`
`3GPP TSG SA WG3 (Security) Meeting #88Bis
`09 – 13 October, 2017, Singapore
`
`Source:
`Title:
`Document for:
`Agenda Item:
`
`Ericsson
`Discussion on protection of Network Steering Information
`Approval
`4.2.15 Others
`
`
`
`S3-172482
`
`revision of S3-17xabc
`
`Decision/action requested
`1
`SA3 received an LS from SA2 in S3-171733/S2-175286 requesting SA3 to consider the requirements in the LS for
`finding a security solution which securs the Network Steering Information from the HPLMN to the UE.
`
`2
`[1]
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`[2]
`
`[3]
`
`[4]
`172866)
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`3
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`
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`References
` S3-171733/S2-175286: LS on PLMN and RAT selection policies for roaming
`
` S3-172034: Securing the Network Steering Information (discussion paper from Samsung in SA3#88)
`
`S3-17xxxx/S1-173478: LS on PLMN and RAT selection policies for roaming
`
`S3-17xxxx/C1-173751: Reply LS to LS on PLMN and RAT selection policies for roaming (S2-175286/C1-
`
`Rationale
`
`3.1 S3-171733/S2-175286: LS on PLMN and RAT selection policies
`for roaming
`An LS from SA2 was received in SA3 #88 in [1]. The LS from SA2 in [1] states that there is a need to define a
`standardized way to allow a given HPLMN to provide its roaming UEs with information about preferred networks and
`RAT depending on the UE current location.
`
`SA2 did submit for consideration the following requirements in [1]:
`
`- a control plane solution is used from the HPLMN to the UE.
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`- VPLMN is able to relay this information to the UE.
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`- VPLMN shall not be able to alter the information sent by the HPLMN; i.e. UE needs to be able to check the
`integrity of the information provided to it.
`
`- UE shall be able to detect if VPLMN alter or remove those information and act accordingly.
`
`SA1 replied in S1-173478 [3] with references to corresponding service requirements in TS 22.261 (subclauses 5.1.2.1
`and 6.19), and TS 22.011 (subclause 3.2.2.8). SA1 requiremements seem to stress that the HPLMN needs to be able to
`steer or redirect the UE for a specific VPLMN at any time.
`
`CT1 indicated in C1-173751 that CT1 is responsible for the stage 2 specification (TS 23.122), and asks SA3 to
`investigate end-to-end security solution based on requirements in [1] before CT1 specifies any solution to the
`requirements.
`
`3.2 S3-172034: Securing the Network Steering Information
`In addition another paper in [2] was submitted to SA3 #88 on this topic.
`
`APPLE 1014
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`1
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`Two different alternatives were discussed in [2]. The two potential security credentials to be considered to secure the
`information from the AUSF (in the HPLMN) to the UE were:
`1. Using HN asymmetric key
`
`2. Using an anchor key resulted from primary authentication.
`
`In the conclusion in [2], the second alternative - Using an anchor key resulted from primary authentication – was
`preferred.
`
`As all operators may not support a HN asymmetric key, using an anchor key resulted from primary authentication,
`seems like the most promising solution.
`
`Detailed proposal
`4
`The following solution is using a key (i.e. configuration key) derived from Kausf resulted from primary authentication
`to secure the Network Steering Information (preferred PLMN and RAT list) from the HPLMN to the UE. The AUSF in
`HPLMN calculates a message authentication code over the Network Steering Information using this configuration key.
`In this proposal, we don't consider encryption even though we acknowledge that it may be required. Confidentiality
`protection requirements should be clarified.
`
`Some additional potential parameters to consider as well could be:
`
`- Configuration key identifier:
`o This identifier could tie the configuration key to the Kausf from which is has been derived. It could
`for example be the RAND;
`
`- Integrity protection algorithm identifier:
`o
`
`If the integrity algorithm is not identified separately, it could be the well-known KDF function
`typically used in 3GPP networks, i.e. HMAC-SHA-256 (cf. 3GPP TS 33.401 Annex A, and TGPP
`TS 33.220 Annex B);
`
`- Counter:
`o
`
`If the same configuration key needs to be used to calculate more than one MAC, then an additional
`counter is preferred as a parameter for detecting replay protection in the UE.
`
`
`Figure 4-1 demonstrates an example of the UE Registration procedure when the AUSF in home network performs
`the integrity protection of the Network Steering Information and includes the security protected Network Steering
`Information over N12 interface to the AMF/SEAF in VPLMN. The AMF/SEAF sends the protected Network
`Steering Information to the UE in a NAS message (e.g. Registration Accept message). Note that the example is an
`optimization, and the HPLMN needs to be able to send the Network Steering Information at any time to the UE, i.e.
`not only during Registration procedure.
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`2
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`UE
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`AMF/
`SEAF
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`1. Registration & authentication
`
`AUSF
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`UDM
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`2. Kausf generated
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`2. Kausf generated
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`3. Network Steering Information
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`4. Kconf derived from
`Kausf. MAC-1 calculation
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`5. Network Steering Information,
`MAC-1
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`6. Network Steering Information, MAC-1
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`7. Kconf derived from Kausf.
