A attaches a digital signature to a document sent to B. Which combination correctly gives the keys used to create and verify the signature and what the digital signature can confirm?

Answer: A. The signature is created with A's private key and B verifies it with A's public key. It confirms that the document was made by A and was not altered on the way

In a digital signature, the signer (A) processes the hash value of the document with A's own private key to create a signature, attaches it to the document and sends it. On receipt, B verifies the signature with A's public key and checks that it corresponds to the hash value computed from the document. Only A holds the private key, so successful verification shows that the document was signed by A (authenticity) and that not a single bit has been changed since it was signed (integrity). It also makes it hard for A to deny having signed it later (non-repudiation). That the public key really belongs to A must be confirmed separately, however, and certificates are used for this. On the other hand, a digital signature does not encrypt the document, so it cannot keep the contents secret; if secrecy is needed, encryption is combined separately. Also, a signature shows who signed, not whether what is written is factually true.

• A. Correct. The signature is created with the private key and verified with the public key, showing who signed and that nothing was altered.
• B. B's public key can be used by anyone, so a signature made with it cannot show who made it. A digital signature also does not keep the contents secret.
• C. The roles of the private and public keys are reversed; only A has A's private key. A signature also cannot show that the contents are true.