To ensure that a digital certificate is valid, the receiver of the certificate must check ________.

• A. the digital signature
• B. the valid period
• C. Both A and B
• D. Neither A nor B

Answer: (C)

Verifying a digital certificate relies on two essential checks: the certificate’s digital signature and its validity period. The digital signature confirms the certificate was issued by a trusted certificate authority and that its contents haven’t been tampered with; you verify it using the CA’s public key. The validity period shows the certificate is currently usable, ensuring the current date falls within the not-before and not-after dates. If either check is skipped, you risk trusting a forged or tampered certificate, or using one that has expired or isn’t yet valid. In practice you’d also check revocation status and the trust chain, but both the signature and the time window must be valid for the certificate to be considered trustworthy.