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import javax.crypto.Cipher;
import javax.crypto.spec.HPKEParameterSpec;
import java.nio.charset.StandardCharsets;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.util.Arrays;
import java.util.HexFormat;

class PackageSnippets {
    public static void main(String[] args) throws Exception {

        // @start region="hpke-spec-example"
        // Recipient key pair generation
        KeyPairGenerator g = KeyPairGenerator.getInstance("X25519");
        KeyPair kp = g.generateKeyPair();

        // The HPKE sender cipher is initialized with the recipient's public
        // key and an HPKEParameterSpec using specified algorithm identifiers
        // and application-supplied info.
        Cipher senderCipher = Cipher.getInstance("HPKE");
        HPKEParameterSpec ps = HPKEParameterSpec.of(
                        HPKEParameterSpec.KEM_DHKEM_X25519_HKDF_SHA256,
                        HPKEParameterSpec.KDF_HKDF_SHA256,
                        HPKEParameterSpec.AEAD_AES_128_GCM)
                .withInfo(HexFormat.of().parseHex("010203040506"));
        senderCipher.init(Cipher.ENCRYPT_MODE, kp.getPublic(), ps);

        // Retrieve the key encapsulation message (from the KEM step) from
        // the sender.
        byte[] kemEncap = senderCipher.getIV();

        // The HPKE recipient cipher is initialized with its own private key,
        // an HPKEParameterSpec using the same algorithm identifiers as used by
        // the sender, and the key encapsulation message from the sender.
        Cipher recipientCipher = Cipher.getInstance("HPKE");
        HPKEParameterSpec pr = HPKEParameterSpec.of(
                        HPKEParameterSpec.KEM_DHKEM_X25519_HKDF_SHA256,
                        HPKEParameterSpec.KDF_HKDF_SHA256,
                        HPKEParameterSpec.AEAD_AES_128_GCM)
                .withInfo(HexFormat.of().parseHex("010203040506"))
                .withEncapsulation(kemEncap);
        recipientCipher.init(Cipher.DECRYPT_MODE, kp.getPrivate(), pr);

        // Encryption and decryption
        byte[] msg = "Hello World".getBytes(StandardCharsets.UTF_8);
        byte[] ct = senderCipher.doFinal(msg);
        byte[] pt = recipientCipher.doFinal(ct);

        assert Arrays.equals(msg, pt);
        // @end
    }
}