Add minimalistic HMAC_DRBG implementation
(copied from ECDSA)
diff --git a/include/polarssl/hmac_drbg.h b/include/polarssl/hmac_drbg.h
new file mode 100644
index 0000000..f86c136
--- /dev/null
+++ b/include/polarssl/hmac_drbg.h
@@ -0,0 +1,112 @@
+/**
+ * \file hmac_drbg.h
+ *
+ * \brief HMAC_DRBG (NIST SP 800-90A)
+ *
+ * Copyright (C) 2014, Brainspark B.V.
+ *
+ * This file is part of PolarSSL (http://www.polarssl.org)
+ * Lead Maintainer: Paul Bakker <polarssl_maintainer at polarssl.org>
+ *
+ * All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 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.
+ */
+#ifndef POLARSSL_HMAC_DRBG_H
+#define POLARSSL_HMAC_DRBG_H
+
+#include "md.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * Simplified HMAC_DRBG context.
+ * No reseed counter, no prediction resistance flag.
+ */
+typedef struct
+{
+ md_context_t md_ctx;
+ unsigned char V[POLARSSL_MD_MAX_SIZE];
+ unsigned char K[POLARSSL_MD_MAX_SIZE];
+} hmac_drbg_context;
+
+/**
+ * \brief HMAC_DRBG initialisation
+ *
+ * \param ctx HMAC_DRBG context to be initialised
+ * \param md_info MD algorithm to use for HMAC_DRBG
+ * \param data Concatenation of entropy string and additional data
+ * \param data_len Length of data in bytes
+ *
+ * \todo Use entropy callback rather than buffer.
+ *
+ * \return 0 if successful, or
+ * POLARSSL_ERR_MD_BAD_INPUT_DATA, or
+ * POLARSSL_ERR_MD_ALLOC_FAILED
+ */
+int hmac_drbg_init( hmac_drbg_context *ctx,
+ const md_info_t * md_info,
+ const unsigned char *data, size_t data_len );
+
+/**
+ * \brief HMAC_DRBG update state
+ *
+ * \param ctx HMAC_DRBG context
+ * \param additional Additional data to update state with, or NULL
+ * \param add_len Length of additional data, or 0
+ *
+ * \note Additional data is optional, pass NULL and 0 as second
+ * third argument if no additional data is being used.
+ */
+void hmac_drbg_update( hmac_drbg_context *ctx,
+ const unsigned char *additional, size_t add_len );
+
+/**
+ * \brief HMAC_DRBG generate random
+ *
+ * Note: Automatically reseeds if reseed_counter is reached. (TODO)
+ *
+ * \param p_rng HMAC_DRBG context
+ * \param output Buffer to fill
+ * \param output_len Length of the buffer
+ *
+ * \return 0 if successful.
+ */
+int hmac_drbg_random( void *p_rng, unsigned char *output, size_t out_len );
+
+/**
+ * \brief Free an HMAC_DRBG context
+ *
+ * \param ctx HMAC_DRBG context to free.
+ */
+void hmac_drbg_free( hmac_drbg_context *ctx );
+
+
+#if defined(POLARSSL_SELF_TEST)
+/**
+ * \brief Checkup routine
+ *
+ * \return 0 if successful, or 1 if the test failed
+ */
+int hmac_drbg_self_test( int verbose );
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* hmac_drbg.h */