Francois Berder / TLS_axTLS

This is a fork of the mbed port of axTLS

Dependents:   TLS_axTLS-Example HTTPSClientExample

Overview

This library is a fork from the mbed port of axTLS. It attempts to :

  • reduce the usage of dynamic memory
  • verify certificates with key size up to 2048 bits
  • provide a simple interface

Encryption

This library uses either RC4 or AES for encryption.

Memory usage

During the establishment of a connection, about 10KB of memory is allocated dynamically (it depends on certificates). Once the connection is established, the memory consumption is relatively low. This means that your program must not use too much static memory or allocate memory before you establish a TLS connection.

Certificates

Certificates are the major source of problem and will often be the reason why your program will crash. Due to memory constraint, there are some limitations on certificates :

  • Each certificate must not be bigger than 2KB
  • TLS client can only handle a chain of up to three certificates (excluding the root certificate). This means that the server must not send more than three certificates.

Also, this library can only load certificates following these specifications :

  • encoded in binary DER format (PKCS1)
  • The public key must use RSA only

Once the connection is established, you should free all loaded certificates by calling CertificateManager::clear(). This will free a few kilobytes (it depends on your certificates). In addition, to enable certificate verification during the connection, this library has a "precomputed mode". This mode uses much less memory than a normal certificate verification.

Normal mode

You need to copy the root certificate in binary-DER format on the mbed. Then in your code, let's say that your root certificate is saved on the mbed as "root.der", assuming that you include CertificateManager.h and that you created a LocalFileSystem, you can load this certificate as this ;

Load root certificate

CertificateManager::add("/local/root.der");
CertificateManager::load();

Do not forget that this mode takes quite a lot of memory ( the memory peak is high while verifying certificates) and will only work if the key size is not bigger than 1024 bits (otherwise it will crash while verifying certificates).

Precomputed mode

In this mode, you need to save the entire chain of certificates (in binary-DER format) including the root certificate on the mbed. In practice, this means that you must first retrieve all certificates that the server sends during a connection and then find the right root certificate. In your code, you must call CertificateManager::add for each certificate and in the right order : from the server certificate to the root certificate. Here is how you shoud load certificates in this mode :

Loadcertificates in precomputed mode

CertificateManager::add("/local/server1.der");
CertificateManager::add("/local/server2.der");
CertificateManager::add("/local/server3.der");
CertificateManager::add("/local/root.der");
CertificateManager::load(true);

Using this mode, you should be able to verify certificates with key size up to 2048 bits.

How do I find these certificates ?

I posted an entry in my notebook detailing how to get certificates from a server. You should be able to get all certificates you need except the root certificate. Here is a way how to get the root certificate on windows :

  1. Open (double-click) the last certificate sent by the server
  2. Go to details panel and click on the entry called Issuer. The first line gives you the name of this certificate and the second line indicates the company who created this certificate
  3. Open firefox
  4. Go to options, advanced panel and click on View Certificates
  5. Go to Authorities panel
  6. Choose the certificate whose name match the issuer of the last certificate sent by the server
  7. Export this certificate to binary-DER format.

Connect to mbed.org !

