Nothing is more important to a customer who is planning to shop at an online retail web site than knowing that all of the personal information such as credit/debit card numbers, name and address is going to be kept confidential. In to prevent phishing and the theft of this information most online retailers use some form of public key cryptography to ensure that even if the information is phished, what the thief comes up with is completely useless.
With public key cryptography there are actually two parts to the system, the first is the public key is that part which is embedded in the website and is responsible for encrypting the information, everyone who visits the site and buys something has access to this key. The second half is the private key and remains the sole possession of the site owner. It is this part of the system that decodes the information and makes it useable, without this key the information remains an indecipherable mess.