You can use the P2SDiagnosticLog Make sure you Enable Diagnostic Logs for VPN Gateway It might take upto 15 minutes for the logs to start generating
Post which, you can use P2SDiagnosticLogs to check what is the cipher used.
This should be available under Message Column.
Kindly let us know if this helps or you need further assistance on this issue.
Thanks, Kapil
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