/// <summary>
/// Decrypt the given ciphertext using the pre-set list of decryption keys
/// Assumes the list of decryption keys has been populated already
/// </summary>
static string DecryptCipherText(string cipherAsHex)
{
if (DecryptionKeys.Count == 0)
{
Console.WriteLine("ERROR: We don't seem to have a list of encryption keys.");
return("ERROR");
}
string decryptedCipher = string.Empty;
byte[] cipherAsBytes = StringToByteArray(cipherAsHex);
// Iterate over the give cipher text and decrypt
for (int i = 0; i < cipherAsBytes.Length; i++)
{
// Find the decrytion key for this position
DecryptionKey key = FindDecryptionKey(i);
if (key == null)
{
// We don't have a key for this position somehow - output ? instead
decryptedCipher += "?";
continue;
}
int xorResult = cipherAsBytes[i] ^ key.Key;
// Convert to ASCII character
char decryptedChar = Convert.ToChar(xorResult);
decryptedCipher += decryptedChar;
}
return(decryptedCipher);
}
static List <DecryptionKey> DecryptionKeys; // List of decryption keys per position (built up along the way)
static void Main(string[] args)
{
// Read the given ciphertexts from the XML file where I store them
CipherTexts = ReadCipherTexts();
DecryptionKeys = new List <DecryptionKey>();
Console.WriteLine("Read {0} ciphertexts from XML.", CipherTexts.Count);
int currentCipherIndex = 0;
// Iterating over Ciphers as Hex Strings
while (currentCipherIndex < CipherTexts.Count)
{
Console.WriteLine("Current Cipher Index: " + currentCipherIndex);
byte[] currentCipherAsBytes = StringToByteArray(CipherTexts[currentCipherIndex]);
int currentByteIndex = 0;
// Iterating over Bytes in the current Cipher
while (currentByteIndex < currentCipherAsBytes.Length)
{
Console.WriteLine("Checking Index: " + currentByteIndex);
// Get the current byte
byte currentByteValue = (byte)currentCipherAsBytes.GetValue(currentByteIndex);
// XOR this byte with the corresponding byte in each ciphertext (until we find a potential key)
for (int i = currentCipherIndex + 1; i < CipherTexts.Count; i++)
{
byte[] cipherAsBytes = StringToByteArray(CipherTexts[i]);
byte byteToXorAgainst;
try
{
byteToXorAgainst = cipherAsBytes[currentByteIndex];
}
catch
{
// No correspdonging byte at this location in this cipher (e.g. cipher could be shorter than the current one)
continue;
}
// XOR
int xor = currentByteValue ^ byteToXorAgainst;
// Check if we got an uppercase ASCII character
if (IsUpperCaseAscii(xor))
{
Console.WriteLine("XOR of current check (" + xor + ") is an uppercase ASCII character, value is: " + Convert.ToChar(xor));
// This means that one of the plain text characters is a space and one is the lowercase ASCII equivalent of this character
// We need to XOR each potential key with an ASCII space
int potentialKey1 = currentByteValue ^ 32;
int potentialKey2 = byteToXorAgainst ^ 32;
// These are our pontetial keys
Console.WriteLine("Potential Key 1 - " + potentialKey1);
Console.WriteLine("Potential Key 2 - " + potentialKey2);
// Check one of these keys is valid - if one isn't we'll assume the other is
Console.WriteLine("Checking Potential Key - " + potentialKey1);
if (CheckPotentialKey(potentialKey1, currentByteIndex))
{
Console.WriteLine("Key is valid - " + potentialKey1);
DecryptionKey key = new DecryptionKey();
key.Key = potentialKey1;
key.Position = currentByteIndex;
DecryptionKeys.Add(key);
}
else
{
Console.WriteLine("Key '" + potentialKey1 + "' not valid, assuming key '" + potentialKey2 + "' is valid");
DecryptionKey key = new DecryptionKey();
key.Key = potentialKey2;
key.Position = currentByteIndex;
DecryptionKeys.Add(key);
}
// Break out of checking further bytes at this position as we've found a valid key
break;
}
}
currentByteIndex++;
}
currentCipherIndex++;
}
// Decrypt the Ciphertexts
Console.WriteLine("Now to decrypt the ciphertexts...");
for (int i = 0; i < CipherTexts.Count; i++)
{
string decryptedCipher = DecryptCipherText(CipherTexts[i]);
Console.WriteLine("Decrypted Ciphertext at Position '" + i + "' - " + decryptedCipher);
}
}