In an IdP or SP Metadata, we can find a KeyDescriptor attribute, containing a X509Certificate.
Sometimes, there is a tag use="signing" / "encryption" / "decryption" / ...
Sometimes there is nothing, only the certificate.
I would like to know what is the purpose of a certificate without any "use". Is it only for establishing the trust between two federate servers? For example, I received a IdP Metadata from an admin, and there is only one certificate without any "use" tag. There is nothing about "signing", "encryption", etc. What does it mean?
Also, I would like to know the exact signification of the tags signing, encryption, etc.