openssl_examples

examples of using OpenSSL

ssl_server_nonblock.c is a simple OpenSSL example program to illustrate the use of memory BIO's (BIO_s_mem) to perform SSL read and write with non-blocking socket IO.

The program accepts connections from SSL clients. To keep it simple only a single live connection is supported. While a client is connected the program will receive any bytes which it sends, unencrypt them and write to stdout, using non-blocking socket reads. It will also read from stdin, encrypt the bytes and send to the client, using non-blocking socket writes.

Note that this program is single threaded. This means it does not have to set up SSL locking. The program does not exit, and so it does not have code to free up the resources associated with the SSL context and library.

ssl_client_nonblock.c is a client version of the same program.

Compilation

To compile the program, use something like:

    gcc ssl_server_nonblock.c -Wall -O0 -g3 -std=c99 -lcrypto -lssl -o ssl_server_nonblock

Or on MacOS:

 gcc -Wall -O0 -g3 -std=c99 -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib -lssl -lcrypto  -o ssl_server_nonblock ssl_server_nonblock.c

Or just try the makefile, for Linux platforms.

On Ubuntu systems you may need to run sudo apt install libssl-dev to install OpenSSL headers.

Running

Running the program requires that a SSL certificate and private key are available to be loaded. These can be generated using the 'openssl' program using these steps:

1. Generate the private key, this is what we normally keep secret:

    openssl genrsa -des3 -passout pass:ABCD -out server.pass.key 2048
    openssl rsa -passin pass:ABCD -in server.pass.key -out server.key
    rm -f server.pass.key

2. Next generate the CSR. We can leave the password empty when prompted (because this is self-sign):

    openssl req -new -key server.key -out server.csr

3. Next generate the self signed certificate:

    openssl x509 -req -sha256 -days 365 -in server.csr -signkey server.key -out server.crt
    rm -f server.csr

The openssl program can also be used to connect to this program as an SSL client. Here's an example command (assuming we're using port 55555):

    openssl s_client -connect 127.0.0.1:55555 -msg -debug -state -showcerts

Flow of encrypted & unencrypted bytes

This diagram shows how the read and write memory BIO's (rbio & wbio) are associated with the socket read and write respectively. On the inbound flow (data into the program) bytes are read from the socket and copied into the rbio via BIO_write. This represents the the transfer of encrypted data into the SSL object. The unencrypted data is then obtained through calling SSL_read. The reverse happens on the outbound flow to convey unencrypted user data into a socket write of encrypted data.

  +------+                                    +-----+
  |......|--> read(fd) --> BIO_write(rbio) -->|.....|--> SSL_read(ssl)  --> IN
  |......|                                    |.....|
  |.sock.|                                    |.SSL.|
  |......|                                    |.....|
  |......|<-- write(fd) <-- BIO_read(wbio) <--|.....|<-- SSL_write(ssl) <-- OUT
  +------+                                    +-----+

          |                                  |       |                     |
          |<-------------------------------->|       |<------------------->|
          |         encrypted bytes          |       |  unencrypted bytes  |