EMAP is an expedited message authentication protocol proposed for vehicular ad hoc networks (VANETs) that replaces the time-consuming certificate revocation list (CRL) checking process with a more efficient revocation checking process using a keyed hash-based message authentication code (HMAC). EMAP uses a novel probabilistic key distribution that enables non-revoked vehicles to securely share and update a secret key. Analysis and evaluation show that EMAP is secure, efficient, and can significantly decrease message loss compared to conventional authentication methods using CRLs.