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


  • : Alice
  • : Bob
  • : Alice’s encryption key
  • : Bob’s decryption key
  • : plaintext message
  • : ciphertext, encrypted with key

Types of attacks

  1. Ciphertext-only attack: knowns but not
  2. Known-plaintext attack: for some knows
  3. Chosen-plaintext attack: knows but not