Import programTLS_axTLS-Example

Establishing a connection to mbed.org using TLS

Last commit 12 Sep 2013 by Francois Berder

Diff: axTLS/crypto/bigint_impl.h

Revision:
0:85fceccc1a7c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/axTLS/crypto/bigint_impl.h	Thu Sep 12 15:18:04 2013 +0000
@@ -0,0 +1,135 @@
+/*
+ * Copyright (c) 2007, Cameron Rich
+ * 
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without 
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, 
+ *   this list of conditions and the following disclaimer.
+ * * Redistributions in binary form must reproduce the above copyright notice, 
+ *   this list of conditions and the following disclaimer in the documentation 
+ *   and/or other materials provided with the distribution.
+ * * Neither the name of the axTLS project nor the names of its contributors 
+ *   may be used to endorse or promote products derived from this software 
+ *   without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef BIGINT_IMPL_HEADER
+#define BIGINT_IMPL_HEADER
+
+/* Maintain a number of precomputed variables when doing reduction */
+#define BIGINT_M_OFFSET     0    /**< Normal modulo offset. */
+#ifdef CONFIG_BIGINT_CRT
+#define BIGINT_P_OFFSET     1    /**< p modulo offset. */
+#define BIGINT_Q_OFFSET     2    /**< q module offset. */
+#define BIGINT_NUM_MODS     3    /**< The number of modulus constants used. */
+#else
+#define BIGINT_NUM_MODS     1    
+#endif
+
+
+/* Architecture specific functions for big ints */
+#if defined(CONFIG_INTEGER_8BIT)
+#define COMP_RADIX          256U       /**< Max component + 1 */
+#define COMP_MAX            0xFFFFU/**< (Max dbl comp -1) */
+#define COMP_BIT_SIZE       8   /**< Number of bits in a component. */
+#define COMP_BYTE_SIZE      1   /**< Number of bytes in a component. */
+#define COMP_NUM_NIBBLES    2   /**< Used For diagnostics only. */
+typedef uint8_t comp;            /**< A single precision component. */
+typedef uint16_t long_comp;     /**< A double precision component. */
+typedef int16_t slong_comp;     /**< A signed double precision component. */
+#elif defined(CONFIG_INTEGER_16BIT)
+#define COMP_RADIX          65536U       /**< Max component + 1 */
+#define COMP_MAX            0xFFFFFFFFU/**< (Max dbl comp -1) */
+#define COMP_BIT_SIZE       16  /**< Number of bits in a component. */
+#define COMP_BYTE_SIZE      2   /**< Number of bytes in a component. */
+#define COMP_NUM_NIBBLES    4   /**< Used For diagnostics only. */
+typedef uint16_t comp;            /**< A single precision component. */
+typedef uint32_t long_comp;     /**< A double precision component. */
+typedef int32_t slong_comp;     /**< A signed double precision component. */
+#else /* regular 32 bit */
+#ifdef WIN32
+#define COMP_RADIX          4294967296i64         
+#define COMP_MAX            0xFFFFFFFFFFFFFFFFui64
+#else
+#define COMP_RADIX          4294967296ULL         /**< Max component + 1 */
+#define COMP_MAX            0xFFFFFFFFFFFFFFFFULL/**< (Max dbl comp -1) */
+#endif
+#define COMP_BIT_SIZE       32  /**< Number of bits in a component. */
+#define COMP_BYTE_SIZE      4   /**< Number of bytes in a component. */
+#define COMP_NUM_NIBBLES    8   /**< Used For diagnostics only. */
+#include <stdint.h>
+typedef uint32_t comp;            /**< A single precision component. */
+typedef uint64_t long_comp;     /**< A double precision component. */
+typedef int64_t slong_comp;     /**< A signed double precision component. */
+#endif
+
+/**
+ * @struct  _bigint
+ * @brief A big integer basic object
+ */
+struct _bigint
+{
+    struct _bigint* next;       /**< The next bigint in the cache. */
+    short size;                 /**< The number of components in this bigint. */
+    short max_comps;            /**< The heapsize allocated for this bigint */
+    int refs;                   /**< An internal reference count. */
+    comp* comps;                /**< A ptr to the actual component data */
+};
+
+typedef struct _bigint bigint;  /**< An alias for _bigint */
+
+/**
+ * Maintains the state of the cache, and a number of variables used in 
+ * reduction.
+ */
+typedef struct /**< A big integer "session" context. */
+{
+    bigint *active_list;                    /**< Bigints currently used. */
+    bigint *free_list;                      /**< Bigints not used. */
+    bigint *bi_radix;                       /**< The radix used. */
+    bigint *bi_mod[BIGINT_NUM_MODS];        /**< modulus */
+
+#if defined(CONFIG_BIGINT_MONTGOMERY)
+    bigint *bi_RR_mod_m[BIGINT_NUM_MODS];   /**< R^2 mod m */
+    bigint *bi_R_mod_m[BIGINT_NUM_MODS];    /**< R mod m */
+    comp N0_dash[BIGINT_NUM_MODS];
+#elif defined(CONFIG_BIGINT_BARRETT)
+    bigint *bi_mu[BIGINT_NUM_MODS];         /**< Storage for mu */
+#endif
+    bigint *bi_normalised_mod[BIGINT_NUM_MODS]; /**< Normalised mod storage. */
+    bigint **g;                 /**< Used by sliding-window. */
+    int window;                 /**< The size of the sliding window */
+    int active_count;           /**< Number of active bigints. */
+    int free_count;             /**< Number of free bigints. */
+
+#ifdef CONFIG_BIGINT_MONTGOMERY
+    uint8_t use_classical;      /**< Use classical reduction. */
+#endif
+    uint8_t mod_offset;         /**< The mod offset we are using */
+} BI_CTX;
+
+#ifndef WIN32
+#define max(a,b) ((a)>(b)?(a):(b))  /**< Find the maximum of 2 numbers. */
+#define min(a,b) ((a)<(b)?(a):(b))  /**< Find the minimum of 2 numbers. */
+#endif
+
+#define PERMANENT           0x7FFF55AA  /**< A magic number for permanents. */
+
+#endif
+
+