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Certificate Validation

Understanding Digital Certificate Validation

During IKE negotiation, the PKI daemon on an SRX Series device validates X509 certificates received from VPN peers. The certificate validation performed is specified in RFC 5280, Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. Basic certificate and certificate chain validations include signature and date validation as well as revocation checks. This topic describes additional digital certificate validations performed by the PKI daemon.

Policy Validation

X509 certificates can include optional policy validation fields. If a policy validation field is present, policy validation is performed for the entire certificate chain including the end entity (EE) certificate and intermediate certificate authority (CA) certificates. Policy validation is not applicable to the root certificate. Policy validation ensures that the EE and intermediate CA certificates have a common policy. If no common policy exists for the certificate chain being validated, certificate validation fails.

Prior to policy validation, a certificate chain containing the self-signed root certificate, intermediate CA certificates, and EE certificate must be built. The policy validation starts with the intermediate CA certificate issued by the self-signed root certificate and continues through the EE certificate.

The following optional certificate fields are used for policy validation:

  • policy-oids

  • requireExplicitPolicy

  • skipCerts

These fields are described in the following sections.

Policy OIDs Configured on SRX Series Devices

In some situations, it might be desirable to only accept certificates with known policy object identifiers (OIDs) from peers. This optional configuration allows certificate validation to succeed only if the certificate chain received from the peer contains at least one policy OID that is configured on the SRX Series device.

On the SRX Series device, policy OIDs are configured in an IKE policy with the policy-oids configuration statement at the [edit security ike policy policy-name certificate] hierarchy level. You can configure up to five policy OIDs. For a peer’s certificate to be validated successfully, the peer’s certificate chain must contain at least one of the policy OIDs configured on the SRX Series device. Note that the policy-oids field in a certificate is optional. If you configure policy OIDs on the SRX Series device but the peer’s certificate chain does not contain any policy OIDs, certificate validation fails.

No Policy OIDs Configured on SRX Series Devices

If no policy OID is configured on the SRX Series device, policy validation starts whenever the requireExplicitPolicy field is encountered in the certificate chain. A certificate can contain one or more certificate policy OIDs. For policy validation to succeed, there must be a common policy OID in the certificate chain.

Figure 3 shows a certificate chain that consists of certificates for a root CA, three intermediate CAs, and an EE. The CA certificate for Int-CA-2 contains the requireExplicitPolicy field; therefore, policy validation starts with Int-CA-2 and continues through EE-1. The certificate for Int-CA-2 contains policy OIDs P1, P2, and P3. The certificate for Int-CA-3 contains policy OIDs P2, P3, and P4. The certificate for EE-1 contains policy OIDs P2 and P5. Because the policy OID P2 is common to the certificates being validated, policy validation succeeds.

Figure 1: Policy Validation with requireExplicitPolicy FieldPolicy Validation with requireExplicitPolicy Field

The optional skipCerts field in an intermediate CA certificate indicates the number of certificates, including the current CA certificate, that are to be excluded from policy validation. If skipCerts is 0, policy validation starts from the current certificate. If skipCerts is 1, the current certificate is excluded from policy validation. The value of the skipCerts field is checked in every intermediate CA certificate. If a skipCerts value is encountered that is lower than the current number of certificates being excluded, the lower skipCerts value is used.

Figure 4 shows a certificate chain consisting of a root CA, four intermediate CAs, and an EE. The skipCerts value in Int-CA-1 is 12, which skips 12 certificates including the certificate for Int-CA-1. However, the skipCerts value is checked in every intermediate CA certificate in the chain. The skipCerts value in Int-CA-2 is 2, which is lower than 12, so now 2 certificates are skipped. The skipCerts value in Int-CA-4 is 5, which is greater than 2, so the Int-CA-4 skipCerts value is ignored.

Figure 2: Policy Validation with skipCerts FieldPolicy Validation with skipCerts Field

When policy OIDs are configured on the SRX Series device, the certificate fields requireExplicitPolicy and skipCerts are ignored.

Path Length Validation

Certificate validation can involve a certificate chain that includes a root CA, one or more optional intermediate CAs, and an EE certificate. The number of intermediate CAs can grow depending upon the deployment scenario. Path length validation provides a mechanism to limit the number of intermediate certificates involved in certificate validation. path-length is an optional field in an X509 certificate. The value of path-length indicates the number of non-self-signed intermediate CA certificates allowed for certificate validation. The last certificate, which is generally the EE certificate, is not included in the path limit. If the root certificate contains a path-length value of 0, no intermediate CA certificates are allowed. If the path-length value is 1, there can be 0 or 1 intermediate CA certificates.

path-length can be present in multiple CA certificates in the certificate chain. The path length validation always begins with the self-signed root certificate. The path limit is decremented by 1 at each intermediate certificate in the chain. If an intermediate certificate contains a path-length value less than the current path limit, the new limit is enforced. On the other hand, if the path-length value is larger than the current path limit, it is ignored.

Figure 5 shows a certificate chain that consists of a root CA, four intermediate CAs, and an EE. The path-length value in Root-CA is 10, therefore the initial path limit of non-self-signed intermediate CA certificates allowed for certificate validation is 10. At Int-CA-1, the path limit is 10-1 or 9. The path-length value in Int-CA-1 is 4, which is less than the path limit of 9, so the new path limit becomes 4. At Int-CA-2, the path limit is 4-1 or 3. The path-length value in Int-CA-2 is 5, which is larger than the path limit of 3, so it is ignored. At Int-CA-3, the path limit is 3-1 or 2. The path-length value in Int-CA-3 is 20, which is larger than the path limit of 2, so it is also ignored.

