We had discussion on the list on whether to include context in the key
derivation, but we never closed on the issue of separating out the MSK and
EMSK derivation. As a result several implementers have gone down the path
of implementing what is in draft 13 and not separating out the derivation.
The main difference is that draft 15 separated out the EMSK and MSK
derivation using two different labels while draft 13 used a single label to
derive key material which is partitioned into two keys. The reason for
the change was to enable different access control for these two different
quantities for different callers, however in practice it is EAP-TLS
application which needs access to both keys that is the caller of the TLS
library so this separation is not particularly useful. Therefore the
recommendation is to align with implementation and derive the MSK and EMSK
by partitioning the key material from the key material produced by a single
label of the exporter function.
Please respond to the list if you support the change below or not to revert
some of the text in the key derivation section. If you object to the
change please state why. Please respond by May 20,2021.
Thanks,
Joe
The proposal is to use the following key derivation which is largely a
reversion to draft 13:
Draft-15 Text:
Type-Code = 0x0D
MSK = TLS-Exporter("EXPORTER_EAP_TLS_MSK",Type-Code,64)
EMSK = TLS-Exporter("EXPORTER_EAP_TLS_EMSK",Type-Code,64)
Method-Id = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id",Type-Code,64)
Session-Id = Type-Code || Method-Id
Proposed New Text:
Type-Code = 0x0D
Key_Material = TLS-Exporter("EXPORTER_EAP_TLS_Key_Material",
Type-Code, 128)
MSK = Key_Material(0, 63)
EMSK = Key_Material(64, 127)
Method-Id = TLS-Exporter("EXPORTER_EAP_TLS_Method-Id", Type-Code, 64)
Session-Id = Type-Code || Method-Id
The rest of the text of the section remains the same as draft-15.
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