Reverse Modulus?

[hipopony66]

I take a number (3) and do the following to it:

Num1= (3+7)%10


This gives us 10%10. The remainded of 10/10 is 0.
Now how can I work backward from this answer to get back to 3. I need to somehow "reverse" the modulus operation. This is for a simple Encryption/Decryption program. Any help appreciated.

[wossname]

You can't.

For the simple reason that 13, 23, 33, 43 and so on would all give you the same answer - 0.

(3+7)%10 = 0
(13+7)%10 = 0
(23+7)%10 = 0
(33+7)%10 = 0
(43+7)%10 = 0

[wossname]

So you see, there are infinitely many X values for any given Y value.

[NotLKH]

Hmmm,

If your encryption is, given Y, F(y) = (Y + (10 - Y)) Mod 10,

Then not only do you have the ultimate secure encryption, but you must also get awesome compression ratios!
It's Brilliant!

[dglienna]

and impossible to decrypt afterwards!

[wossname]

No, what you actually have is a very poor hash function.

SHA is teh h@sh pwner.

[Acidic]

It's something I'd never thought of to use in encryption, and it is a very nice one way encryption method, but it can be bruteforced quite easily. It really is quite nice though.

[NotLKH]

Originally posted by dglienna
and impossible to decrypt afterwards!

Build it as a Webbrowser app!
Then, to decrypt, just hit the back button!

[alkatran]

Originally posted by NotLKH
pathfinder

We'll meet again! You just wait!