Explain how a sender creates a digital signature and how a recipient verifies it, including the role of the document and the two keys.

### The concept A **hash** is a compact value calculated from a document’s contents; in the example, the named algorithm is SHA-256. The sender’s signing command uses the document, its hash calculation, and the sender’s private key to produce a signature file. The sender shares the document and signature with the recipient and provides the corresponding public key. The recipient’s verification command checks the signature against the supplied document using that public key. If the document changes, its calculated hash no longer corresponds to the value represented by the original signature, so verification should fail. The private key and public key are related but have different jobs: the private key creates the signature, and the public key checks it. This workflow does not encrypt the document or establish message freshness. ### What a good answer covers - The sender signs with the private key and keeps it protected. - The signature is checked against the actual document, using a hash calculation. - The recipient uses the corresponding public key to verify. - Changes to the document should cause verification to fail. - Signing does not itself provide confidentiality or freshness. ### Terms - **Hash**: a compact value calculated from data for use in checking the signed content. - **SHA-256**: Secure Hash Algorithm 256, the hash algorithm used in the example. - **Private key**: the key used by the sender to create the signature. - **Public key**: the paired key used by the recipient to verify the signature. - **Confidentiality**: protection that prevents unauthorized people from reading data. - **Freshness**: evidence that a message is new rather than a reused message. ### Remember Describe the document, the private-key signing step, and the public-key verification step in that order.