1
|
= Specification for IMSI Pseudonymization on the Radio Interface for 2G and Above
|
2
|
|
3
|
== Introduction
|
4
|
|
5
|
A long-standing issue in the 3GPP specifications is, that mobile phones and
|
6
|
other mobile equipment (ME) have to send the International Mobile Subscriber
|
7
|
Identity (IMSI) unencrypted over the air. Each IMSI is uniquely identifying the
|
8
|
person who bought the associated Subscriber Identity Module (SIM) used in the
|
9
|
ME. Therefore most people can be uniquely identified by recording the IMSI that
|
10
|
their ME is sending. Efforts are made in the 2G and above specifications to
|
11
|
send the IMSI less often, by using the Temporary Mobile Subscriber Identity
|
12
|
(TMSI) where possible.
|
13
|
|
14
|
But this is not enough. So-called IMSI catchers were invented and are used to
|
15
|
not only record IMSIs when they have to be sent. But also to force ME to send
|
16
|
their IMSI by immitating a Base Transceiver Station (BTS). IMSI catchers have
|
17
|
become small and affordable, even criminals actors without much budget can use
|
18
|
them to track anybody with a mobile phone.
|
19
|
|
20
|
The solution presented in this document is to periodically change the IMSI of
|
21
|
the ME to a new pseudonymous IMSI allocated by the Home Location Register (HLR)
|
22
|
or Home Subscriber Service (HSS). The only component that needs to be changed
|
23
|
in the network besides the SIM is the HLR/HSS, therefore it should be possible
|
24
|
even for a Mobile Virtual Network Operator (MVNO) to deploy this privacy
|
25
|
enhancement.
|
26
|
|
27
|
== Location Updating
|
28
|
|
29
|
=== Regular
|
30
|
|
31
|
The SIM is provisioned with the IMSI (3GPP TS 23.008 section 2.1.9) and
|
32
|
cryptographic keys, that it uses to authenticate with the network. In the
|
33
|
Remote Access Network (RAN), the IMSI is sent over the air interface and then
|
34
|
transmitted to the Core Network (CN), where it is validated by the HLR/HSS.
|
35
|
The involved components vary by the generation of the network and whether the
|
36
|
SIM is attempting a Circuit Switched (CS) or Packet Switched (PS) connection.
|
37
|
But the principle is the same and looks like <<figure-imsi-regular>> for 2G CS
|
38
|
Location Updating with IMSI.
|
39
|
|
40
|
The IMSI is transmitted in the Location Updating Request from ME. The VLR
|
41
|
needs an authentication challenge specific to the secret keys on the SIM to
|
42
|
authenticate the SIM, and looks the authentication challenges up by the IMSI.
|
43
|
If the VLR does not have any more authentication challenges for the IMSI (as it
|
44
|
happens when the VLR sees the IMSI for the first time), the VLR requests new
|
45
|
authentication challenges from the HLR. Then the HLR verifies that the IMSI is
|
46
|
known and, if it is unknown, sends back an error that will terminate the
|
47
|
Location Updating procedure.
|
48
|
|
49
|
After the VLR found the authentication challenge, it authenticates the SIM, and
|
50
|
performs a Classmark Enquiry and Physical Channel Reconfiguration. Then the VLR
|
51
|
has the required information to finish the Location Updating, and continues
|
52
|
with an Update Location Request procedure with the HLR. Afterwards, the VLR
|
53
|
assigns a new TMSI with the Location Updating Accept, which is acknowledged by
|
54
|
the TMSI Reallocation Complete. In following Location Updates with the same
|
55
|
MSC, the ME sends the TMSI instead of the IMSI in the Location Updating
|
56
|
Request.
|
57
|
|
58
|
[[figure-imsi-regular]]
|
59
|
.Location Updating in 2G CS with IMSI
|
60
|
["mscgen"]
|
61
|
----
|
62
|
msc {
|
63
|
hscale="1.75";
|
64
|
ME [label="ME"], BTS [label="BTS"], BSC [label="BSC"], MSC [label="MSC/VLR"],
|
65
|
HLR [label="HLR"];
|
66
|
|
67
|
// BTS <=> BSC: RSL
|
68
|
// BSC <=> MSC: BSSAP, RNSAP
|
69
|
// MSC <=> HLR: MAP (process Update_Location_HLR, 3GPP TS 29.002)
|
70
|
|
71
|
ME => BTS [label="Location Updating Request"];
|
72
|
BTS => BSC [label="Location Updating Request"];
|
73
|
BSC => MSC [label="Location Updating Request"];
|
74
|
|
75
|
--- [label="VLR requests new authentication challenges for this IMSI if necessary"];
|
76
|
MSC => HLR [label="Send Auth Info Request"];
|
77
|
MSC <= HLR [label="Send Auth Info Result"];
|
78
|
---;
|
79
|
|
80
|
BSC <= MSC [label="Authentication Request"];
|
81
|
BTS <= BSC [label="Authentication Request"];
|
82
|
ME <= BTS [label="Authentication Request"];
|
83
|
ME => BTS [label="Authentication Response"];
|
84
|
BTS => BSC [label="Authentication Response"];
|
85
|
BSC => MSC [label="Authentication Response"];
|
86
|
BSC <= MSC [label="Classmark Enquiry"];
|
87
|
BTS <= BSC [label="Classmark Enquiry"];
|
88
|
ME <= BTS [label="Classmark Enquiry"];
|
89
|
ME => BTS [label="Classmark Change"];
|
90
|
BTS => BSC [label="Classmark Change"];
|
91
|
BSC => MSC [label="Classmark Update"];
|
92
|
BSC <= MSC [label="Physical Channel Reconfiguration"];
|
93
|
BTS <= BSC [label="Ciphering Mode Command"];
|
94
|
ME <= BTS [label="Ciphering Mode Command"];
|
95
|
ME => BTS [label="Ciphering Mode Complete"];
|
96
|
BTS => BSC [label="Ciphering Mode Complete"];
|
97
|
BSC => MSC [label="Ciphering Mode Complete"];
|
98
|
|
99
|
MSC => HLR [label="Update Location Request"];
|
100
|
MSC <= HLR [label="Insert Subscriber Data Request"];
|
101
|
MSC => HLR [label="Insert Subscriber Data Result"];
|
102
|
MSC <= HLR [label="Update Location Result"];
|
103
|
|
104
|
BSC <= MSC [label="Location Updating Accept"];
|
105
|
BTS <= BSC [label="Location Updating Accept"];
|
106
|
ME <= BTS [label="Location Updating Accept"];
|
107
|
ME => BTS [label="TMSI Reallocation Complete"];
|
108
|
BTS => BSC [label="TMSI Reallocation Complete"];
|
109
|
}
|
110
|
----
|
111
|
|
112
|
=== With IMSI Pseudonymization
|
113
|
|
114
|
==== SIM Provisioning
|
115
|
|
116
|
==== Successful Location Update With Pseudonymous IMSI
|
117
|
|
118
|
==== Next Pseudonymous IMSI Arrives Via SMS
|
119
|
|
120
|
==== Error Handling
|
121
|
|
122
|
===== SMS is Lost
|
123
|
|
124
|
===== SMS Arrives Late
|
125
|
|
126
|
== Implementation Notes
|
127
|
|
128
|
=== Source Code for Reference Implementation
|
129
|
|
130
|
=== ATT = 0 required
|
131
|
|
132
|
=== Warning the User if the IMSI Does Not Change
|
133
|
|
134
|
=== End to End Encryption of SMS
|
135
|
|
136
|
=== User-configurable Minimum Duration Between IMSI Changes
|