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/*
* Transposition cipher cracker
*
* Algorithm used to find positions:
*
* n = keylength
* t = textlength
* a = t / n
* b = t % n
* d = accumulated rest terms
* k = wanted plain text position
* loc(k) = a * perm(k % n) + d(perm(k % n)) + k/n
*
* By Ben Ruijl, 2009
*
* compile: g++ -O2 col.cpp
*/

#include <iostream>
#include <cstring>

#define MAXKEY 20

const char* buffer = "irtyhockeibtdaamoelatcnsronhoniirttcacdeiunsihaioarnndgpruphahirgtoarnmclspstwe";
int buflength;
const char* crib = "computer";
int criblength;

void print(int* perm, int n)
{
int a = buflength / n;
int b = buflength % n;

//invert perm
int invperm[MAXKEY];

for (int i = 0; i < n; i++)
invperm[perm] = i;

int d[MAXKEY] = {0};
for (int i = 1; i < n; i++)
{
d = d[i -1];

if (invperm[i - 1] < b)
d++;
}

std::cout << "Found: ";

for (int i = 0; i < buflength; i++)
std::cout << buffer[a * perm[i % n] + d[perm[i % n]] + i / n];

std::cout << std::endl;
}


bool checkperm(int* perm, int n)
{
int cribpos = 0;

int a = buflength / n;
int b = buflength % n;

//invert perm
int invperm[MAXKEY];

for (int i = 0; i < n; i++)
invperm[perm] = i;

int d[MAXKEY] = {0};
for (int i = 1; i < n; i++)
{
d = d[i -1];

if (invperm[i - 1] < b)
d++;
}


for (int i = 0; i < buflength; i++)
{
if (buffer[a * perm[i % n] + d[perm[i % n]] + i / n] == crib[cribpos])
cribpos++;
else
cribpos = 0;

if (cribpos == criblength)
{
print(perm, n); // print the found text
return true;
}
}

return false;
}

// University of Exeter algorithm
// a fast algorithm, we don't care about sorting it like a dictionary
void permute(int *v, const int start, const int n)
{
if (start == n - 1)
checkperm(v, n);
else
{
for (int i = start; i < n; i++)
{
int tmp = v;

v = v[start];
v[start] = tmp;
permute(v, start + 1, n);
v[start] = v;
v = tmp;
}
}
}

int main(int argv, char** argc)
{
int perm[MAXKEY];

for (int i = 0; i < MAXKEY; i++)
perm = i;

buflength = strlen(buffer);
criblength = strlen(crib);

int curkey = 2; // start key

while (curkey < MAXKEY)
{
std::cout << "Testing key " << curkey << std::endl;
permute(perm, 0, curkey); // permutate keys
curkey++;
}

return 0;
}