Question

Try to make it as simple as you can and explain as much as it needed.

1. Define Integrity and Nonrepudiation? (2 points)

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2. What are the differences between Stream Cipher and Block Cipher? (2 points)

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3. What is Access Control and why is it important? (2 points)

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4. Message Authentication Code ensures authentication or integrity or both? Justify and explain your answer. (3 points)

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5. What are the weaknesses of DES? Why triple DES is better than Double DES? (3 points)

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Answer 1

(1) - Integrity is a fundamental requirement of a trustworthy identity infrastructure. Identity systems exchange credentials as well as messages and transactions regarding attributes, provisioning information, and other data. Trusting that the contents have not been tampered with is important. As an example, consider a document representing identity credentials. To trust those credentials, we must be able to verify they are authentic and have not been changed.

Non-Repudiation, is an assurance mechanism that that provides proof of the integrity and origin of data. An authentication that the message was sent by the send that can be asserted to be genuine with high assurance. This term was most often used in Business to Business transactions where the sender wanted assurance that an electronic purchase order for goods and services could not be repudiated later by the person who sent in a purchase order. A good example of non-repudiation is the blockchain, the mechanism by which Bitcoins are moved from one person to another.

(2) -

S.NO	BLOCK CIPHER	STREAM CIPHER
1.	Block Cipher Converts the plain text into cipher text by taking plain text’s block at a time.	Stream Cipher Converts the plain text into cipher text by taking 1 byte of plain text at a time.
2.	Block cipher uses either 64 bits or more than 64 bits.	While stream cipher uses 8 bits.
3.	The complexity of block cipher is simple.	While stream cipher is more complex.
4.	Block cipher Uses confusion as well as diffusion.	While stream cipher uses only confusion.
5.	In block cipher, reverse encrypted text is hard.	While in stream cipher, reverse encrypted text is easy.
6.	The algorithm modes which are used in block cipher are: ECB (Electronic Code Book) and CBC (Cipher Block Chaining).	The algorithm modes which are used in stream cipher are: CFB (Cipher Feedback) and OFB (Output Feedback).
7.	Block cipher works on transposition techniques like Caesar cipher, polygram substitution cipher, etc.	While stream cipher works on substitution techniques like rail-fence technique, columnar transposition technique, etc.
8.	Block cipher is slow as compared to stream cipher.	While stream cipher is fast in comparison to block cipher.

(3) - Access control is a security system that enables an organization or authority to control access at specific dates and time to certain areas of a community, building, or residence. It can also be described as a security technique that regulates who or what can view or use resources in a computing environment. It is a fundamental concept in security that minimizes risk to the business or organization.

Importance -

• Protecting your Employees – provide a safe working environment
• Restrict unauthorized access and reduce theft – control who has access to where and at what time
• Provide remote access – does a contractor need to access building but no-one is on site to let them in? You can provide access to your site remotely and you always know who is coming and going
• Eliminate key problems – no more keys, you can use swipe cards, tags, fingerprints or even your mobile phone
• Electronic visitor, history logging and reports – every access is logged and recorded. Reports allow you to see who is where in your building in case of an emergency.

(4) - Message Authentication Code(MAC) ensures both authentication and integrity. It uses a cryptographic checksum on data that uses a session key to detect both accidental and intentional modifications of the data. A MAC requires two inputs: a message and a secret key known only to the originator of the message and its intended recipient(s). This allows the recipient of the message to verify the integrity of the message and authenticate that the messege's sender has the shared secret key. If a sender doesn’t know the secret key, the hash value would then be different, which would tell the recipient that the message was not from the original sender.

(5) - Weakness of DES - Weakness has been found in the design of the cipher:
a. Two chosen input to an S-box can create the same output.
b. The purpose of initial and final permutation is not clear.

Triple DES is better than Double DES because it is much more secured than the original DES because it uses 112 and 168 bit keys respectively. It expands the size of the key by running the algorithm in succession with three different keys. It makes 48 passes through the algorithm. The resulting key is 168 bits; this can be hard to implement, so there is also a two-key option provided in 3DES that runs through a method called Encrypt-Decrypt-Encrypt (EDE):