Remote keyless entry message authen-tication

A remote keyless entry system includes a key fob and a vehicle. The key fob generates a secured message with a plaintext space and a ciphertext space. The vehicle determines whether the key fob is authorized based on first information in the plaintext space 404, decrypts an encrypted value in the ciphertext space based on a predicted full counter value 410, and determines whether the key fob is authorized based on second information in the encrypted value 412, 414. The key fob stores a counter value and a key fob identifier. A processor of the key fob generates an encrypted counter value using an encryption key and encryption algorithm. The key fob processor generates an authentication tag via a bitwise XOR with the encrypted counter value on a first portion of the counter value and a first portion of the key fob identifier. The authentication tag is included in the ciphertext space of the message. A second portion of the counter value and second portion of the key fob ID is included in the plaintext space. The second portion of the counter value is used by the vehicle to predict the full counter value that is used to decrypt the ciphertext.