Enrolment Principles

Requirements for Enrolment Procedure

The main guideline for the specification of a base station enrolment procedure was to allow a plug-and-play deployment of base stations with:

• usage of an existing standardized protocol for certificate enrolment,

• minimization of manual interaction,

• no need for pre-provisioning of operator-specific data in factory,

• no need for security-relevant provisioning on installation site,

• authentication of a base station to the RA of the operator based on a vendor-signed base station certificate,

• secure provisioning of a base station certificate signed by the operator PKI and

• secure provisioning of an operator root certificate.

An extensive threat and risk analysis for the different possible solutions was carried out.

As part of this analysis, the following topics were discussed and resolved. Two solution variants for the provisioning of operator root certificates (see later in this Section) were accepted in order to allow for different trade-offs between risk and complexity depending on the deployment scenarios.

Selection of Enrolment Protocol

Clause 7.2 of [TS33.310] requires the support of the Certificate Management Protocol ver-sion 2 (CMPv2) [RFC4210] for the lifecycle management of network element certificates.

Therefore the selection of CMPv2 also for base station enrolment was a natural choice.

Communication Channel between the Base Station and RA

The enrolment is done end-to-end between the base station and the RA of the operator.

The CMPv2 protocol provides means for proof of origin and integrity protection of the

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messages, so no additional security tunnel for the communication is needed. As only public data is transferred in a CMP protocol exchange, confidentiality protection is not necessary for the communication either.

Authentication of Base Station by the Operator Network

The enrolment of the base station to the operator PKI is based on a vendor-provided base station identity. The base station authenticates itself to the operator network during the enrolment procedure using a vendor-provided public–private key pair installed in the base station before enrolment, and a certificate on the base station identity and the public key signed by a vendor CA. In the terminology of the CMPv2 specification, this enrolment is an in-band initialization using an external identity certificate according to Appendix E.7 of [RFC4210]. The authenticating entity in the operator network (e.g. the RA) must be provided with the root certificate of the vendor PKI to be able to authenticate the vendor-provided base station identity. This provisioning of the vendor root certificate must occur before the enrolment procedure in a trustworthy manner.

Proof of Possession of the Private Key

The base station must provide the RA/CA with a proof of possession for the private key which belongs to the public key to be certified. This is accomplished by usage of the Proof-of-Possession information elements within the CMPv2 messages. This private–public key pair may differ from the one used in authenticating the base station to the operator network.

Authorization of Base Station Enrolment

Authorization of the enrolment for each base station is not in the scope of the specifica-tion [TS33.310]. Nevertheless the RA has to be informed via management means of the vendor-provided base station identities expected to be enrolled. This management data may include the identity of the base station meant to be used in the operator infrastructure (which may differ from the identity put into the base station in the factory) so that it can be provided to the base station as part of the CMP protocol run. This identity may also be provided to the base station after its installation, but before certificate enrolment – for example in a DHCP response sent when the base station first attaches to the network.

Provisioning of the Operator Root Certificate

The authentication of the operator network by the base station during enrolment would require a pre-provisioned operator root certificate in the base station. This, in turn, would require provisioning of the operator root certificate in the factory (contradicting the third requirement from the bulleted list above), the installation of the operator root certificate on-site at installation time (which would have some unwanted security implications), or some complex cross-signing relations between vendors and operators (where the base station would be provided with a vendor root certificate in the factory, and the operator root certificate would be cross-signed by this vendor root certificate).

Solutions with cross-signing were ruled out, as they would not allow the authentication of single operators (as any cross-signed operator root certificate would be accepted by the base station), and their added benefit would not outweigh the incurred complexity of handling of the many trust relations necessary between a vendor and their many customers. Thus only provisioning of the operator root certificate before or during the CMPv2 protocol run is specified. If there is no operator root certificate provisioned at all, the base station shall assume the enrolment procedure to have failed.

Provisioning of Operator Root Certificate during CMP Run

This is the plug-and-play solution without any operator-specific pre-provisioning and security-relevant interaction on installation site. The base station extracts the operator root certificate from the CMP response message from the RA/CA. As there may be multiple certificates in the CMP response, the selection of the operator root certificate is done based on the following two criteria: (i) the certificate is a self-signed certificate (only root certificates are self-signed) and (ii) the newly generated base station certificate can be validated by this root certificate, possibly via a chain of intermediate certificates also contained in the CMP response. The risk of not performing an authentication of the operator network at this time was seen as tolerable, as it is still enforced that only allowed base stations may enrol with the operator network. As a base station needs the cooperation of core network elements to be able to establish a connection with the UE, and only a base station with a certificate of the intended operator may connect to their network, a base station with a wrong certificate may never impersonate the intended operator network towards the UE. In addition, architectural provisions below the IP layer may reduce such risks, such as the deployment of virtual LANs.

Provisioning of Operator Root Certificate before CMP Run

This means that the operator root certificate is provisioned to the base station either in the factory or by service personnel on installation site. Both variants violate one of the requirements given here; that is, the operator-independent delivery of base stations from the factory, or by having the need to perform security-relevant actions at installation site.

On the other hand, this allows the authentication of the operator network during enrolment if the operator is willing to accept the higher complexity of the delivery process or the additional trust into the manual installation procedure. If the base station is provisioned with the operator root certificate before the start of the enrolment procedure, it must use this root certificate for authenticating the RA/CA during enrolment.

Renewal of Base Station Key Pair or Operator Certificate

During the lifetime of a base station, the operator may choose to renew the private–public key pair of the base station, or he may issue certificates with a lifetime shorter than the expected device lifetime. In both cases, the key update message exchange specified in the CMPv2 protocol suite is used. The main difference to the initial enrolment of the base station is that, in this case, the base station authenticates itself to the RA/CA based on the (old) operator certificate, and not the vendor certificate. This implies that all authen-tications performed during the renewal process are based on the operator root certificate.

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Vendor Base Station Certificate

After successful initial enrolment of the base station, the vendor base station certificate is no longer needed according to the specification in [TS33.310]. It is left to vendor policy whether the vendor-provided certificate and the private key are deleted after initial operator enrolment, or whether they are kept to allow a return to the pristine state of the base station. Starting from this pristine state, the base station could then be enrolled to another operator’s network. Also a completely new enrolment to the same operator’s network could be performed, if needed.