SSL Programming Tutorial

Initializing the SSL Library

Before you can call any other OpenSSL APIs in the SSL application programs, you must perform initialization using the following SSL APIs.

SSL_library_init(); /* load encryption & hash algorithms for SSL */         	
SSL_load_error_strings(); /* load the error strings for good error reporting */

The SSL_library_init() API registers all ciphers and hash algorithms used in SSL APIs. The encryption algorithms loaded with this API are DES-CBC, DES-EDE3-CBC, RC2 and RC4 (IDEA and RC5 are not available in HP SSL for OpenVMS); and the hash algorithms are MD2, MD5, and SHA. The SSL_library_init() API has a return value that is always 1 (integer).

SSL applications should call the SSL_load_error_strings() API. This API loads error strings for SSL APIs as well as for Crypto APIs. Both SSL and Crypto error strings need to be loaded because many SSL applications call some Crypto APIs as well as SSL APIs.

Creating and Setting Up the SSL Context Structure (SSL_CTX)

The first step after the intialization is to choose an SSL/TLS protocol version. Do this by creating an SSL_METHOD structure with one of the following APIs. The SSL_METHOD structure is then used to create an SSL_CTX structure with the SSL_CTX_new() API.

For every SSL/TLS version, there are three types of APIs to create an SSL_METHOD structure: one for both client and server, one for server only, and one for client only. SSLv2, SSLv3, and TLSv1 APIs correspond with the same name protocols. Table 4-2 “ Types of APIs for SSL_METHOD Creation” shows the types of APIs.

	NOTE: There is no SSL protocol version named SSLv23. The SSLv23_method() API and its variants choose SSLv2, SSLv3, or TLSv1 for compatibility with the peer.

Consider the incompatibility among the SSL/TLS versions when you develop SSL client/server applications. For example, a TLSv1 server cannot understand a client-hello message from an SSLv2 or SSLv3 client. The SSLv2 client/server recognizes messages from only an SSLv2 peer. The SSLv23_method() API and its variants may be used when the compatibility with the peer is important. An SSL server with the SSLv23 method can understand any of the SSLv2, SSLv3, and TLSv1 hello messages. However, the SSL client using the SSLv23 method cannot establish connection with the SSL server with the SSLv3/TLSv1 method because SSLv2 hello message is sent by the client.

The SSL_CTX_new() API takes the SSL_METHOD structure as an argument and creates an SSL_CTX structure.

In the following example, an SSL_METHOD structure that can be used for either an SSLv3 client or SSLv3 server is created and passed to SSL_CTX_new(). The SSL_CTX structure is initialized for SSLv3 client and server.

meth = SSLv3_method();
ctx = SSL_CTX_new(meth);

Setting Up the Certificate and Key

“Certificates for SSL Applications” discussed how the SSL client and server programs require you to set up appropriate certificates. This setup is done by loading the certificates and keys into the SSL_CTX or SSL structures. The mandatory and optional certificates are as follows:

Loading a CA Certificate

To verify a certificate, you must first load a CA certificate (because the peer certificate is verified against a CA certificate). The SSL_CTX_load_verify_locations() API loads a CA certificate into the SSL_CTX structure.

The prototype of this API is as follows:

int SSL_CTX_load_verify_locations(SSL_CTX *ctx, const char *CAfile, const char *CApath);

The first argument, ctx, points to an SSL_CTX structure into which the CA certificate is loaded. The second and third arguments, CAfile and CApath, are used to specify the location of the CA certificate. When looking up CA certificates, the OpenSSL library first searches the certificates in CAfile, then those in CApath.

The following rules apply to the CAfile and CApath arguments:

• If the certificate is specified by CAfile (the certificate must exist in the same directory as the SSL application), specify NULL for CApath.

• To use the third argument, CApath, specify NULL for CAfile. You must also hash the CA certificates in the directory specified by CApath. Use the Certificate Tool (described in Chapter 3) to perform the hashing operation.

Setting Up Peer Certificate Verification

The CA certificate loaded in the SSL_CTX structure is used for peer certificate verification. For example, peer certificate verification on the SSL client is performed by checking the relationships between the CA certificate (loaded in the SSL client) and the server certificate.

For successful verification, the peer certificate must be signed with the CA certificate directly or indirectly (a proper certificate chain exists). The certificate chain length from the CA certificate to the peer certificate can be set in the verify_depth field of the SSL_CTXand SSL structures. (The value in SSL is inherited from SSL_CTX when you create an SSL structure using the SSL_new() API). Setting verify_depth to 1 means that the peer certificate must be directly signed by the CA certificate.

