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TLS socket on mbed OS
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README.md Update README.md Oct 25, 2018
TLSSocket.cpp fixup! More fiddles Jul 4, 2018
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TLSSocketWrapper.cpp Introduce transport modes that allow controlling underlying socket. Sep 28, 2018
TLSSocketWrapper.h Introduce transport modes that allow controlling underlying socket. Sep 28, 2018
mbed_lib.json Refactor TLSSocket to abstract socket interface Jun 28, 2018
tlssocket.png

README.md

DEPRECATED

This repository is deprecated. The code has been integrated into Mbed OS, so no developent will happen here anymore.

TLSSocket is now in Mbed OS master, and will be published in Mbed OS 5.11

TLSSocket

TLSSocket provides the ability to easily secure any stream based socket connection, for example TCP stream. This allows existing protocol libraries to be used through secure connections.

TLSSocket is inheriting the Socket class, which allows any application that uses Socket to use TLSSocket instead. Secure socket both uses Socket interface as its transport layer and implements it. This makes it transport independent and there is no direct dependency to IP stack. For example, we can use HTTP library and give TLSSocket for it, to covert it to HTTPS.

Usage example

TLSSocket API follows Socket API so it is easy to use after setting up:

TLSSocket *socket = new TLSSocket();
socket->open(network)
socket->set_root_ca_cert(certificate);

socket->connect(HOST_NAME, PORT)

socket->send(data, size);

Please note that internal TLS structures require over 1 kB of RAM, so each TLSSocket should be allocated from heap using new command, instead of using stack or statically allocating it.

Design

Internally TLSSocket consist of two classes TLSSocketWrapper and TLSSocket as shown in the following diagram:

TLSSocket UML

The TLSSocketWrapper is able to use any Socket as its transport. TLSSocket is a helper that uses directly TCPSocket for its transport, making it easy to adopt existing TCP based applications to TLS.

One use case of TLSSocketWrapper is that existing TCP socket can be upgraded to TLS, by wrapping it like this:

TCPSocket connection(net);
connection.connect(SERVER, PORT);

// First talk with the server without encryption
connection.send("STARTTLS\r\n", 10);

// Wrap the TCP into TLS object
TLSSocketWrapper tls = new TLSSocketWrapper(connection, SERVER, TLSSocketWrapper::TRANSPORT_CLOSE);

// Initiate TLS handshake
tls.connect();

// Now the secure connection can be used like regular socket
tls.send("HELLO", 5);

API

TLSSocketWrapper implements Mbed OS Socket API and extends it with functions that allow configuring security certificates. Please note that for most of the use cases, you are using these methods through TLSSocket class.

Configuring certificates

TLSSocketWrapper provides following API to set server certificate. You can use either BASE64 formatted PEM certificate, or binary DER certificates. Later form of these functions just assumes root_ca_pem or client_cert_pem to be standard C string and counts its lenght and passes to method which takes just void* and len.

/** Sets the certification of Root CA.
 *
 * @param root_ca Root CA Certificate in any mbed-TLS supported format.
 * @param len     Length of certificate (including terminating 0 for PEM).
 */
nsapi_error_t TLSSocketWrapper::set_root_ca_cert(const void *root_ca, size_t len);

/** Sets the certification of Root CA.
 *
 * @param root_ca_pem Root CA Certificate in PEM format
 */
nsapi_error_t TLSSocketWrapper::set_root_ca_cert(const char *root_ca_pem);

If client authentication is required, following API allows you to set the client certificate and private key:

/** Sets client certificate, and client private key.
 *
 * @param client_cert Client certification in any mbed-TLS supported format.
 * @param client_private_key Client private key in PEM format.
 */
nsapi_error_t TLSSocketWrapper::set_client_cert_key(const void *client_cert_pem, size_t client_cert_len,
                                                    const void *client_private_key_pem, size_t client_private_key_len);

/** Sets client certificate, and client private key.
 *
 * @param client_cert_pem Client certification in PEM format.
 * @param client_private_key Client private key in PEM format.
 */
nsapi_error_t TLSSocketWrapper::set_client_cert_key(const char *client_cert_pem, const char *client_private_key_pem);

Socket API

TLSSocketWrapper implements Mbed OS Socket API as follows.

virtual nsapi_error_t close();

Destroys the memory allocated by TLS library. Also closes the transport socket, unless transport mode is set to TRANSPORT_KEEP or TRANSPORT_CONNECT.

virtual nsapi_error_t connect(const SocketAddress &address);

Initiates the TCP connection and continues to TLS hanshake. If transport mode is either TRANSPORT_KEEP or TRANSPORT_CLOSE, TCP is assumed to be open and state directly goes into TLS handshake. This is currently forced to blocking mode. After succesfully connecting, you can set it to non-blockin mode.

virtual nsapi_size_or_error_t send(const void *data, nsapi_size_t size);
virtual nsapi_size_or_error_t recv(void *data, nsapi_size_t size);
virtual nsapi_size_or_error_t sendto(const SocketAddress &address, const void *data, nsapi_size_t size);
virtual nsapi_size_or_error_t recvfrom(SocketAddress *address, void *data, nsapi_size_t size);

These work as expected, but SocketAddress parameters are ignored. TLS connection cannot change the peer. Also recvfrom() call does not set the peer address.

Mbed TLS error codes MBEDTLS_ERR_SSL_WANT_READ and MBEDTLS_ERR_SSL_WANT_WRITE are translated to NSAPI_ERROR_WOULD_BLOCK before passing to user.

MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY is ignored and zero is returned to user (connection closed). Other error codes are passed through.

virtual nsapi_error_t bind(const SocketAddress &address);
virtual void set_blocking(bool blocking);
virtual void set_timeout(int timeout);
virtual void sigio(mbed::Callback<void()> func);
virtual nsapi_error_t setsockopt(int level, int optname, const void *optval, unsigned optlen);
virtual nsapi_error_t getsockopt(int level, int optname, void *optval, unsigned *optlen);

These are passed through to transport socket.

virtual Socket *accept(nsapi_error_t *error = NULL);
virtual nsapi_error_t listen(int backlog = 1);

These are returning NSAPI_ERROR_UNSUPPORTED as TLS socket cannot be set to listening mode.

Transport modes

TLSSocketWrapper has four modes that are given in the constructor and affect how the transport Socket is used in connection and closing phases.

Mode Behaviour on trasport socket
TRANSPORT_KEEP Keep the transport as it is. Does not call connect() or close() methods.
TRANSPORT_CONNECT_AND_CLOSE Both connect() and close() are called. (default)
TRANSPORT_CONNECT Call connect() but do not close the connection when finished.
TRANSPORT_CLOSE Call close() when connection is finished.

Default mode is TRANSPORT_CONNECT_AND_CLOSE.

Advanced usage: using internal Mbed TLS structures

User may choose to use internal Mbed TLS structures to configure the TLS instance. This is supported by exposing some Mbed TLS structures like this:

mbedtls_x509_crt *get_own_cert();
int set_own_cert(mbedtls_x509_crt *);
mbedtls_x509_crt *get_ca_chain();
void set_ca_chain(mbedtls_x509_crt *);
mbedtls_ssl_config *get_ssl_config();
void set_ssl_config(mbedtls_ssl_config *);

For guidance of how to use these, please refer to Mbed TLS documentation.

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