Encryption

A process of converting the original representation of the information (plaintext) into an alternative form (ciphertext). Ideally, only authorized parties can decipher a ciphertext back to plaintext and access the original information.

An encryption algorithm comprises

• a method for encrypting data
• a method for decrypting data
• a secret key used in the decryption/encryption method

The two types of encryption are

1. Asymmetric Key Cryptography (sometimes called public-key cryptography)
2. Symmetric Key Cryptography

Trapdoor: a mathematical function that is easy to go one way but hard to go the other way (an effectively one-way function)

• Common functions include RSA (prime factorization) and ECC
• RSA for example, is a trapdoor because multiplying primes is easy but factoring the result back into its component primes is hard.
• The bigger the spread between the difficulty of going one direction in a Trapdoor Function and going the other, the more secure a cryptographic system based on it will be

Language

• $A$: Alice
• $B$: Bob
• $K_A$: Alice’s encryption key
• $K_B$: Bob’s decryption key
• $m$: plaintext message
• $K_A(m)$: ciphertext, encrypted with key $K_A$
• $K_B(K_A(m))=m$

Types of attacks

1. Ciphertext-only attack: knowns $K_A(m)$ but not $m$
2. Known-plaintext attack: for some $m$ knows $K_A(m)$
3. Chosen-plaintext attack: knows $K_A$ but not $K_B$