The SSL_CTX_set_verify() API allows you to set the verification flags in the SSL_CTX structure and a callback function for customized verification as its third argument. (Setting NULL to the callback function means the built-in default verification function is used.) In the second argument of SSL_CTX_set_verify(), you can set the following macros:

• SSL_VERIFY_NONE

• SSL_VERIFY_PEER

• SSL_VERIFY_FAIL_IF_NO_PEER_CERT

• SSL_VERIFY_CLIENT_ONCE

The SSL_VERIFY_PEER macro can be used on both SSL client and server to enable the verification. However, the subsequent behaviors depend on whether the macro is set on a client or a server. For example:

/* Set a callback function (verify_callback) for peer certificate */ /* verification */ SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, verify_callback); /* Set the verification depth to 1 */ SSL_CTX_set_verify_depth(ctx,1);

You can verify a peer certificate in another, less common way - by using the SSL_get_verify_result() API. This method allows you to obtain the peer certificate verification result without using the SSL_CTX_set_verify() API.

Call the following two APIs before you call the SSL_get_verify_result() API:

1. Call SSL_connect() (in the client) or SSL_accept() (in the server) to perform the SSL handshake. Certificate verification is performed during the handshake. SSL_get_verify_result() cannot obtain the result before the verification process.

2. Call SSL_get_peer_certificate() to explicitly obtain the peer certificate. The X509_V_OK macro value is returned when a peer certificate is not presented as well as when the verification succeeds.

The following code shows how to use SSL_get_verify_result() in the SSL client:

SSL_CTX_set_verify_depth(ctx, 1); err = SSL_connect(ssl); if(SSL_get_peer_certificate(ssl) != NULL) { if(SSL_get_verify_result(ssl) == X509_V_OK)

BIO_printf(bio_c_out, "client verification with SSL_get_verify_result() succeeded.\n"); else{ BIO_printf(bio_err, "client verification with SSL_get_verify_result() failed.\n"); exit(1); } } else BIO_printf(bio_c_out, -the peer certificate was not presented.\n-);

Example 1: Setting Up Certificates for the SSL Server

The SSL protocol requires that the server set its own certificate and key. If you want the server to conduct client authentication with the client certificate, the server must load a CA certificate so that it can verify the client-s certificate.

The following example shows how to set up certificates for the SSL server:

/* Load server certificate into the SSL context */ if (SSL_CTX_use_certificate_file(ctx, SERVER_CERT, SSL_FILETYPE_PEM) <= 0) }

ERR_print_errors(bio_err); /* == ERR_print_errors_fp(stderr); */ exit(1); } /* Load the server private-key into the SSL context */ if (SSL_CTX_use_PrivateKey_file(ctx, SERVER_KEY, SSL_FILETYPE_PEM) <= 0) {

ERR_print_errors(bio_err); /* == ERR_print_errors_fp(stderr); */ exit(1); } /* Load trusted CA. */ if (!SSL_CTX_load_verify_locations(ctx,CA_CERT,NULL)) { ERR_print_errors(bio_err); /* == ERR_print_errors_fp(stderr); */ exit(1); } /* Set to require peer (client) certificate verification */ SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, verify_callback); /* Set the verification depth to 1 */ SSL_CTX_set_verify_depth(ctx,1);

Example 2: Setting Up Certificates for the SSL Client

Generally, the SSL client verifies the server certificate in the process of the SSL handshake. This verification requires the SSL client to set up its trusting CA certificate. The server certificate must be signed with the CA certificate loaded in the SSL client in order for the server certificate verification to succeed.

The following example shows how to set up certificates for the SSL client:

/*----- Load a client certificate into the SSL_CTX structure -----*/ if(SSL_CTX_use_certificate_file(ctx,CLIENT_CERT, SSL_FILETYPE_PEM) <= 0){ ERR_print_errors_fp(stderr); exit(1); } /*----- Load a private-key into the SSL_CTX structure -----*/ if(SSL_CTX_use_PrivateKey_file(ctx,CLIENT_KEY, SSL_FILETYPE_PEM) <= 0){ ERR_print_errors_fp(stderr); exit(1); } /* Load trusted CA. */ if (!SSL_CTX_load_verify_locations(ctx,CA_CERT,NULL)) { ERR_print_errors_fp(stderr); exit(1); }

Creating and Setting Up the SSL Structure

Call SSL_new() to create an SSL structure. Information for an SSL connection is stored in the SSL structure. The protocol for the SSL_new() API is as follows:

ssl = SSL_new(ctx);

A newly created SSL structure inherits information from the SSL_CTX structure. This information includes types of connection methods, options, verification settings, and timeout settings. No additional settings are required for the SSL structure if the appropriate initialization and configuration have been done for the SSL_CTX structure.

