Encryption and Cryptography

Correct Answer: Support for key agility

In designing a cryptographic system, the aspect that would ensure that an encryption scheme supports a variable length key to enhance security is 'Support for key agility.' Key agility refers to the capability of a cryptographic algorithm or system to efficiently switch between different key sizes or keys without significantly affecting performance. This feature allows a system to adapt to varying security requirements, by enabling it to use longer keys when higher security is needed, without the need to redesign the system. Employment of fixed block sizes typically refers to the handling of input data rather than to key length, thus this choice does not directly suggest variable length key support. Implementation of a static algorithm selection suggests using one cryptographic algorithm, and does not imply support for variable key lengths. Inclusion of an elliptic curve digital signature algorithm (ECDSA) also does not ensure variable key length support for encryption. ECDSA is a specific type of digital signature algorithm that uses elliptic curve cryptography, and its presence in a system does not necessarily mean that the system's encryption scheme can support a range of key sizes.

Correct Answer: Certificate Authority issuance of digital certificates

The most effective mechanism to validate the integrity of a public key within a public key infrastructure (PKI) is through the issuance of digital certificates by a trusted Certificate Authority (CA). This practice ensures that a third party can authenticate the binding between a public key and the identity of its holder. The CA verifies the identity of the entity and then issues a certificate that associates their identity with their public key securely. Self-signed certificates, by contrast, offer a lower level of trust because they are created and signed by the entity itself rather than a trusted third party. While they can be used in closed environments or for internal testing, they are not widely regarded as a robust security mechanism for public key validation in open or large-scale settings. Using a public registry of hashes for public keys could provide a way to validate the integrity of the keys, but it does not deal with the problem of verifying the identity of the key owner. Without this layer of trust, it is still possible for a malicious actor to insert fake keys into the registry. Lastly, employing secret key cryptography for digital signature generation does not relate to the integrity of public keys within a PKI. Secret key cryptography uses the same key for both encryption and decryption and is not typically related to the digital signature schemes used in PKIs, which rely on a public/private key pair.

Correct Answer: Frequency analysis

The correct answer is Frequency analysis. This technique involves analyzing the frequency of letters or groups of letters in a piece of ciphertext. The idea is that, in any given language, certain letters and combinations of letters occur with predictable frequencies. By comparing these frequencies with the frequencies in the ciphertext, an attacker can make educated guesses about the content of the plaintext, and potentially the key used to encrypt it. This is particularly effective against simple substitution ciphers and some other classical encryption methods. Ciphertext-only attack is a broader term that refers to the scenario where the attacker has access only to a set of ciphertexts without any corresponding plaintext. While frequency analysis could be employed in a ciphertext-only attack, the term itself does not specifically denote the statistical technique used to decipher a key. Chosen-plaintext attack is when an attacker can encrypt plaintexts of their choosing and then study the ciphertexts to deduce the key or some other aspect of the encryption scheme. This involves actively interacting with the encryption system, not just passively analyzing patterns. Known-plaintext attack involves an attacker who has both the plaintext and its corresponding ciphertext. This knowledge can then be used to uncover the key or further details about the encryption algorithm. This type of attack is more about leveraging specific pairs of known plaintext and ciphertext rather than statistical analysis of patterns.