crypto/hpke: new package

// This program demonstrates the new crypto/hpke package that was added in Go 1.27.1.
// It generates a DHKEM key pair, encrypts a message with the public key,
// and then decrypts it with the private key. The output shows the original
// plaintext and the decrypted plaintext to confirm that the round‑trip
// succeeded.

package main

import (
	"crypto/hpke"
	"crypto/ecdh"
	"fmt"
)

func main() {
	// Create a DHKEM instance for the X25519 curve.
	kem := hpke.DHKEM(ecdh.X25519())

	// Generate a new key pair.
	priv, err := kem.GenerateKey()
	if err != nil {
		panic(err)
	}
	pub := priv.PublicKey()

	// Choose a KDF and AEAD. Here we use HKDF‑SHA256 and AES‑128‑GCM.
	kdf := hpke.HKDFSHA256()
	aead := hpke.AES128GCM()

	// The sender creates a context with the recipient's public key.
	enc, sender, err := hpke.NewSender(pub, kdf, aead, []byte("info"))
	if err != nil {
		panic(err)
	}

	// Encrypt a message.
	plaintext := []byte("Hello, HPKE!")
	ciphertext, err := sender.Seal(nil, plaintext)
	if err != nil {
		panic(err)
	}

	// The recipient creates a context with the encapsulated key and its private key.
	recipient, err := hpke.NewRecipient(enc, priv, kdf, aead, []byte("info"))
	if err != nil {
		panic(err)
	}

	// Decrypt the message.
	decrypted, err := recipient.Open(nil, ciphertext)
	if err != nil {
		panic(err)
	}

	fmt.Printf("original: %s\n", plaintext)
	fmt.Printf("decrypted: %s\n", decrypted)
}