Updated the README file.

README.md

@@ -6,7 +6,9 @@ title: "![SlunkCrypt](etc/img/SlunkCrypt-Logo.png)"
 Introduction
 ============
 
-SlunkCrypt is an experimental cryptography library and command-line tool. See [*encryption algorithm*](#encryption-algorithm) for details.
+SlunkCrypt is an experimental cross-platform cryptography library and command-line tool. A fully-featured GUI is provided for the Windows platform.
+
+Please refer to the section [*encryption algorithm*](#encryption-algorithm) for more details!
 
 
 Legal Warning
@@ -15,6 +17,45 @@ Legal Warning
 Use of SlunkCrypt may be illegal in countries where encryption is outlawed. We believe it is legal to use SlunkCrypt in many countries all around the world, but we are not lawyers, and so if in doubt you should seek legal advice before downloading it. You may find useful information at [cryptolaw.org](http://www.cryptolaw.org/), which collects information on cryptography laws in many countries.
 
 
+System Requirements
+===================
+
+The SlunkCrypt library and the command-line application currently run on the following platforms:
+
+* **Microsoft Windows** (Windows XP SP-3, or later) — 32-Bit (i686) and 64-Bit (AMD64)
+* **Linux** (kernel version 3.17, or later) — 32-Bit (i686) and 64-Bit (AMD64)
+* **Various BSD flavors** (tested on FreeBSD 13 and OpenBSD 7) — 32-Bit (i686) and 64-Bit (AMD64)
+* **Oracle Solaris** (tested on version 11.4) — 32-Bit (i686) and 64-Bit (AMD64)
+
+The SlunkCrypt GUI application currently runs on the following platforms:
+
+* **Microsoft Windows** with .NET Framework 4.5 — can be installed on Windows Vista, or later
+
+
+GUI Usage
+=========
+
+This is how the graphical user interface (GUI) for SlunkCrypt looks on [Windows 11](https://i.pinimg.com/736x/0f/57/4c/0f574c445b497cc4748a9e1ab7491582--taps-funny-stuff.jpg):
+
+![](etc/img/SlunkCrypt-GUI.jpg)
+
+Prerequisites
+-------------
+
+Please be sure to install the **.NET Framework 4.5**, or any later *.NET Framework 4.x* version, before running the SlunkCrypt GUI application:  
+<https://dotnet.microsoft.com/download/dotnet-framework>
+
+***Note:*** If you are running Windows 8 or later, then almost certainly a suitable version of the .NET Framework is already installed &#128526;
+
+Settings
+--------
+
+The following settings can be adjusted in the `slunkcrypt-gui.exe.config` configuration file:
+
+- **`DisableBusyIndicator`:**  
+  If set to `true`, the “busy indicator” animation will be *disabled* on application startup &mdash; default value: `false`.
+
+
 Command-line Usage
 ==================
 
@@ -143,19 +184,17 @@ Here are some examples on how to use the SlunkCrypt command-line application:
 Encryption algorithm
 ====================
 
-The SlunkCrypt algorithm is based on concepts of the well-known [**Enigma**](https://en.wikipedia.org/wiki/Enigma_machine) machine, but with various improvements:
+The SlunkCrypt algorithm is based on core concepts of the well-known [**Enigma**](https://en.wikipedia.org/wiki/Enigma_machine) machine but with numerous improvements, largely inspired by R. Anderson's [***“A Modern Rotor Machine”***](https://rdcu.be/cBo8y):
 
-- The original Enigma machine had only *three* (or somtimes *four*) rotors, plus a static "reflector" wheel. In SlunkCrypt, we uses **256** simulated rotors for an improved security.
+- The original Enigma machine had only *three* (or, in some models, *four*) rotors, plus a static "reflector" wheel. In SlunkCrypt, we uses **256** simulated rotors for an improved security. Furthermore, the original Enigma machine supported only 26 distinct symbols, i.e. the letters `A` to `Z`. In SlunkCrypt, we use **256** distinct symbols, i.e. the byte values `0x00` to `0xFF`, which allows the encryption (and decryption) of arbitrary streams of bytes, rather than just plain text. Of course, SlunkCrypt can encrypt (and decrypt) text files as well.
 
