reversing the Mod operator

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I am writing an encryption program that takes a digit and yields its modulus.
For example, 7 Mod 4 yields 3. How can I reverse this procedure and get the
original digit back in order to write a decryption program?
 
I am writing an encryption program that takes a digit and yields its modulus.
For example, 7 Mod 4 yields 3. How can I reverse this procedure and get the
original digit back in order to write a decryption program?
Click to expand...

Since there can be same result for multiple numbers (e.g. 7 MOD 4 = 3
and 3 MOD 4 = 3) there is no way.

BTW. Why would you even consider this? You have lots of crypting
algorithms already available that do good job.
 
This is for a class that I am taking. I'm just learning to program. The
instructions said "Your program should read a four-digit Integer entered by
the user and encrypt it as follows: Replace each digit by (the sum of that
digit and 7) modulo 10. Then write a program that inputs an encryption
four-digit Integer and decrypts it to form the original number." Do you think
I could be miss-understanding the instructions? Or are you reading it as the
same meaning that I am?
 
I agree, for crypting.

Take a look at the post by "IZZY" about 128 bit encryption function he
posted.

Search this newsgroup for
Variable = EncryptString128Bit(txt_Password.Text, EncryptionKey)

Miro
 
Candace said:
This is for a class that I am taking. I'm just learning to program. The
instructions said "Your program should read a four-digit Integer entered by
the user and encrypt it as follows: Replace each digit by (the sum of that
digit and 7) modulo 10. Then write a program that inputs an encryption
four-digit Integer and decrypts it to form the original number." Do you think
I could be miss-understanding the instructions? Or are you reading it as the
same meaning that I am?
Click to expand...

Try looking at the problem a little differently, and I think you'll get
it.

What Mod 10 actually does to a whole number is remove everything except
the rightmost digit, right?

Suppose your original digit is 5. You have to add 7. Try incrementing
5 by 1, seven times, in your head - but remember, you can never reach
double digits. When you pass nine, start back at zero. 5 becomes 6,
7, 8, 9, 0, 1, 2. So 2 is your encrypted version of 5.

To decrypt: 2 becomes 1, 0, 9, 8, 7, 6, 5.

Now see if you can find a mathematical way to decrypt a single digit in
*one* line of code. If you can't, I'll tell you - but you learn more
this way; gotta exercise the brain to get really good at this stuff. :)
 
I see. This is a classic Caesar encryption, a method that is more than
2000 years old. :)

The encryption works by shifting the alphabet a certain number of steps,
for example using a shift of +1 would turn "BAR" into "CBS". To decrypt
you simply use the negative shift, i.e. the shift -1 turns "CBS" into "BAR".

The characters that are shifted outside of the alphabet just wraps over
at the other end, that's where the modulo comes in.

With this lesson of history and cryptography, I hope that you understand
what your program is actually supposed to be doing, only using digits
instead of letters. :)
 
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