JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD MCA IV INFORMATION SECURITY Previous Question Paper (October/ November - 2020 - R15)
Answer any five questions. All questions carry equal marks.
Main
Define Caesar cipher? Calculate the encryption and decryption for the following plain text P= “COME TO MY HOME” by using ceaser cipher with Key k=3.
Answer
The Caesar cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. The transformation is defined by the mathematical function:
where x is the numerical value of the plaintext letter (A=0, B=1, ..., Z=25) and k is the shift key. The decryption process reverses this shift:
For the plaintext "COME TO MY HOME" with a key k=3, the encryption process maps each letter as follows:
- C(2) + 3 = 5 (F)
- O(14) + 3 = 17 (R)
- M(12) + 3 = 15 (P)
- E(4) + 3 = 7 (H)
- T(19) + 3 = 22 (W)
- O(14) + 3 = 17 (R)
- M(12) + 3 = 15 (P)
- Y(24) + 3 = 27 mod 26 = 1 (B)
- H(7) + 3 = 10 (K)
- O(14) + 3 = 17 (R)
- M(12) + 3 = 15 (P)
- E(4) + 3 = 7 (H)
The resulting ciphertext is "FRPH WR PB KRPH". To decrypt, each letter is shifted back by 3 positions (e.g., F(5) - 3 = 2 (C), R(17) - 3 = 14 (O), and so on), successfully restoring the original message "COME TO MY HOME".
Specify and contrast all kinds of cipher techniques in the cryptography.
Answer
Cipher techniques are broadly categorized into Substitution Ciphers and Transposition Ciphers.
Substitution ciphers replace plaintext characters with other characters, symbols, or numbers. They are subdivided into Monoalphabetic ciphers, where a single substitution alphabet is used for the entire message (e.g., Caesar cipher), and Polyalphabetic ciphers, which use multiple substitution alphabets to obscure frequency patterns (e.g., Vigenère cipher).
Transposition ciphers do not change the characters themselves but rearrange their positions according to a specific rule or key. Common examples include the Rail Fence cipher and Columnar transposition. Unlike substitution, transposition ciphers preserve the frequency distribution of the original plaintext letters.
The primary contrast is that substitution ciphers alter the identity of the characters, whereas transposition ciphers alter the order of the characters. Modern symmetric ciphers, such as AES, combine both techniques in multiple rounds to achieve high levels of diffusion and confusion.
Mention the key principles of security.
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Differentiate between AES and DES in a brief manner.
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Explain how key is generated in the RSA algorithm.
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Differentiate block cipher and stream cipher techniques.
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Explain key distribution of symmetric key.
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Define authentication service. And explain x.509 authentication services in a detail.
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Describe why Kerberos is more secure than the other security mechanisms?
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Describe the following terms in detail:
Whirlpool
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knapsack algorithm
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Demonstrate combining security associations.
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Discuss about the IP security architecture in detail.
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Differentiate socket layer security and transport security.
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Demonstrate techniques used to avoid guessable password.
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