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I have a customer who insists upon using only certificate-authority signed key-pair certificates for encrypting SAML2 assertions.

In this use case, I hold the private key in my Service Provider, and we furnish the public key to the Identity Provider. For most customers using this mode of SAML2, they just accept the self-signed public key we provide.

To me it seems the trust relationship is between me (the SP operator) and the customer (the IdP operator) and a CA doesn't add anything to that trust relationship.

What am I missing about extra security provided by a signed certificate in this case?

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I agree that self-signed certificates are sufficient, but the CA approach does add some incremental security in how that trust relationship is maintained, especially when you consider the rollover scenario.

Imagine this scenario:

  1. OJones@sp.com sends a public key to TSmith@idp.com and the relationship is established
  2. Two years later, TSmith@idp.com receives an email from GStevens@sp.com which says "I now manage the SAML configuration previously managed by OJones. Here is a new public key to replace the one that's expiring soon"

How do they know that GStevens is authorized to do this? If both old & new certificates are CA-signed, they can examine the properties and verify it was issued to the right organization. Some systems are also configured to trust based on CA issuer and properties.

This assumes, of course, that the SP organization has good control over its CA process.

It also assumes the public keys are being exchanged "out of band". Automatic metadata processing changes the risk calculations.

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