WGU Introduction-to-Cryptography Online Access

A digital signature is a cryptographic mechanism that enables a recipient to verify who created a message (authenticity) and that the message has not been altered (integrity). It is typically built using asymmetric cryptography: the signer uses a private key to sign a hash (digest) of the message, producing a signature. Anyone with the signer’s public key can verify that the signature matches the message digest, confirming the signature was created by the corresponding private key and that the content remains unchanged. Digital signatures do not primarily provide confidentiality; the signed message may still be readable unless separately encrypted. They also support nonrepudiation in many operational contexts because a valid signature can be strong evidence that the private key holder authorized the signed data, assuming key protection and policy controls. Common digital signature algorithms include RSA-PSS, ECDSA, and EdDSA. Certificates (X.509) are often used to bind public keys to identities, allowing verifiers to trust the claimed signer. Therefore, the best definition is a technique to verify authenticity and integrity.

OCSP is the primary online alternative to CRLs for checking whether a certificate has been revoked. With a CRL, a relying party periodically downloads a list of revoked certificate serial numbers published by the issuing CA (or CRL distribution point). That approach can be bandwidth-heavy, introduces latency between revocation and client awareness, and can result in clients using stale revocation data if updates are infrequent. OCSP improves this by allowing a client (or a server on the client’s behalf) to query an OCSP responder in near real time about the status of a specific certificate (good, revoked, or unknown). In practice, many TLS deployments use OCSP stapling, where the server periodically fetches a signed OCSP response from the CA’s responder and “staples” it to the TLS handshake, reducing client-side network calls and improving privacy (the CA doesn’t learn which site the client is visiting). Thus, OCSP provides a more timely, certificate-specific revocation status mechanism than CRLs while preserving the CA’s signed assurance.

An IV (Initialization Vector) is a value used to ensure that encrypting identical plaintext under the same key produces different ciphertexts, preventing pattern leakage. In many secure designs, the IV must be unique (and often unpredictable) per encryption operation. A common way to ensure uniqueness is to incorporate a nonce—a “number used once.” A nonce can be random, pseudo-random, or a counter-based value depending on the mode and security requirements. For example, CTR mode uses a nonce combined with a counter to produce unique input blocks; GCM uses a nonce/IV to ensure unique authentication and encryption behavior. The encryption key should remain stable across many operations and should not be used as the “randomizer” for IV generation; mixing key material into IV creation in an ad hoc way can create reuse or correlation issues. Plaintext and algorithm do not provide the needed uniqueness property. The nonce concept is specifically about ensuring one-time uniqueness of the starting value so that IV reuse does not repeat keystream blocks (stream modes) or reveal plaintext equality (CBC/CTR). Therefore, the correct choice is Nonce.

A polyalphabetic substitution cipher uses multiple substitution alphabets rather than a single fixed mapping. The classic cipher that uses a keyword to select shifting alphabets across the message is the Vigenère cipher. In Vigenère, each plaintext letter is shifted by an amount determined by the corresponding key letter (repeating the keyword as needed). For example, a keyword like “YELLOW” is aligned under the plaintext; each key character defines a Caesar shift (A=0, B=1, …) applied to the plaintext character, producing ciphertext. This rotation of alphabets across positions makes Vigenère more resistant to simple frequency analysis than monoalphabetic substitution, because the same plaintext letter may encrypt to different ciphertext letters depending on its position relative to the key. The Pigpen cipher is a symbol substitution cipher, Caesar is monoalphabetic with a single shift, and Playfair is a digraph substitution cipher using a 5×5 key square, not the repeating-key polyalphabetic method described. Therefore, the correct cipher is Vigenère.