Add more to README

README.md

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 # EasyRSA
 
 Simple and Secure Wrapper for PHPSecLib
+
+## Motivation
+
+Although the long-term security of RSA is questionable (at best) given the
+advances in index calculus attacks, there are many issues with how RSA is
+implemented in popular PHP cryptography libraries that make it vulnerable to
+attacks *today*.
+
+Thanks to the folks who developed [PHPSecLib](https://github.com/phpseclib/phpseclib),
+it's possible to use secure RSA in PHP. However, it's not user-friendly enough
+for the average PHP developer to use to its full potential. So we took it upon
+ourselves to offer a user-friendly interface instead.
+
+## How to use this library?
+
+### Generating RSA key pairs
+
+You can generate 2048-bit keys (or larger) using EasyRSA. The default size is 2048.
+
+```php
+use \ParagonIE\EasyRSA\EasyRSA;
+
+list($secretKey, $publicKey) = EasyRSA::generateKeyPair(4096);
+```
+
+### Encrypting/Decrypting a Message
+
+```php
+$ciphertext = EasyRSA::encrypt($message, $publicKey);
+
+$plaintext = EasyRSA::decrypt($ciphertext, $secretKey);
+```
+
+### Signing/Verifying a Message
+
+```php
+$signature = EasyRSA::sign($message, $secretKey);
+
+if (EasyRSA::verify($message, $signature, $publicKey)) {
+    // Signature is valid!
+}
+```
+
+## What Does it Do Under the Hood?
+
+* Encryption
+    * Generates an ephemeral encryption key
+    * Encrypts your plaintext message using [defuse/php-encryption](https://github.com/defuse/php-encryption)
+      (authenticated symmetric-key encryption)
+    * Encrypts the ephemeral key with your RSA public key, using PHPSecLib
+      (RSAES-OAEP + MGF1-SHA256)
+    * Calculates a checksum of both encrypted values (and a version tag)
+* Authentication
+    * Signs a message using PHPSecLib (RSASS-PSS + MGF1-SHA256)