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

axTLS/ssl/openssl.c

Committer:
feb11
Date:
2013-09-12
Revision:
0:85fceccc1a7c

File content as of revision 0:85fceccc1a7c:

/*
 * 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.
 */

/*
 * Enable a subset of openssl compatible functions. We don't aim to be 100%
 * compatible - just to be able to do basic ports etc.
 *
 * Only really tested on mini_httpd, so I'm not too sure how extensive this
 * port is.
 */

#include "config.h"

#ifdef CONFIG_OPENSSL_COMPATIBLE
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include "os_port.h"
#include "ssl.h"

#define OPENSSL_CTX_ATTR  ((OPENSSL_CTX *)ssl_ctx->bonus_attr)

static char *key_password = NULL;

void *SSLv23_server_method(void) { return NULL; }
void *SSLv3_server_method(void) { return NULL; }
void *TLSv1_server_method(void) { return NULL; }
void *SSLv23_client_method(void) { return NULL; }
void *SSLv3_client_method(void) { return NULL; }
void *TLSv1_client_method(void) { return NULL; }

typedef void * (*ssl_func_type_t)(void);
typedef void * (*bio_func_type_t)(void);

typedef struct
{
    ssl_func_type_t ssl_func_type;
} OPENSSL_CTX;

SSL_CTX * SSL_CTX_new(ssl_func_type_t meth)
{
    SSL_CTX *ssl_ctx = ssl_ctx_new(0, 5);
    ssl_ctx->bonus_attr = malloc(sizeof(OPENSSL_CTX));
    OPENSSL_CTX_ATTR->ssl_func_type = meth;
    return ssl_ctx;
}

void SSL_CTX_free(SSL_CTX * ssl_ctx)
{
    free(ssl_ctx->bonus_attr);
    ssl_ctx_free(ssl_ctx);
}

SSL * SSL_new(SSL_CTX *ssl_ctx)
{
    SSL *ssl;
    ssl_func_type_t ssl_func_type;

    ssl = ssl_new(ssl_ctx, -1);        /* fd is set later */
    ssl_func_type = OPENSSL_CTX_ATTR->ssl_func_type;

#ifdef CONFIG_SSL_ENABLE_CLIENT
    if (ssl_func_type == SSLv23_client_method ||
        ssl_func_type == SSLv3_client_method ||
        ssl_func_type == TLSv1_client_method)
    {
        SET_SSL_FLAG(SSL_IS_CLIENT);
    }
    else
#endif
    {
        ssl->next_state = HS_CLIENT_HELLO;
    }

    return ssl;
}

int SSL_set_fd(SSL *s, int fd)
{
    s->client_fd = fd;
    return 1;   /* always succeeds */
}

int SSL_accept(SSL *ssl)
{
    while (ssl_read(ssl, NULL) == SSL_OK)
    {
        if (ssl->next_state == HS_CLIENT_HELLO)
            return 1;   /* we're done */
    }

    return -1;
}

#ifdef CONFIG_SSL_ENABLE_CLIENT
int SSL_connect(SSL *ssl)
{
    return do_client_connect(ssl) == SSL_OK ? 1 : -1;
}
#endif

void SSL_free(SSL *ssl)
{
    ssl_free(ssl);
}

int SSL_read(SSL *ssl, void *buf, int num)
{
    uint8_t *read_buf;
    int ret;

    while ((ret = ssl_read(ssl, &read_buf)) == SSL_OK);

    if (ret > SSL_OK)
    {
        memcpy(buf, read_buf, ret > num ? num : ret);
    }

    return ret;
}

int SSL_write(SSL *ssl, const void *buf, int num)
{
    return ssl_write(ssl, buf, num);
}

int SSL_CTX_use_certificate_file(SSL_CTX *ssl_ctx, const char *file, int type)
{
    return (ssl_obj_load(ssl_ctx, SSL_OBJ_X509_CERT, file, NULL) == SSL_OK);
}

int SSL_CTX_use_PrivateKey_file(SSL_CTX *ssl_ctx, const char *file, int type)
{
    return (ssl_obj_load(ssl_ctx, SSL_OBJ_RSA_KEY, file, key_password) == SSL_OK);
}

int SSL_CTX_use_certificate_ASN1(SSL_CTX *ssl_ctx, int len, const uint8_t *d)
{
    return (ssl_obj_memory_load(ssl_ctx, 
                        SSL_OBJ_X509_CERT, d, len, NULL) == SSL_OK);
}

