siv: introduce API for SIV

Add a header file for Synthetic Initialization Vector (SIV) cipher mode,
which will be used by NTS.

siv.h

@@ -0,0 +1,70 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2019
+ * 
+ * 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.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for Synthetic Initialization Vector (SIV) ciphers.
+
+  */
+
+#ifndef GOT_SIV_H
+#define GOT_SIV_H
+
+/* Maximum key length of all supported SIVs */
+#define SIV_MAX_KEY_LENGTH 32
+
+/* Maximum difference between lengths of ciphertext and plaintext */
+#define SIV_MAX_TAG_LENGTH 16
+
+/* Identifiers of SIV algorithms following the IANA AEAD registry */
+typedef enum {
+  AEAD_AES_SIV_CMAC_256 = 15,
+  AEAD_AES_SIV_CMAC_384 = 16,
+  AEAD_AES_SIV_CMAC_512 = 17,
+  AEAD_AES_128_GCM_SIV = 30,
+  AEAD_AES_256_GCM_SIV = 31,
+} SIV_Algorithm;
+
+typedef struct SIV_Instance_Record *SIV_Instance;
+
+extern SIV_Instance SIV_CreateInstance(SIV_Algorithm algorithm);
+
+extern void SIV_DestroyInstance(SIV_Instance instance);
+
+extern int SIV_GetKeyLength(SIV_Algorithm algorithm);
+
+extern int SIV_SetKey(SIV_Instance instance, const unsigned char *key, int length);
+
+extern int SIV_GetTagLength(SIV_Instance instance);
+
+extern 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);
+
+extern 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);
+
+#endif