cryptography?

it cannot be implemented using standard communication media. Requires a dedicated, special purpose ‘quantum channel‘ between sender and receiver. Costs are very high so routine unlikely

What are some itesm present in a digital certificate?

Identity of owner

owner public key

CA name

Digital signature of CA