PKI for 3GPP

The following provides information on the 3GPP CMP standard and enrollment using certificates through CMP with 3GPP.

ENTERPRISE

To find out more about how to configure CMP for 3GPP and common operations and test, see 3GPP CMP Operations.

This describes the 3GPP CMP standard and EJBCA's integration with it. It contains extracts from the 3GPP specification 3GPP TS 33.310. An entire example of a CMP setup is described in the 3GPP standards, refer to 3GPP Specifications.

Terminology

The following terms are important for understanding the underlying process of 3GPP (33.310).

Term	Meaning
MNO	Mobile Network Operator. The MNO is the customer of this PKI solution, i.e. the MNO will be the CA. The MNO buys eNodeB's from a Vendor.
eNodeB	A base station in 4G and 5G networks, i.e. a device.
Vendor	A vendor of eNodeB's, i.e. a telco equipment manufacturer such as Ericsson.
Vendor CA	A CA operated by the Vendor. In this context, we only see certificates from the Vendor CA and are not setting up the Vendor CA.
Operator CA	A CA operated by the MNO, issuing certificates to eNodeB's in order for these to communicate in the MNOs network. In this context we are setting up the Operator CA.

Overview

The main purpose of using 3GPP CMP is to allow an eNodeB (base station in 4G and 5G networks) to automatically provision itself with an operator certificate from the Mobile Network Operator (MNO) without the eNodeB manufacturer and the MNO sharing PKIs.

This is accomplished by the eNodeB using a vendor-issued certificate from the manufacturer's PKI, to authenticate to the MNO upon installation, authorizing the issuance of an Operator certificate from the MNOs PKI. The Operator certificate is then used to connect the eNodeB to the MNOs network.

Generalized Workflow

This section describes the general 3GPP CMP workflow, purely for overview purposes. The EJBCA specific workflow is slightly modified, see EJBCA Specific Workflow.

The following figure shows the general deployment architecture for certificate enrollment of a device at an operator PKI.

The device is pre-provisioned with a public-private key pair by the vendor, and has the vendor-signed certificate of its public key pre-installed.

On initial contact to the operator network, the device establishes a communication channel to the RA/CA of the vendor. Using a CMPv2, a request for a certificate is sent to the RA/CA. The network authenticates the messages from the device based on the vendor-signed certificate of the device and the vendor root certificate pre-installed in the network. The device checks the integrity protection on the messages from the RA/CA based on the operator root certificate provisioned in the device. In a response message, the device receives the operator-signed certificate. During the execution of the CMPv2 protocol, the device has to provide a successful proof of possession of the private key associated to the public key in order to be certified.

The operator root certificate may be provisioned in the device prior to or during the CMPv2 protocol run. The protection of the operator root certificate during provisioning may be decided by the operator security policy. If an operator root certificate provisioned prior to the CMPv2 protocol run is available, the device shall use it. Otherwise, the device shall use the operator root certificate provisioned during the CMPv2 run. If no operator root certificate is provisioned at all, then the device shall abort the procedure.

If the operator wants to renew the device certificate, the same procedure will be executed with the old operator-signed device certificate taking the place of the vendor-signed certificate of the initial enrollment.

The following figure describes the general message flow in both cases:

The device discovers the RA/CA address.
The device generates the private/public key pair to be enrolled in the operator CA, if this is not pre-provisioned.
The device generates the Initialization Request (IR). The CertReqMsg inside the request specifies the requested certificate. If the suggested identity is known to the device, it includes this in the subject field. To provide proof of possession, the device generates the signature for the POPOSigningKey field of the CertReqMsg using the private key related to the public key to be certified by the RA/CA. The device signs the request using the vendor provided public key, and includes the digital signature in the PKIMessage. Its own vendor signed certificate and any intermediate certificates are included in the extraCerts field of the PKIMessage carrying the initialization request.
The device sends the signed initialization request message to the RA/CA.
The RA/CA verifies the digital signature on the initialization request message against the vendor root certificate using the certificate(s) sent by the device. The RA/CA also verifies the proof of the possession of the private key for the requested certificate.
The RA/CA generates the certificate for the device. If the suggested identity of the device is not included in the initialization request message, the RA/CA determines the suggested identity, based on the vendor provided identity contained in the device certificate. The RA/CA may also replace a suggested identity sent by the device with another identity based on local information.
The RA/CA generates an Initialization Response (IP) which includes the issued certificate. The RA/CA signs the response with the RA/CA private key (or the private key for signing CMP messages, if separate), and includes the signature, the RA/CA certificate(s) and the operator root certificate in the PKIMessage. The appropriate certificate chains for authenticating the RA/CA certificate(s) are included in the PKIMessage.
The RA/CA sends the signed initialization response to the device.
If the operator root certificate is not pre-provisioned to the device, the device extracts the operator root certificate from the PKIMessage. The device authenticates the PKIMessage using the RA/CA certificate and installs the device certificate on success.
The device creates and signs the CertificateConfirm (certconf) message.
The device sends the PKIMessage that includes the signed CertificateConfirm to the RA/CA.
The RA/CA authenticates the PKI Message that includes the CertificateConfirm.
The RA/CA creates and signs a Confirmation message (pkiconf).
The RA/CA sends the signed PKIMessage including the pkiconf message to the device.
The device authenticates the pkiconf message.

EJBCA Specific Workflow

To use EJBCA with the 3GPP CMP profile, the generalized setup described above is slightly adjusted.

The 3GPP CMP profile is supported by EJBCA Enterprise operating in Client mode. In this case, devices are in direct contact with EJBCA and each device has a corresponding end entity in EJBCA.

Additionally, there is an option to use EJBCA in RA mode, for indirect communication between EJBCA and the device via a third entity acting as an RA. In this case, the RA has a corresponding end entity in EJBCA and is given the necessary privileges to process CMP requests on behalf of the device.

Direct CA - Device Communication

In the case of direct contact between EJBCA and the device, EJBCA operates in client mode. Each device has a corresponding end entity in EJBCA. The initial enrollment request/certification request is sent by the device to EJBCA as a CMP request signed by the key provided to the device by the vendor. The vendor issued certificate is attached to the request in the extraCerts field. EJBCA will authenticate the request by checking that the certificate in extraCerts was issued by the vendor CA (an external CA in EJBCA). If the authentication succeeds, EJBCA will issue a certificate for the device and includes it in the CMP response message.

For future communication, when the device needs to update its certificate, the old EJBCA obtained certificate is used to authenticate the update or key renewal request. This is typically done by the device signing the update request with its private key and attaching its certificate (the one to be renewed) in the extraCerts field in the CMP message.

For the required configuration, see 3GPP CMP Operations.

CA - Device Communication Through a Bespoke RA

The 3GPP CMP standard specifies a usable profile for direct CA-to-device communication. Although this is the primary use case for the 3GPP specification, solutions are often found in a use case where a trusted central point, a registration authority, or a security gateway needs to issue certificates to end entities under its control. For such use cases, you can use CMP in RA mode, a profile of CMP not described in the 3GPP standard.

In the case of indirect communication between EJBCA and the device, EJBCA operates in RA mode. The device communicates with the CA through a third device/organization that acts as an RA. The RA has a corresponding end entity in EJBCA with an issued certificate. This certificate is transported to the RA manually. The RA is also registered in EJBCA as an administrator and is given the necessary privileges to process CMP requests on behalf of the device.

Both initialization/certification requests and certificate update requests regarding the devices, are expected to be signed by the RA. Any changes or updates of the RA certificate are expected to be performed directly in EJBCA and transported to the RA manually. For the required configuration, see 3GPP CMP Operations.