Over-the-Air Encryption

The SIM/USIM encryption works as follows (a GSM/TETRA example):
1. A random challenge is sent from the network of 128 bits.
2. The handset encrypts the challenge using an algorithm known as A3 held on
the smart card and the key K: of 128 bits also on the smart card.
3. The handset sends back a signed response (S-RES 32 or 64 bit).
4. S-RES is passed through the A8 algorithm on the smart card to derive the key
Kc (54 bits + stuffer bits making up a 64-bit word), which is stored in the nonvolatile
memory on the SIM.
5. Kc is multiplied with a 22-bit word representing the frame number using the
A5 algorithm to produce 114 ciphered bits.
6. The 114 ciphered bits are Exclusive OR’d with 114 coded bits (2 × 57 coded bits
are contained in each bit burst).
7. A5 is embedded in the handset/BTS/Node B hardware.
To provide subscriber identity protection, the IMSI is replaced with a Temporary
Mobile Subscriber Identity number (TMSI) when the handset initially talks to the network
(before encryption is enabled). The TMSI is a product of the IMSI and the location
area identity (LAI).