Content for TS 33.310 Word version: 18.2.0
5.2 Use cases
5.2.1 Operator Registration: Creation of interconnect agreement
5.2.2 Establishment of secure communications
5.2.2.1 NDS/IP case
5.2.2.1.1 NDS/IP case for the Za interface
5.2.2.1.2 NDS/IP case for the Zb-interface
5.2.2.2 TLS case
5.2.3 Operator deregistration: Termination of interconnect agreement
5.2.3a Interconnection CA registration
5.2.3b Interconnection CA deregistration
5.2.3c Interconnection CA certification creation
5.2.3d Interconnection CA certification revocation
5.2.3e Interconnection CA certification renewal
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5.2 Use cases p. 16
5.2.1 Operator Registration: Creation of interconnect agreement p. 16
SEGs or TLS entities of two different security domains need to establish a secure connection, when the operators make an interconnect agreement. The first technical step in creating the interconnect agreement between domains is the creation of cross-certificates by the Interconnection CAs of the two domains.
Inter-operator cross-certification can be done using different protocols, but the certification authority shall support the PKCS#10 method for certificate requests as specified in RFC 2986. The SEG CA, TLS client CA and TLS server CA create a PKCS#10 certificate request, and send it to the other operator's Interconnection CA. The method for transferring the PKCS#10 request is not specified, but the transfer method shall be secure. The PKCS#10 can be transferred e.g. HTTPS, in a flash drive, or be send in a signed email. The PKCS#10 request contains the public key of the authority and the name of the authority requesting the cross-certificate. When the Interconnection CA accepts the request, a new cross-certificate is created for the requesting CA. The Interconnection CA shall make the new cross-certificate available to SEGs and TLS entities in its own domain that need to use it. Cross-certificates on the other domain's SEG CA's are stored in a local CR (Certificate Repository) which all SEGs that need to communicate with the other domains shall access using LDAP as specified in RFC 2252. Cross-certificates on TLS client CAs and TLS server CAs are made available to TLS entities, e.g. by storing them in a file of trusted CAs on the TLS entity, or by storing them in a local CR (Certificate Repository) which all TLS entities that need to communicate with the other domain shall access e.g. using LDAP as specified in RFC 2252.
The cross-certification is a manual operation, and thus PKCS#10 is a suitable solution for the interconnect agreement.
Creation of an interconnect agreement only involves use of the private keys of the Interconnection CAs. There is no need for the operators to use the private keys of their respective SEG CAs, TLS client CAs or TLS server CAs in forming an interconnect agreement.
When creating the new cross-certificate, the Interconnection CA should use basic constraint extension (according to Section 4.2.1.9 of RFC 5280) and set the path length to zero. This inhibits the new cross-certificate to be used in signing new CA certificates. The validity of the certificate should be set sufficiently long. The cross-certification process needs to be done again when the validity of the cross-certificate is ending.
When the new cross-certificate is available to the SEG, all that needs to be configured in the SEG is the DNS name or IP address of the peering SEG gateway. The authentication can be done based on the created cross-certificates.
When the new cross-certificate is available to a TLS entity, it allows that TLS entity to authenticate TLS entities in the peering network. Authentication is done based on the created cross-certificates.
The certificate hierarchy in the case of two peering operators is illustrated in Figure 3.
5.2.2 Establishment of secure communications p. 17
5.2.2.1 NDS/IP case |R7| p. 17
5.2.2.1.1 NDS/IP case for the Za interface |R8| p. 17
After establishing an interconnect agreement and finishing the required preliminary certificate management operations as specified in clause 5.2.1, the operators configure their SEGs for SEG-SEG connection, and the SAs are established as specified by NDS/IP [1].
In each connection configuration, the remote SEG DNS name or IP address is specified. Only the local Interconnection CA and SEG CA are configured as trusted CAs. Because of the cross-certification, any operator whose SEG CA has been cross-certified can get access using this VPN connection configuration.
The following is the flow of connection negotiation from the point of view of Operator A's SEG (initiator). Operator B's SEG (responder) shall behave in a similar fashion. In case of any failure in following steps, SEG A will treat this as an error and abort the procedure.
- During connection initiation, the initiating Operator A's SEG A provides its own SEG certificate and the corresponding digital signature in the IKE_AUTH exchange for IKEv2;
- SEG A receives the remote SEG B certificate and signature;
- SEG A verifies the remote SEG B signature;
- SEG A checks the validity of the SEG B certificate by a revocation check to Operator B's CRL databases or OCSP server. If a SEG cannot successfully perform the revocation check, it shall treat this as an error and abort tunnel establishment;
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SEG A verifies the SEG B certificate by executing the following actions:
- SEG A fetches the cross-certificate for Operator B's SEG CA from Operator A's Certificate Repository or from a local cache.
