8 Security Aspects
完成条件
8.1 User Identity and Authentication
For
confidentiality reasons, information should be accessible to authorized parties
only. Which part or amount of data should be protected can differ extremely: it
can include all data or even cover their existence, or it can only include
specific parts of the data. The authorization is established with a contract
and stored in an authentication center. Based on this, an authentication can
identify if the source of information is correct. This is done when
establishing a connection, but can also be initiated during an ongoing
connection, e.g. with respect to handover.
The SIM card plays a vital role with respect to security. It needs to be tamper resistant, protected by a PIN code, and removable from the mobile equipment. It contains secret keys and specific algorithms.
A mobile station consists of hardware, protected against abuse by an International Mobile Equipment Identity (IMEI) and a SIM card with user specific data. IMEIs are stored centralized in Dublin at the Equipment Identity Register (EIR) containing three different lists: white - all terminals, gray – malfunctioning terminals (optional) and black – barred terminals. At attach or after a request the hardware can be checked. Also the equipment identifies itself to the network. This feature should enable barring of stolen mobile stations, but unfortunately not all operators deliver the data to the EIR making it less effective.
The user accesses his/her SIM (Subscriber Identity Module) with a Personal Identity Number (PIN). This procedure has no access to the air interface. The PIN is not secure enough for authentication to the network over the air interface, because once getting access to the PIN the network access would be possible.
For user authentication to the network another authentication mechanism is used. As in the fixed network each user or better each SIM has a telephone number associated. For confidentiality reasons GSM distinguishes between telephone number MSISDN (Mobile Subscriber ISDN (Integrated Service Digital Network)) and user identity 128 bit IMSI (International Mobile Subscriber Identity). Only the MSISDN has to be available for public. The unique assignment between MSISDN and IMSI is stored in the Home Location Register (HLR) and the IMSI used for identification is not public. This makes it harder to impersonate a foreign identity, because from the MSISDN the user identity cannot be derived. The IMSI is stored in the SIM as well for identification to the network.
The IMSI and its corresponding secret key Ki, a shared key of 128 bit, is the quintessence of the protection of the user identity according to authentication and confidentiality discussed in the following paragraphs.
Each IMSI gets assigned a secret authentication key Ki. This key is top confidential and will never be transmitted over the air interface. On the network side Ki is stored in the Authentication Center (AuC) of the home network, on the user’s side it is stored in the SIM. This is the basis for authentication as follows.
The goal of the authentication is to protect the network against unauthorized use and the creation of a session key for the following communication. The subscriber is identified by the IMSI/TMSI, followed by a challenge response authentication based on a long term secret key (Ki) shared by SIM and AuC.
The SIM card plays a vital role with respect to security. It needs to be tamper resistant, protected by a PIN code, and removable from the mobile equipment. It contains secret keys and specific algorithms.
Authentication
has different stages:The user identifies him-/herself to
his/her SIM card, i.e. the SIM is protected against unauthorized use. Then the
user identifies him-/herself to the network via SIM. The mobile equipment also
is identified.
A mobile station consists of hardware, protected against abuse by an International Mobile Equipment Identity (IMEI) and a SIM card with user specific data. IMEIs are stored centralized in Dublin at the Equipment Identity Register (EIR) containing three different lists: white - all terminals, gray – malfunctioning terminals (optional) and black – barred terminals. At attach or after a request the hardware can be checked. Also the equipment identifies itself to the network. This feature should enable barring of stolen mobile stations, but unfortunately not all operators deliver the data to the EIR making it less effective.
The user accesses his/her SIM (Subscriber Identity Module) with a Personal Identity Number (PIN). This procedure has no access to the air interface. The PIN is not secure enough for authentication to the network over the air interface, because once getting access to the PIN the network access would be possible.
For user authentication to the network another authentication mechanism is used. As in the fixed network each user or better each SIM has a telephone number associated. For confidentiality reasons GSM distinguishes between telephone number MSISDN (Mobile Subscriber ISDN (Integrated Service Digital Network)) and user identity 128 bit IMSI (International Mobile Subscriber Identity). Only the MSISDN has to be available for public. The unique assignment between MSISDN and IMSI is stored in the Home Location Register (HLR) and the IMSI used for identification is not public. This makes it harder to impersonate a foreign identity, because from the MSISDN the user identity cannot be derived. The IMSI is stored in the SIM as well for identification to the network.
The IMSI and its corresponding secret key Ki, a shared key of 128 bit, is the quintessence of the protection of the user identity according to authentication and confidentiality discussed in the following paragraphs.
Each IMSI gets assigned a secret authentication key Ki. This key is top confidential and will never be transmitted over the air interface. On the network side Ki is stored in the Authentication Center (AuC) of the home network, on the user’s side it is stored in the SIM. This is the basis for authentication as follows.
The goal of the authentication is to protect the network against unauthorized use and the creation of a session key for the following communication. The subscriber is identified by the IMSI/TMSI, followed by a challenge response authentication based on a long term secret key (Ki) shared by SIM and AuC.