/// <summary>
/// Because the PPPv3 spec says that individual passcodes can not
/// cross crypto/counter boundaries we must calculate how many
/// passcodes we can generate out of a given block of 128 bits
/// </summary>
/// <returns></returns>
public int CalculatePasscodesPerBlock(int passcodeLength, int alphabetLength)
{
byte[] bits128 = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
BigInteger allBits = new BigInteger(bits128);
int charactersPerBlock = 0;
while (allBits > alphabetLength)
{
allBits.ModDivInPlace(alphabetLength);
charactersPerBlock++;
}
int codesPerBlock = charactersPerBlock / passcodeLength;
return codesPerBlock;
}
/// <summary>
/// Calculates the passcode for the given index
/// </summary>
/// <param name="passcodeOrdinal"></param>
/// <returns></returns>
public string GetPasscode(BigInteger passcodeOrdinal)
{
if (passcodeOrdinal > Int128Max)
throw new ArgumentOutOfRangeException("passcodeOrdinal", String.Format("Passcode Ordinals can not exceed 128 bits!", string.Empty));
byte[] crypto = GetCryptoBlockForCounter(_sequenceKey, passcodeOrdinal);
StringBuilder passcode = new StringBuilder(_passcodeLength);
BigInteger working = new BigInteger(crypto);
while (passcode.Length < _passcodeLength)
{
int index = working.ModDivInPlace(_alphabet.Count);
passcode.Append(_alphabet[index]);
}
return passcode.ToString();
}
/// <summary>
/// Each passcode is generated from a seperate 128 bit crypto operation, meaning that
/// passcodes can not exceed a certain length for a given alphabet.
/// </summary>
/// <param name="alphabetLength"></param>
/// <returns></returns>
public static int CalculateMaxPasscodeLength(int alphabetLength)
{
BigInteger allBits = new BigInteger(bits128);
int blocks = 0;
while (allBits > alphabetLength)
{
allBits.ModDivInPlace(alphabetLength);
blocks++;
}
return blocks;
}