Documentation on EncryptionMethod negotiation between SP and IdP

[Jose Hales-Garcia]

Hi, 

Where do I look for documentation on the negotiation the SP and IdP
undergo in choosing the cipher used in decrypting messages?
In particular I'm interested to know how Shibboleth SP 3.0.4 deals with
the cipher the IdP sends it in the EncryptionMethod? 

In an experimental client we're testing, I see the IdP is sending the
EncryptionMethod of "aes128-gcm".  Our production Shibboleth SP 3.0.4
seems to be handling this cipher, or negotiating another one.  I want to
better understand how Shibboleth SP 3.0.4 is negotiating ciphers, in
order to apply that technique to the experimental client. 

Thank you in advance,
Jose
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