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Encryption Cryptography Caesor's Code-System Security-Exam Paper, Exams of Cryptography and System Security

Bengal Engineering & Science UniversityCryptography and System Security

This exam paper is related to System Security. This paper held at Bengal Engineering and Science University. It includes: Symmetric, Encryption, Public, Channel, Caesar, Cipher, Substitution, Counter, Mode, OFB, Confusion

Typology: Exams

2011/2012

Uploaded on 07/17/2012

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1. (45pts) Answer TRUE or FALSE to each of the questions below?
•Example: In order to communicate securely using symmetric encryption, two parties
must share a common key using a public channel. FALSE (In the example the reason
is underlined.)
•A cipher is an invertible function. TRUE ****
•The Caesar’s cipher is a poly-alphabetic substitution cipher. FALSE ***
•A block cipher is a stateless function, while a stream cipher is a finite-state machine.
TRUE ****
•Character frequency analysis is an example of a ciphertext-only attack. TRUE ****
•A simple substitution cipher can be broken using exhaustive key search in very short
time. FALSE ****
•The CFB mode of operation can use the block cipher encryption function both to encrypt
and decrypt messages. TRUE ****
•The Counter mode of operation is more efficiently parallelizable than other encryption
modes. TRUE ****
•The ECB mode of operation is not recommended for the transmission of long messages.
TRUE ****
•The OFB mode of operation is self-synchronizing, recovering automatically from trans-
mission losses. FALSE ****
•In CBC mode, the initialization vector IV is sent as the first block of the cipher-text.
TRUE ****
•One-time pad encryption with a randomly generated keystream is secure even if the
adversary has infinite computational power. TRUE ****
•Certain block cipher encryption modes can be used to implement stream ciphers. TRUE ****
•The DES S-Box implements a permutation. FALSE ***
•The OFB mode of operation has no error propagation: If a ciphertext bit is transmitted
incorrectly only the same bit of the plaintext is decrypted incorrectly. TRUE ****.
•If the random pad is very long, then it can be used multiple times without risk that the
adversary will decrypt messages. FALSE ***
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  1. (45pts) Answer TRUE or FALSE to each of the questions below?
    • Example: In order to communicate securely using symmetric encryption, two parties must share a common key using a public channel. FALSE (In the example the reason is underlined.)
    • A cipher is an invertible function. TRUE ****
    • The Caesar’s cipher is a poly-alphabetic substitution cipher. FALSE ***
    • A block cipher is a stateless function, while a stream cipher is a finite-state machine. TRUE ****
    • Character frequency analysis is an example of a ciphertext-only attack. TRUE ****
    • A simple substitution cipher can be broken using exhaustive key search in very short time. FALSE ****
    • The CFB mode of operation can use the block cipher encryption function both to encrypt and decrypt messages. TRUE ****
    • The Counter mode of operation is more efficiently parallelizable than other encryption modes. TRUE ****
    • The ECB mode of operation is not recommended for the transmission of long messages. TRUE ****
    • The OFB mode of operation is self-synchronizing, recovering automatically from trans- mission losses. FALSE ****
    • In CBC mode, the initialization vector IV is sent as the first block of the cipher-text. TRUE ****
    • One-time pad encryption with a randomly generated keystream is secure even if the adversary has infinite computational power. TRUE ****
    • Certain block cipher encryption modes can be used to implement stream ciphers. TRUE ****
    • The DES S-Box implements a permutation. FALSE ***
    • The OFB mode of operation has no error propagation: If a ciphertext bit is transmitted incorrectly only the same bit of the plaintext is decrypted incorrectly. TRUE ****.
    • If the random pad is very long, then it can be used multiple times without risk that the adversary will decrypt messages. FALSE ***
  1. (30 pts) Underline the word that makes the sentence true.
    • Example: The { Vigen´ere, Hill } cipher can be broken via linear cryptanalysis (solving linear equations relating the plaintext to the ciphertext).
    • The keyword length of a Vigen´ere cipher may be found using { the Kasiski method, the index of coincidence method (IC), either using Kasiski or IC methods}.
    • { Diffusion, Confusion } is a property of block ciphers that can be describe as follows: Each bit of the cipher-text is a function of all bits of the plain-text.
    • The effective length of the DES key is { 56, 64 } bits.
    • The { Hill, Playfair } cipher uses a 5×5 square to encrypt two characters of the plaintext at a time.
    • The enigma machine implements a { poly-alphabetic substitution, Vigen`ere } cipher.
    • The { CBC, CFB } mode of encryption uses the block cipher function to generate a keystream that is XORed with the plaintext.
  2. (40 pts) Define the following concepts precisely and completely.
    • Example: Confusion: is a property of block ciphers, meaning that the computed cipher- text is not statistically related to the key in a simple way.
    • Chosen plaintext attack: The adversary chooses some plaintexts and obtains the corresponding ciphertexts, hoping that the information will help him recover either the key or the decryption of a target ciphertext.
    • Brute-force attack: The adversary tries to decrypt ciphertext with all available keys, until the plaintext is recognized (either by its statistical properties or because the at- tacker has plaintext-ciphertext pairs).
    • Security through obscurity: When a system is designed to make analysis difficult, so that vulnerabilities in the system may remain undetected.
    • Differential of two equal-length bit strings: another bit string of the same length. The bits of the differential with value 1 correspond to positions where the argument bit strings have opposite values.
    • Error propagation rate: Given that one bit of ciphertext is transmitted incorrect, the error propagation rate is given by how many bits of plaintext are corrupted after decryption.
  1. (20pts) Since DES has too short keys (56-bits), Alice decides to use a quick scheme to combine DES with XOR to achieve 120-bit encryption. She picks a 56-bit key K 1 for regular DES and a 64-bit key K 2. To encrypt a block b, she first encrypts it using DES, then XORs the result with the second key. Mathematically, Alice’s encryption scheme is:

Alice-Encrypt(K 1 , K 2 , b) = DES(K 1 , b) ⊕ K 2.

Show that Bob can exhaustively search for the key K 1 without having to worry about the value of K 2 , and therefore the scheme is not much secure than basic DES, as far as exhaustive key search goes. Hint. Assume that Bob has two plaintext-ciphertext blocks:

b 1 , Alice-Encrypt(K 1 , K 2 , b 1 ) = DES(K 1 , b 1 ) ⊕ K 2 , and

b 2 , Alice-Encrypt(K 1 , K 2 , b 2 ) = DES(K 1 , b 2 ) ⊕ K 2 , and Show that Bob can craft a value that depends only on b 1 , b 2 , and K 1 , but not K 2. Bob can compute Alice-Encrypt(K 1 , K 2 , b 1 ) ⊕ Alice-Encrypt(K 1 , K 2 , b 2 ) = DES(K 1 , b 1 )⊕ DES(K 1 , b 2 ). The last value depends only on K 1 , so can be exhaustively searched using ap- proximately 2^55 tries, as in regular DES. Once the value of K 1 is known, DES(K 1 , b 1 ) can be computed and K 2 recovered as K 2 = Alice-Encrypt(K 1 , K 2 , b 1 ) ⊕DES(K 1 , b 1 ).