common: new file hmac for hmac calculation.

Blinded paths will want this too; it's mainly copied from sphinx.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/hmac.c

@@ -0,0 +1,36 @@
+#include <bitcoin/privkey.h>
+#include <ccan/array_size/array_size.h>
+#include <ccan/mem/mem.h>
+#include <common/hmac.h>
+#include <wire/wire.h>
+
+void hmac(const void *src, size_t slen,
+	  const void *key, size_t klen,
+	  struct hmac *hmac)
+{
+	crypto_auth_hmacsha256_state state;
+
+	crypto_auth_hmacsha256_init(&state, memcheck(key, klen), klen);
+	crypto_auth_hmacsha256_update(&state, memcheck(src, slen), slen);
+	crypto_auth_hmacsha256_final(&state, hmac->bytes);
+}
+
+void subkey_from_hmac(const char *prefix,
+		      const struct secret *base,
+		      struct secret *key)
+{
+	struct hmac h;
+	hmac(base->data, sizeof(base->data), prefix, strlen(prefix), &h);
+	BUILD_ASSERT(sizeof(h.bytes) == sizeof(key->data));
+	memcpy(key->data, h.bytes, sizeof(key->data));
+}
+
+void towire_hmac(u8 **pptr, const struct hmac *hmac)
+{
+	towire_u8_array(pptr, hmac->bytes, ARRAY_SIZE(hmac->bytes));
+}
+
+void fromwire_hmac(const u8 **ptr, size_t *max, struct hmac *hmac)
+{
+	fromwire_u8_array(ptr, max, hmac->bytes, ARRAY_SIZE(hmac->bytes));
+}