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

axTLS/ssl/crypto_misc.h

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.
*/

/**
 * @file crypto_misc.h
 */

#ifndef HEADER_CRYPTO_MISC_H
#define HEADER_CRYPTO_MISC_H

#ifdef __cplusplus
extern "C" {
#endif

#include "config.h"
#include "crypto.h"
#include "bigint.h"

/**************************************************************************
 * X509 declarations 
 **************************************************************************/
#define X509_OK                             0
#define X509_NOT_OK                         -1
#define X509_VFY_ERROR_NO_TRUSTED_CERT      -2
#define X509_VFY_ERROR_BAD_SIGNATURE        -3      
#define X509_VFY_ERROR_NOT_YET_VALID        -4
#define X509_VFY_ERROR_EXPIRED              -5
#define X509_VFY_ERROR_SELF_SIGNED          -6
#define X509_VFY_ERROR_INVALID_CHAIN        -7
#define X509_VFY_ERROR_UNSUPPORTED_DIGEST   -8
#define X509_INVALID_PRIV_KEY               -9
#define X509_KEY_SIZE_TOO_BIG               -10

/*
 * The Distinguished Name
 */
#define X509_NUM_DN_TYPES                   3
#define X509_COMMON_NAME                    0
#define X509_ORGANIZATION                   1
#define X509_ORGANIZATIONAL_UNIT            2

#include <time.h>

struct PrecomputedCertificate;
typedef struct PrecomputedCertificate PrecomputedCertificate;


struct _x509_ctx
{
    char *ca_cert_dn[X509_NUM_DN_TYPES];
    char *cert_dn[X509_NUM_DN_TYPES];
    char **subject_alt_dnsnames;
    time_t not_before;
    time_t not_after;
    uint8_t *signature;
    uint16_t sig_len;
    uint8_t sig_type;
    RSA_CTX *rsa_ctx;
    bigint *digest;
    struct _x509_ctx *next;
};

typedef struct _x509_ctx X509_CTX;
typedef struct 
{
    X509_CTX *cert[CONFIG_X509_MAX_CA_CERTS];
} CA_CERT_CTX;


int x509_new(const uint8_t *cert, int *len, X509_CTX **ctx);
void x509_free(X509_CTX *x509_ctx);
int x509_verify(PrecomputedCertificate *cert);
bigint *sig_verify(BI_CTX *ctx, const uint8_t *sig, int sig_len,
        bigint *modulus, bigint *pub_exp);

#ifdef CONFIG_SSL_FULL_MODE
void x509_print(const X509_CTX *cert, CA_CERT_CTX *ca_cert_ctx);
const char * x509_display_error(int error);
#endif

/**************************************************************************
 * ASN1 declarations 
 **************************************************************************/
#define ASN1_INTEGER            0x02
#define ASN1_BIT_STRING         0x03
#define ASN1_OCTET_STRING       0x04
#define ASN1_NULL               0x05
#define ASN1_PRINTABLE_STR2     0x0C
#define ASN1_OID                0x06
#define ASN1_PRINTABLE_STR2     0x0C
#define ASN1_PRINTABLE_STR      0x13
#define ASN1_TELETEX_STR        0x14
#define ASN1_IA5_STR            0x16
#define ASN1_UTC_TIME           0x17
#define ASN1_UNICODE_STR        0x1e
#define ASN1_SEQUENCE           0x30
#define ASN1_CONTEXT_DNSNAME    0x82
#define ASN1_SET                0x31
#define ASN1_V3_DATA            0xa3
#define ASN1_IMPLICIT_TAG       0x80
#define ASN1_CONTEXT_DNSNAME    0x82
#define ASN1_EXPLICIT_TAG       0xa0
#define ASN1_V3_DATA            0xa3

#define SIG_TYPE_MD2            0x02
#define SIG_TYPE_MD5            0x04
#define SIG_TYPE_SHA1           0x05

int get_asn1_length(const uint8_t *buf, int *offset);
int asn1_get_private_key(const uint8_t *buf, int len, RSA_CTX **rsa_ctx);
int asn1_next_obj(const uint8_t *buf, int *offset, int obj_type);
int asn1_skip_obj(const uint8_t *buf, int *offset, int obj_type);
int asn1_get_int(const uint8_t *buf, int *offset, uint8_t **object);
int asn1_version(const uint8_t *cert, int *offset, X509_CTX *x509_ctx);
int asn1_validity(const uint8_t *cert, int *offset, X509_CTX *x509_ctx);
int asn1_name(const uint8_t *cert, int *offset, char *dn[]);
int asn1_public_key(const uint8_t *cert, int *offset, X509_CTX *x509_ctx);
#ifdef CONFIG_SSL_CERT_VERIFICATION
int asn1_signature(const uint8_t *cert, int *offset, X509_CTX *x509_ctx);
int asn1_find_subjectaltname(const uint8_t* cert, int offset);
int asn1_compare_dn(char * const dn1[], char * const dn2[]);
#endif /* CONFIG_SSL_CERT_VERIFICATION */
int asn1_signature_type(const uint8_t *cert, 
                                int *offset, X509_CTX *x509_ctx);

/**************************************************************************
 * MISC declarations 
 **************************************************************************/
#define SALT_SIZE               8

extern const char * const unsupported_str;

typedef void (*crypt_func)(void *, const uint8_t *, uint8_t *, int);
typedef void (*hmac_func)(const uint8_t *msg, int length, const uint8_t *key, 
        int key_len, uint8_t *digest);

int get_file(const char *filename, uint8_t **buf);

#if defined(CONFIG_SSL_FULL_MODE) || defined(WIN32) || defined(CONFIG_DEBUG)
EXP_FUNC void STDCALL print_blob(const char *format, const uint8_t *data, int size, ...);
#else
    #define print_blob(...)
#endif

EXP_FUNC int STDCALL base64_decode(const char *in,  int len,
                    uint8_t *out, int *outlen);

#ifdef __cplusplus
}
#endif

#endif