A lock box or key safe is used to store the key to a door in a semi-secure fashion. In the type shown here, the box is opened by pushing exactly the correct digits, then sliding a lever to open. If the wrong combination is entered, the lock must be reset before trying again. On this type of lock, the order of the digits does not matter; i.e., the combination 3924 is the same as 2349.
The combination can be any number of digits up to 10, but most people use a 4-digit combination. Below is a list of possible combinations for such a lock.
0123 0124 0125 0126 0127 0128 0129 0134 0135 0136 0137 0138 0139 0145 0146 0147 0148 0149 0156 0157 0158 0159 0167 0168 0169 0178 0179 0189 0234 0235 0236 0237 0238 0239 0245 0246 0247 0248 0249 0256 0257 0258 0259 0267 0268 0269 0278 0279 0289 0345 0346 0347 0348 0349 0356 0357 0358 0359 0367 0368 0369 0378 0379 0389 0456 0457 0458 0459 0467 0468 0469 0478 0479 0489 0567 0568 0569 0578 0579 0589 0678 0679 0689 0789 1234 1235 1236 1237 1238 1239 1245 1246 1247 1248 1249 1256 1257 1258 1259 1267 1268 1269 1278 1279 1289 1345 1346 1347 1348 1349 1356 1357 1358 1359 1367 1368 1369 1378 1379 1389 1456 1457 1458 1459 1467 1468 1469 1478 1479 1489 1567 1568 1569 1578 1579 1589 1678 1679 1689 1789 2345 2346 2347 2348 2349 2356 2357 2358 2359 2367 2368 2369 2378 2379 2389 2456 2457 2458 2459 2467 2468 2469 2478 2479 2489 2567 2568 2569 2578 2579 2589 2678 2679 2689 2789 3456 3457 3458 3459 3467 3468 3469 3478 3479 3489 3567 3568 3569 3578 3579 3589 3678 3679 3689 3789 4567 4568 4569 4578 4579 4589 4678 4679 4689 4789 5678 5679 5689 5789 6789
Three are 210 possible combinations, meaning that it would take someone about half an hour to hack such a lock simply by trying numbers from the list. Some combinations are more likely, such as 1234, the house number, or an important year, but that’s beyond the scope of this article.
The code below was used to generate the list. It could easily be modified to generate a list of all 1024 possible combinations. Also, some lock boxes have an additional # and * key, like a telephone keypad. A list of combinations could be generated for such a lock by changing nButtons to 10 and MaxCombos to 4096. Note that the additional keys would print as hexadecimal A and B, which you would map to the extra keys.
// This quick program will calculate the possible combinations
// for a 10-digit lockbox. Order of the digits does not matter,
// and no digit may be repeated.
// To compile
// gcc combo.c -o combo
// Roderick; 26-AUG-2025.
#include <stdio.h>
#define nButtons 10
//#define MaxCombos (2^nButtons)
#define MaxCombos 1024
int main() {
int ComboNumber; // Combination #1, #2, #3, etc.
int OutputLineNumber = 1;
for (ComboNumber=0; ComboNumber<MaxCombos; ComboNumber++) {
int n, nDigits, tComboNumber;
tComboNumber = ComboNumber;
// see if we have a 4-digit combination
nDigits = 0;
for (n=0; n<nButtons; n++) {
if (tComboNumber & 1) nDigits++;
tComboNumber >>= 1;
}
// if the combination is not 4 digits, we're not interested
// if we are interested in all combinations regardless of
// digit count, take out this condition
if (nDigits == 4) {
tComboNumber = ComboNumber;
// DEBUG printf("Combo #%d: ", ComboNumber);
// DEBUG printf("%d. ", OutputLineNumber);
for (n=0; n<nButtons; n++) {
if (tComboNumber & 1) printf("%1X", n);
tComboNumber >>= 1;
}
printf("\n");
OutputLineNumber++;
}
}
}