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In IKEv1 (RFC 2409), preshared secret is involved to key derivation where IKEv2 (RFC 7296) use it for only authentication. When we consider post-quantum security, this property makes IKEv1 suitable if preshared key has sufficient entropy. Therefore post-quantum extension of IKEv2 proposed in (RFC 8784) which is basicly adding an additional secret that is shared between the initiator and the responder in addition to the authentication method that is already provided within IKEv2.

Why pre-shared key is not involved to key derivation in IKEv2? Are there any disadvantage of using it as in IKEv1?

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Are there any disadvantage of using it as in IKEv1?

Well, there were a couple of things; the most glaring one was determining why the negotiation failed.

What happened if two IKEv1 implementations tried to negotiate with different PSKs? What happened was that they derived different keys, and so it would fail - with no indication that the failure reason was the preshared keys were different (rather than, say, the DH operation came up with different values for some reason).

While mismatched PSKs were the most common reason for not being able to decrypt the initial message, it wasn't the only one. So, IKEv2 reworked how PSKs were used in order to make it clear why negotiation failed.

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