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 ;

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 :

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.

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