-- The original Enigma machine supported only 26 distinct symbols, i.e. the letters `A` to `Z`. In SlunkCrypt, we use **256** distinct symbols, i.e. the byte values `0x00` to `0xFF`, which allows the encryption (and decryption) of arbitrary streams of bytes, rather than just plain text. Of course, SlunkCrypt can encrypt (and decrypt) text files as well.
+- In the original Enigma machine, the signal passes through the rotors *twice*, once in forward direction and then again in backwards direction &ndash; thus the "reflector" wheel. This way, the Enigma's encryption was made *involutory*, i.e. encryption and decryption were the same operation. While this was highly convenient, it also severely weakened the cryptographic strength of the Enigma machine, because the number of possible permutations was reduced drastically! This is one of the main reasons why the Enigma machine eventually was defeated. In SlunkCrypt, the signal passes through the simulated rotors just *once*, in order to maximize the number of possible permutations. This eliminates the most important known weakness of the Enigma machine. Obviously, in SlunkCrypt, separate "modes" for encryption and decryption need to be provided, since encryption and decryption *no* longer are the same operation.
 
-- In the original Enigma machine, the signal passes through the rotors *twice*, once in forward direction and then again in backwards direction &ndash; thus the "reflector" wheel. This way, the Enigma's encryption was made *involutory*, i.e. encryption and decryption were the same operation. While this was highly convenient, it also severely weakened the cryptographic strength of the Enigma machine, because the number of possible permutations was reduced drastically! This is one of the main reasons why the Enigma machine eventually was defeated. In SlunkCrypt, the signal passes through the simulated rotors just *once*, in order to maximize the number of possible permutations. This eliminates the most important known weakness of the Enigma machine. Obviously, in SlunkCrypt, separate modes for encryption and decryption need to be provided, because encryption and decryption *no* longer are the same.
+- In the original Enigma machine, the rightmost rotor was moved, by one step, after every symbol. Meanwhile, all other rotors were moved, by one step, *only* when their right-hand neighbor had completed a full turn &ndash; much like the odometer in a car. The fact that most of the rotors remained in the same "static" position most of the time was an important weakness of the Enigma machine. Also, the sequence of the Enigma's rotor positions started to repeat after only 16,900 characters. SlunkCrypt employs an improved stepping algorithm, based on a ***linear-feedback shift register* (LSFR)**, ensuring that *all* rotors move frequently and in a "randomized" unpredictable pattern. The rotor positions of SlunkCrypt practically *never* repeat.
-
-- In the original Enigma machine, *only* the rightmost rotor was stepped after every symbol. The other rotors were stepped infrequently. Specifically, if one rotor had completed a full turn, it caused the next rotor to move by one step &ndash; much like the odometer in a car. The fact that most of the rotors remained in the same "static" position for a long time was another important weakness of the Enigma machine that ultimately lead to its demise. Furthermore, the positions of the Enigma's rotors would start to repeat after "only" 16,900 characters. In SlunkCrypt, an improved stepping algorithm is used, which ensures that *all* rotors are stepped often. At the same time, the rotor positions in SlunkCrypt *never* repeat (practically).
 
 - The internal wiring of each of the original Enigma machine's rotors was *fixed*. Each rotor "type" came with a different internal wiring (i.e. permutation). Some models had up to eight rotor "types" to choose from, but only three or four rotors were used at a time. Nonetheless, the internal wiring (i.e. permutation) of each of the supplied rotors was **not** modifiable. This severely restricted the key space of the Enigma machine, as far as the rotors are concerned, because *only* the order of the rotors and the initial position of each rotor could be varied. In SlunkCrypt, a fully *randomized* wiring (i.e. permutation) is generated from the password for each of the 256 simulated rotors. The initial rotor positions are *randomized* as well.
 
-- SlunkCrypt does **not** currently implement the *plugboard* of the original Enigma machine. That is because, even though the plugboard has a large key space, it is just a *fixed* substitution cipher that does *not* contribute too much to the cryptographic strength of the Enigma machine. In fact, the plugboard could be "erased" by Welchman's [diagonal board](https://en.wikipedia.org/wiki/Bombe#Stecker_values).
+- SlunkCrypt does **not** currently implement the *plugboard* (“Steckerbrett”) of the original Enigma machine. That is because, even though the plugboard has a large key space, it is just a *fixed* substitution cipher that does *not* contribute much to the cryptographic strength of the Enigma machine. In fact, the plugboard could be "erased" by Welchman's [diagonal board](https://en.wikipedia.org/wiki/Bombe#Stecker_values).
 
 
 Programming Interface (API)