Jawaharlal Nehru Technological University Hyderabad B.Tech III CRYPTOGRAPHY AND NETWORK SECURITY Previous Question Paper (January/February - 2021 - R16)

Security is essential in modern computing to protect information from unauthorized access, modification, or destruction. As data becomes the primary asset of organizations, security ensures the CIA Triad: Confidentiality (preventing unauthorized disclosure), Integrity (preventing unauthorized modification), and Availability (ensuring timely access to authorized users).

Security attacks are generally categorized into two types:

1. Passive Attacks: These involve monitoring or eavesdropping on transmissions to obtain information without altering the system resources. Examples include traffic analysis and release of message contents. They are difficult to detect, so the focus is on prevention through encryption.
2. Active Attacks: These involve modification of the data stream or creation of a false stream. They include masquerade (pretending to be an authorized entity), replay (capturing and re-transmitting data), modification of messages, and denial of service (disrupting system availability). Detection and recovery are the primary goals here.

Security services, as defined by X.800, are mechanisms designed to counter these attacks:

1. Authentication: Ensures the identity of the communicating entity is genuine.
2. Access Control: Limits and controls access to host systems and applications.
3. Data Confidentiality: Protects data from unauthorized disclosure via encryption.
4. Data Integrity: Ensures data received is exactly as sent, without alteration.
5. Non-repudiation: Prevents either sender or receiver from denying a transmitted message.
6. Availability Service: Protects a system to ensure it remains accessible to authorized users despite attacks.

A block cipher is a symmetric-key cryptographic algorithm that operates on fixed-length groups of bits, called blocks, rather than individual bits or characters. It uses an invariant transformation algorithm and a secret key to map a plaintext block of size n bits to a ciphertext block of the same size. If the input data is not a multiple of the block size, padding is applied.

Because a block cipher can only encrypt a single block at a time, modes of operation are used to define how to repeatedly apply the cipher's single-block operation to securely transform amounts of data larger than the block size.

1. Electronic Codebook (ECB) Mode: Each plaintext block is encrypted independently using the same key. While simple and parallelizable, it is insecure for long messages because identical plaintext blocks result in identical ciphertext blocks, revealing patterns.
2. Cipher Block Chaining (CBC) Mode: Each plaintext block is XORed with the previous ciphertext block before being encrypted. An Initialization Vector (IV) is used for the first block. This ensures that identical plaintext blocks produce different ciphertext blocks, providing semantic security.
3. Cipher Feedback (CFB) Mode: The cipher turns the block cipher into a self-synchronizing stream cipher. The previous ciphertext block is encrypted and then XORed with the current plaintext block. This allows encryption of data smaller than the block size without padding.
4. Output Feedback (OFB) Mode: Similar to CFB, but it encrypts the output of the previous encryption step rather than the ciphertext itself. This creates a keystream independent of the plaintext, making it resilient to transmission errors as bit errors do not propagate.
5. Counter (CTR) Mode: This mode turns a block cipher into a stream cipher by encrypting successive values of a counter. It is highly efficient because it allows for random access and parallel processing of blocks, as the encryption of each block is independent of the others.