A tag affixable to a vehicle for obtaining vehicle telematics data, the tag including: a battery for powering the tag, wherein the tag is not connected to the vehicle's computer or power systems; an accelerometer to measure acceleration of the tag and thereby of the vehicle; a processor running executable code to process acceleration data provided by the accelerometer; a memory for storing acceleration data provided by the accelerometer; and a communication module for short range wireless communication with an untrusted mobile communications device located in the vehicle via a short range wireless communications protocol, the communication module configured to: a) transmit the tag's unique identifier and a sequence of time stamped acceleration data to the untrusted mobile communications device which is configured to forward the data to a server, the transmitted data being digitally signed by the tag using a secret key; b) receive data from the server forwarded by the untrusted mobile communications device, the received data being digitally signed using the secret key; and the tag being configured to verify the authenticity of the received data using the secret key, wherein every tag has a secret internal ID number (S_ID) built into the tag hardware, the secret key being derived from the tag's secret internal ID number (S_ID) and its MAC address and the mapping between the secret internal ID number (S_ID) and the MAC address is known to the server, such that it may verify the authenticity of the received data using the secret key. a battery for powering the tag, wherein the tag is not connected to the vehicle's computer or power systems; an accelerometer to measure acceleration of the tag and thereby of the vehicle; a processor running executable code to process acceleration data provided by the accelerometer; a memory for storing acceleration data provided by the accelerometer; and a communication module for short range wireless communication with an untrusted mobile communications device located in the vehicle via a short range wireless communications protocol, the communication module configured to: a) transmit the tag's unique identifier and a sequence of time stamped acceleration data to the untrusted mobile communications device which is configured to forward the data to a server, the transmitted data being digitally signed by the tag using a secret key; b) receive data from the server forwarded by the untrusted mobile communications device, the received data being digitally signed using the secret key; and the tag being configured to verify the authenticity of the received data using the secret key, a) transmit the tag's unique identifier and a sequence of time stamped acceleration data to the untrusted mobile communications device which is configured to forward the data to a server, the transmitted data being digitally signed by the tag using a secret key; b) receive data from the server forwarded by the untrusted mobile communications device, the received data being digitally signed using the secret key; and the tag being configured to verify the authenticity of the received data using the secret key, wherein every tag has a secret internal ID number (S_ID) built into the tag hardware, the secret key being derived from the tag's secret internal ID number (S_ID) and its MAC address and the mapping between the secret internal ID number (S_ID) and the MAC address is known to the server, such that it may verify the authenticity of the received data using the secret key.