ITPEC IP April 2026 Question 35

A and C — a digital signature proves who sent the e-mail and that it was not altered, but does not hide its content.

The sender computes a hash (message digest) of the e-mail and encrypts it with the sender's private key; that is the signature. The recipient decrypts the signature with the sender's public key, taken from a certificate issued by a certificate authority in the PKI, and compares it with a hash computed from the received e-mail.

• •A — appropriate: only the holder of the private key can make a signature that the sender's public key verifies, so the sender is confirmed
• •B — inappropriate: signing leaves the body readable; keeping it secret on the way needs encryption with the recipient's public key
• •C — appropriate: any change to the e-mail changes its hash, so the comparison fails and the tampering shows

Answer (b)

Why not others:

• •(a) A, B — includes B, confidentiality, which a signature does not give, and omits C
• •(c) B, C — includes B and omits A, confirming the sender
• •(d) A, B, C — includes B

Key rule: A digital signature gives authentication and integrity (and non-repudiation), never confidentiality: sign with the sender's private key, encrypt with the recipient's public key.

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