Update the General and Overview sections of README.md

1 files changed, 51 insertions, 6 deletions

diff --git a/README.md b/README.md
index 7201b88..fef620a 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,54 @@ It requires no additional libraries.
 
 ## General
 
-TODO
+The main reason for writing this library was the need to login into my
+FreeBSD servers using the [opiepasswd]
+(https://www.freebsd.org/cgi/man.cgi?query=opiepasswd&sektion=1&manpath=freebsd-release-ports)
+as described in [FreeBSD Handbook]
+(https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/one-time-passwords.html).
+
+I decided to license the library under the
+[Simplified BSD License / 2-clause BSD license](LICENSE) and not under the
+(L)GPL-3 as I usually do.
+
+I hope that somebody will find it useful.
+
+
+## Overview of RFC-2289
+
+RFC-2289 describes a one-time password authentication system (OTP):
+
+"The system provides authentication for system access (login) and other
+applications requiring authentication that is secure against passive attacks
+based on replaying captured reusable passwords. OTP evolved from the S/KEY
+(S/KEY is a trademark of Bellcore) One-Time Password System that was released
+by Bellcore."
+
+"One form of attack on networked computing systems is eavesdropping on
+network connections to obtain authentication information such as the
+login IDs and passwords of legitimate users. Once this information is
+captured, it can be used at a later time to gain access to the
+system. One-time password systems are designed to counter this type
+of attack, called a 'replay attack'."
+
+The authentication system described in RFC-2289 "uses a secret
+pass-phrase to generate a sequence of one-time (single use)
+passwords.  With this system, the user's secret pass-phrase never
+needs to cross the network at any time such as during authentication
+or during pass-phrase changes. Thus, it is not vulnerable to replay
+attacks.  Added security is provided by the property that no secret
+information need be stored on any system, including the server being
+protected."
+
+"There are two entities in the operation of the OTP one-time password
+system. The **generator** must produce the appropriate one-time password
+from the user's secret pass-phrase and from information provided in
+the **challenge** from the **server**. The server must send a challenge that
+includes the appropriate generation parameters to the generator, must
+verify the one-time password received, must store the last valid
+one-time password it received, and must store the corresponding one-
+time password sequence number. The server must also facilitate the
+changing of the user's secret pass-phrase in a secure manner."
 
 
 ## Examples
@@ -18,15 +65,13 @@ TODO
 
 ## Author
 
-Simeon Simeonov - sgs AT pichove D0T org - sgs @ Freenode
+Simeon Simeonov - sgs @ Freenode
 
 
-## [License][license]
+## [License](LICENSE)
 
 Copyright (c) 2020, Simeon Simeonov
 All rights reserved.
 
-[licensed][license] under the BSD 2-clause.
+[licensed](LICENSE) under the BSD 2-clause.
 SPDX-License-Identifier: BSD-2-Clause-FreeBSD
-
-[license]: LICENSE