static bool Func2(byte[] cipherText)
{
byte[] plainText = Utility.Decrypt(cipherText, _keyBytes, _iv, CipherMode.CBC);
byte[] unpaddedPlainText = Challenge15.StripPkcs7Padding(plainText);
string s = Encoding.ASCII.GetString(unpaddedPlainText);
string[] words = s.Split(';');
return(words.Contains("admin=true"));
}
private static void DoOne()
{
// Create ciphertext and an IV
Func1(out byte[] cipherText, out byte[] iv);
// Prepare a buffer for the decoded plain text
byte[] decodedPlainText = new byte[cipherText.Length];
for (int i = 0; i < cipherText.Length; i += BLOCK_SIZE)
{
byte[] bytes = Utility.Concat(
new byte[BLOCK_SIZE],
Utility.Pluck(cipherText, i, BLOCK_SIZE));
for (int j = BLOCK_SIZE - 1; j >= 0; j--)
{
var candidates = new List <byte>();
for (int k = 0x00; k <= 0xFF; k++)
{
bytes[j] = (byte)k;
for (int x = j + 1; x < BLOCK_SIZE; x++)
{
bytes[x] = (byte)(
decodedPlainText[i + x] ^
(BLOCK_SIZE - j) ^
(i == 0 ? iv[x] : cipherText[i - BLOCK_SIZE + x]));
}
if (Func2(bytes, iv))
{
candidates.Add((byte)k);
}
}
if (candidates.Count != 1)
{
throw new Exception("Expected single candidate");
}
decodedPlainText[i + j] = (byte)(
(BLOCK_SIZE - j) ^
(i == 0 ? iv[j] : cipherText[i - BLOCK_SIZE + j]) ^
candidates[0]);
}
}
byte[] strippedPlainText = Challenge15.StripPkcs7Padding(decodedPlainText);
// Finally present the output
string base64String = Encoding.ASCII.GetString(strippedPlainText);
byte[] base64Bytes = Convert.FromBase64String(base64String);
string finalString = Encoding.ASCII.GetString(base64Bytes);
Console.WriteLine(finalString);
}
private static bool Func2(byte[] cipherText, byte[] iv)
{
byte[] plainText = Utility.Decrypt(cipherText, _keyBytes, iv, CipherMode.CBC);
return(Challenge15.IsValidPkcs7Padding(plainText));
}