You can modify the default values in the SSL structure using SSL APIs. To do this, use variants of the APIs that set attributes of the SSL_CTX structure. For example, you can use SSL_CTX_use_certificate() to load a certificate into an SSL_CTX structure, and you can use SSL_use_certificate() to load a certificate into an SSL structure.

Setting Up the TCP/IP Connection

Although SSL works with some other reliable protocols, TCP/IP is the most common transport protocol used with SSL.

The following sections describe how to set up TCP/IP for the SSL APIs. This configuration is the same as in many other TCP/IP client/server application programs; it is not specific to SSL API applications. In these sections, TCP/IP is set up with the ordinary socket APIs, although it is also possible to use OpenVMS system services.

Creating and Setting Up the Listening Socket (on the SSL Server)

The SSL server needs two sockets as an ordinary TCP/IP server—one for the SSL connection, the other for detecting an incoming connection request from the SSL client.

In the following code, the socket() function creates a listening socket. After the address and port are assigned to the listening socket with bind(), the listen() function allows the listening socket to handle an incoming TCP/IP connection request from the client.

listen_sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP); CHK_ERR(listen_sock, "socket"); memset(&sa_serv, 0, sizeof(sa_serv)); sa_serv.sin_family = AF_INET; sa_serv.sin_addr.s_addr = INADDR_ANY; sa_serv.sin_port = htons(s_port); /* Server Port number */ err = bind(listen_sock, (struct sockaddr*)&sa_serv,sizeof(sa_serv)); CHK_ERR(err, "bind"); /* Receive a TCP connection. */ err = listen(listen_sock, 5); CHK_ERR(err, "listen");

Creating and Setting Up the Socket (on the SSL Client)

On the client, you must create a TCP/IP socket and attempt to connect to the server with this socket. To establish a connection to the specified server, the TCP/IP connect() function is used. If the function succeeds, the socket passed to the connect() function as a first argument can be used for data communication over the connection.

sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); CHK_ERR(sock, "socket");

memset (&server_addr, '\0', sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_port = htons(s_port); /* Server Port number */ server_addr.sin_addr.s_addr = inet_addr(s_ipaddr); /* Server IP */ err = connect(sock, (struct sockaddr*) &server_addr, sizeof(server_addr)); CHK_ERR(err, "connect");

Establishing a TCP/IP Connection (on the SSL Server)

To accept an incoming connection request and to establish a TCP/IP connection, the SSL server needs to call the accept() function. The socket created with this function is used for the data communication between the SSL client and server. For example:

sock = accept(listen_sock, (struct sockaddr*)&sa_cli, &client_len); BIO_printf(bio_c_out, "Connection from %lx, port %x\n", sa_cli.sin_addr.s_addr, sa_cli.sin_port);

Setting Up the Socket/Socket BIO in the SSL Structure

After you create the SSL structure and the TCP/IP socket (sock), you must configure them so that SSL data communication with the SSL structure can be performed automatically through the socket.

The following code fragments show the various ways to assign sock to ssl. The simplest way is to set the socket directly into the SSL structure, as follows:

SSL_set_fd(ssl, sock);

A better way is to use a BIO structure, which is the I/O abstraction provided by OpenSSL. This way is preferable because BIO hides details of an underlying I/O. As long as a BIO structure is set up properly, you can establish SSL connections over any I/O.

The following two examples demonstrate how to create a socket BIO and set it into the SSL structure.

 sbio=BIO_new(BIO_s_socket());
 BIO_set_fd(sbio, sock, BIO_NOCLOSE);
 SSL_set_bio(ssl, sbio, sbio);

In the following example, the BIO_new_socket() API creates a socket BIO in which the TCP/IP socket is assigned, and the SSL_set_bio() API assigns the socket BIO into the SSL structure. The following two lines of code are equivalent to the preceding three lines:

 sbio = BIO_new_socket(socket, BIO_NOCLOSE);
 SSL_set_bio(ssl, sbio, sbio);

	NOTE: If there is already a BIO connected to ssl, BIO_free() is called (for both the reading and writing side, if different).

SSL Handshake

The SSL handshake is a complicated process that involves significant cryptographic key exchanges. However, the handshake can be completed by calling SSL_accept() on the SSL server and SSL_connect() on the SSL client.