Figure 3: Path Length ValidationPath Length Validation

Key Usage

The key usage field in an EE or CA certificate defines the purpose of the key contained in the certificate.

  • For EE certificates, if the key usage field is present but the certificate does not contain digitalSignature or nonrepudiation flags, the certificate is rejected. If the key usage field is not present, then key usage is not checked.

  • For CA certificates, the key can be used for certificate or CRL signature validation. Because the PKI daemon is responsible for both X509 certificate validation and CRL downloads, key usage must be checked before validating the certificate or CRL.

    In certificate signature validation, the keyCertSign flag indicates that a CA certificate can be used for certificate signature validation. If this flag is not set, certificate validation is terminated.

    In Phase 1 negotiations of CRL signature validation, participants check the certificate revocation list (CRL) to see if certificates received during an IKE exchange are still valid. The CRL is periodically downloaded for CA profiles configured with CRL as the certificate revocation check. Downloaded CRL files must be verified before they are downloaded into the device. One of the verification steps is to validate the CRL signature using a CA certificate. The downloaded CRL is signed with the CA certificate’s private key and it must be verified with the CA certificate’s public key stored in the device. The key usage field in the CA certificate must contain the CRLSign flag to verify the downloaded CRL. If this flag is not present, the CRL is discarded.

Issuer and Subject Distinguished Name Validation

Signature validation is performed for certificates received from a peer as well as for the CRL file downloaded from a CA server. Signature validation involves looking up the CA certificate in a CA database based on the issuer’s distinguished name (DN) in the certificate or the CRL being verified.

Figure 6 shows the lookup for CA certificates based on the issuer DN. In the EE certificate, the issuer DN is CA-1, which is the subject DN of the intermediate CA certificate in the chain. In the intermediate CA certificate, the issuer DN is CA-Root, which is the subject DN of the self-signed Root-CA certificate in the chain. In the CRL, the issuer DN is CA-Root, which is the subject DN of the self-signed Root-CA certificate.

Figure 4: Issuer and Subject DN ValidationIssuer and Subject DN Validation

The lookup for the issuer or subject DN must follow these rules for attribute values:

  • Attribute values encoded in different ASN.1 types (for example, PrintableString and BMPString) are assumed to represent different strings.

  • Attribute values encoded in PrintableString types are not case-sensitive. These attribute values are compared after removing leading and trailing white spaces and converting internal substrings of one or more consecutive white spaces to a single space.

  • Attribute values encoded in types other than PrintableString are case-sensitive.

Example: Validating Digital Certificate by Configuring Policy OIDs on an SRX Series Device

In some situations, it might be desirable to only accept certificates with known policy object identifiers (OIDs) from peers. This optional configuration allows certificate validation to succeed only if the certificate chain received from the peer contains at least one policy OID that is configured on the SRX Series device. This example shows how to configure policy OIDs in the IKE policy on an SRX Series device.

You must ensure that at least one of the policy OIDs configured on the SRX Series device is included in a peer’s certificate or certificate chain. Note that the policy-oids field in a peer’s certificate is optional. If you configure policy OIDs in an IKE policy and the peer’s certificate chain does not contain any policy OIDs, certificate validation for the peer fails.

Requirements

Before you begin:

  • Ensure that you are using Junos OS Release 12.3X48-D10 or later for SRX Series devices.

  • Configure an IPsec VPN tunnel. See IPsec VPN with Autokey IKE Configuration Overview. The complete IKE phase 1 and phase 2 VPN tunnel configuration is not shown in this example.

Overview

This example shows an IKE policy configuration where policy OIDs 2.16.840.1.101.3.1.48.2 and 5.16.40.1.101.3.1.55.2 are specified. The IKE policy ike_cert_pol references the IKE proposal ike_cert_prop, which is not shown. The local certificate on the SRX Series device is lc-igloo-root.

Configuration

Procedure

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure policy OIDs for certificate validation:

  1. Configure the IKE policy:

Results

From configuration mode, confirm your configuration by entering the show security ike policy ike_cert_pol command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying the CA Certificate

Purpose

Display the CA certificate configured on the device.

Action

From operational mode, enter the show security pki ca-certificate ca-profile ca-tmp command.

Verifying Policy OID Validation

Purpose

If the peer’s certificate is successfully validated, IKE and IPsec security associations are established. If the validation of the peer’s certificate fails, no IKE security association is established.

Action

From operational mode, enter the show security ike security-associations and show security ipsec security-associations commands.

Meaning

The show security ike security-associations command lists all active IKE Phase 1 SAs. If no SAs are listed, there was a problem with Phase 1 establishment. In this case, check for the PKID_CERT_POLICY_CHECK_FAIL message in the system logs. This message indicates that the peer’s certificate chain does not contain a policy OID that is configured on the SRX Series device. Check the policy-oids values in the peer’s certificate chain with the values configured on the SRX Series device.

It might also be that the peer’s certificate chain does not contain any policy-oids fields, which are optional fields. If this is the case, certificate validation fails if there are any policy OIDs configured on the SRX Series device.