@Test
    void rfc_5869_test_vector_case_1() {
        var ikm  = unhex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
        var salt = unhex("000102030405060708090a0b0c");
        var info = unhex("f0f1f2f3f4f5f6f7f8f9");

        var hkdf = HKDF.extractedFrom(salt, ikm);
        var okm = hkdf.expand(42, info);
        assertEquals("3cb25f25faacd57a90434f64d0362f2a" +
                     "2d2d0a90cf1a5a4c5db02d56ecc4c5bf" +
                     "34007208d5b887185865",
                     hex(okm));
    }
HKDF tests that ensure that the output of our own implementation matches the test
vectors given in <a href="https://tools.ietf.org/html/rfc5869">RFC-5869</a>.

We don't expose the internal PRK (pseudo-random key) value of the HKDF itself,
so we don't test it explicitly. The actual OKM (output keying material) inherently
depends on it, so its correctness is verified transitively.

@author vekterli
security-utils/src/test/java/com/yahoo/security/HKDFTest.java