Introduction to Cryptography (D334)

Correct Answer B. shifts the alphabet by 3 letters

Explanation

The Caesar cipher is one of the simplest and oldest substitution ciphers. It works by shifting the letters of the alphabet by a fixed number of positions. In its classic form, this shift is by 3 letters, so A becomes D, B becomes E, and so on. It is a form of monoalphabetic substitution cipher.

Why other options are wrong

A. uses a cryptographic machine with multiple interlocking discs

This describes the Enigma machine, which was a complex mechanical cipher machine used in World War II. The Caesar cipher does not use any machine but simply shifts letters.

C. uses a codebook of truly random data to mix with the plaintext

This description more closely matches a one-time pad or random substitution cipher, not the Caesar cipher. The Caesar cipher relies on a fixed, predictable shift, not random data.

D. uses two keys - one for encryption and the other for decryption

This describes asymmetric encryption systems, like RSA, where there are distinct keys for encryption and decryption. The Caesar cipher uses one key (the shift) for both encryption and decryption.

Correct Answer D. Allows only the correct receiver to decode the data

Explanation

In a public key encryption system, data is encrypted with the receiver’s public key and can only be decrypted by the receiver using their private key. This ensures that only the correct receiver, who holds the corresponding private key, can decode the transmitted data.

Why other options are wrong

A. Allows only the correct sender to decode the data

This is incorrect because the sender does not need to decode the data in public key encryption. The sender encrypts the data using the receiver's public key, and only the receiver can decrypt it using their private key.

B. Does not encode the data before transmitting it

This is incorrect. In a public key encryption system, data is encoded (encrypted) before transmission to ensure privacy and security.

C. Allows anyone to decode the transmission

This is false because public key encryption relies on the private key being kept secret. Only the recipient, who possesses the private key, can decode the message, not everyone.

Correct Answer C. To flatten the frequency distribution of the ciphertext characters

Explanation

Polyalphabetic ciphers were designed to address the weaknesses of monoalphabetic ciphers, particularly their vulnerability to frequency analysis. In a monoalphabetic cipher, each letter in the plaintext is replaced with a specific letter in the ciphertext, which means that frequent letters in the plaintext (like 'E' or 'T') will appear frequently in the ciphertext. Polyalphabetic ciphers use multiple cipher alphabets, making it much harder to identify patterns and flattening the frequency distribution of ciphertext characters.

Why other options are wrong

A. To reduce the size of the key to something that could be easily remembered

Polyalphabetic ciphers actually use larger and more complex keys than monoalphabetic ciphers, and the aim was not to make the key easier to remember. The motivation was to make the encryption stronger by using different alphabets, not to simplify the key management.

B. To overcome the fact that nearly all monoalphabetic ciphers had already been used

While it’s true that polyalphabetic ciphers were developed to enhance security, the main reason was to reduce the vulnerability of monoalphabetic ciphers to frequency analysis. The idea was to make the cipher stronger, not simply because monoalphabetic ciphers were already in use.

D. To increase the size of the key space

Increasing the key space could be a result of using polyalphabetic ciphers, but this was not the primary motivation. The primary goal was to make cryptanalysis more difficult by flattening the frequency distribution, thus improving the security of the encryption system.

Correct Answer C. Frequency analysis

Explanation

Frequency analysis is a cryptanalysis technique that studies the frequency of letters or groups of letters in ciphertext. It is especially effective against classical ciphers like substitution ciphers, where patterns from the original plaintext remain visible in the ciphertext. By identifying recurring patterns, attackers can make educated guesses about the mapping between plaintext and ciphertext letters.

Why other options are wrong

A. Rainbow table

Rainbow tables are used for cracking hashed passwords by precomputing and storing hash values for a range of inputs. This method is not used for analyzing ciphertext patterns or letter combinations but rather for matching known hash values with their plaintext inputs.

B. Dictionary attack

A dictionary attack involves trying a list of likely passwords or phrases (such as words from a dictionary) to gain unauthorized access. It does not analyze ciphertext structure or recurring letter patterns but relies on guessing likely values.

