Checksum for savestates
Moderator: Moderators
Re: Checksum for savestates
Tecmo super bowl uses a simple two byte checksum. If the current checksum doesn't match the saved checksum it wipes the entire SRAM.
Re: Checksum for savestates
I've never even heard of those screens. Got some examples?Pokun wrote:I love these screens since they show up so rarely.
Re: Checksum for savestates
I read that Adler32 has some weaknesses in that it's pretty easy to have the same hash value if only a few bytes are changed around.calima wrote:The cc65 runtime includes crc32 and adler32. Adler32 is faster.
CRC32 in cc65 requires 4 x 256 bytes in RAM. I wouldn't want to use an implementation that needs so many RAM values.
My game "City Trouble":
Gameplay video: https://youtu.be/Eee0yurkIW4
Download (ROM, manual, artworks): http://www.denny-r-walter.de/city.html
Gameplay video: https://youtu.be/Eee0yurkIW4
Download (ROM, manual, artworks): http://www.denny-r-walter.de/city.html
Re: Checksum for savestates
If you want a cryptographic hash, neither CRC32 nor adler32 are suitable.
If you don't want a cryptographic hash, you don't need to worry about byte shuffling. That isn't the kind of failure mode that will come from save RAM corruption.
If you don't want a cryptographic hash, you don't need to worry about byte shuffling. That isn't the kind of failure mode that will come from save RAM corruption.
- rainwarrior
- Posts: 8731
- Joined: Sun Jan 22, 2012 12:03 pm
- Location: Canada
- Contact:
Re: Checksum for savestates
That's a cryptographic weakness, i.e. it's easy for an attacker to modify the data without changing the hash. That's a different problem than just detecting errors in transmission.DRW wrote:I read that Adler32 has some weaknesses in that it's pretty easy to have the same hash value if only a few bytes are changed around.
You don't need to worry about attacks, though, users can already modify stuff easily in this case.
Your problem is just error detection, which Adler32 is actually designed for and relatively good at.
Edit: LOL ninja'd by lidnariq making the exact same distinction i did.
Re: Checksum for savestates
I don't need a cryptographic hash or any protection against hackers, only one where even a small difference already produces a completely different result to reliably spot corrupted data.
Regarding adler32, Wikipedia says:
If byte n of the input is incremented by k, byte n + 1 is decremented by 2 x k and Byte n + 2 is incremented by k, then s1 (the sum of all bytes) and s2 (the sum of all in-between values of s1) remain unchanged.
So, doesn't this description mean that if three bytes next to each other are corrupted in this specific way, that the hash value still remains the same?
Regarding adler32, Wikipedia says:
The German Wikipedia mentions this:https://en.wikipedia.org/wiki/Adler-32 wrote:Adler-32 has a weakness for short messages with few hundred bytes, because the checksums for these messages have a poor coverage of the 32 available bits.
If byte n of the input is incremented by k, byte n + 1 is decremented by 2 x k and Byte n + 2 is incremented by k, then s1 (the sum of all bytes) and s2 (the sum of all in-between values of s1) remain unchanged.
So, doesn't this description mean that if three bytes next to each other are corrupted in this specific way, that the hash value still remains the same?
My game "City Trouble":
Gameplay video: https://youtu.be/Eee0yurkIW4
Download (ROM, manual, artworks): http://www.denny-r-walter.de/city.html
Gameplay video: https://youtu.be/Eee0yurkIW4
Download (ROM, manual, artworks): http://www.denny-r-walter.de/city.html
Re: Checksum for savestates
You're describing a complex enough process that it can basically be assumed to not happen by chance.
- rainwarrior
- Posts: 8731
- Joined: Sun Jan 22, 2012 12:03 pm
- Location: Canada
- Contact:
Re: Checksum for savestates
When wikipedia says it's "weak" it's a relative comparison against similar 32-bit checksums.
e.g. if you were comparing it against pretty much any 16-bit checksum it's rather strong.
For protection against errors in an NES save game, a 32-bit checksum is almost absurdly overpowered. 8 bits is probably already more than sufficient?
Think about it this way, if you had some sort of ideal hash (e.g. truly "cryptographically secure"), an 8-bit hash should give you a 1 in 256 chance of detecting an error. That's pretty good already, isn't it? Like the "weakness" of Adler32 is sort of like comparing a 1 in 4 billion chance to a 1 in 1 billion chance; the question is whether the result is still good enough, the magnitudes matter here. (...and the only result of failing to identify the error is just a save game with some wrong data in it, rather than just being destroyed. Not really a big deal?)
Here's some info on just how "weak" that weakness isn't:
https://guru.multimedia.cx/crc32-vs-adler32/
e.g. if you were comparing it against pretty much any 16-bit checksum it's rather strong.
For protection against errors in an NES save game, a 32-bit checksum is almost absurdly overpowered. 8 bits is probably already more than sufficient?