- SEG A checks the validity of the cross-certificate for Operator B's SEG CA by a revocation check to Operator A's Interconnection CA CRL database or OCSP server. If a SEG cannot successfully perform the revocation check, it shall treat this as an error and abort tunnel establishment;
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SEG A verifies the cross-certificate for Operator B's SEG CA using Operator A's Interconnection CA's certificate. Operator A's Interconnection CA's certificate shall be verified if the Interconnection CA is not a top-level CA, otherwise the Interconnection CA's public key is implicitly trusted.
- SEG A verifies the SEG B certificate using cross-certificate for Operator B's SEG CA.
5.2.2.1.2 NDS/IP case for the Zb-interface |R8| p. 18
In this case there is no need for cross-certification. Both end entity certificates belong to the same administrative domain and thus authorization check resolves to the same top level CA.
The following is the flow of connection negotiation from the point of view of NE-A (initiator). NE-B (or SEG-B) from the same domain (responder) shall behave in a similar fashion. In case of any failure in following steps, NE A will treat this as an error and abort the procedure.
- During connection initiation, the initiating Operator A's NE-A provides its own NE certificate and the corresponding digital signature in the IKE_AUTH exchange for IKEv2;
- NE A receives the NE B (or SEG B) certificate and signature;
- NE A verifies the NE B (or SEG B) signature;
- NE A checks the validity of the NE B (or SEG B) certificate by a revocation check to the CRL databases or OCSP server of the same domain. If a NE cannot successfully perform the CRL check, it shall treat this as an error and abort tunnel establishment;
- NE A verifies the NE B (or SEG B) certificate using Operator NE CA certificate.
5.2.2.2 TLS case |R9| p. 18
After establishing a interconnect agreement and finishing the required preliminary certificate management operations as specified in clause 5.2.1, the operators configure their TLS entities for secure interconnection. The exact process for establishing the TLS connections is dependent on the application protocol and is outside the scope of this specification. However, the general flow is described in the remainder of this clause.
The local Interconnection CA and TLS client/server CAs are configured as trusted CAs in the TLS entity typically by storing them in a file of trusted CAs on the TLS entity. The cross-certificates on the TLS client/server CAs of the remote operator are also made available to the TLS entity, e.g. by storing them in a file of trusted CAs on the TLS entity, or by storing them in a local CR (Certificate Repository) which all TLS entities that need to communicate with the other domain shall access e.g. using LDAP. Because of the cross-certification, any operator whose TLS client CA or TLS server CA has been cross-certified by another operator can establish TLS connections with that other operator.
The following is the connection establishment from the point of view of a TLS client in Operator A (TLSa) and a TLS server in Operator B (TLSb). The case where the TLS client is in Operator B and the TLS server is in Operator A is treated in a similar fashion. The flow is based on the TLS handshake protocol as described in RFC 8446. In case of any failure in following steps, TLSa or TLSb will treat this as an error and abort the procedure.
- During connection initiation, the TLSa sends a ClientHello message to TLSb. TLSb responds with a ServerHello message followed by a Certificate message, an optional CertificateRequest message, and other additional messages depending on the TLS version and options. The Certificate message will contain TLSb's certificate (or certificate chain)that was issued by Operator B's TLS server CA. The CertificateRequest message is sent if TLSb wants to authenticate TLSa using certificates in TLS, TLSa may otherwise be authenticated at a later stage using the application layer.
- TLSa receives the messages from TLSb
- TLSa verifies the received TLS messages using TLSb's public key
- TLSa checks the validity of TLSb's certificate by a revocation check to Operator B's CRL databases or OCSP server. If a TLS peer cannot successfully perform the revocation check, it shall treat this as an error and abort the TLS handshake
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TLSa verifies TLSb's certificate using the cross-certificate for Operator B's TLS server CA by executing the following actions:
- TLSa fetches the cross-certificate for Operator B's TLS server CA from Operator A's Certificate Repository, from a local cache of the Certificate Repository on TLSa, or from a local certificate store on TLSa if a separate Certificate Repository is not used.
- TLSa checks the validity of the cross-certificate for Operator B's TLS server CA by a revocation check to Operator A's Interconnection CA CRL database or OCSP server. If a TLS peer cannot successfully perform the revocation check, it shall treat this as an error and abort the TLS handshake;
- TLSa verifies the cross-certificate for Operator B's TLS server CA using Operator A's Interconnection CA's certificate if the Interconnection CA is not a top-level CA, otherwise the Interconnection CA's public key is implicitly trusted.
- TLSa verifies TLSb's certificate using the cross-certificate for Operator B's TLS server CA.
- If TLSb requested a certificate using the CertificateRequest message, then TLSa responds with a Certificate message followed by a CertificateVerify message and a Finished message. The Certificate and CertificateVerify messages are only sent if the Server requests a certificate. If present, the Certificate message will contain TLSa's certificate (or certificate chain) that was issued by Operator A's TLS client CA. The CertificateVerify message is used to provide explicit verification of a client certificate.
