Which of the following defines keyspace? It is the range of all possible keys that can be used to construct a cryptosystem. It involves applying an algorithm to plaintext, producing ciphertext. It enables stronger cryptography with shorter keys. It ensures that only the intended recipient can read the data.

The term "keyspace" refers to the first option: the range of all possible keys that can be used to construct a cryptosystem. This encompasses all potential combinations of keys that could be used in the encryption and decryption process in a cryptographic system. A larger keyspace increases the security of the system by making it more resistant to attacks such as brute force, where an attacker attempts to try every possible key until the correct one is found.

Extra: Cryptography is the practice of secure communication in the presence of third parties. To achieve this security, algorithms (a set of mathematical operations) are used to transform readable data, or plaintext, into an unreadable format, known as ciphertext. A key is a piece of information that determines the functional output of the cryptographic algorithm. When a key is applied to plaintext using a cryptographic algorithm, the result is ciphertext that appears random and meaningless to anyone who does not have the corresponding decryption key.

The keyspace essentially represents the total number of possible keys that could be used with the encryption algorithm. For example, if an encryption algorithm uses a 128-bit key, the keyspace consists of 2^128 (or approximately 3.4 x 10^38) different possible keys. A larger keyspace increases the difficulty of an attacker guessing the correct key through brute force attacks. However, simply having a large keyspace does not necessarily mean a cryptosystem is secure. The algorithm must also be designed to withstand various types of cryptographic attacks.

It's important for students learning about cryptography to understand that while a large keyspace is beneficial, the actual security depends on other factors as well, such as the cryptographic algorithm's resistance to cryptanalysis (the study of methods for obtaining the meaning of encrypted information, without access to the secret key). Additionally, the management of keys, including secure generation, distribution, and storage, plays a critical role in the overall security of a cryptosystem.