I have a simple html login form
Go through http://codeigniter.com/user_guide/libraries/encryption.html . Everything about the encryption and decryption is described here.
Performs the data encryption and returns it as a string. Example:
$pass_word = $this->encrypt->encode(input->post('pass_word'));
Decrypts an encoded string. Example:
$encrypted_pass_word = 'APANtByIGI1BpVXZTJgcsAG8GZl8pdwwa84';
$plaintext_pass_word = $this->encrypt->decode($encrypted_pass_word);
You can optionally pass your encryption key via the second parameter if you don't want to use the one in your config file. Go through the link to read the detail explanation about it.