Introduction

Cryptography can be defined as the practice and study of techniques for secure communication and data protection where we expect the presence of adversaries and third parties. In other words, these adversaries should not be able to disclose or alter the contents of the messages

Terms

• Plaintext is the original, readable message or data before it’s encrypted. It can be a document, an image, a multimedia file, or any other binary data.
• Ciphertext is the scrambled, unreadable version of the message after encryption. Ideally, we cannot get any information about the original plaintext except its approximate size.
• Cipher is an algorithm or method to convert plaintext into ciphertext and back again. A cipher is usually developed by a mathematician.
• Key is a string of bits the cipher uses to encrypt or decrypt data. In general, the used cipher is public knowledge; however, the key must remain secret unless it is the public key in asymmetric encryption. We will visit asymmetric encryption in a later task.
• Encryption is the process of converting plaintext into ciphertext using a cipher and a key. Unlike the key, the choice of the cipher is disclosed.
• Decryption is the reverse process of encryption, converting ciphertext back into plaintext using a cipher and a key. Although the cipher would be public knowledge, recovering the plaintext without knowledge of the key should be impossible (infeasible).

Symmetric Encryption

Symmetric encryption, also known as symmetric cryptography, uses the same key to encrypt and decrypt the data, as shown in the figure below. Keeping the key secret is a must; it is also called private key cryptography. Furthermore, communicating the key to the intended parties can be challenging as it requires a secure communication channel. Maintaining the secrecy of the key can be a significant challenge, especially if there are many recipients. The problem becomes more severe in the presence of a powerful adversary; consider the threat of industrial espionage, for instance.

Assymetric Encryption

Unlike symmetric encryption, which uses the same key for encryption and decryption, asymmetric encryption uses a pair of keys, one to encrypt and the other to decrypt, as shown in the illustration below. To protect confidentiality, asymmetric encryption or asymmetric cryptography encrypts the data using the public key; hence, it is also called public key cryptography.

XOR Operation