basic: add hmac_sha256 implementation

Based on the FIPS 198 specification. Not optimized and probably
completely unsafe, to be used only for non-strong-cryptographic
purposes when OpenSSL cannot be used.

src/basic/hmac.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#define SHA256_DIGEST_SIZE 32
+
+/* Unoptimized implementation based on FIPS 198. 'res' has to be allocated by
+ * the caller. Prefer external OpenSSL functions, and use this only when
+ * linking to OpenSSL is not desireable (eg: libsystemd.so). */
+void hmac_sha256(const void *key, size_t key_size, const void *input, size_t input_size, uint8_t res[static SHA256_DIGEST_SIZE]);