public bool IsValidCipherKey(PlayfairKey cipherKey)
{
if (cipherKey == null ||
string.IsNullOrWhiteSpace(cipherKey.Value) ||
cipherKey.Value.Equals(Alphabet) ||
cipherKey.Value.Contains(OmittedCharacter) ||
cipherKey.Value.Length != Alphabet.Length ||
!cipherKey.Value.Equals(PlayfairUtil.GetSanitisedString(cipherKey.Value)) ||
cipherKey.Value.Any(c => !char.IsLetter(c) || !char.IsUpper(c)))
{
return(false);
}
// Once every other condition is met, need to ensure all characters are unique.
var observedCharacters = new HashSet <char>();
foreach (var character in cipherKey.Value)
{
if (observedCharacters.Contains(character))
{
return(false);
}
observedCharacters.Add(character);
}
return(true);
}
public PlayfairKey GenerateCipherKey(string memorableKey)
{
// Filter out all the unnecessary/redundant/unsupported characters.
var sanitisedKey = GetSanitisedKey(PlayfairUtil.GetSanitisedString(memorableKey));
return(new PlayfairKey
{
// If necessary amount of characters is present then key is done, alternatively fill in missing values.
Value = sanitisedKey.Length == Alphabet.Length
? sanitisedKey
: GetCompleteCipherKey(sanitisedKey)
});
}
public IEnumerable <Digraph> GetCipherTextDigraphs(string cipherText)
{
if (!PlayfairUtil.IsValidCipherText(cipherText))
{
throw new ArgumentException("Invalid cipher text.");
}
var digrapths = new List <Digraph>();
for (var i = 0; i < cipherText.Length; i += PlayfairUtil.DigrathDenominator)
{
digrapths.Add(new Digraph(cipherText[i], cipherText[i + 1]));
}
Digraths = digrapths;
return(digrapths);
}
public string ReplaceMostCommonDigraphs(string cipherText, IEnumerable <Digraph> replacementDigraphs)
{
if (!PlayfairUtil.IsValidCipherText(cipherText))
{
throw new ArgumentOutOfRangeException(nameof(cipherText));
}
replacementDigraphs = replacementDigraphs as Digraph[] ?? replacementDigraphs.ToArray();
if (replacementDigraphs == null || !replacementDigraphs.Any())
{
throw new ArgumentOutOfRangeException(nameof(replacementDigraphs));
}
// Get all digraths and count their occurance count
var analysedDigraphs = new Dictionary <string, AnalysedDigrath>();
var cipherTextDigraphs = _digraphGenerator.GetCipherTextDigraphs(cipherText);
foreach (var digraph in cipherTextDigraphs)
{
if (analysedDigraphs.ContainsKey(digraph.ToString()))
{
// Update the occurance count if digraph has already been observed.
analysedDigraphs[digraph.ToString()].OccurenceCount++;
}
else
{
// Add new digraph to the dictionary and count the initial occurance.
analysedDigraphs.Add(digraph.ToString(), new AnalysedDigrath
{
Digraph = digraph,
OccurenceCount = 1
});
}
}
// Commentted out as this metric isn't currently needed. Left within the function for easy future
// implementation.
/* Calculate digram frequency & order digrams by their occurance.
* var cipherTextDigramCount = cipherText.Length / DigramDenominator;
* foreach (var analysedDigram in analysedDigrams)
* {
* analysedDigram.Value.CalculateFrequency(cipherTextDigramCount);
* }
*/
// Orders digraphs by descending order of occurance count and extracts amount to match method input.
var orderedAnalysedDigraphs = analysedDigraphs
.OrderByDescending(kv => kv.Value.OccurenceCount)
.Take(replacementDigraphs.Count())
.ToArray();
// Create a replacement map for the most common digraphs.
var digraphReplacementMap = new Dictionary <string, Digraph>();
for (var i = 0; i < replacementDigraphs.Count(); i++)
{
digraphReplacementMap.Add(orderedAnalysedDigraphs[i].Key, replacementDigraphs.ElementAt(i));
}
// Reconstruct the cipher text whilst replacing the most common digraphs within original cipher text with
// most frequent digraphs in the assumed language.
var sb = new StringBuilder(string.Empty);
foreach (var digraph in cipherTextDigraphs)
{
sb.Append(digraphReplacementMap.ContainsKey(digraph.ToString())
? digraphReplacementMap[digraph.ToString()]
: digraph);
}
return(sb.ToString());
}
public void IsCipherTextValid_InvalidCipherText_IsFalse(string cipherText)
{
Assert.IsFalse(PlayfairUtil.IsValidCipherText(cipherText));
}