What is encryption?

Encryption is the process of converting readable information (plaintext) into an unreadable format (ciphertext) using a cryptographic algorithm and a key. The goal is to ensure data confidentiality so that only authorized persons can read it, using the appropriate key for decryption.

There are two main types of encryption:

1. Symmetric encryption: The same key is used for both encryption and decryption. It is fast, but the problem lies in secure key transmission. Examples: AES, DES.

   <?php
   // Example of symmetric encryption (simplified, for demonstration purposes only)
   $key = 'key123';
   $plaintext = 'Secret message';
   
   // Assuming a simple encryption function (not for production use)
   function simple_encrypt($data, $key) {
       return base64_encode($data ^ $key); // XOR operation example
   }
   
   // Assuming a simple decryption function (not for production use)
   function simple_decrypt($data, $key) {
       return base64_decode($data) ^ $key; // XOR operation example
   }
   
   $ciphertext = simple_encrypt($plaintext, $key);
   echo "Encrypted: " . $ciphertext . "\n";
   
   $decryptedtext = simple_decrypt($ciphertext, $key);
   echo "Decrypted: " . $decryptedtext . "\n";
   ?>
   

2. Asymmetric encryption (public key encryption): Uses a pair of keys: a public key for encryption and a private key for decryption. The public key can be freely distributed, while the private key is kept secret. It is slower than symmetric encryption. Examples: RSA, ECC.

   <?php
   // Example of asymmetric encryption (simplified, for demonstration purposes only)
   // In practice, PHP functions like openssl_public_encrypt, openssl_private_decrypt, etc., are used.
   
   // Key generation (complex process)
   // $privateKey = ...
   // $publicKey = ...
   
   $plaintext = 'Another secret message';
   
   // Encryption with public key
   // $encrypted = openssl_public_encrypt($plaintext, $ciphertext, $publicKey);
   // echo "Encrypted (asymmetric): " . base64_encode($ciphertext) . "\n";
   
   // Decryption with private key
   // $decrypted = openssl_private_decrypt($ciphertext, $decryptedtext, $privateKey);
   // echo "Decrypted (asymmetric): " . $decryptedtext . "\n";
   
   // In real code, OpenSSL functions would be used.
   ?>
   

Encryption is used to protect data at rest and in transit, for example, in SSL/TLS protocols of websites.