SSL Handshake on the SSL Server

The SSL_accept() API waits for an SSL handshake initiation from the SSL client. Successful completion of this API means that the SSL handshake has been completed.

err = SSL_accept(ssl);

SSL Handshake on the SSL Client

The SSL client calls the SSL_connect() API to initiate an SSL handshake. If this API returns a value of 1, the handshake has completed successfully. The data can now be transmitted securely over this connection.

err = SSL_connect(ssl);

Performing an SSL Handshake with SSL_read and SSL_write (Optional)

Optionally, you can call SSL_write() and SSL_read() to complete the SSL handshake as well as perform SSL data exchange. With this approach, you must call SSL_set_accept_state() before you call SSL_read() on the SSL server. You must also call SSL_set_connect_state()before you call SSL_write() on the client. For example:

/* When SSL_accept() is not called, SSL_set_accept_state() */ /* must be called prior to SSL_read() */ SSL_set_accept_state(ssl); /* When SSL_connect() is not called, SSL_set_connect_state() */ /* must be called prior to

SSL_write() */ SSL_set_connect_state(ssl);

Obtaining a Peer Certificate (Optional)

Optionally, after the SSL handshake, you can obtain a peer certificate by calling SSL_get_peer_certificate(). This API is often used for straight certificate verification, such as checking certificate information (for example, the common name and expiration date).

peer_cert = SSL_get_peer_certificate(ssl);

Transmitting SSL Data

After the SSL handshake is completed, data can be transmitted securely over the established SSL connection. SSL_write() and SSL_read() are used for SSL data transmission, just as write() and read() or send() and recv() are used for an ordinary TCP/IP connection.

Sending Data

To send data over the SSL connection, call SSL_write(). The data to be sent is stored in the buffer specified as a second argument. For example:

err = SSL_write(ssl, wbuf, strlen(wbuf));

Receiving Data

To read data sent from the peer over the SSL connection, call SSL_read(). The received data is stored in the buffer specified as a second argument. For example:

err = SSL_read(ssl, rbuf, sizeof(rbuf)-1);

Using BIOs for SSL Data Transmission (Optional)

Instead of using SSL_write() and SSL_read(), you can transmit data by calling BIO_puts() and BIO_gets(), and BIO_write() and BIO_read(), provided that a buffer BIO is created and set up as follows:

BIO *buf_io, *ssl_bio; char rbuf[READBUF_SIZE]; char wbuf[WRITEBUF_SIZE] buf_io = BIO_new(BIO_f_buffer()); /* create a buffer BIO */ ssl_bio = BIO_new(BIO_f_ssl()); /* create an ssl BIO */ BIO_set_ssl(ssl_bio, ssl, BIO_CLOSE); /* assign the ssl BIO to SSL */ BIO_push(buf_io, ssl_bio); /* add ssl_bio to buf_io */ ret = BIO_puts(buf_io, wbuf); /* Write contents of wbuf[] into buf_io */ ret = BIO_write(buf_io, wbuf, wlen); /* Write wlen-byte contents of wbuf[] into buf_io */ ret = BIO_gets(buf_io, rbuf, READBUF_SIZE); /* Read data from buf_io and store in rbuf[] */ ret = BIO_read(buf_io, rbuf, rlen); /* Read rlen-byte data from buf_io and store rbuf[] */

Closing an SSL Connection

When you close an SSL connection, the SSL client and server send close_notify messages to notify each other of the SSL closure. You use the SSL_shutdown() API to send the close_notify alert to the peer.

The shutdown procedure consists of two steps:

The following rules apply to closing an SSL connection:

The SSL client or server that initiates the SSL closure calls SSL_shutdown() either once or twice. If it calls the API twice, one call sends the close_notify alert and one call receives the response from the peer. If the initator calls the API only once, the initiator does not receive the close_notify alert from the peer. (The initiator is not required to wait for the responding alert.)

The peer that receives the alert calls SSL_shutdown() once to send the alert to the initiating party.

Resuming an SSL Connection

You can reuse the information from an already established SSL session to create a new SSL connection. Because the new SSL connection is reusing the same master secret, the SSL handshake can be performed more quickly. As a result, SSL session resumption can reduce the load of a server that is accepting many SSL connections.

Perform the following steps to resume an SSL session on the SSL client:

If the SSL client calls SSL_get1_session() and SSL_set_session(), the SSL server can accept a new SSL connection using the same session without calling special APIs to resume the session. The server does this by following the steps discussed in “Creating and Setting Up the SSL Structure ”, “Setting Up the TCP/IP Connection”, “Setting Up the Socket/Socket BIO in the SSL Structure”, “SSL Handshake”, and “Transmitting SSL Data”.

