@Test
    void passes_rfc_9180_dhkem_x25519_hkdf_sha256_hkdf_sha256_aes_gcm_128_base_test_vector() {
        byte[] info = unhex("4f6465206f6e2061204772656369616e2055726e");
        byte[] pt   = unhex("4265617574792069732074727574682c20747275746820626561757479");
        byte[] aad  = unhex("436f756e742d30");
        var kpR     = receiverRfc9180BaseTestVectorKeyPair();

        var kem = Kem.dHKemX25519HkdfSha256(new Kem.UnsafeDeterminsticKeyPairOnlyUsedByTesting(ephemeralRfc9180BaseTestVectorKeyPair()));
        var ciphersuite = Ciphersuite.of(kem, Kdf.hkdfSha256(), Aead.aes128Gcm());

        var hpke = Hpke.of(ciphersuite);
        var s = hpke.sealBase(pk(kpR), info, aad, pt);

        // The "enc" output is the ephemeral public key
        var expectedEnc = "37fda3567bdbd628e88668c3c8d7e97d1d1253b6d4ea6d44c150f741f1bf4431";
        assertEquals(expectedEnc, hex(s.enc()));

        var expectedCiphertext = "f938558b5d72f1a23810b4be2ab4f84331acc02fc97babc53a52ae8218a355a9" +
                                 "6d8770ac83d07bea87e13c512a";
        assertEquals(expectedCiphertext, hex(s.ciphertext()));

        byte[] openedPt = hpke.openBase(s.enc(), sk(kpR), info, aad, s.ciphertext());
        assertEquals(hex(pt), hex(openedPt));
    }
<a href="https://www.rfc-editor.org/rfc/rfc9180.html">RFC 9180</a> test vector, Base mode

Appendix A.1.1

DHKEM(X25519, HKDF-SHA256), HKDF-SHA256, AES-128-GCM

Only tests first encryption, i.e. sequence number 0.
security-utils/src/test/java/com/yahoo/security/HpkeTest.java