DH4096_RFC3526 NIKE produces 519 byte keys sometimes

[david415]

Currently this tests fails for our diffiehellman implementation:

func TestDHKeyLengths(t *testing.T) {
	s := Scheme()

	for i := 0; i < 1000; i++ {
		pubkey1, privkey1, err := s.GenerateKeyPair()
		require.NoError(t, err)

		require.Equal(t, s.PublicKeySize(), len(pubkey1.Bytes()))
		require.Equal(t, s.PrivateKeySize(), len(privkey1.Bytes()))
	}
}

which produces test outputs like this:

go test -v
=== RUN   TestDHKeyLengths
    dh_test.go:17: 
        	Error Trace:	/home/human/code/hpqc/nike/diffiehellman/dh_test.go:17
        	Error:      	Not equal: 
        	           	expected: 520
        	           	actual  : 519
        	Test:       	TestDHKeyLengths
--- FAIL: TestDHKeyLengths (4.87s)