We’ve already touched on how subscribers are authenticated to the network, how the network is authenticated to subscribers.
Those functions are done “in the clear” meaning anyone listening can get a copy of the data transmitted, and responses could be spoofed or faked.
To prevent this, we want to ensure the data is ciphered (encrypted) and the integrity of the data is ensured (no one has messed with our packets in transmission or is sending fake packets).
Ciphering of Messages
Before being transmitted over the Air interface (Uu) each packet is encrypted to prevent eavesdropping.
This is done by taking the plain text data and a ciphering sequence for that data of the same length as the packet and XORing two.
The terminal and the eNodeB both generate the same ciphering sequence for that data.
This means to get the ciphered version of the packet you simply XOR the Ciphering Sequence and the Plain text data.
To get the plain text from the ciphered packet you simply XOR the ciphered packet and ciphering sequence.
The Ciphering Sequence is made up of parts known only to the Terminal and the Network (eNB), meaning anyone listening can’t deduce the same ciphering sequence.
The Ciphering Sequence is derived from the following input parameters:
- Key Kenc
- Packet Number
- Bearer Number
- Direction (UL/DL)
- Packet Size
Is is then ciphered using a ciphering algorithm, 3GPP define two options – AES or SNOW 3G. There is an option to not generate a ciphering sequence at all, but it’s not designed for use in production environments for obvious reasons.
Ciphering Sequences are never reused, the packet number increments with each packet sent, and therefore a new Cipher Sequence is generated for each.
Someone listening to the air interface (Uu) may be able to deduce packet size, direction and even bearer, but without the packet number and secret key Kenc, the data won’t be readable.
Data Integrity
By using the same ciphering sequence & XOR process outlined above, we also ensure that data has not been manipulated or changed in transmission, or that it’s not a fake message spoofing the terminal or the eNB.
Each frame contains the packet and also a “Message Authentication Code” or “MAC” (Not to be confused with media access control), a 32 bit long cryptographic hash of the contents of the packet.
The sender generates the MAC for each packet and appends it in the frame,
The receiver looks at the contents of the packet and generates it’s own MAC using the same input parameters, if the two MACs (Generated and received) do not match, the packet is discarded.
This allows the receiver to detect corrupted packets, but does not prevent a malicious person from sending their own fake packets,
To prevent this the MAC hash function requires other input parameter as well as the packet itself, such as the secret key Kint, packet number, direction and bearer.
By adding this we ensure that the packet was sourced from a sender with access to all this data – either the terminal or the eNB.
Nick