3.14 X509

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Version 3.14 introduces significant changes to pluto's X509 certificate handling. Prior to 3.14, pluto contained its own functions for certificate and crl parsing and verification. This code is now removed and has been replaced by the NSS library certificate functions.

Database changes

Starting with the NSS database, pluto uses an SQL format database in /etc/ipsec.d. This format allows pluto to pick up changes to the database without requiring a restart. When upgrading libreswan to 3.14, the 'ipsec checknss' command run on service startup will attempt to upgrade the existing DBM format database. - ipsec checknss first detects if an upgrade is needed and runs a certutil --upgrade-merge on the database. If /etc/ipsec.d/nsspassword is present it will also preserve the database password from that file. - Any certificates and crls in /etc/ipsec.d/cacerts and /etc/ipsec.d/crls will be imported into the upgraded database. If these are duplicate, or old, it won't replace anything current in the database.

IMPORTANT: When using crlutil or certutil on the upgraded database, you must always prefix the database path with 'sql:'. For example, to list all certificates: 

certutil -L -d sql:/etc/ipsec.d

Alternately, you can set the NSS_DEFAULT_DB_TYPE environment variable to 'sql:' and use certutil/crlutil as normal.

Certificate changes

Pluto now uses CERT_Certificate structures instead of its internal x509_cert_t structure. The certificates received over the IKE exchange are verified with a call to CERT_PKIXVerifyCert, verifying the certificate down to a root CA that is located in the NSS database. Since the IKE exchange can include intermediate certificates, a temporary import is performed on all certificate payloads received, which places the intermediates into the NSS ephemeral cache. Doing this ensures that the intermediate certs are also verified and used to complete the chain.

The use of NSS certificates now considers the database trust arguments. Before, this did not matter to pluto as the NSS certificates were only converted to the internal pluto certificate when loaded.

The following trust strings should be used:

  • End cert with private key: u,u,u
  • Root CA: CT,,
  • Intermediate CA: ,,
  • Trusted Peer cert (no private key), OCSP responder cert: P,,

For example: 

 [root@east ~]# certutil -L -d sql:/etc/ipsec.d
 
 Certificate Nickname                                         Trust Attributes
                                                              SSL,S/MIME,JAR/XPI
 
 east                                                         u,u,u
 Libreswan test CA for mainca - Libreswan                     CT,, 
 north                                                        P,,  
 hashsha2                                                     P,,  
 west-ec                                                      P,,  
 nic                                                          P,,

The 'CT,,' trust flags on a CA ensure that the CA is valid for verifying a certificate with the serverAuth/clientAuth EKU extensions. While these are not particularly relevant to the IKE and IPsec standards, the NSS library requires a server or client profile type to be specified during verification. Some clients (Windows) requires client certificates to contain the serverAuth EKU, and due to this NSS API limitation, there will be a failover verification attempt for the serverAuth profile. This way, pluto is able to handle certificates with either serverAuth or clientAuth (or both) set.

For best compatibility with the NSS verification and other clients, at least the digitalSignature and nonRepudiation extensions should be set on end certificates.

CA certificates must have a basicConstraints extension specifying that it is a CA.

CRL changes

Previously, CRLs were placed into /etc/ipsec.d/crls and pluto loaded them internally on startup. The /etc/ipsec.d/crls directory is no longer used in 3.14 and it is required that CRLs are imported into the database. This can be done with: 

crlutil -I -i <file> -d sql:/etc/ipsec.d

(TODO: add ipsec importcrl)

When the NSS database contains a valid CRL for a CA used during the IKE exchange, the CRL status is used automatically during certificate verification. Before verifying the certificate there is a check for the issuer's CRL. The behavior of pluto has not changed in this regard - If a CRL is expired or missing, pluto queues a crl fetch request to be processed during the next check interval (crlcheckinterval). The fetch thread uses curl to retrieve the current CRL at the CA's CRL URI. The fetched blob is passed to a spawned helper program '_import_crl' that attempts to import the blob as a CRL into the NSS database. Assuming that the fetch resulted in an updated CRL, it is now cached and available for any subsequent exchanges.

The configuration option "strictcrlpolicy" has been renamed to "crl_strict"

In strict mode (crl_strict), a missing or expired CRL results in a rejection. If a peer's exchange fails while using a good certificate due to the CRL expiring in strict mode, setting a crlcheckinterval that triggers between the peer's retransmit attempts will allow a fetch to update the CRL and authorize the exchange. The peer will then only notice a delay in connection.

