IPA IP 2020 Question 100

It becomes impossible to check that the e-mail came from the correct counterpart — a certificate the sender made for themselves proves nothing about who owns the key.

The receiver verifies the signature with the public key in the sender's certificate. That shows the e-mail was signed with the matching private key and has not been altered since. Who holds that key is vouched for by the certificate authority: it checks the applicant's identity before signing the certificate, and the receiver trusts the CA. A self-created certificate is signed only by its own subject, so an impersonator can make one under anyone's name; the signature still verifies, but the link to the real sender is gone.

Answer (a)

Why not others:

• •(b) — spying on the e-mail is a matter of confidentiality; a signature does not encrypt the mail with either kind of certificate, encryption does
• •(c) — losing mail in transit is a delivery matter of mail servers and networks, unaffected by certificates
• •(d) — the certificate does not make content more fragile, and the signature still detects any change to the signed content

Key rule: A digital signature binds a message to a key; a certificate from a trusted CA binds that key to a person, so without the CA the signature proves integrity but not identity.

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