Following are the common security problems involved in communications and email:
Eavesdropping: It is the act of secretly listening to private information through telephone lines, e-mail, instant
messaging, and any other method of communication considered private.
Identity theft: It is the act of obtaining someone's username and password to access his/her email servers for
reading email and sending false email messages. These credentials can be obtained by eavesdropping on
SMTP, POP, IMAP, or Webmail connections.
Message modification: The person who has system administrator permission on any of the SMTP servers can
visit anyone's message and can delete or change the message before it continues on to its destination. The
recipient has no way of telling that the email message has been altered.
False message: It the act of constructing messages that appear to be sent by someone else.
Message replay: In a message replay, messages are modified, saved, and re-sent later.
Message repudiation: In message repudiation, normal email messages can be forged. There is no way for the
receiver to prove that someone had sent him/her a particular message. This means that even if someone has
sent a message, he/she can successfully deny it.
Answer option B is incorrect. A message digest is a number that is created algorithmically from a file and
represents that file uniquely.

Client-server networking is also known as client-server computing. It is a distributed application architecture
that partitions tasks or workloads between service providers (servers) and service requesters, called clients.
Often clients and servers operate over a computer network on separate hardware. A server machine is a high-
performance host that is running one or more server programs which share its resources with clients. A client
does not share any of its resources, but requests a server's content or service function. Clients therefore
initiate
communication sessions with servers which await (listen to) incoming requests.
Answer options D and B are incorrect. Peer-to-peer (P2P) computing or networking is a distributed application
architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent
participants in the application. They are said to form a peer-to-peer network of nodes. Peer-to-peer networking
(also known simply as peer networking) differs from client-server networking, where certain devices have the
responsibility to provide or "serve" data, and other devices consume or otherwise act as "clients" of those
servers.

Asymmetric encryption, also known as public-key cryptography, uses a pair of keys-a public key and a private key-to encrypt and decrypt data. This method is widely used for securing small amounts of data, such as digital signatures. In asymmetric encryption:
* The public key is used to encrypt the data.
* The private key is used to decrypt the data.
Digital signatures utilize asymmetric encryption to ensure the integrity and authenticity of a message. When a sender signs a document with their private key, the recipient can verify the signature using the sender's public key, confirming that the document was indeed signed by the sender and has not been altered.
References:
* EC-Council Certified Network Defender (CND) Study Guide
* Cryptography and Network Security Principles