siv: add gnutls support

Add support for the AES-SIV-CMAC cipher in gnutls using the AEAD
interface. It should be available in gnutls-3.6.14.

This will enable NTS support on systems that have a pre-3.6 version of
Nettle, without falling back to the internal SIV implementation.

siv_gnutls.c

@@ -0,0 +1,237 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2020
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ **********************************************************************
+
+  =======================================================================
+
+  SIV ciphers using the GnuTLS library
+  */
+
+#include "config.h"
+
+#include "sysincl.h"
+
+#include <gnutls/crypto.h>
+
+#include "logging.h"
+#include "memory.h"
+#include "siv.h"
+
+struct SIV_Instance_Record {
+  gnutls_cipher_algorithm_t algorithm;
+  gnutls_aead_cipher_hd_t cipher;
+};
+
+/* ================================================== */
+
+static int instance_counter = 0;
+static int gnutls_initialised = 0;
+
+/* ================================================== */
+
+static void
+init_gnutls(void)
+{
+  int r;
+
+  if (gnutls_initialised)
+    return;
+
+  r = gnutls_global_init();
+  if (r < 0)
+    LOG_FATAL("Could not initialise %s : %s", "gnutls", gnutls_strerror(r));
+
+  DEBUG_LOG("Initialised");
+  gnutls_initialised = 1;
+}
+
+/* ================================================== */
+
+static void
+deinit_gnutls(void)
+{
+  assert(gnutls_initialised);
+  gnutls_global_deinit();
+  gnutls_initialised = 0;
+  DEBUG_LOG("Deinitialised");
+}
+
+/* ================================================== */
+
+static gnutls_cipher_algorithm_t
+get_cipher_algorithm(SIV_Algorithm algorithm)
+{
+  switch (algorithm) {
+    case AEAD_AES_SIV_CMAC_256:
+      return GNUTLS_CIPHER_AES_128_SIV;
+    default:
+      return 0;
+  }
+}
+
+/* ================================================== */
+
+SIV_Instance
+SIV_CreateInstance(SIV_Algorithm algorithm)
+{
+  gnutls_cipher_algorithm_t calgo;
+  SIV_Instance instance;
+
+  calgo = get_cipher_algorithm(algorithm);
+  if (calgo == 0)
+    return NULL;
+
+  if (instance_counter == 0)
+    init_gnutls();
+
+  /* Check if the cipher is actually supported */
+  if (gnutls_cipher_get_tag_size(calgo) == 0)
+    return NULL;
+
+  instance = MallocNew(struct SIV_Instance_Record);
+  instance->algorithm = calgo;
+  instance->cipher = NULL;
+
+  instance_counter++;
+
+  return instance;
+}
+
+/* ================================================== */
+
+void
+SIV_DestroyInstance(SIV_Instance instance)
+{
+  if (instance->cipher)
+    gnutls_aead_cipher_deinit(instance->cipher);
+  Free(instance);
+
+  instance_counter--;
+  if (instance_counter == 0)
+    deinit_gnutls();
+}
+
+/* ================================================== */
+
+int
+SIV_GetKeyLength(SIV_Algorithm algorithm)
+{
+  gnutls_cipher_algorithm_t calgo = get_cipher_algorithm(algorithm);
+
+  if (calgo == 0)
+    return 0;
+
+  return gnutls_cipher_get_key_size(calgo);
+}
+
+/* ================================================== */
+
+int
+SIV_SetKey(SIV_Instance instance, const unsigned char *key, int length)
+{
+  gnutls_aead_cipher_hd_t cipher;
+  gnutls_datum_t datum;
+  int r;
+
+  if (length <= 0 || length != gnutls_cipher_get_key_size(instance->algorithm))
+    return 0;
+
+  datum.data = (unsigned char *)key;
+  datum.size = length;
+
+  /* Initialise a new cipher with the provided key (gnutls does not seem to
+     have a function to change the key directly) */
+  r = gnutls_aead_cipher_init(&cipher, instance->algorithm, &datum);
+  if (r < 0) {
+    DEBUG_LOG("Could not initialise %s : %s", "cipher", gnutls_strerror(r));
+    return 0;
+  }
+
+  /* Replace the previous cipher */
+  if (instance->cipher)
+    gnutls_aead_cipher_deinit(instance->cipher);
+  instance->cipher = cipher;
+
+  return 1;
+}
+
+/* ================================================== */
+
+int
+SIV_GetTagLength(SIV_Instance instance)
+{
+  return gnutls_cipher_get_tag_size(instance->algorithm);
+}
+
+/* ================================================== */
+
+int
+SIV_Encrypt(SIV_Instance instance,
+            const unsigned char *nonce, int nonce_length,
+            const void *assoc, int assoc_length,
+            const void *plaintext, int plaintext_length,
+            unsigned char *ciphertext, int ciphertext_length)
+{
+  size_t clen = ciphertext_length;
+
+  if (nonce_length < 1 || assoc_length < 0 ||
+      plaintext_length < 0 || ciphertext_length < 0)
+    return 0;
+
+  assert(assoc && plaintext);
+
+  if (gnutls_aead_cipher_encrypt(instance->cipher,
+                                 nonce, nonce_length, assoc, assoc_length, 0,
+                                 plaintext, plaintext_length, ciphertext, &clen) < 0)
+    return 0;
+
+  if (clen != ciphertext_length)
+    return 0;
+
+  return 1;
+}
+
+/* ================================================== */
+
+int
+SIV_Decrypt(SIV_Instance instance,
+            const unsigned char *nonce, int nonce_length,
+            const void *assoc, int assoc_length,
+            const unsigned char *ciphertext, int ciphertext_length,
+            void *plaintext, int plaintext_length)
+{
+  size_t plen = plaintext_length;
+
+  if (nonce_length < 1 || assoc_length < 0 ||
+      plaintext_length < 0 || ciphertext_length < 0)
+    return 0;
+
+  assert(assoc && plaintext);
+
+  if (gnutls_aead_cipher_decrypt(instance->cipher,
+                                 nonce, nonce_length, assoc, assoc_length, 0,
+                                 ciphertext, ciphertext_length, plaintext, &plen) < 0)
+    return 0;
+
+  if (plen != plaintext_length)
+    return 0;
+
+  return 1;
+}