Add existing HPKE project files

crypto/ecdh/interfaces.go

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+// 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)
+}

crypto/ecdh/stubs.go

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+package ecdh
+
+import "crypto/ecdh"
+
+// This file contains stubs to allow importing only this package instead of
+// crypto/ecdh, to minimize the diff.
+
+type Curve = ecdh.Curve
+type PrivateKey = ecdh.PrivateKey
+type PublicKey = ecdh.PublicKey
+
+func X25519() Curve { return ecdh.X25519() }
+func P256() Curve   { return ecdh.P256() }
+func P384() Curve   { return ecdh.P384() }
+func P521() Curve   { return ecdh.P521() }