Added Simple SSL Echo Client/Server to demos.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #17260)

(cherry picked from commit 801c638)

demos/README.txt

@@ -50,3 +50,6 @@ signature:
 EVP_Signature_demo.c   Compute and verify a signature from multiple buffers
 rsa_pss_direct.c       Compute and verify an RSA-PSS signature from a hash
 rsa_pss_hash.c         Compute and verify an RSA-PSS signature over a buffer
+
+sslecho:
+main.c                 Simple SSL echo client/server.

demos/sslecho/A-SSL-Docs.txt

@@ -0,0 +1,20 @@
+Useful Links:
+
+OpenSSL API Documentation: https://www.openssl.org/docs
+
+Github: https://github.com/openssl/openssl
+
+OpenSSL Wiki: https://wiki.openssl.org/index.php/Main_Page
+
+Original Simple Server: https://wiki.openssl.org/index.php/Simple_TLS_Server
+
+---------------------------------------------------------------
+
+Generate self signed cert and key 'pem' files (good for 10 years):
+
+openssl req -newkey rsa:4096 -x509 -sha256 -days 3650 -nodes -out cert.pem -keyout key.pem
+
+You can just hit carriage returns to accept the default values, except for "Common Name"; you
+should enter 'localhost', or an actual hostname.
+
+The same keys can be used for both communicating instances; same or different machines.

demos/sslecho/README.md

@@ -0,0 +1,26 @@
+OpenSSL Simple Echo Client/Server
+=================================
+
+This project implements a simple echo client/server.
+
+It is a console application, with command line parameters determining the mode
+of operation (client or server). Start it with no parameters to see usage.
+
+The server code was adapted from the Simple TLS Server on the OpenSSL Wiki.
+The server code was modified to perform the echo function, and client code
+was added to open a connection with the server and to send keyboard input
+to the server.
+
+The new client code illustrates that:
+
+- Connection to the SSL server starts as a standard TCP 'connect'.
+- Once connected with TCP, the client 'upgrades' to SSL using
+  SSL_connect().
+- When the SSL connection completes, data is sent and received using
+  SSL_write() and SSL_read().
+- Pretty simple.
+
+The cert.pem and key.pem files included are self signed certificates with the
+"Common Name" of 'localhost'.
+
+Best to create the 'pem' files using an actual hostname.

demos/sslecho/cert.pem

@@ -0,0 +1,32 @@
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+-----END CERTIFICATE-----

demos/sslecho/key.pem

@@ -0,0 +1,52 @@
+-----BEGIN PRIVATE KEY-----
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+-----END PRIVATE KEY-----