RPKI Validation Reconsidered

The allocations recorded in the RPKI change as a result of resource transfers. For example, the CAs involved in transfer might choose to modify CA certificates in an order that causes some of these certificates to “over-claim” temporarily. A certificate is said to “over-claim” if it includes INRs not contained in the INRs of the CA that issued the certificate in question. It may also happen that a child CA does not voluntarily request a shrunk resource certificate when resources are being transferred or reclaimed by the parent. Furthermore operational errors that may occur during management of RPKI databases also may create CA certificates that, temporarily, no longer encompass all of the INRs of subordinate certificates. Consider the following sequence:

Here Certificate 2 from the previous example was re-issued by TA to CA1 and the prefix 198.51.100.0/24 was removed. However, CA1 failed to re-issue a new Certificate 3 to CA2. As a result Certificate 3 is now over-claiming and considered invalid; by recursion the embedded Certificate 4 used for ROA1 is also invalid. And ROA1 is invalid because the specified prefix contained in the ROA is no longer encompassed by a valid embedded EE certificate, as required by However, it should be noted that ROA1 does not make use of any of the address resources that were removed from CA1’s certificate, and thus it would be desirable if ROA1 could still be viewed as valid. Technically CA1 should re-issue a Certificate 3 to CA2 without 198.51.100.0/24, and then ROA1 would be considered valid according to . But as long as CA1 does not take this action, ROA1 remains invalid. It would be preferable if ROA1 could be considered valid, since the assertion it makes was not affected by the reduced scope of CA1’s certificate.

The problem described above can be considered as a low probability problem today. However the potential impact on routing security would be high if an over-claiming occurred near the apex of the RPKI hierarchy, as this would invalidate the entirety of the sub-tree located below this point. The changes proposed here to the validation procedure in do not change the probability of this problem, but they do limit the impact to just the over-claimed resources. This revised validation algorithm is intended to avoid causing CA certificates to be treated as completely invalid as a result of over-claims. However, these changes are designed to not degrade the security offered by the RPKI. Specifically, ROAs and router certificates will be treated as valid only if all of the resources contained in them are encompassed by all superior certificates along a path to a trust anchor. The way this is achieved conceptually is by maintaining Verified Resource Set (VRS) for each certificate that is separate from the INRs found in the resource extension in the certificate.

The following is an amended specification to be used in place of section 7.2 of . The following algorithm is employed to validate CA and EE resources certificates. It is modeled on the path validation algorithm from , but modified to make use of the IP Address Delegation and AS Identifier Delegation Extensions from . There are two inputs to the validation algorithm: a trust anchor a certificate to be validated The algorithm is initialized with two new variables for use in the RPKI: Validated Resource Set-IP (VRS-IP) and Validated Resource Set-AS (VRS-AS). These sets are used to track the set of INRs (IP address space and AS Numbers) that are considered valid for each CA certificate. The VRS-IP and VRS-AS sets are initially set to the IP Address Delegation and AS Identifier Delegation values, respectively, from the trust anchor used to perform validation. This path validation algorithm verifies, among other things, that a prospective certification path (a sequence of n certificates) satisfies the following conditions: for all 'x' in {1, ..., n-1}, the subject of certificate 'x' is the issuer of certificate ('x' + 1); certificate '1' is issued by a trust anchor; certificate 'n' is the certificate to be validated; and for all 'x' in {1, ..., n}, certificate 'x' is valid. Certificate validation requires verifying that all of the following conditions hold, in addition to the certification path validation criteria specified in Section 6 of [RFC5280]. The signature of certificate x (x>1) is verified using the public key of the issuer’s certificate (x-1), using the signature algorithm specified for that public key (in certificate x-1). The current time lies within the interval defined by the NotBefore and NotAfter values in the Validity field of certificate x. The Version, Issuer, and Subject fields of certificate x satisfy the constraints established in Section 4.1-4.7 of this specification. Certificate x contains all the extensions that MUST be present, as defined in Section 4.8 of this specification. The value(s) for each of these extensions MUST be satisfy the constraints established for each extension in the respective sections. Any extension not identified in Section 4.8 MUST NOT appear in certificate x. Certificate x MUST NOT have been revoked, i.e., it MUST NOT appear on a CRL issued by the CA represented by certificate x-1 Compute the VRS-IP and VRS-AS set values as indicated below: If the IP Address Delegation extension is present in certificate x, compute the intersection of the resources between this extension and the value of the VRS-IP computed for certificate x-1. If the IP Address Delegation extension is absent in certificate x, set the VRS-IP to NULL. If the AS Identifier Delegation extension is present in certificate x, compute the intersection of the resources between this extension and the value of the VRS-AS computed for certificate x-1 If the AS Identifier Delegation extension is absent in certificate x, set the VRS-AS to NULL. If x = n (i.e., this is the certificate being validated), then: If IP Address Delegation extension is present, it is replaced with the intersection of the values from that extension and the current value of the VRS-IP. If an AS Identifier Delegation extension is present, it is replaced with the intersection of the values from that extension and the current value of the VRS-IP. If an RP is caching the results of validation, these values MAY be stored along with the certificate, to facilitate incremental validation based on cached results. These rules allow a CA certificate to contain resources that are not present in (all of) the certificates along the path from the trust anchor to the CA certificate. If none of the resources in the CA certificate are present in all certificates along the path, no subordinate certificates could be valid. However, the certificate is not immediately rejected as this may be a transient condition. Not immediately rejecting the certificate does not result in a security problem because the associated VRS sets accurately reflect the resources validly associated with the certificate in question.

Section 4 of currently has the following text on the validation of resources on a ROA: The IP address delegation extension [RFC3779] is present in the end-entity (EE) certificate (contained within the ROA), and each IP address prefix(es) in the ROA is contained within the set of IP addresses specified by the EE certificate's IP address delegation extension. The following is an amended specification to be used in place of this text. The IP address delegation extension [RFC3779] is present in the end-entity (EE) certificate (contained within the ROA), and each IP address prefix(es) in the ROA is contained within the VRS-IP set that is specified as an outcome of EE certificate validation. Note that this ensures that ROAs can be valid only, if all IP address prefixes in the ROA are encompassed by the VRS-IP of all certificates along the path to the trust anchor used to verify it. Operators MAY issue separate ROAs for each IP address prefix, so that the loss of on IP address prefix from the VRS-IP of any certificate along the path to the trust anchor would not invalidate authorizations for other IP address prefixes.

BGPsec Router Certificate Validation is defined in section 3.3 of . Path validation defined section 7 of is used as the first step in validation, and a number of additional constraints are applied. We request that the authors add the following constraint: The VRS-AS of BGPsec Router Certificates MUST encompass all ASNs in the AS Resource Identifier Delegation extension. Furthermore we request that the authors include text instructing operators that they MAY issue separate BGPsec Router Certificates for different ASNs, so that the loss of on ASN from the VRS-AS of any certificate along the path to the trust anchor would not invalidate router keys for other ASNs.

Consider the following example under the amended approach: