private Characteristic FindBestCharacteristic(int round, List <Differential> differentialsList, int outputDiff, Characteristic res)
{
//end of rekursion
if (round == 0)
{
return(res);
}
//contains the active SBoxes in the round
bool[] activeSBoxes = new bool[CipherThreeConfiguration.SBOXNUM];
//check active SBoxes
int[] outputDiffs = new int[CipherThreeConfiguration.SBOXNUM];
for (int i = 0; i < CipherThreeConfiguration.SBOXNUM; i++)
{
outputDiffs[i] = GetSubBlockFromBlock(outputDiff, i);
if (outputDiffs[i] > 0)
{
activeSBoxes[i] = true;
}
else
{
activeSBoxes[i] = false;
}
}
//resultList
List <Characteristic> diffList = new List <Characteristic>();
//prepare the arrayOfDifferentialLists
List <Differential>[] arrayOfDifferentialLists = new List <Differential> [CipherThreeConfiguration.SBOXNUM];
int comb = 1;
for (int b = 0; b < CipherThreeConfiguration.SBOXNUM; b++)
{
if (activeSBoxes[b])
{
arrayOfDifferentialLists[b] = new List <Differential>(differentialsList.Count);
differentialsList.ForEach((item) =>
{
arrayOfDifferentialLists[b].Add((Differential)item.Clone());
});
List <Differential> diffsToRemove = new List <Differential>();
for (int j = 0; j < arrayOfDifferentialLists[b].Count; j++)
{
if (arrayOfDifferentialLists[b][j].OutputDifferential != outputDiffs[b])
{
diffsToRemove.Add(arrayOfDifferentialLists[b][j]);
}
}
foreach (var curDiff in diffsToRemove)
{
arrayOfDifferentialLists[b].Remove(curDiff);
}
comb *= arrayOfDifferentialLists[b].Count;
}
else
{
arrayOfDifferentialLists[b] = new List <Differential>();
}
}
for (int c = 0; c < comb; c++)
{
Differential[] curDiffSBoxes = new Differential[CipherThreeConfiguration.SBOXNUM];
//calc indices
int indexNo = 0;
int j = c;
while (j > 0)
{
if (arrayOfDifferentialLists[indexNo].Count > 0)
{
int index = j % arrayOfDifferentialLists[indexNo].Count;
j = j / arrayOfDifferentialLists[indexNo].Count;
curDiffSBoxes[indexNo] = arrayOfDifferentialLists[indexNo][index];
}
indexNo++;
}
//zero case
if (c == 0)
{
for (int i = 0; i < CipherThreeConfiguration.SBOXNUM; i++)
{
if (activeSBoxes[i])
{
curDiffSBoxes[i] = arrayOfDifferentialLists[i][0];
}
}
}
//check null values
for (int z = 0; z < CipherThreeConfiguration.SBOXNUM; z++)
{
if (curDiffSBoxes[z] == null)
{
curDiffSBoxes[z] = new Differential()
{
Count = 0,
InputDifferential = 0,
OutputDifferential = 0,
Probability = -1
};
}
}
//calc conditions
bool satisfied = true;
for (int i = 0; i < CipherThreeConfiguration.SBOXNUM; i++)
{
if (curDiffSBoxes[i].OutputDifferential != outputDiffs[i])
{
satisfied = false;
}
}
//check if conditions are satisfied
if (!satisfied)
{
continue;
}
//copy object
Characteristic characteristic = res.Clone() as Characteristic;
//calculate inputDifference
int inputDiff = 0;
for (int i = CipherThreeConfiguration.SBOXNUM - 1; i >= 0; i--)
{
inputDiff = inputDiff ^ curDiffSBoxes[i].InputDifferential;
if ((i - 1) >= 0)
{
inputDiff = inputDiff << CipherThreeConfiguration.BITWIDTHCIPHERFOUR;
}
}
//outputDifference for previous round
int outputDiffPreviousRound = inputDiff;
//calc new prob
// ReSharper disable once CompareOfFloatsByEqualityOperator
if (characteristic != null && characteristic.Probability != -1)
{
for (int i = 0; i < CipherThreeConfiguration.SBOXNUM; i++)
{
if (curDiffSBoxes[i].Count == 0)
{
continue;
}
characteristic.Probability = characteristic.Probability * (curDiffSBoxes[i].Count / 16.0);
}
}
else
{
double value = 1.0;
for (int i = 0; i < CipherThreeConfiguration.SBOXNUM; i++)
{
if (curDiffSBoxes[i].Count == 0)
{
continue;
}
value = value * (curDiffSBoxes[i].Count / 16.0);
}
if (characteristic != null)
{
characteristic.Probability = value;
}
}
//store result
if (characteristic != null)
{
characteristic.InputDifferentials[round - 1] = inputDiff;
characteristic.OutputDifferentials[round - 1] = outputDiff;
//go one round deeper
Characteristic retval = FindBestCharacteristic(round - 1, differentialsList,
outputDiffPreviousRound, characteristic);
//check if there is a result
if (retval != null)
{
diffList.Add(retval);
}
}
}
//search for the best result
Characteristic best = new CipherThreeCharacteristic();
foreach (var curDiffs in diffList)
{
if (best.Probability < curDiffs.Probability)
{
best = curDiffs;
}
}
return(best);
}
