This document discusses message digests and digital signatures. It begins by introducing message digests or hashes, which represent a fixed-length string that uniquely identifies a message. It then discusses digital signatures, where a message digest is encrypted with a sender's private key. This allows a recipient to verify the integrity and authenticity of the message by decrypting the signature with the sender's public key and comparing it to a new message digest. The document outlines the steps for digital signatures and notes they can provide integrity, authentication, and non-repudiation, but not privacy. It explains each of these properties in more detail.