auth: add HMAC implementation

This allows for checking the SHA-512 implementation against Wycheproof
via the HMAC tests.

impl_hmac.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2024 Lucas Gabriel Vuotto <lucas@lgv5.net>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "lilcrypto.h"
+
+
+#define HMAC_BLOCKSZ_MAX	128
+
+
+struct hmac_ctx {
+	struct lc_hash_ctx	*hctx;
+	size_t			 blocksz;
+	uint8_t			 key[HMAC_BLOCKSZ_MAX];
+};