Which scenario would necessitate placing a certificate on the Certificate Revocation List (CRL)?

A certificate should be placed on the Certificate Revocation List (CRL) when the private key associated with that certificate is compromised. The private key is crucial for maintaining the integrity and security of the certificate, which is used to establish secure communications and authenticate identities. If the private key is compromised, an attacker could impersonate the certificate holder or decrypt messages intended for them, thereby undermining the security protocols in place.

Revoking the certificate and adding it to the CRL prevents any further use of that certificate, alerting users and systems that the certificate can no longer be trusted. This helps maintain the overall security of the network by ensuring that only valid and secure certificates are in circulation.

While the other scenarios—such as the expiration of the certificate validity period, the revelation of an encryption key algorithm, or the signature key size being disclosed—may pose security concerns, they do not necessitate immediate revocation through the CRL in the same direct manner as a compromised private key. The primary risk lies in the ability of an attacker to misuse the compromised key, which is why this specific situation leads to a certificate being listed on the CRL.