1. A method in a device that provides data, the method comprising: receiving by the device from a previous device a request for device data that includes a public key and a homomorphic encryption of received data based on the public key; combining by the device the received public key with a device public key into a combined public key; generating by the device a homomorphic encryption of device data to be added to the request using the combined public key; combining by the device the generated homomorphic encryption of the device data with the homomorphic encryption of the received data into a homomorphic encryption of the combined data; and forwarding by the device to a next device the combined public key and the homomorphic encryption of the combined data wherein (G,·) is a group and gεG and the homographic encryption of the device data is represented by the following: 
 x i =g r i 
 y i =( g s ·g s i ) r i ·g m i =g r i s+s ) ·g m i where r i is a device-specific number, g s is the received public key, g s+s i is the combined public key, s and s i are secrets for the received and device public keys, and m i is the device data and wherein (g r , g rs ·g m ) is the homomorphic encryption of the received data and where the combined homomorphic encryption of the data is represented by the following: 
 x′=g r+r i 
 y′=g (r+r i )(s+s i ) ·g m+m i where x′ is a first component and y′ is a second component of the combined homomorphic encryption represented by (x′, y′).