X.509 Certificates in the InCommon Federation

The use of self-signed certificates in InCommon Federation metadata is strongly recommended. Certificates signed by a Certificate Authority (CA) are allowed, and in most situations will work just fine, but the use of certificates other than self-signed certificates is discouraged. See the Background information and the Interoperability notes below for further discussion.

As of January 2010, *InCommon does not issue certificates* signed by the InCommon CA. We will transition the entire federation to *self-signed certificates* by the end of 2012. See \[3\] for more detail.

Background

In the base SAML metadata specification \[1\], a certificate signing authority (CA) has no assumed relevance to the trust model that secures the interactions among a federation's participants. In fact, certificates signed by a CA are discouraged since they can create interoperability issues in certain situations and lead to configurations that mistakenly establish trust based on the certificate signer. Allowing self-signed certificates simplifies the work of participants who may be required to join multiple federations, or who support local systems that are not enrolled in the Federation.

InCommon conforms to the _SAML V2.0 Metadata Interoperability Profile_ \[2\] from OASIS. Participant site administrators securely transmit X.509 certificates and metadata to InCommon. InCommon signs the entire metadata file, securing the keys of its participants whether those keys are bound to self-signed certificates or certificates signed by a CA. The critical element in the certificate is the public key, which is associated with the participant's "entityID." The other elements in the certificate are irrelevant for security and trust processing. Theoretically, if all the relevant software systems could accept a public key without a certificate wrapper, InCommon would only need to include the public key of each endpoint. As it is, the certificate is a practical shell for the public key, the critical element being that the key is bound to a particular entity in the metadata.

Requirements

InCommon sets the following security and trust parameters around certificates that are included in Federation metadata:

1. The use of self-signed certificates in Federation metadata is strongly RECOMMENDED.
2. RSA keys with a minimum size of 2048 bits must be used for all new certificates introduced into Federation metadata.
3. Certificates with keys less than 2048 bits will not be allowed in Federation metadata.
   • All participants must migrate old 1024-bit keys out of metadata and upgrade to 2048-bit keys by December 2012.
4. We recommend that participants submit a new certificate with a new 2048-bit key every 3 years.
5. Expired certificates should not be introduced into Federation metadata.

6. A certificate in metadata that expires should be removed once the *rollover process* to a new key has been completed \[2\].

7. InCommon does not validate Subject information in self-signed certificates because this information is irrelevant to the federated security context. However, at its own discretion, InCommon will reject metadata submissions if that submission contains a certificate with fields that contain egregiously misrepresentated Subject information as decided by InCommon on a case by case basis. Generally, subject information should express a somewhat reasonable relationship between the certificate and your organization.

Interoperability

Consider the following interoperability issues as you set up and maintain your deployment:

• A potential federation partner (especially a partner not using the Shibboleth software) may question the use of self-signed certificates. As discussed in the Background section, there are, in fact, fewer interoperability issues with self-signed certificates compared to CA-signed certificates.

• The Shibboleth software does not check the expiration dates of certificates \[4\], but *expired certificates often cause interoperability issues* with other software and with some versions of Apache used in the deployment of the Shibboleth IdP. InCommon recommends that you plan ahead and migrate to an unexpired certificate well ahead of your certificate's expiration date.

• For key management purposes, InCommon allows multiple certificates per endpoint at any time. (You can log in to the site administration tool, select a particular endpoint, and associate more than one certificate with that endpoint for the purposes of migrating from one certificate to another.) Bear in mind, however, that some SAML implementations do not support multiple keys properly and you may want to test this capability with non-Shibboleth partners. For example, EZProxy supports metadata, but is known to ignore additional keys beyond the first.
• If the certificate will be used for TLS/SSL server authentication, the certificate's CN (and/or subjectAltName) value should match the server's hostname. This is especially true for IdPs but may also be true in certain advanced scenarios where the SP acts as a SOAP responder.

Obtaining a Self-signed Certificate

For those using the Shibboleth SP, the self-signed certificate generated during installation of the software (or subsequently using the keygen shell/batch script) is generally suitable for use within the federation.

The self-signed certificate generated during the installation of the Shibboleth IdP may be suitable if the hostname it deduces matches the one you expect to publish in your metadata. This will often not be the case, so use caution.

Use the following command to generate a self-signed certificate with OpenSSL:

openssl req -new -x509 -days 1095 -keyout key.pem -out cert.pem -newkey rsa:2048 -subj "/CN=hostname.example.org"

References

\[1\] _Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0_ [http://saml.xml.org/saml-specifications]
\[2\] _SAML V2.0 Metadata Interoperability Profile_ [http://wiki.oasis-open.org/security/SAML2MetadataIOP]
\[3\] [X.509 Certificates in the Federation Metadata|http://internet2.na6.acrobat.com/p46467886/]: A technical webinar presented by the _InCommon Technical Advisory Committee_ (October 22, 2009)
\[4\] _The Shibboleth ExplicitKey Trust Engine_ [https://spaces.at.internet2.edu/display/SHIB2/ExplicitKeyTrustEngine]