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SAML Certificates in Federation Metadata

This article discusses the use of X.509

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certificates in Federation metadata. It has security implications so please read it carefully.

A SAML entity uses public key cryptography to secure the data transmitted to trusted partners. Public keys are published in the form of X.509 certificates in metadata whereas the corresponding private keys are held securely by the entity. These keys are used for message-level signing and encryption, and to create secure back channels for transporting SAML messages over TLS. They are not used for browser-facing TLS transactions on port 443. See the Key Usage topic for more information.

The InCommon Federation is based on the Explicit Key Trust Model, one of several possible metadata trust models. Consequently, the use of long-lived, self-signed certificates in metadata is strongly recommended. Certificates signed by a Certification 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.

Any certificates you want to use with your SAML software are uploaded via the Federation Manager. Typically only one certificate is required per entity but multiple certificates may be uploaded and used as needed. In particular, multiple certificates may be used to facilitate the controlled rollover of expired certificates or compromised keys. To avoid interoperability problems, refer to the Certificate Migration topic for recommended guidelines regarding the rollover process.

It is easy to create a self-signed certificate with the OpenSSL command-line tool. Before you do this, however, take a moment to consider how best to handle the all-important private key.

Contents

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 registered 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 via the administrative web interface. 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 an entity via its entity ID. 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 entity. As it is, the certificate is a convenient container for the public key, the critical element being that the key is bound to a particular entity in the metadataAs of January 2010, InCommon does not issue certificates signed by the InCommon CA. We will transition the entire federation to self-signed certificates over the next two years.

Requirements

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

• The use of long-lived, self-signed certificates in Federation metadata is strongly RECOMMENDED.
  • Certificates with lifetimes of at least 10 years are RECOMMENDED to avoid unnecessary technically-imposed deadlines on key rollover.
  • Certificates SHOULD expire before 2038 to avoid the so-called Year 2038 problem.
• RSA keys with a minimum size of

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• 2048 bits MUST be used for all new certificates introduced into Federation metadata.

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  • • New certificates with

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  • • key sizes less than

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  • • 2048 bits are not allowed in Federation metadata

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  • • .

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  • • Certificates with keys greater than 2048 bits are NOT RECOMMENDED since such keys force relying parties to perform unnecessary computation.
  • Expired certificates SHOULD NOT be introduced into Federation metadata. An expired certificate in metadata SHOULD be removed once a certificate migration process to a new certificate has been completed.
    • A certificate's expiration date has nothing to do with the security of the corresponding private key, which is an ongoing concern.
  • If a private key is lost or stolen, immediate steps MUST be taken to configure a new private key and to introduce the corresponding public key certificate into metadata. Since there are no other known attacks on RSA 2048-bit keys, generating a new private key for any other purpose is NOT RECOMMENDED.
  • Service providers MUST include an encryption key in SP metadata.
    • The encryption key is used by IdPs to encrypt SAML V2.0 assertions transmitted to the SP

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  • • .
  • InCommon does not validate Subject information in self-signed certificates because this information is irrelevant

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  • from a security perspective. However, at its

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  • discretion, InCommon will reject metadata submissions if that submission contains a certificate with fields that contain egregiously

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  • misrepresented Subject information as decided by InCommon on a case-by-case basis. Generally,

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  • Subject information should express a somewhat reasonable relationship between the certificate and

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  • the organization.

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  interop interop

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  Interoperability

  To increase the chances of Consider the following interoperability issues as you set up and maintain your deployment interoperating:

  • 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 (such as AD FS 2.0 and the OIOSAML Java SP) and with older versions of Apache used to deploy 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 role descriptor at any time. (You can log into the administrative interface, select a particular role, and associate more than one certificate with that role for the purposes of migrating from one certificate to another.) Bear in mind, however, that some SAML implementations do not support
  • If the certificate will be used for TLS/SSL server authentication (e.g., an IdP's SOAP endpoint), make sure your certificate's CN (or subjectAltName) value matches the intended hostname. This will maximize the chances that your implementation will work. This TLS/SSL configuration is left to your discretion and responsibility. InCommon highlights this point as one that may likely cause problems if not met.
  • Shibboleth does not check expiration dates of certificates, but this practice often causes interoperability issues with other software, and with some older (generally now unsupported) versions of Apache used in the deployment of the Shibboleth IdP.
  • For key management, InCommon allows multiple certificates per end point at any time. You can log in to the site administration tool, select the particular endpoint, then choose the one or more certificates you want to associate with this endpoint. This is helpful for migrating from one certificate to another during a finite period of time.
  • However, bear in mind that not every SAML implementation supports multiple keys properly and you may want to test this capability with any your non-Shibboleth partners. For example, EZProxy supports metadata, but :
    • EZProxy is known to ignore additional keys beyond the first.
  • 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.

  InCommon No Longer Issues Certificates (as of January 2010)

  As stated above, we are moving the federation toward self-signed certificates in the metadata. As your InCommon certificates expire, you should:

  1. Submit a self-signed certificate containing a 2048-bit key.
  2. For key management and migration, InCommon allows multiple certificates per end point at any time. Select the new certificate from your list of submitted certs, while keeping the old certificate associated with the endpoint for as long as your transition process requires. Before doing so, you may need to verify that non-Shibboleth deployments you interact with will accept metadata containing multiple keys.

  If necessary, you may continue using the expired certificate. Shibboleth, when used with metadata of the form used by InCommon, does not enforce the expiration dates of certificates. However, other software often will check expiration dates, therefore, we recommend planning ahead and migrating to a self-signed certificate well ahead of your certificate's expiration.

  Backgrounder: Security and Certificate Authorities

  Wiki Markup

  In the base SAML metadata specification \[1\], a certificate signing authority has no assumed relevance to the trust model that secures the interactions among a federation's participants. 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 they are represented in the context of self-signed certificates or certificates signed by an authority. 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 end point. 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.

  Furthermore, 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.

  Third-party certificates signed by an authoritative CA are discouraged since they can create interoperability issues in certain cases, and lead to configurations that mistakenly rely on the certificate signer to establish trust in the certificate. Where necessary they can be accommodated because of constraints imposed on participants from other sources.

  Where Do I Get My 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, but this depends on your need for a TLS/SSL certificate and whether the hostname it deduces matches the one you expect to publish in your metadata. This will often not be the case, so use caution.

  If you need to generate your own, an example of doing so using OpenSSL follows:

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

  References

  • • AD FS 2.0 will not consume an <md:EntityDescriptor> element containing more than one encryption key.
  • At the deployer's convenience, a single certificate may be bound to multiple SPs in InCommon metadata. However, some implementations (e.g., AD FS 2.0) do not allow the same certificate to be used by two distinct entities.
  • If the certificate will be used for TLS 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.
  • Avoid certificates with special certificate extensions, since some implementations will actually try to use them. For example, AD FS 2.0 will attempt to access the CRL at the location given in the CRL Distribution Point certificate extension.

  References

  [1] Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0 Wiki Markup\[1\] _Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0_ [http://saml.xml.org/saml-specifications] \
  [2\] _SAML V2] 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) (October 22, 2009)
  [4] The Shibboleth ExplicitKey Trust Engine https://wiki.shibboleth.net/confluence/display/SHIB2/ExplicitKeyTrustEngine
  ]5] Shibboleth Security and Networking https://wiki.shibboleth.net/confluence/x/VoEOAQ