`MAC-1 verification,
`MAC-2 calculation
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`8. Network Steering ACK, MAC-2
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`10. MAC-2 verification
`
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`Figure 4-1: Signalling flow showing provisioning of the Network Steering Information from home PLMN to UE
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`1. The UE registers to the VPLMN, and is authenticated by AUSF.
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`2. UE and AUSF generate Kausf.
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`3. A node in the HPLMN (e.g. UDM) sends the Network Steering Information to AUSF. This example assumes that
`AUSF protects the Network Steering Information, however, it could be some other node, e.g. Policy Control
`Function.
`
`4. AUSF derives the configuration key (Kconf) from the home network root key (Kausf), and calculates the Message
`Authentication Code (MAC-1) over Network Steering Information.
`
`5. AUSF forwards the protected Network Steering Information to AMF/SEAF.
`
`6.AMF/SEAF forwards the protected Network Steering Information to the UE. This message could be confidentiality
`protected over the air with NAS security. Depending on the final architecture, the Network Steering Information
`could be piggybacked e.g. in Registration Accept message.
`
`7.The UE derives the configuration key (Kconf) from the home network root key (Kausf), and verifies the MAC-1.
`Depending on the final protocol design, UE may send an acknowledgement message (“Network Steering ACK”) to
`the HPLMN, and protect that information with the MAC-2.
`
`8. The UE sends the protected ACK message to the AMF/SEAF.
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`9.The AMF/SEAF forwards the protected ACK to the AUSF.
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`10. AUSF verifies the MAC-2 in the protected Network Steering ACK message.
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`11. AUSF forwards the ACK to the original source of the Network Steering Information.
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`Figure 4-2 demonstrates the case when a node other than AUSF (e.g. the Policy Control Function, PCF) is in charge of
`delivering the Network Steering Information.
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`3
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`
`
`
`
`UE
`
`AMF/
`SEAF
`
`1. Registration & authentication
`
`AUSF
`
`PCF
`
`2. Kausf generated
`
`2. Kausf generated
`
`3. Key request
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`4. Kconf derived from Kausf.
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`7. Network Steering Information, MAC-1
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`6. MAC-1 calculation
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`8. Kconf derived from Kausf.
`MAC-1 verification,
`MAC-2 calculation
`
`10. MAC-2 verification
`
`
`Figure 4-2: Signalling flow showing provisioning of the Network Steering Information from home PLMN to UE
`
`1. The UE registers to the VPLMN, and is authenticated by AUSF.
`
`2. UE and AUSF generate Kausf.
`
`3. A node in the HPLMN (e.g. PCF) sends a key request to AUSF. This example assumes that AUSF only derives
`further keys from the home network root key (Kausf), and acts as an key management server, and distributes such
`keys in HPLMN.
`
`4. AUSF derives the configuration key (Kconf) from the home network root key (Kausf).
`
`5. AUSF sends the Key response with the configuration key (Kconf) to the PCF.
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`6. The PCF constructs the Network Steering Information, and protects it with MAC-1.
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`7. The PCF sends the protected Network Steering Information to the UE. There may be intermediate nodes between
`the PCF and the UE.
`
`8.The UE derives the configuration key (Kconf) from the home network root key (Kausf), and verifies the MAC -1.
`Depending on the final protocol design, UE may send an acknowledgement message (“Network Steering ACK”) to
`the PCF, and protect that information with the MAC-2.
`
`9. The UE sends the protected ACK message to the PCF. There may be intermediate nodes between the node and the
`UE.
`
`10. The PCF verifies the MAC-2 in the protected Network Steering ACK message.
`
`The requirement of the UE detecting the removal of Network Steering Information by the VPLMN is very difficult to
`solve. This would require that the UE is able to expect such message to arrive, and AUSF would need to send the
`message (with the MAC) even when nothing needs to be configured. This is not efficient, and would not guarantee the
`delivery at any time but only when the UE expects them to arrive. Instead, we would propose the use of the
`acknowledge message back to the HPLMN, so that at least HPLMN is able to detect the failure of delivery.
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`4
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`
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`Detailed proposal
`4
`It is proposed to send an LS back to SA2 and CT1, and report the following.
`- SA3 has agreed that if end-to-end solution is required, a node in the HPLMN (e.g. AUSF or PCF) could send an
`integrity protected Network Steering Information to the UE. The solution would be based on the Kausf,
`direved from the primary authentication. This key would be known only by the UE and the HPLMN.
`
`- The solution could be enhanced with end-to-end encryption if SA2 has a requirement of hiding the Network
`Steering Information from the VPLMN. Confidentiality protection over the air interface can be achieved by
`NAS security. However, the usage of any form of confidentiality protection may be subject to regional or
`national regulatory policies.
`
`- SA3 has not found satisfactory solution for the requirement of UE detecting the removal of Network Steering
`Information by the VPLMN. Instead, SA3 would propose the usage of acknowledge message back to the
`HPLMN so that at least HPLMN knows if the UE received the information. The acknowledge information
`would need to be integrity protected by the UE.
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`5
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