Your information is not entirely accurate:
Yes, use the corresponding passphrase...it seems you're improperly trying to translate a hextet into a passphrase instead (it already has a passphrase ).
(I have a feeling you're trying to make up a passphrase from a set of bytes though...and haven't got to your moment)
@mk24 fruther elaborates in the next post:
I hope this helps.
Edit: (The other idea is you're trying to hack someone's AP from the key seen on air...hence...you'd definitely understand why the solution is use the passphrase. I assume you're just being scholarly, though. )
See: https://www.cisco.com/assets/sol/sb/WAP321_Emulators/WAP321_Emulator_v1.0.0.3/help/Wireless05.html
Can you show the RFC that says it can be any string?
This reference does not cite an RFC - https://en.wikipedia.org/wiki/Service_set_(802.11_network)#Service_set_identifier