Validate Certificate
Learn how you can dynamically update the CA certificates.
Digital Certificate Validation
During IKE negotiation, the PKI daemon on an SRX Series Firewall 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 Firewalls
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 Firewall.
On the SRX Series Firewall, 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 Firewall.
Note that the policy-oids field in a certificate is
optional. If you configure policy OIDs on the SRX Series Firewall but the peer’s
certificate chain does not contain any policy OIDs, certificate validation
fails.
No Policy OIDs Configured on SRX Series Firewalls
If no policy OID is configured on the SRX Series Firewall, 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 1 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.
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 2 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.
When policy OIDs are configured on the SRX Series Firewall, 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 3 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.
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 4 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.
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: Validate Digital Certificate by Configuring Policy OIDs on an SRX Series Firewall
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 Firewall. This example shows how to configure policy OIDs in the IKE policy on an SRX Series Firewall.
You must ensure that at least one of the policy OIDs configured on the SRX Series Firewall 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 Firewalls.
Configure an IPsec VPN tunnel. See IPsec VPN 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 Firewall 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.
set security ike policy ike_cert_pol mode main set security ike policy ike_cert_pol proposals ike_cert_prop set security ike policy ike_cert_pol certificate local-certificate lc-igloo-root set security ike policy ike_cert_pol certificate policy-oids 2.16.840.1.101.3.1.48.2 set security ike policy ike_cert_pol certificate policy-oids 5.16.40.1.101.3.1.55.2
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 Use the CLI Editor in Configuration Mode in the CLI User Guide.
To configure policy OIDs for certificate validation:
Configure the IKE policy:
content_copy zoom_out_map[edit security ike policy ike_cert_pol] user@host# set mode main user@host# set proposals ike_cert_prop user@host# set certificate local-certificate lc-igloo-root user@host# set certificate policy-oids 2.16.840.1.101.3.1.48.2 user@host# set certificate policy-oids 5.16.40.1.101.3.1.55.2
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.
user@host# show security ike policy ike_cert_pol
mode main;
proposals ike_cert_prop;
certificate {
local-certificate lc-igloo-root;
policy-oids [ 2.16.840.1.101.3.1.48.2 5.16.40.1.101.3.1.55.2 ];
}
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.
user@host> show security pki ca-certificate ca-profile ca-tmp detail
Certificate identifier: ca-tmp
Certificate version: 3
Serial number: 00000047
Issuer:
Organization: U.S. Government,
Organizational unit: DoD, Organizational unit: Testing, Country: US,
Common name: Trust Anchor
Subject:
Organization: U.S. Government,
Organizational unit: Dod, Organizational unit: Testing, Country: US,
Common name: CA1-PP.01.03
Subject string:
C=US, O=U.S. Government, OU=Dod, OU=Testing, CN=CA1-PP.01.03
Validity:
Not before: 01- 1-1998 12:01 UTC
Not after: 01- 1-2048 12:01 UTC
?Public key algorithm: rsaEncryption(1024 bits)
30:81:89:02:81:81:00:cb:fd:78:0c:be:87:ac:cd:c0:33:66:a3:18
9e:fd:40:b7:9b:bc:dc:66:ff:08:45:f7:7e:fe:8e:d6:32:f8:5b:75
db:76:f0:4d:21:9a:6e:4f:04:21:4c:7e:08:a1:f9:3d:ac:8b:90:76
44:7b:c4:e9:9b:93:80:2a:64:83:6e:6a:cd:d8:d4:23:dd:ce:cb:3b
b5:ea:da:2b:40:8d:ad:a9:4d:97:58:cf:60:af:82:94:30:47:b7:7d
88:c3:76:c0:97:b4:6a:59:7e:f7:86:5d:d8:1f:af:fb:72:f1:b8:5c
2a:35:1e:a7:9e:14:51:d4:19:ae:c7:5c:65:ea:f5:02:03:01:00:01
Signature algorithm: sha1WithRSAEncryption
Certificate Policy:
Policy Identifier = 2.16.840.1.101.3.1.48.2
Use for key: CRL signing, Certificate signing
Fingerprint:
e0:b3:2f:2e:a1:c5:ee:ad:af:dd:96:85:f6:78:24:c5:89:ed:39:40 (sha1)
f3:47:6e:55:bc:9d:80:39:5a:40:70:8b:10:0e:93:c5 (md5)
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.
user@host> show security ike security-associations node0: -------------------------------------------------------------------------- Index State Initiator cookie Responder cookie Mode Remote Address 821765168 UP 88875c981252c1d8 b744ac9c21bde57e IKEv2 192.0.2.2 1106977837 UP 1a09e32d1e6f20f1 e008278091060acb IKEv2 198.51.100.202
user@host> show security ipsec security-associations node0: -------------------------------------------------------------------------- Total active tunnels: 2 ID Algorithm SPI Life:sec/kb Mon lsys Port Gateway <213909506 ESP:aes-cbc-192/sha256 8cb9e40a 1295/ unlim - root 500 192.0.2.2 >213909506 ESP:aes-cbc-192/sha256 8271d2b2 1295/ unlim - root 500 192.0.2.2 <218365954 ESP:aes-cbc-192/sha256 d0153bc0 1726/ unlim - root 1495 198.51.100.202 >218365954 ESP:aes-cbc-192/sha256 97611813 1726/ unlim - root 1495 198.51.100.202
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 Firewall. Check the policy-oids values
in the peer’s certificate chain with the values configured on the SRX Series Firewall.
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 Firewall.
