Thread: Caesar's encoding problem

I have an exercise that is based in Caesar's encoding method.
I have to encode ASCII characters from:
32 to 127 with any key.
key may be [1, any integer].

I made two functions one for encoding end the second for decoding ....when i encode a word i cant get the original back after decoding....can you help?

I think MOD is the problem..

This is the encode function:

Code:

char *Ceasar_enc(char text[],int key) {
char *p=text;
long int temp;
 
 while(*p){	
/*    127 - 32= 96 ascii values plus 32  to start from 32 not    from 0 and end to ascii char 127 not 96
*/
  temp=(*p+key)%96+32;
  *p++=(char)temp;
 }

This is the decode function-i didnt write the decode formula because every time i write one i get faulse results

Code:

char *Ceasar_dec(char text[],int key) {
char *p=text;
long temp;

 while(*p){
  /* i cant find the correct decode formula */
  *p++=(char)temp;
 } 
 
 return text;
}

> temp=(*p+key)%96+32;
Write it out in long form

Code:

temp = *p;
temp = temp + key;
temp = temp % 96;
temp = temp + 32;

So to reverse it, we do

Code:

temp = *p;
temp = temp - 32;
// getting the idea yet?

no i dont understand
sorry

when the key +ascii value are bigger than 127 then:result mod 96
how i can get the original number?

The unsigned char datatype allows values from 0 to 255 - cast to unsigned char if you're worried about signed arithmetic.

Code:

char *Ceasar_enc(char text[],int key) {
char *p=text;
long int temp;
 
 while(*p){	 
  temp=(*p+key)%96+32;
  *p++=(char)temp;
 } 
 
 return text;
}
char *Ceasar_dec(char text[],int key) {
char *p=text;
long temp;

 while(*p){
  temp=*p;
  temp=abs(temp-32-key);
  temp=temp%96;
  *p++=(char)temp;
 } 
 
 return text;
}

input:theo
encoded output:5)&0

then when i use the second function i get:♣☼

i'm worried about the key..the key is any integer so i must use mod arithmetic

p.s:i encode characters in the interval 32-127(ascii values)
so an usnigned char array is useless...

thanks my friend but is working for keys 1-90 ,
for keys 91,92,93,94,95,96 i get chinese why?

If you haven't redone your algorithm yet, here's my version

Restate the encoding process:
Take any lower case letter ASCII value and add the key value.
Modulo the sum of that to get a range of 0 to 95 inclusive.
Now add 32 to the modulus result to get range of 32 to 127
using any key that is any positive integer greater than 0.
The result is called temp and will be casted back to a char
for the visual expression of the encoded message.

The following formula does that okay.

temp = (ASCIIvalueOfChar + key) % 96 + 32;

Develop some test cases:

let: ASCIIvalueOfChar = 97;
then:
temp = 34 if key = 1
temp = 32 if key = 95
temp = 32 if key = 191
temp = 32 if key = 287

Now reverse the process to decode:

Using the above test cases the trick is to get back to 97
if temp is 32 and key is 95, 191, or 287.

To solve this start out by finding mod.

mod = temp - 32 = (ASCIIvalueOfChar + key) % 96;

then restate the equation like this

ASCIIvalueOfChar + key = x*96 + mod; where x is a positive integer

and then this

ASCIIvalueOfChar = x*96 + mod - key;

now find x such that ASCIIvalueOfChar is in range 97 to 122;

Code:

for(x = 1; ASCIIvalueOfChar < 97 || ASCIIvalueOfChar > 122; ++x)
ASCIIvalueOfChar = x * 96 + mod - key;