Previously pluto allowed CRLs with an MD5 signature algorithm. These are now rejected by NSS. You must use CRLs with a signature algorithm of at least SHA-1.

OCSP support

Switching to the NSS cert library now gives us OCSP support.

To configure OCSP support, the following 'config setup' options are provided:

  • ocsp_enable
    • enable or disable OCSP checks within NSS. yes/no. Default == no
  • ocsp_uri
  • ocsp_trust_name
    • set to the NSS nickname of the certificate used by the OCSP responder to sign the responses. Default == unset
    • this option goes together with ocsp_uri to enable a default responder.
  • ocsp_timeout
    • set to the maximum time (in seconds) that pluto will block waiting on the OCSP response. Default == 2 seconds.
  • ocsp_strict
    • 'yes' sets OCSP checking to "strict" which configures CERT_SetOCSPFailureMode with ocspMode_FailureIsVerification
    • 'no' (the default) sets it to ocspMode_FailureIsNotAVerificationFailure (perhaps this should not be default)

The following comment from NSS sources explains the failure modes: 

/*                                                                    
 * ocspMode_FailureIsVerificationFailure:                             
 * This is the classic behaviour of NSS.                              
 * Any OCSP failure is a verification failure (classic mode, default).
 * Without a good response, OCSP networking will be retried each time 
 * it is required for verifying a cert.                               
 *                                                                    
 * ocspMode_FailureIsNotAVerificationFailure:                         
 * If we fail to obtain a valid OCSP response, consider the           
 * cert as good.                                                      
 * Failed OCSP attempts might get cached and not retried until        
 * minimumSecondsToNextFetchAttempt.                                  
 * If we are able to obtain a valid response, the cert                
 * will be considered good, if either status is "good"                
 * or the cert was not yet revoked at verification time.              
 *                                                                    
 * Additional failure modes might be added in the future.             
 */

When enabling OCSP it is recommended to set ocsp_uri and ocsp_trust_name together in order to enable a default responder. Without it, NSS will rely solely on OCSP information contained in the certificates. Similar to the problem of configuring CRL fetches that rely on the IPsec connectivity, configuring an OCSP URI with an address that is only reachable through the IPsec tunnel can be problematic. You should test carefully and adjust the ocsp_timeout option if you are using OCSP responders over an IPsec tunnel or the internet, but for best results you should specify a default responder with an address on the LAN of the ipsec host.

An OCSP responder certificate can be a CA, or a seperate certificate issued just for the responder's OCSP service, and it must be pre-loaded into the NSS database before starting pluto. The OCSP responder certificate should have the OCSP Responder Certificate EKU extension, and an OCSP authInfo ext. with the URI of the OCSP responder.

For example, the test harness has a certificate 'nic' used for OCSP. Its extensions are: 

        Signed Extensions:
            Name: Certificate Basic Constraints
            Data: Is not a CA.

            Name: Certificate Key Usage
            Usages: Digital Signature
                    Non-Repudiation
                    Certificate Signing
                    CRL Signing 

            Name: Extended Key Usage
                TLS Web Server Authentication Certificate
                TLS Web Client Authentication Certificate
                Code Signing Certificate
                OCSP Responder Certificate

            Name: Authority Information Access
            Method: PKIX Online Certificate Status Protocol
            Location: 
                URI: "http://nic.testing.libreswan.org:2560" 

            Name: CRL Distribution Points
            Distribution point:
                URI: "http://nic.testing.libreswan.org/revoked.crl"

The CRL Signing/Certificate Signing KU extensions and Code Signing EKU may also be required. This text should be updated when this is confirmed.

OCSP requests are HTTP GET type through the NSS library. The NSS state maintains an OCSP cache that can be cleared for a running instance of pluto with 'ipsec auto --purgeocsp'.

Test coverage

WIP..

Test list:

  • nss-cert-0x
  • nss-cert-chain-0x
  • nss-cert-crl-0x
  • nss-cert-ocsp-0x

1. Requires ocspd installed on nic (available in fedora repo)

    • For CRL tests a fetch URL is provided by nic through launching a python SimpleHTTPServer from nicinit

2. pyOpenSSL on testing _host_ system needs a patch to support creating SHA-1 signed CRL. https://nohats.ca/ftp/pyOpenSSL/pyOpenSSL-0.14-4.fc21.noarch.rpm

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