Introduction to Cryptography (D334)

Correct Answer B. The original data before any encryption is applied

Explanation

Plaintext refers to the original, readable data that is to be encrypted before any cryptographic algorithm is applied to it. This data could be a message, file, or any form of information that needs to be kept confidential. Once encrypted, the plaintext becomes ciphertext, which is unreadable without the appropriate decryption process. Plaintext is the "raw" data that forms the basis of cryptographic processes.

Why other options are wrong

A. The encrypted output of a cryptographic algorithm

This is incorrect. The encrypted output of a cryptographic algorithm is called ciphertext, not plaintext. Plaintext is the original data before encryption.

C. A method used to analyze encrypted messages

This is incorrect. The method used to analyze encrypted messages is called cryptanalysis, not plaintext. Plaintext refers to the unencrypted data before encryption.

D. A type of key used for decryption

This is incorrect. The key used for decryption is simply called a decryption key, not plaintext. Plaintext refers to the original data that needs to be encrypted or has been decrypted

Correct Answer B. Brute Force Attack

Explanation

A brute force attack is a cryptographic technique where an attacker systematically tries every possible combination of letters or numbers until they find the correct one. This is one of the most direct and simple methods of cracking ciphers, as it does not rely on understanding the encryption algorithm but rather exhaustively testing all possibilities. Brute force attacks are commonly used by hackers to gain unauthorized access to systems and data, especially when encryption keys are weak or the algorithm is vulnerable.

Why other options are wrong

A. Exhaustive Search

While "exhaustive search" may sound similar to brute force, it typically refers to a broader technique that may not always involve the same exhaustive and direct approach of testing all combinations as a brute force attack. Brute force is a specific form of exhaustive search that aims directly to break ciphers.

C. Permutation and Combination

Permutation and combination are mathematical principles used in probability and combinatorics, not directly a cryptographic attack method. These terms refer to the arrangement and selection of items in different ways, which may be part of understanding key structures, but they do not describe the method of testing combinations in brute force attacks.

D. Recursive Brute Force

Recursive brute force is not a standard cryptographic term. A brute force attack can sometimes be recursive, but the term itself does not describe the simple technique of testing all possible combinations for a key. Recursive brute force would refer to applying brute force iteratively in a recursive manner, but the general term for the attack remains brute force.

Correct Answer B. Caesar

Explanation

The Caesar cipher is one of the earliest and simplest forms of encryption. It works by shifting the letters of the alphabet by a fixed number of positions. For example, with a shift of 3, A becomes D, B becomes E, and so on. Named after Julius Caesar, who reportedly used it in his private correspondence, it demonstrates the basic principles of substitution ciphers.

Why other options are wrong

A. Roman

While Julius Caesar was Roman, "Roman" is not the name of a cipher technique. This term refers more generally to the civilization or era, not a specific encryption method. It is too vague to describe the precise nature of the cipher.

C. Augustus

Augustus was another Roman emperor, but no known cipher technique is named after him. Associating a cipher with his name is historically inaccurate. The Caesar cipher is specifically attributed to Julius Caesar, not Augustus.

D. Romani

"Romani" refers to a group of people, not a cipher technique. It has no historical or technical relevance to classical encryption methods. Using this term in the context of cipher techniques is incorrect and misleading.

Correct Answer B. A cipher that rearranges all the plaintext characters

Explanation

A transposition cipher works by rearranging the positions of the characters in the plaintext to create ciphertext. Unlike substitution ciphers that replace characters, transposition ciphers simply change their order. The goal is to make the original message unreadable without the correct key or process to reverse the transposition.

Why other options are wrong

A. A cipher that hides all the characters in a picture

This is incorrect. This describes a technique more related to steganography, where data is hidden in images, not a transposition cipher.

C. A cipher that replaces characters with different characters or symbols

This is incorrect. This describes a substitution cipher, not a transposition cipher. In substitution ciphers, characters are replaced by others, not rearranged.

D. A cipher that uses a one-way operation to create unreadable text

This is incorrect. A one-way operation typically refers to hash functions, not transposition ciphers. Transposition ciphers involve rearranging text and are reversible with the right key or method.

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.

