Perform 2-RTT Handshake to upgrade to PQ when possible #4526
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Resolved issues:
N/A
Description of changes:
Updates s2n to always prefer upgrading to a PQ Hybrid KeyShare whenever possible, even if doing so would require a 2 round trip handshake. s2n's current behavior is to choose the best KeyShare option possible in a 1-RTT handshake first, and only if a 1-RTT is not possible will a KeyShare algorithm that requires a 2-RTT handshake be chosen. After this change, s2n's server-side negotiation logic will consider a Hybrid PQ KeyShare as higher priority than completing the handshake in 1-RTT. This is to ensure that Hybrid PQ KeyShares will be guaranteed to be negotiated during any transition periods between one PQ KeyShare algorithm and another PQ KeyShare algorithm (where a newer PQ algorithm isn't mutually supported yet, but the older PQ algorithm can be negotiated in 2 round trips).
There is further discussion of this change in the IETF mailing list, and in a draft RFC by David Benjamin from Google:
Call-outs:
N/A
Testing:
Unit Tests
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.