Add existing HPKE project files

crypto/ecdh/interfaces.go

@@ -0,0 +1,15 @@
+// Package ecdh defines an additional interface that will be added to the
+// crypto/ecdh package in Go 1.26+.
+package ecdh
+
+import "crypto/ecdh"
+
+// KeyExchanger is an interface for an opaque private key that can be used for
+// key exchange operations. For example, an ECDH key kept in a hardware module.
+//
+// It is implemented by [ecdh.PrivateKey].
+type KeyExchanger interface {
+	PublicKey() *ecdh.PublicKey
+	Curve() ecdh.Curve
+	ECDH(*ecdh.PublicKey) ([]byte, error)
+}