/// <summary>Decrypt a Book Cipher using a string representation of integer cipher numbers and a set of key words.</summary>
/// <param name="cipherNums">an ordered list of 1-based cipher numbers (word indexes) in string format</param>
/// <param name="keyWords">an ordered list of key words used to encrypt the document.</param>
/// <param name="letterSel">a choice of first letter or last letter from key word to encrypt document</param>
/// <param name="throwOnOutOfRange">a flag to raise an exception if a number exceeds the range of key text. If false, a question-mark will be returned.</param>
/// <returns>a string representation of decrypted text without any punctuation or spacing</returns>
/// <remarks>A zero or negative number will still raise an exception regardless of the throwOnOutOfRange value.</remarks>
public static string Decrypt(IEnumerable <string> cipherNums, IEnumerable <string> keyWords, LetterSelection letterSel = LetterSelection.FirstLetter, OutOfBoundsIndex outOfRangeHandler = OutOfBoundsIndex.ThrowException)
{
int num = 0;
var numArray = cipherNums.ToArray();
int[] nums = new int[numArray.Length];
// validation of cipher numbers being valid string representations of integers
for (int ndx = 0; ndx < numArray.Length; ndx++)
{
var trimmedNum = numArray[ndx].Trim();
if (int.TryParse(trimmedNum, out num) == false)
{
throw new FormatException($"Cipher number {ndx + 1} has an invalid number ({trimmedNum}).");
}
nums[ndx] = num;
}
return(Decrypt(nums, keyWords, letterSel, outOfRangeHandler));
}
/// <summary>Decrypt a Book Cipher using a list of integer cipher numbers and a set of key words.</summary>
/// <param name="cipherNums">an ordered list of 1-based cipher numbers (word indexes)</param>
/// <param name="keyWords">an ordered list of key words used to encrypt the document.</param>
/// <param name="letterSel">a choice of first letter or last letter from key word to encrypt document</param>
/// <param name="throwOnOutOfRange">a flag to raise an exception if a number exceeds the range of key text. If false, a question-mark will be returned.</param>
/// <returns>a string representation of decrypted text without any punctuation or spacing</returns>
/// <remarks>A zero or negative number will still raise an exception regardless of the throwOnOutOfRange value.</remarks>
public static string Decrypt(IEnumerable <int> cipherNums, IEnumerable <string> keyWords, LetterSelection letterSel = LetterSelection.FirstLetter, OutOfBoundsIndex outOfRangeHandler = OutOfBoundsIndex.ThrowException)
{
string[] wdArray = keyWords.ToArray();
char[] charList = new char[wdArray.Length];
for (int ndx = 0; ndx < wdArray.Length; ndx++)
{
if (wdArray[ndx] == null)
{
throw new ArgumentException($"Word {ndx + 1} is null.");
}
wdArray[ndx] = wdArray[ndx].Trim();
if (wdArray[ndx] == string.Empty)
{
throw new ArgumentException($"Word {ndx + 1} is empty.");
}
int letterNdx = (letterSel == LetterSelection.FirstLetter) ? 0 : wdArray[ndx].Length - 1;
charList[ndx] = (wdArray[ndx])[letterNdx];
}
return(Decrypt(cipherNums, charList, outOfRangeHandler));
}
/// <summary>Encrypt text using a set of key words from a book or document</summary>
/// <param name="plainTextWords">an ordered list of text to encrypt</param>
/// <param name="bookText">an ordered list of words used to encrypt; repeats are okay and recommended.</param>
/// <param name="delimiter">the delimeter to use for separating numbers; typical is comma or space</param>
/// <param name="letterSel">a choice of first letter or last letter of key word to encrypt document</param>
/// <returns>a string of encrypted text</returns>
public static string Encrypt(string plainText, IEnumerable <string> bookText, char delimiter, LetterSelection letterSel = LetterSelection.FirstLetter)
{
Random rnd = new Random();
var lookup = new Dictionary <char, List <int> >();
int cnt = 0;
foreach (var wd in bookText)
{
if (string.IsNullOrEmpty(wd))
{
throw new ArgumentException($"Word {cnt + 1} is null or empty.");
}
var oneWord = wd.Trim().ToLower();
if (oneWord == string.Empty)
{
throw new ArgumentException($"Word {cnt + 1} is whitespace.");
}
int letterNdx = (letterSel == LetterSelection.FirstLetter) ? 0 : oneWord.Length - 1;
char c = oneWord[letterNdx];
if (lookup.ContainsKey(c))
{
lookup[c].Add(cnt + 1); // add to list
}
else
{
var intList = new List <int> {
cnt + 1
};
lookup.Add(c, intList);
}
cnt++;
}
// now each letter has an associated list of word indexes (which can be chosen at random) to encrypt each letter of the plaintext
var sb = new StringBuilder();
bool firstNum = true;
for (int ndx = 0; ndx < plainText.Length; ndx++)
{
char c = (plainText.Substring(ndx, 1).ToLower())[0];
if (char.IsLetter(c))
{
if (lookup.ContainsKey(c) == false)
{
throw new ArgumentOutOfRangeException($"letter {c} is not found in any words of book text.");
}
var intList = lookup[c];
var numChoices = intList.Count;
// todo: replace Random with crypto-quality PNG
int wdChoiceNdx = (numChoices == 1) ? 0 : rnd.Next(intList.Count);
if (firstNum == false)
{
sb.Append($"{delimiter}");
}
sb.Append($"{intList[wdChoiceNdx]}");
firstNum = false;
}
}
return(sb.ToString());
}
