A service that enables PayPal customers to use their mobile phones to authenticate transactions through a service that lets customers use SMS messaging to get a randomly generated access code to log into their accounts. The PayPal SMS Security Key is an extension of PayPal's current Security Key service, which uses a hardware token. However, unlike the current service, which charges customers $5 for the token, the new service is free. Customers will have to pay their carriers' charges for SMS services.