Cryptography

May 20, 2023

Cryptography is the practice of securing communication from adversaries. It involves the use of mathematical techniques to transform messages and data into a form that is unreadable to anyone who does not possess the key to decrypt it. Cryptography is used in a variety of applications, including secure communication, digital signatures, and authentication.

Cryptography algorithms can be divided into two main categories: symmetric and asymmetric.

Symmetric Cryptography

In symmetric cryptography, the same key is used for both encryption and decryption. The sender encrypts the message using the key, and the receiver decrypts the message using the same key. The key must be kept secret by both parties to ensure the security of the communication.

Symmetric cryptography is fast and efficient, making it ideal for encrypting large amounts of data. However, it suffers from the key distribution problem – how to securely distribute the key to the receiver without it being intercepted by an adversary.

Asymmetric Cryptography

Asymmetric cryptography, also known as public-key cryptography, uses a pair of keys: a public key and a private key. The public key can be freely distributed, while the private key is kept secret by the owner. Messages encrypted with the public key can only be decrypted with the corresponding private key.

Asymmetric cryptography solves the key distribution problem, as the public key can be freely distributed without compromising the security of the communication. However, it is slower and more computationally expensive than symmetric cryptography.

Cryptographic Hash Functions

Cryptographic hash functions are a type of one-way function that takes an input message and produces a fixed-size output, known as a hash. The output is unique to the input, and any slight change in the input will result in a vastly different output.

Hash functions are used for a variety of purposes, including data integrity checks and password storage. In password storage, the password is hashed and stored in a database, rather than the plaintext password. When a user enters their password, it is hashed and compared to the stored hash. This ensures that even if the database is compromised, the attackers cannot easily retrieve the plaintext passwords.

Digital Signatures

Digital signatures are used to ensure the integrity and authenticity of messages. A digital signature is created by taking a hash of the message and encrypting it with the sender’s private key. The receiver can then verify the authenticity of the message by decrypting the signature with the sender’s public key and comparing it to the hash of the message.

Digital signatures are commonly used in electronic transactions, such as online banking and e-commerce, to ensure that messages have not been tampered with and are from the intended sender.

Authentication

Authentication is the process of verifying the identity of a user or device. Cryptography is used in authentication protocols to ensure that the user or device is who they claim to be.

One example of an authentication protocol is the Transport Layer Security (TLS) protocol, used to secure communication between web browsers and servers. During the TLS handshake, the server sends its public key to the browser, which encrypts a random number with the key and sends it back to the server. The server decrypts the message with its private key and verifies that the random number matches the one it sent. This ensures that the browser is communicating with the intended server and not an imposter.

Cryptography and Privacy

Cryptography plays a crucial role in protecting privacy in the digital age. End-to-end encryption, where messages are encrypted on the sender’s device and decrypted on the recipient’s device, ensures that only the intended recipient can read the message. This prevents third parties, such as service providers and governments, from intercepting and reading messages.

However, cryptography also poses challenges to law enforcement and national security agencies, who may need to access encrypted messages to prevent or investigate crimes. This has led to debates around the balance between privacy and security, and the use of backdoors and other methods to allow access to encrypted messages.