	NOTE: Calling SSL_free() results in the failure of the SSL session to resume, even if you saved the SSL session with SSL_get1_session().

Renegotiating the SSL Handshake

SSL renegotiation is a new SSL handshake over an already established SSL connection. Because the renegotiation messages (including types of ciphers and encryption keys) are encrypted and then sent over the existing SSL connection, SSL renegotiation can establish another SSL session securely. SSL renegotiation is useful in the following situations, once you have established an ordinary SSL session:

SSL renegotiation can be initiated by either the SSL client or the SSL server. Initiating an SSL renegotiation on the client requires a different set of APIs (on both the initiating SSL client and the accepting server) from the APIs required for the initiation on the SSL server (in this case, on the initiating SSL server and the accepting SSL client).

The following sections discuss the required APIs for both situations.

	NOTE: SSLv2 cannot perform SSL renegotiation. Use SSLv3 or TLSv3 for this operation.

SSL Renegotiation Initiated by the SSL Server

To initiate an SSL renegotiation from the SSL server, call SSL_renegotiate() once and SSL_do_handshake() twice.

The SSL_renegotiate() API sets flags for SSL renegotiation. This API does not actually initiate the renegotiation. The flags turned on by SSL_renegotiate() inform SSL_do_handshake() that it needs to perform SSL renegotiation with the SSL client. The SSL_do_handshake() API performs an actual SSL handshake. The first call sends a -Server Hello- message to the SSL client.

If the first call succeeds, the client has agreed to perform an SSL renegotiation. The server then sets the SSL_ST_ACCEPT state in the SSL structure and calls SSL_do_handshake() again to complete the rest of the renegotiation.

The following code fragment shows how these APIs are used:

printf("Starting SSL renegotiation on SSL server (initiating by SSL server)"); if(SSL_renegotiate(ssl) <= 0){ printf("SSL_renegotiate() failed\n"); exit(1); } if(SSL_do_handshake(ssl) <= 0){ printf("SSL_do_handshake() failed\n"); exit(1); } ssl->state = SSL_ST_ACCEPT; if(SSL_do_handshake(ssl) <= 0){ printf("SSL_do_handshake() failed\n"); exit(1); }

The following code shows the APIs called by the SSL client when the renegotiation is initiated by the server:

printf("Starting SSL renegotiation on SSL client (initiating by SSL server)"); /* SSL renegotiation */ err = SSL_read(ssl, buf, sizeof(buf)-1);

As the example shows, SSL_READ() performs data exchange, and can also handle connection-related functions such as renegotiation.

SSL Renegotiation Initiated by the SSL Client

The SSL client can also initiate SSL renegotiation. In this case, the setup on the client initiating the renegotiation is similar to that on a server initiating the renegotiation. To complete this operation, the SSL client calls SSL_renegotiate() and SSL_do_handshake() only once. SSL_renegotiate() simply sets the flags for SSL renegotiation, and a single call of SSL_do_handshake() covers the entire renegotiation.

printf("Starting SSL renegotiation on SSL client (initiating by SSL client)"); if(SSL_renegotiate(ssl) <= 0){ printf("SSL_renegotiate() failed\n"); exit(1); } if(SSL_do_handshake(ssl) <= 0){ printf("SSL_do_handshake() failed\n"); exit(1); }

The following code shows the APIs called by the SSL server when the renegotiation is initiated by the client. (These are the same APIs that are called by the SSL client when the renegotiation is initiated by the server.)

printf("Starting SSL renegotiation on SSL server (initiating by SSL client)"); /* SSL renegotiation */ err = SSL_read(ssl, buf, sizeof(buf)-1);

Again in this example, SSL_READ() is handling the data exchange and connection renegotiation.

Finishing the SSL Application

When you finish an SSL application program, the major task is to free (deallocate) the data structures that were created and used in the application program. The APIs for deallocation usually contain the _free suffix, whereas the APIs that create a new data structure contain the_new suffix.

You must free data structures that you explicitly created in the SSL application program. Data structures that were created inside another structure with an xxx_new() API are automatically deallocated when the structure is deallocated with the corresponding xxx_free() API. For example, a BIO structure created with SSL_new() is freed when you call SSL_free(); you do not need to call BIO_free() to free the BIO inside the SSL structure. However, if the application program called BIO_new() to allocate a BIO structure, you must free that structure with BIO_free().

	NOTE: You must call SSL_shutdown() before you call SSL_free().