private static char GetSquareChar(int charOneIndex, int charTwoIndex, PlayfairKey key,
bool isDecryption = false, bool isSecondChar = false)
{
int row;
if (isDecryption)
{
if (isSecondChar)
{
row = GetCharRowIndex(charTwoIndex) * CipherGridDimension;
return(key.Value[row + GetCharColumnIndex(charOneIndex)]);
}
row = GetCharRowIndex(charOneIndex) * CipherGridDimension;
return(key.Value[row + GetCharColumnIndex(charTwoIndex)]);
}
if (isSecondChar)
{
row = GetCharRowIndex(charTwoIndex) * CipherGridDimension;
return(key.Value[row + GetCharColumnIndex(charOneIndex)]);
}
// Calculation inside Convert works out the row multiplier and multiplication provides the actual row index.
row = GetCharRowIndex(charOneIndex) * CipherGridDimension;
return(key.Value[row + GetCharColumnIndex(charTwoIndex)]);
}
public string DecryptMessage(string cipherText, PlayfairKey cipherKey)
{
if (!_keyManagement.IsValidCipherKey(cipherKey))
{
throw new ArgumentException("Invalid cipher key.");
}
return(Decrypt(_digrathGenerator.GetCipherTextDigraphs(cipherText), cipherKey));
}
public string EncryptMessage(string plainText, PlayfairKey cipherKey)
{
if (!_keyManagement.IsValidCipherKey(cipherKey))
{
throw new ArgumentException("Invalid cipher key.");
}
var sanitizedMessage = PlayfairUtil.GetSanitisedString(plainText);
var digraths = _digrathGenerator.GetMessageDigraths(sanitizedMessage);
return(Encrypt(digraths, cipherKey));
}
/// <summary>
/// Based on the current index and whether attempting to encrypt/decrypt appropriate character is retrieved
/// from the cipher key value. This method applies when both of the characters are on the same row.
/// </summary>
/// <param name="currentIndex">Current index of the character attempting to encrypt/decrypt.</param>
/// <param name="key">Cipher key being used for encryption/decryption.</param>
/// <param name="isDecryption">Flag determining whether to use decryption rule set. If not set to <c>true</c>,
/// encryption mode is assumed.</param>
/// <returns>Encrypted/Decrypted character corresponding to conditions specified.</returns>
private static char GetCharacterSameRow(int currentIndex, PlayfairKey key, bool isDecryption = false)
{
// Decryption; Character immediately to the left.
if (isDecryption)
{
// If left-most column: use end column character, otherwise use character to the left of current index.
return(GetCharColumnIndex(currentIndex) == 0
? key.Value[currentIndex + (CipherGridDimension - 1)]
: key.Value[currentIndex - 1]);
}
// Encryption; If character is right-most use first first character within the row, otherwise use
// character immediately to the right.
return(GetCharColumnIndex(currentIndex + 1) == 0
? key.Value[currentIndex - (CipherGridDimension - 1)]
: key.Value[currentIndex + 1]);
}
/// <summary>
/// Based on the current index and whether attempting to encrypt/decrypt appropriate character is retrieved
/// from the cipher key value. This method applies when both of the characters are in the same column.
/// </summary>
/// <param name="currentIndex">Current index of the character attempting to encrypt/decrypt.</param>
/// <param name="key">Cipher key being used for encryption/decryption.</param>
/// <param name="isDecryption">Flag determining whether to use decryption rule set. If not set to <c>true</c>,
/// encryption mode is assumed.</param>
/// <returns>Encrypted/Decrypted character corresponding to conditions specified.</returns>
private static char GetCharacterSameColumn(int currentIndex, PlayfairKey key, bool isDecryption = false)
{
// Decryption; Character immediately above.
if (isDecryption)
{
// If top-most row: use bottom row, otherwise use character immediately above. The 4 multiplier makes
// it easy to transition from top-most to bottom row within the key.
return(currentIndex < CipherGridDimension
? key.Value[(CipherGridDimension * 4) + currentIndex]
: key.Value[currentIndex - CipherGridDimension]);
}
// Encryption; If character is on the bottom-most row use character from the top row of the same column,
// otherwise use character from the row below.
return(GetCharRowIndex(currentIndex) == (CipherGridDimension - 1)
? key.Value[currentIndex - (CipherGridDimension * 4)]
: key.Value[currentIndex + CipherGridDimension]);
}
private static string PerformGenericPlayfair(IEnumerable <Digraph> digraphs, PlayfairKey key,
bool isDecrypt = false)
{
var sb = new StringBuilder(string.Empty);
foreach (var digraph in digraphs)
{
// Find digraph character index values.
var charOneIndex = 0;
var charTwoIndex = 0;
for (var i = 0; i < key.Value.Length; i++)
{
if (key.Value[i].Equals(digraph.CharacterOne))
{
charOneIndex = i;
}
else if (key.Value[i].Equals(digraph.CharacterTwo))
{
charTwoIndex = i;
}
}
if (AreOnTheSameRow(charOneIndex, charTwoIndex))
{
sb.Append(GetCharacterSameRow(charOneIndex, key, isDecrypt));
sb.Append(GetCharacterSameRow(charTwoIndex, key, isDecrypt));
}
else if (AreInTheSameColumn(charOneIndex, charTwoIndex))
{
sb.Append(GetCharacterSameColumn(charOneIndex, key, isDecrypt));
sb.Append(GetCharacterSameColumn(charTwoIndex, key, isDecrypt));
}
else
{
sb.Append(GetSquareChar(charOneIndex, charTwoIndex, key, isDecrypt));
sb.Append(GetSquareChar(charOneIndex, charTwoIndex, key, isDecrypt, true));
}
}
return(sb.ToString());
}
/// <summary>
/// Performs decryption.
/// </summary>
/// <param name="digraphs">Cipher text digraphs.</param>
/// <param name="key">Encryption cipher key.</param>
/// <returns>Decrypted cipher text.</returns>
private static string Decrypt(IEnumerable <Digraph> digraphs, PlayfairKey key)
{
return(PerformGenericPlayfair(digraphs, key, true).ToLower());
}
/// <summary>
/// Performs the encryption.
/// </summary>
/// <param name="digraths">Plain text digraphs.</param>
/// <param name="key">Encryption cipher key.</param>
/// <returns>Encrypted cipher text.</returns>
private static string Encrypt(IEnumerable <Digraph> digraths, PlayfairKey key)
{
return(PerformGenericPlayfair(digraths, key));
}