Cryptography Basics

Cryptography is to hide information before sending and reveal it at recipient. Figure 1 is an illustration of encryption and decryption process.  

Fig. Crypto Basics

Alice uses an encryption key to encryption the plaintext. The encrypted text (ciphertext) is then sent to Bob. Bob decrypts the ciphertext with a key to reveal the plaintext Alice sent to him. 

Here the encryption key and decryption key (could be one) are important. They need to be strong and convenient. 

Common secret key (one key) encryption algorithm: DES (1976, 56 bits key length, vulnerabilities exist), AES (256-bit key length, current).

Common public key (two keys) encryption algorithm: RSA (current).

Ideally, keys should keep changing to make them impossible to guess or deduce. Random numbers are used to generate keys but only truly random number are considered ideal. 

Quantum Random Number Generators (RNG) use the randomness of photons, it is impossible for observing or predicting the outcome.   

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