- TLSb receives the messages from TLSa.
- If TLSb requested a certificate using the CertificateRequest message, then TLSb verifies the CertificateVerify message using TLSa's public key.
- If TLSb requested a certificate using the CertificateRequest message, then TLSb checks the validity of TLSa's certificate by a revocation check to Operator A's CRL databases or OCSP server. If a TLS entity cannot successfully perform both revocation checks, it shall treat this as an error and abort the TLS handshake.
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If TLSb requested a certificate using the CertificateRequest message, then TLSb validates TLSa's certificate using the cross-certificate for Operator A's TLS client CA by executing the following actions:
- TLSb fetches the cross-certificate for Operator A's TLS client CA from Operator B's Certificate Repository, from a local cache of the Certificate Repository on TLSb, or from a local certificate store on TLSb if a separate Certificate Repository is not used.
- TLSb checks the validity of the cross-certificate for Operator A's TLS client CA by a revocation check to Operator B's Interconnection CA CRL database or OCSP server. If a TLS entity cannot successfully perform the revocation check, it shall treat this as an error and abort the TLS handshake.
- TLSb verifies the cross-certificate for Operator A's TLS client CA using Operator B's Interconnection CA's certificate if the Interconnection CA is not a top-level CA, otherwise the Interconnection CA's public key is implicitly trusted.
- TLSb verifies TLSa's certificate using the cross-certificate for Operator A's TLS client CA.
5.2.3 Operator deregistration: Termination of interconnect agreement p. 20
When an interconnect agreement is terminated or due to an urgent service termination need, all concerned SEG peers shall remove the IPsec SAs using device-specific management methods, while all concerned TLS entities shall terminate any ongoing TLS sessions with the peer network and not permit those sessions to be resumed (e.g. by prohibiting TLS session resumption).
Each concerned operator shall also list the cross-certificate created for the Interconnection CA, SEG CA, TLS client CA and TLS server CA of the terminated operator in his own local CRL or OCSP server.
5.2.3a Interconnection CA registration |R7| p. 20
In principle only one Interconnection CA shall be used within the operator's network, but using more than one Interconnection CA is possible (in which case the public keys of all the operator's interconnection CAs should be installed in the operator's SEGs or TLS entities). The involved actions in Interconnection CA registration are those as described in the cross-certification part of clause 5.2.1: 'Operator Registration: creation of interconnect agreement'. Such a situation may exist if the Interconnection CA functions are to be moved from one responsible organisation to another (e.g. outsourcing of CA services).
5.2.3b Interconnection CA deregistration |R7| p. 20
If an Interconnection CA is removed from the network, it shall be assured that all certificates that have been issued by that CA to SEG or TLS CAs, and have not expired yet, shall be listed in the CRLs or OCSP servers.
5.2.3c Interconnection CA certification creation |R7| p. 20
The Interconnection CA certificate may not be the top-level CA of the operator, which means that the Interconnection CA certificate is not self-signed. If the Interconnection CA certificate is self-signed then it needs to be securely transferred to each SEG or TLS entity and stored within secure memory otherwise it can be managed in the same way as a SEG or TLS entity certificate.
The Interconnection CA certificate shall have a 'longer' lifetime than SEG CA or TLS CA certificates in order to avoid the cross-certification actions that are needed each time an Interconnection CA certificate has to be renewed.
5.2.3d Interconnection CA certification revocation |R7| p. 20
If an Interconnection CA key pair gets compromised then a hacker could use the keys to issue himself SEG CA or TLS CA certificates which in turn could be used to issue SEG or TLS entity certificates. Since however the trusted Interconnection CA certificates are stored locally on the SEG or TLS entity device or in a dedicated repository (i.e. received Interconnection CA certificates within the IKE payload or TLS handshake shall not be accepted), the hacker also needs to compromise the SEG, TLS entity, or the local repository to be able to set up a secure connection.
Existing secure connections need not be torn down. The old cross-certificates - and any other certificates - issued by the Interconnection CA shall be taken out of service by listing them in the Interconnection CA's CRL or OCSP server (provided the operator still has the key available to sign) and removing them from the dedicated repository. If the Interconnection CA certificate is self-signed then it shall be removed from each of the operator's SEGs and TLS entities. If the Interconnection CA certificate is issued by a higher level CA of the operator, then it shall be revoked by this higher level CA.
The operator has to create a new Interconnection CA key pair, perform the actions as described within clause 5.2.3c for Interconnection CA certification creation, and perform the actions as described within clause 5.2.1 to generate new cross-certificates for all his interconnected networks SEG CAs or TLS CAs.
5.2.3e Interconnection CA certification renewal |R7| p. 21
The Interconnection CA certificate has to be renewed before the old Interconnection CA certificate expires. The renewing of an Interconnection CA certificate involves repeating the actions as described in clause 5.2.3c. This should be done before the old certificate expires.
