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How would I make a password system for a single-player game like the ones used in older console games?

For example, Mega Man X saves work by giving you a series of numbers that you can enter later to load your save.

Accumulator
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    Don't do what Donkey Kong (NES) did - which is let you save to the cartridge but NOT PRESERVE THE LIVES COUNT - so you'd have loads of lives from the first level (like 24) save, go to bed, WAKE UP WITH 3 - I hated that – Alec Teal Jul 26 '15 at 02:08
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    Pedantic note: a password system does not save a game state, it loads a predefined one. – Lilienthal Jul 26 '15 at 10:08
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    @Lilienthal not necessarily. Maybe it is just some sort of "dump" of some variables state, shuffled with a deterministic two-way algorythm. – o0'. Jul 26 '15 at 12:10
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    @Lohoris Yes, if you have sufficient entropy in your password you can consider it a sort of saved state that's close enough to resemble current savegames. After all, both require a way to export and import the game's state. However, the entropy required for anything resembling a true save (exact location/checkpoint, lives, enemies slain, ammo counts, ....) would require both a very long password and a very complex algorithm. For cases where the decision to use passwords over savegames makes sense (if there even are any), the maxim should hold true. – Lilienthal Jul 26 '15 at 15:06
  • You really ought not to. – Schilcote Jul 26 '15 at 16:39
  • @AlecTeal Sounds like a feature for unlimited lives to me! (save and restart when at 1 life). – NESPowerGlove Jul 26 '15 at 18:32
  • @NESPowerGlove only in the same way that hitting "Game over" and not throwing the game in the bin is infinite lives. – Alec Teal Jul 26 '15 at 19:27
  • @Lilienthal that would be stupid. Also empirically false. Rock & Roll Racing (another NES game) "saved" your inventory and balance AND game level in a password. I really doubt they enumerated ALL possible states! – Alec Teal Jul 26 '15 at 19:44
  • @Schilcote I'm trying to make a Mega Man X game, so... – Accumulator Jul 26 '15 at 19:57
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    @AlecTeal What exactly whould be "stupid"? I'm evaluating a password against the amount of data stored in contemporary games, where they are clearly unsuitable for the vast majority. – Lilienthal Jul 26 '15 at 20:56

4 Answers4

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First, break down your game state (or rather, the aspects of the state you want to save). In the case of a Mega Man style game, you might track which of the end-level bosses you've killed, the number of energy-tank-like powerups you have, and so on.

Pack all of that data into a bit field, that is, assign an appropriate number of bits to each value:

  • Killed boss 1 (one bit)
  • Killed boss 2 (one bit)
  • Killed boss 3 (one bit)
  • Killed boss 4 (one bit)
  • Energy tanks (x of 5 total) (3 bits)
  • Unlocked some achievement (one bit)

Our example has 8 bits total, which means a single character can represent a password. In practice, your game is likely to have more state and thus require more total bits and thus more characters. As noted in the comments here and elsewhere in this question, this approach works for "retro" games or other games where the size of the captured game state is reasonable. Beyond a certain point, you may discover that the complexity of the passwords necessary to encode your state is too large.

To decrease the likelyhood of casual observation cracking the password, you can transform the bit layout so that you introduce dummy bits (which have no effect on the game state but which make the password look different when you interpret all the bits as characters) or run it through some reversible hash-like operation to scramble the bits around so that all the "killed box X" bits are not right next to eachother, making corresponding sequential-state passwords look very different, or introduce checksum values.

If you dig around, there's a fair bit of information on the systems employed for some of the more-popular passworded games out there:

You could read up on those for additional inspiration.