Think about it this way, if you had some sort of ideal hash (e.g. truly "cryptographically secure"), an 8-bit hash should give you a 1 in 256 chance of detecting an error. That's pretty good already, isn't it? Like the "weakness" of Adler32 is sort of like comparing a 1 in 4 billion chance to a 1 in 1 billion chance; the question is whether the result is still good enough, the magnitudes matter here. (...and the only result of failing to identify the error is just a save game with some wrong data in it, rather than just being destroyed. Not really a big deal?)
Here's some info on just how "weak" that weakness isn't:
https://guru.multimedia.cx/crc32-vs-adler32/
Re: Checksum for savestates
Not really. There were devices, namely some late Game Copier models, that you can save the state of games to FDS disks, to be restored for later play. It's correct that battery save files normally don't contain (nearly) complete dumps of the system's states though, so indeed the terms have to be used carefully to avoid confusion. This applies to many later consoles using memory cards too.Bregalad wrote:DRW wrote:Savestates is an emulator-only thing.
I think in systems where storage is less of a concern, for example, PCs, some games probably dump (nearly) everything to their save files (so these files are indeed savestates), but for console games, especially on those retro systems, it's usually not cost effective to have the hardware or memory chips to hold the relatively huge states of games.
Re: Checksum for savestates
Yeah those devices are what I meant by special hardware. Modern flashcarts may also allow saving the state of the machine to a certain extent.
Western PC RPGs tend to allow you to store any item in any drawer or chest in the game and remembers them when saving, but even for that it may be enough if it only needs to remember changes I guess.
Western PC RPGs tend to allow you to store any item in any drawer or chest in the game and remembers them when saving, but even for that it may be enough if it only needs to remember changes I guess.
Probably about any Dragon Quest game that uses a battery for saving (I and II uses passwords, although English Dragon Warrior I and II might have the screen). Joy Mecha Fight is another one I remember (when I bought it the battery was already old), I just tried it in an emulator by manually corrupting the save in a hex editor. It's just a text box that tells you the data corrupted in Japanese and some sound plays.Sumez wrote:I've never even heard of those screens. Got some examples?Pokun wrote:I love these screens since they show up so rarely.
Re: Checksum for savestates
CRC16 should be good enough for practical error detection in most cases I can think of, and Greg Cook's tableless CRC16 is fast enough (66 cycles per byte).
Re: Checksum for savestates
i have made an adler32 online hash generator and the link is https://hash.onlinetoolsland.com/adler-32/,if you want to try it ,that will be good
Re: Checksum for savestates
What's the license on that calculation script? (Or should I call Hormel?)
In any case, I took this opportunity to write an (untested) implementation of Adler-32 based on the description at Wikipedia. I will warn that it's weak for messages smaller than about 1000 bytes because the sum won't reach the wrap at the prime modulus 65521.
EDIT: The difference between this and the version bundled with cc65 is that 1. mine uses the fact that 65536-65521 fits into 8 bits, and 2. mine doesn't call a bunch of stack manipulation in the inner loop.
In any case, I took this opportunity to write an (untested) implementation of Adler-32 based on the description at Wikipedia. I will warn that it's weak for messages smaller than about 1000 bytes because the sum won't reach the wrap at the prime modulus 65521.
Code: Select all
MOD_ADLER = 65521
.proc adler32
src = locals+$00
negcountlo = locals+$02
negcounthi = locals+$03
a_lo = locals+$04
a_hi = locals+$05
b_lo = locals+$06
b_hi = locals+$07
lda #1
sta a_lo
lsr a
sta a_hi
sta b_lo
sta b_hi
ldy src
sta src
byteloop:
; a += *src++
lda (src),y
iny
bne :+
inc src+1
:
clc
adc a_lo
sta a_lo
lda #0
adc a_hi
sta a_hi
; if (a >= 65521) a -= 65521
bcs a_overflow
lda a_lo
adc #65536-MOD_ADLER
lda a_hi
adc #0
bcc a_no_overflow
a_overflow:
; carry always set
lda a_lo
adc #65536-MOD_ADLER-1 ; compensate for carry
sta a_lo
lda a_hi
adc #0
sta a_hi
clc
a_no_overflow:
; b += a
lda b_lo
adc a_lo
sta b_lo
lda b_hi
adc a_hi
sta b_hi
; if (b >= 65521) b -= 65521
bcs b_overflow
lda b_lo
adc #65536-MOD_ADLER
lda b_hi
adc #0
bcc b_no_overflow
b_overflow:
; carry always set
lda b_lo
adc #65536-MOD_ADLER-1 ; compensate for carry
sta b_lo
lda b_hi
adc #0
sta b_hi
b_no_overflow:
inc negcountlo
bne byteloop
inc negcounthi
bne byteloop
sty src
rts
.endproc
Re: Checksum for savestates
tepples, you of all people should have remembered the posts above