The message or the data that Alice wants to share to Bob is called the plaintext. And in cryptography, the goal is to protect the plaintext against the unauthorized thief. The ciphertext is the coded message that gets sent through the medium, which Bob and Eve receive. Cipher is an algorithm for transforming the plaintext to ciphertext, and it typically uses a key for the transformation. It is important to note that the key is only known to Alice and Bob. The process of converting the plaintext to ciphertext is called encryption, while the reverse process of converting the ciphertext to plaintext is decryption. Let's apply the terms that we just defined when Alice wants to communicate the message, the plaintext, to Bob in the presence of the attacker Eve. Alice has the plaintext and uses an encryption cipher with the plaintext input to generate the ciphertext. The ciphertext gets shared via the medium and Bob accesses the ciphertext and decrypts it to retrieve the plain text. Although Eve can also receive the cyphertext, it does not have the key to correctly decrypt the cyphertext, and cannot learn the plaintext message. While the term cryptography Is to study the encryption and decryption techniques and focus on the Alice and Bob's perspective, it really does not make sense to analyze the security without considering the attacker Eve's perspective. Studying codebreaking from Eve's perspective is called cryptanalysis. While the term cryptology embodies both cryptography, to secure, and cryptanalysis, to break security. The advances in cryptography comes from studying both the Alice and Bob's perspective and Eve's perspective. When somebody says that he or she works in cryptography, that person generally means cryptology and include cryptanalysis. In the rest of the module, we will also look at the attacker's perspective, and model Eve, who's a threat to the cryptographic data protection.