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    Remember to make sure that the "save state" password is all easily type-able and, preferably, unambiguous (eg., use '0' or 'O' but not both). – minnmass Jul 25 '15 at 21:16
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    @minnmass bonus points if you implicitly convert similar characters to your standard, so that typing 'O' instead of '0' still translates to '0'. –  Jul 26 '15 at 01:37
  • "..and run it through some reversible hash-like operation.." - Or take the hash and append some of its bits to the end of the password. When used like this, it's called a checksum. – BlueRaja - Danny Pflughoeft Jul 26 '15 at 05:25
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    A checksum character might also be useful to prevent typos. – March Ho Jul 26 '15 at 06:04
  • From a practical perspective this is a horrendous idea, why link your password size to your storage size? Why invent your own encryption system when there are plenty of pre built solutions? – NPSF3000 Jul 26 '15 at 09:59
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    @NPSF3000 Because in this case the "password" isn't an authentication measure; the "password" is the storage medium itself. – Schilcote Jul 26 '15 at 16:41
  • @Schilcote that's not clear to me from the question that that's a requirement, and lacking such a requirement I don't see how it's a particularly good implementation idea. – NPSF3000 Jul 26 '15 at 22:16
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    @NPSF3000 if it's not clear to you from the question then you need to reread the question or learn the background material. – hobbs Jul 27 '15 at 00:32
  • Note that if the game is linear you only need to store which boss was most recently killed, so "Killed boss 0 .. 4" is actually 3 bits. In fact, with linear games, all you need to store is the most recent level id, their stats/inventory/quest-flags/achievements, and any information relevant to that level (any non-respawning monsters killed, most recent save point, etc). Given the level id, "killed the boss' becomes just one bit. – Dewi Morgan Jul 27 '15 at 02:02
  • @hobbs the question is "How would I make a password system for a single-player game? For example, Mega Man X saves work by giving you a series of numbers that you can enter later to load your save." Not once does it state that the password must be the save, nor does it state that the game he's building is suitable for the method proposed here. – NPSF3000 Jul 27 '15 at 03:01
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    @NPSF3000 OP clearly says "Retro Style" and references Megaman to confirm, which is not ambiguous. Learn about the subject matter, understand that this is not related to security, and please stop trying to defend what started as an honest and forgivable ignorance. – MickLH Jul 27 '15 at 15:58
  • The "retro-style" reference was in the original question, but removed by an edit before NPSF3000 answered, so it's understandable that it was not clear; the relevant information has since been restored to the question, so we can stop the pile-on now, thanks. –  Jul 27 '15 at 16:02
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Those games did only save a state I believe. The password just references a level to load. Just use a dictionary for something like this.

If it is a little bit more complex like having a certain weapon or booster you can encrypt the state to a short (but long enough) hash code.

But I strongly advice you to not use this. Password will spread fast on the Internet and soon everyone had won your game.

Madmenyo
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    Passwords spreading on the internet can be good. Cheating can be fun, and in a single-player game, players only cheat themselves. – Anko Jul 25 '15 at 19:14
  • @Anko Yeah, I dislike cheating and barely did it so I am biased. But from my own experience my games would gather dust a lot quicker and sooner when I cheated my way trough. – Madmenyo Jul 25 '15 at 19:17
  • "... just references a level to load." Not true in all cases by far. Some passwords also reference bosses, killed, powerups gained, etc. Secondly, advising that the password can be spread is one thing - an issue in our hyper-connected world. Saying that cheating options should be removed because YOU don't like cheating is another. What you like isn't exactly what others like. Its not about what you like, it's the consumer. Cheating/shortcuts are an important option - especially for single player games. – WernerCD Jul 26 '15 at 03:00
  • @WernerCD firstly I talk about more complex systems I just start with a simple password system. I am NOT claiming anything. Secondly, I am not saying anything about cheating in the answer. I am just giving my BIASED reply on a comment and even saying that I am biased and aware of your point that it is not what I like. But if nobody says what he likes or dislikes how would you know what others like? I still think a system like that is due time and 21 century cheating is achieved differently. Next time you should read more carefully before you write a comment like that IMHO. – Madmenyo Jul 26 '15 at 08:59
  • If you have savegames, people will share savegames, too. – Dewi Morgan Jul 27 '15 at 01:47
  • @DewiMorgan true, but to a much lesser extent. It's just to easy to write a password, fight the end boss and see the end game. If the game has no real replay value the game will lose popularity. A password system is a old system used when it was necessary to store a password because saving a complete game was not possible due to size or read only memory. It might still work, I did not do any research on the subject other then my own experience. – Madmenyo Jul 27 '15 at 06:58
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Well you could basically do it this way:

Bosses killed = 7

Amount of coins = 36

Owns Sword = 0(No)

Owns Heart = 1(Yes)

Current Level = 6

Current World = 9

Health = 100

Code = 7V36R0A1T6O9A100

Basically every letter separate The Types

aanda tv
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While solutions like Josh Petrie's character-to-state system have benefits (small save size, works on all copies of the game), they couple the games design and state system to be intrinsically to the password system. Any change to the password or state or design and the entire system falls apart.

