On the SitePoint PHP blog today David Brumbaugh shows you how to encrypt large messages with phpseclib and asymmetric keys. phpseclib is a PHP library specifically designed to handle encryption and decryption in an easy-to-use way.
Most of us understand the need to encrypt sensitive data before transmitting it. Encryption is the process of translating plaintext (i.e. normal data) into ciphertext (i.e. secret data). During encryption, plaintext information is translated to ciphertext using a key and an algorithm. To read the data, the ciphertext must be decrypted (i.e. translated back to plaintext) using a key and an algorithm. [...] A core problem to be solved with any encryption algorithm is key distribution. How do you transmit keys to those who need them in order to establish secure communication? The solution to the problem depends on the nature of the keys and algorithms.
He talks some about the difference between symmetric and asymmetric algorithms and some advice about the selection of the right one (or ones) to use in your app. He also talks briefly about the problem with RSA keys, mostly that it has limits on the amount of text it can encrypt. His solution is to "encrypt the message with a symmetric key, then asymmetrically encrypt the key and attach it to the message". He explains the encryption/decryption process step by step and starts in showing the code to make phpseclib do the work. He shows how to generate the keys, build the encrypt function and the decrypt function with about 30 lines of code each.