D. Exhaustive search

Also known as brute force, exhaustive search tries all possible keys or combinations until the correct one is found. This method does not involve pattern recognition or analysis of letter frequency and is generally much more computationally intensive.

Explanation

Correct Answer Is:

Correct Answer C. Frequency Analysis

Explanation

Frequency analysis is the best method for breaking substitution ciphers. This technique relies on the fact that in any language, certain letters appear more frequently than others. By analyzing the frequency of letters in the ciphertext and comparing them to common letter frequencies in the language of the plaintext, cryptanalysts can often identify patterns and deduce the substitution key.

Why other options are wrong

A. Differential Analysis

This is incorrect. Differential analysis is more effective against block ciphers and focuses on how differences in the input affect the output. It is not suitable for breaking simple substitution ciphers.

B. Punch Card

This is incorrect. Punch cards are a form of data storage and are not directly used for breaking substitution ciphers. They were used historically for data processing, not cryptanalysis.

D. Meet-in-the-middle attack

This is incorrect. A meet-in-the-middle attack is used in scenarios like double encryption with two keys, typically in block ciphers, not in substitution ciphers. It does not apply to the simpler structure of substitution ciphers.

Correct Answer C. Cryptography

Explanation

Cryptography is the science of securing information by transforming it into a secure format. This field includes various techniques and principles for ensuring confidentiality, integrity, and authenticity of data. Through methods such as encryption and the use of ciphers, cryptography enables secure communication over potentially insecure channels.

Why other options are wrong

A. Encryption

Encryption is one of the methods used in cryptography, but it is not the overarching process or field itself. It refers specifically to the act of converting plaintext into ciphertext, not the entire discipline concerned with secure communication.

B. Decryption

Decryption is the reverse of encryption, where ciphertext is transformed back into readable plaintext. While essential to cryptography, it is just one component and does not represent the full range of activities and principles covered by cryptography.

D. Ciphers

Ciphers are algorithms or techniques used to perform encryption and decryption. They are tools within the field of cryptography, not the field itself. Confusing a tool with the discipline it serves can lead to a limited understanding of the scope of cryptography.

Correct Answer C. A cryptographic system should remain secure even if its algorithm is publicly known.

Explanation

Kerckhoffs' Principle asserts that the security of a cryptographic system should not depend on the secrecy of the algorithm, but rather on the secrecy of the key. This principle encourages transparency in the cryptographic algorithm, which can be publicly analyzed and tested for flaws while still maintaining security as long as the key is kept secret.

Why other options are wrong

A. A cryptographic system must be kept secret to ensure its security.

This statement contradicts Kerckhoffs' Principle. The security of a system should not rely on keeping the system itself secret but rather on the security of the key used in the system.

B. The security of a cryptographic system relies solely on the secrecy of the key.

While the key is a critical component, this statement oversimplifies the concept. It neglects the fact that the algorithm itself should also be robust and well-tested, as Kerckhoffs' Principle suggests that the system should remain secure even if the algorithm is public.

D. The complexity of a cryptographic algorithm is the primary factor for its security.

While complexity can play a role in security, Kerckhoffs' Principle emphasizes that security should primarily depend on the secrecy of the key, not the complexity of the algorit. hmOverly complex algorithms can lead to implementation errors and vulnerabilities, making simplicity sometimes a better design choice.

Correct Answer C. The public key is used to encrypt plaintext and the private key is used to decrypt ciphertext.

Explanation

In the RSA algorithm, encryption is performed using the recipient’s public key. The sender uses the recipient's public key to encrypt the plaintext message. To decrypt the ciphertext, the recipient uses their private key. This ensures that only the recipient can decrypt the message, as only they possess the private key.

Why other options are wrong

A. The public key is used to encrypt plaintext and to decrypt ciphertext.

This is incorrect because, in RSA, the public key is used only for encryption. The decryption process requires the private key.

B. The private key is used to encrypt plaintext and to decrypt ciphertext.

This is incorrect because, in RSA, the private key is used for decryption, not encryption. Encryption is done using the public key.

D. The private key is used to encrypt plaintext and the private key is used to decrypt ciphertext.

This is incorrect because the private key is only used for decryption in RSA. Encryption requires the use of the public key.