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Unit 04 BASIC CRYPTOGRAPHY | Computer System Security AKTU Btech

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Important Questions For Computer System Security: 
*Unit-01     *Unit-02    
*Unit-03    *Unit-04 
*Unit-05    *Short-Q/Ans
*Question-Paper with solution 21-22 

Q1. What is the principle of public key cryptography? Discuss the applications for public key cryptography.

Ans. Principle of public key cryptography :

1. In an effort to address the most challenging issues with symmetric encryption, the idea of public key cryptography emerged:

i. Two communicants already share a key, which has been distributed to them

ii. The use of a key distribution center.

2. The capacity to retain complete secrecy over the communication is negated by the second issue, which goes against the entire foundation of cryptography.

Applications for public key cryptography : The use of public key cryptography is classified into three categories :

a . Encryption/decryption : With the recipient’s public key, a communication is encrypted by the sender.

b. Digital signature: A communication is signed by the sender using its private key. A cryptographic algorithm is used to sign the message or a short block of data that functions as part of the message.

c. Key exchange: To exchange a session key, two parties work in concert. The use of one or both parties’ private keys is included in a number of different strategies.

 Q2. What are the advantages and disadvantages of RSA?

Ans. Advantages of RSA :

1. Convenience : It fixes the issue with sharing the encryption key.

2. Provides message authentication : Digital signatures can be used with public key encryption, allowing the recipient of a message to confirm that it came from a certain sender.

3. Detection of tampering : Public key encryption with digital signatures enables the receiver to determine whether the communication was changed while in transit. A message that has been digitally signed cannot be changed without the signature becoming invalid.

4. Provides non-repudiation : Physically signing a document and digitally signing a message are connected. The communication has been acknowledged, thus the sender cannot retract it.

Disadvantages of RSA :

1. Public keys should/must be authenticated : Everyone must confirm that their public keys belong to them because no one can be certain that a public key belongs to the person it claims to represent.

2. Slow : Comparatively speaking to symmetric encryption, public key encryption is slow. Not practical for use in mass message decryption.

3. Uses more computer resources : Compared to single-key encryption, it takes a lot more computer hardware.

4. Widespread security compromise is possible : The entirety of a person’s correspondence can be read by an attacker if they discover their private key.

5. Loss of private key may be irreparable : All received messages cannot be decoded if a private key is lost.

Q3. Explain key generation algorithm, signing algorithm, signature verification algorithm in digital signature.

Ans. 1. Key generation algorithms :

  • a. Digital signatures are electronic signatures that guarantee a certain sender sent the communication.
  • b. Authenticity and integrity should be guaranteed while conducting digital transactions since otherwise the data may be manipulated or a third party may pretend to be the sender and expect a response.

2. Signing algorithms :

  • a. Email programmes and other signing algorithms establish a one-way hash of the electronic data that needs to be signed in order to create a digital signature.
  • b. The hash value is subsequently encrypted by the signing algorithm using the private key (signature key).
  • c. The digital signature consists of this encrypted hash as well as other details like the hashing technique.
  • d. This digital signature is appended with the data and sent to the verifier.
  • e. Due to the fact that a hash function may transform any arbitrary input into a much shorter fixed length value, it is preferable to encrypt the hash rather than the full message or document. The digital signature consists of this encrypted hash as well as other details like the hashing technique.
  • f. This saves time because only a small hash value needs to be signed rather than a lengthy message, and hashing is also significantly quicker than signing.

3. Signature verification algorithms :

  • a Verifier receives digital signature along with the data.
  • b. It then processes the digital signature and the public key (verification key) using the verification algorithm and produces some value.
  • c. The received data is also subjected to the same hash function, which produces a hash value. The output of the verification process is then compared to the hash value. The digital signature is legitimate if they are both equivalent; else, it is invalid.

Q4. Write a short note on Message Digest (MD) hash function.


  • 1. A message of any length can be entered into the MD hashing algorithm, which is a one-way cryptographic function, and it outputs a fixed-length digest value that can be used to verify the validity of the original message.
  • 2. A secure cryptographic hash algorithm for validating digital signatures is the MD hash function.
  • 3. For purposes other than ensuring data integrity and spotting unintended data corruption, MD has been deprecated.
  • 4. Any message digest function aims to generate digests that seem random.
  • 5. To be considered cryptographically secure, the hash function should meet two requirements:
    • i. It is impossible for an attacker to generate a message matching a specific hash value.
    • ii. It is impossible for an attacker to create two messages that produce the same hash value.

Q5. Discuss public key distribution. Describe the various schemes μsed for public key distribution.


  • 1. Public key servers are used in public key cryptography to distribute public keys.
  • 2. When a key pair is created, one key is kept secret while the other, known as the public key, is uploaded to a server and made accessible to everyone so they can send the user a private, encrypted message.

Schemes used for the distribution of public keys are as follows :

1. Public announcement :

  • a . The public key should be public, meaning that a user can send it to any other user or broadcast it to a large community. This is the basic goal of public key encryption.
  • b. The forgery issue is the key issue. In other words, the key can be faked while it is being transferred.

2. Public directory :

  • a. The name and public key entries for each user are preserved and distributed by a reputable authority in a dynamic directory known as public directory.
  • b. This method presupposes that the authority’s private key corresponds to its public key, which is known to no one but the authority.
  • c. Each user has to register his or her public key with the directory authority.
  • d. The user has the option to replace the key with a new one of their choosing.

3. Public key authority :

  • a. If the authority’s private key is stolen in a public directory scheme, data loss may occur.
  • b. Therefore, a tighter control over the distribution of public keys from the directory needs to be provided in order to ensure higher security for public key distribution.
  • c. In this scenario, a central authority manages a dynamic directory of all users’ public keys. Only the authority’s public key is known to the user; the private key that corresponds to it is kept a secret by the authority.

Q6. What is Transport Layer Security (TLS) ?


  • 1. A technology known as Transport Layer Security (TLS) offers communication security for client/server applications that interact with one another over the Internet.
  • 2. It makes it possible for data being transported between multiple Internet nodes to be transmitted privately and with integrity.
  • 3 . TLS is a successor to the Secure Socket Layer (SSL) protocol.
  • 4. The protocol known as Transport Layer Security (TLS) ensures data integrity, privacy, and authentication between two interacting computer applications.
  • 5. For web browsers and other applications that require data to be shared securely across a network, such as web browsing sessions, file transfers, VPN connections, remote desktop sessions, and Voice over IP, it is the most extensively used security protocol (VoIP).
  • 6 . Widely used for internet communications and online transactions, TLS is a cryptographic technology that offers end-to-end communications security over networks.
  • 7. TLS essentially makes it possible to access programmes securely, transfer data, and conduct the majority of Internet-based communication.
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