This morning, my family and I went swimming at the community swimming center. Unfortunately, I couldn't find my key-based lock that I normally take. However, I did find my Master combination lock, but couldn't recall the combination. Fortunately, I knew how to find it. I took this lock with me to lock my personal items in the locker while swimming around in the pool.
While swimming, I started thinking about ways to better recall lock combinations in the future. The obvious choice is to encrypt it, so I could engrave the encrypted combination on the lock. However, it needs to be simple enough to do in my head should I temporarily forget it while swimming, and easy enough to recall if I haven't used the lock in a few years. Thankfully, this can be done easily enough with modulo addition and subtraction.
Before beginning, you need a 6-digit PIN that you won't easily forget. Tempting enough, dates can easily be in 6-digits, and something like a birthday or an anniversary are not hard to remember. Unfortunately, if someone knows you, and knows these dates, they can easily reverse the process to open the lock. So, as tempting as dates are, don't use them. Instead, you should probably use a 6-digit PIN, that only you would know, and always know. So, knowing this, let's see how this works.
You need to be familiar with modulus math, aka "clock math". The idea, is that after a certain maximum, the numbers reset back to 0. For example, 00:00 is midnight while 23:59 is the minute before. As soon as the hour is "24", then it resets back to 0, for a full 24-hour day. You could call telling time "mod 24 math". For combination locks, we're going to be using "mod 40 math", if the maximum number on your combination lock is "40", on "mod 60 math" if the max is "60", and so forth.
Suppose the combination to your lock is "03-23-36", and suppose your 6-digit PIN is "512133". Let's encrypt the combination with our PIN, by using "mod 40 subtraction". We'll use subtraction now, because most people have an easier time with addition than subtraction. When you are trying to rediscover your combination, you'll take your encrypted number, and do "mod 40 addition" to reverse it, and bring it back to the original combination lock numbers.
Here it is in action:
Encrypting the original combination 03 23 36 <- original combination - 51 21 33 <- secret PIN = -------- -48 02 03 = -------- 32 02 03 <- encrypted after "mod 40"
Because the first number in our combination is "03", and we are subtracting off "51", we end up with "-48". As such, we need to add "40" until our target new number is in the range of [0, 40), or "0 <= n < 40". This gives us "32" as the result. The rest of the numbers fell within that range, so no adjusting was necessary. I can then engrave "32-02-03" on the bottom of the lock, so when I hold the lock up while in a locker, the text is readable. Okay, that's all fine and dandy, but what about reversing it? Taking the encrypted combination, and returning to the original combination? This is where "mod 40 addition" comes in. For example:
Decrypting the encrypted combination 32 02 03 <- encrypted combination + 51 21 33 <- secret PIN = -------- 83 23 36 = -------- 03 23 36 <- original combination after "mod 40"
Notice that this time, the first number in our "mod 40 addition" is "83". So, we subtract of "40" until our original combination number is in the range of [0,40), or "0 <= n < 40", just like when doing "mod 40 subtraction" to create the new combination lock values. At worst case, you'll have to subtract a "40" only three times per number. On thing to watch out for, is that your encrypted combination numbers are far enough away from the original, that trying out the encrypted combination, won't accidentally open the lock, due to their proximity to the original numbers. If only one number is substantially off, that should be good enough to prevent an accidental opening. I want to come back to dates however, and why not to use them. Not only do they fall victim to a targeted attack, but they also have an exceptionally small key space. Assuming there are only 365 days per year, and assuming the attacker has a good idea of your age, plus or minus five years, that's a total of 3,650 total keys that must be tried following the common convention of "MM-DD-YY". It could be greatly reduced, if the attacker has a better handle on when you were born. If a 6-digit PIN is chosen instead, then the search space has 1,000,000 possible PINs. This is greater than the 64,000 possible maximum combination numbers a 40-digit Master lock could have, which puts the attacker on a brute force search for the original combination, if they aren't aware that Master combination locks can be broken in 8 tries or less.