int SSL_CTX_set_session_id_context(SSL_CTX *ctx, const unsigned char *sid_ctx,
                                            unsigned int sid_ctx_len)
{
    return 1;
}

int SSL_CTX_set_default_verify_paths(SSL_CTX *ctx)
{
    return 1;
}

int SSL_CTX_use_certificate_chain_file(SSL_CTX *ssl_ctx, const char *file)
{
    return (ssl_obj_load(ssl_ctx, 
                        SSL_OBJ_X509_CERT, file, NULL) == SSL_OK);
}

int SSL_shutdown(SSL *ssl)
{
    return 1;
}

/*** get/set session ***/
SSL_SESSION *SSL_get1_session(SSL *ssl)
{
    return (SSL_SESSION *)ssl_get_session_id(ssl); /* note: wrong cast */
}

int SSL_set_session(SSL *ssl, SSL_SESSION *session)
{
    memcpy(ssl->session_id, (uint8_t *)session, SSL_SESSION_ID_SIZE);
    return 1;
}

void SSL_SESSION_free(SSL_SESSION *session) { }
/*** end get/set session ***/

long SSL_CTX_ctrl(SSL_CTX *ctx, int cmd, long larg, void *parg)
{
    return 0;
}

void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
                                 int (*verify_callback)(int, void *)) { }

void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth) { }

int SSL_CTX_load_verify_locations(SSL_CTX *ctx, const char *CAfile,
                                           const char *CApath)
{
    return 1;
}

void *SSL_load_client_CA_file(const char *file)
{
    return (void *)file;
}

void SSL_CTX_set_client_CA_list(SSL_CTX *ssl_ctx, void *file) 
{ 

    ssl_obj_load(ssl_ctx, SSL_OBJ_X509_CERT, (const char *)file, NULL);
}

void SSLv23_method(void) { }

void SSL_CTX_set_default_passwd_cb(SSL_CTX *ctx, void *cb) { }

void SSL_CTX_set_default_passwd_cb_userdata(SSL_CTX *ctx, void *u) 
{ 
    key_password = (char *)u;
}

int SSL_peek(SSL *ssl, void *buf, int num)
{
    memcpy(buf, ssl->bm_data, num);
    return num;
}

void SSL_set_bio(SSL *ssl, void *rbio, void *wbio) { }

long SSL_get_verify_result(const SSL *ssl)
{
    return ssl_handshake_status(ssl);
}

int SSL_state(SSL *ssl)
{
    return 0x03; // ok state
}

/** end of could do better list */

void *SSL_get_peer_certificate(const SSL *ssl)
{
    return &ssl->ssl_ctx->certs[0];
}

int SSL_clear(SSL *ssl)
{
    return 1;
}


int SSL_CTX_check_private_key(const SSL_CTX *ctx)
{
    return 1;
}

int SSL_CTX_set_cipher_list(SSL *s, const char *str)
{
    return 1;
}

int SSL_get_error(const SSL *ssl, int ret)
{
    ssl_display_error(ret);
    return 0;   /* TODO: return proper return code */
}

void SSL_CTX_set_options(SSL_CTX *ssl_ctx, int option) {}
int SSL_library_init(void ) { return 1; }
void SSL_load_error_strings(void ) {}
void ERR_print_errors_fp(FILE *fp) {}

#ifndef CONFIG_SSL_SKELETON_MODE
long SSL_CTX_get_timeout(const SSL_CTX *ssl_ctx) { 
                            return CONFIG_SSL_EXPIRY_TIME*3600; }
long SSL_CTX_set_timeout(SSL_CTX *ssl_ctx, long t) { 
                            return SSL_CTX_get_timeout(ssl_ctx); }
#endif
void BIO_printf(FILE *f, const char *format, ...)
{
    va_list(ap);
    va_start(ap, format);
    vfprintf(f, format, ap);
    va_end(ap);
}

void* BIO_s_null(void) { return NULL; }
FILE *BIO_new(bio_func_type_t func)
{
    if (func == BIO_s_null)
        return fopen("/dev/null", "r");
    else
        return NULL;
}

FILE *BIO_new_fp(FILE *stream, int close_flag) { return stream; }
int BIO_free(FILE *a) { if (a != stdout && a != stderr) fclose(a); return 1; }



#endif