A far simpler solution would be to build your save system as per normal, and simply add a password system over the top. This can be as simple as saving the password in the save (possibly doubling as file name) and validating it on load, or as complex as using one of the many encryption libraries around (which is pretty trivial).

There seems to be some confusion around this answer.

1) The op has simply asked for a password that let's them load a save... there is no requirement that the password be the save.

2) Many games today have significant save storage requirement, we just built a quick save/load system internally that was producing 10MB files... for a quiz game! Josh Petrie's save system, while valid is very limited - as soon as the store system becomes non-trivial the passwords would have to be of enormous length to persist all pertinent data.

For example, what happens if there are 30 bosses? 20 achievements? Are we halfway through an achievement? What level is he at? How many bits are we putting aside for health, lives, coins, xp? What if the snapshot is not of state between levels... but half-way through a level - the positions of the player, enemies, projectiles, destructed/altered terrain suddenly add up to a lot of state that needs to be stored.

For perspective, assuming a 30 character alphabet and a ten letter password we have roughly 50 bits worth of information to store. It's not uncommon to see a game/engine use up to 320 bits just to store a single object's position, rotation and scaling. Sure, you can do all kinds of optimisation tricks to do more with less... but ultimately you're using up the time dedicated to making the game fun - one of the worst optimisations you can make.

NPSF3000
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    I think the problem here, and with your comment above, is that cryptographic security is not the goal of saving the game's state as bits within a password. Passwords composed in a such a way are easily reversible but difficult to guess hashes, and nothing more. And for that same reason, it doesn't cause undue coupling in the way you are imagining; if the game state becomes more or less complicated, so does the hash (which does invalidate previous hashes), but the system works equally well before and after. – Seth Battin Jul 26 '15 at 12:54
  • @Seth Battin "it doesn't cause undue coupling in the way you are imagining" Oh cool! So a game I am working on right now had a save file measured in the megabytes... all I need to do is do some basic obfuscation and display it on a screen and voila! We now have a perfectly good password system? – NPSF3000 Jul 26 '15 at 22:18
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    It sounds like your game is not appropriate for this method. – Seth Battin Jul 26 '15 at 23:59
  • @NPSF3000 What's the point of passwords in this case? If it's to prevent others using the same machine from using their savegames without permission, why not just have a once-per-game login with username/pass? If it's to specify which save they want, that's a "savegame id", not a password, and should likely be made friendlier. As it is, if I read you right, you're requiring a different, non-user-specified password every time they load a new save? WHY? What useful advantage does that give to anybody? – Dewi Morgan Jul 27 '15 at 01:55
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    @DewiMorgan I'm not requiring anything, I'm purely giving an answer to op's question: "How would I make a password system for a single-player game? For example, Mega Man X saves work by giving you a series of numbers that you can enter later to load your save." My solution does that, without coupling the save system and the password system together. The top voted answer has the same downsides as mine, with the addition that it requires the entire game to be designed to suit the method. – NPSF3000 Jul 27 '15 at 02:59
  • @SethBattin incorrect, the game is suitable, it is the currently proposed method is not. – NPSF3000 Jul 27 '15 at 03:00
  • @NPSF3000 You still aren't explaining the advantage of a hybrid "savefile+password" system as you describe in your answer, over a savefile without a password. – Dewi Morgan Jul 27 '15 at 06:24
  • @DewiMorgan Morgan why do I have to explain the advantage of having a password with the save file? It's the requirement of the question, not an additional feature I'm trying to sell. – NPSF3000 Jul 27 '15 at 06:50
  • Because as I (mis?)read it, having a save file seems to completely remove any point in having a password. So, an edit to clarify how the password+savefile system might be useful would make all the difference between a non-answer, and an upvotably insightful answer! Since you're making a game like this yourself, you're probably the best placed people around to explain why a game designer would go with this savegame model... at least if that wouldn't reveal NDA'd info or something? – Dewi Morgan Jul 27 '15 at 07:31
  • Let us continue this discussion in chat. – Dewi Morgan Jul 27 '15 at 07:41