Editing Key-agreement protocol (section)

=== Types of key agreement ===

Boyd et al.<ref name="Boyd2020">{{cite book |last1=Boyd |first1=Colin |last2=Mathuria |first2=Anish |last3=Stebila |first3=Douglas |title=Protocols for Authentication and Key Establishment |series=Information Security and Cryptography |date=2020 |doi=10.1007/978-3-662-58146-9 |isbn=978-3-662-58145-2 |url=https://link.springer.com/book/10.1007/978-3-662-58146-9 |language=en}}</ref> classify two-party key agreement protocols according to two criteria as follows:
# whether a pre-shared key already exists or not
# the method of generating the [[session key]].

The pre-shared key may be shared between the two parties, or each party may share a key with a trusted third party. If there is no secure channel (as may be established via a pre-shared key), it is impossible to create an authenticated session key.<ref>{{cite journal |last1=Boyd |first1=C. |title=Security architectures using formal methods |journal=IEEE Journal on Selected Areas in Communications |date=June 1993 |volume=11 |issue=5 |pages=694–701 |doi=10.1109/49.223872 |url=https://gnusha.org/~nmz787/pdf/Security%20Architectures%20Using%20Formal%20Methods.pdf}}</ref>

The session key may be generated via: key transport, key agreement and hybrid. If there is no trusted third party, then the cases of key transport and hybrid session key generation are indistinguishable. SKA is concerned with protocols in which the session key is established using only symmetric primitives.