Correct Answer C. By applying an encryption algorithm to the plaintext

Explanation

Ciphertext is produced by applying an encryption algorithm to plaintext. This process transforms readable data into an unreadable format to protect it from unauthorized access. The encryption algorithm uses a key to encode the data, ensuring only those with the correct key can decrypt and access the original message.

Why other options are wrong

A. By applying a decryption algorithm to the plaintext

Decryption algorithms are used to convert ciphertext back into plaintext, not the other way around. Using a decryption algorithm on plaintext would not generate ciphertext and goes against the basic principles of encryption processes.

B. By using a hashing function on the plaintext

Hashing and encryption are different. Hashing creates a fixed-size representation of data (a hash), which cannot be reversed to retrieve the original data. Encryption, on the other hand, is a reversible process intended to hide data and later restore it, making hashing unsuitable for generating ciphertext.

D. By converting the plaintext into a binary format

Converting plaintext into binary is simply changing the data's representation, not securing it. Binary conversion does not involve any cryptographic transformation or protection, so it does not result in ciphertext.

Correct Answer B. It uses a predefined set of codewords to represent entire words or phrases for message encryption.

Explanation

A codebook cipher works by using a predefined codebook that contains a set of codewords or code symbols that represent entire words or phrases. When encrypting a message, each word or phrase in the plaintext is replaced with its corresponding codeword from the codebook. This method can be more efficient than substituting individual letters and is often used for encoding entire messages or phrases.

Why other options are wrong

A. It replaces each letter in the plaintext with a corresponding letter from a fixed alphabet.

This describes a substitution cipher, not a codebook cipher. In a substitution cipher, individual letters are substituted, whereas a codebook cipher replaces entire words or phrases.

C. It employs a mathematical algorithm to transform plaintext into ciphertext.

This is a general description of many modern encryption algorithms, but it does not specifically apply to codebook ciphers, which rely on a fixed set of codewords rather than an algorithmic transformation.

D. It relies on a single key for both encryption and decryption processes.

This describes symmetric encryption, not a codebook cipher. While some encryption systems do use a single key, a codebook cipher primarily depends on a pre-shared codebook, not on a key for encryption and decryption.

Correct Answer B. By revealing the most common letters in a given ciphertext

Explanation

Letter frequency analysis is a common technique used in cryptanalysis to break substitution ciphers. It relies on the observation that certain letters or combinations of letters appear more frequently in the language of the plaintext than others. By analyzing the frequency of letters in the ciphertext, cryptanalysts can often make educated guesses about which ciphertext letters correspond to common plaintext letters, helping them to gradually decipher the message.

Why other options are wrong

A. By providing a method to encrypt messages more securely

Letter frequency analysis is a cryptanalysis technique, not an encryption method. It is used to break ciphers, not to make encryption more secure.

C. By ensuring that all letters are used equally in encryption

While some encryption methods like the Vigenère cipher may attempt to distribute letters more uniformly, frequency analysis focuses on identifying the unequal distribution of letters in substitution ciphers and exploiting this for decryption, not ensuring equal use of letters.

D. By generating random keys for symmetric encryption

Frequency analysis is unrelated to key generation. It is a technique for analyzing patterns in ciphertext, not for creating encryption keys.

Correct Answer D. Because TLS is deprecated, SSL should be used instead.

Explanation

The statement that "because TLS is deprecated, SSL should be used instead" is false. TLS (Transport Layer Security) is not deprecated, and SSL (Secure Sockets Layer) is actually considered outdated and insecure. TLS has replaced SSL for secure communications over the internet due to SSL’s vulnerabilities.

Why other options are wrong

A. Developing your own cryptographic algorithm is considered an insecure practice.

This is true because creating your own cryptographic algorithm is considered risky. Cryptographic algorithms need to undergo extensive peer review and analysis to ensure they are secure. A poorly designed custom algorithm is often vulnerable to attacks.

B. Cryptographic algorithms become trusted only after years of scrutiny and repelling attacks.

This is true. Cryptographic algorithms gain trust through extensive testing and scrutiny by the cryptographic community over time. This includes analysis for potential vulnerabilities and attacks.

C. The ability to use ever-faster hardware has enabled attackers to defeat some cryptographic methods.

This is also true. As hardware performance improves, previously secure cryptographic methods may become vulnerable to attacks, such as brute-force attacks, due to the increased computational power available to attackers.