encryption behaviour unexpected | Together C & C++ | Page 1

slate wagon Oct 10, 2022, 6:05 PM

#

Hi there trying to make a text cipher for cs50 ( coding beginner) , i didn't want to cheat and watch a guide . i Thought i had figured out a solution but i am getting unexpected behaviour.

the program takes a command line argument i.e the key for the cipher
followed by the message to be encrypted

i am subtracting the key from the regular alphabet to get the distance between ascii characters so i can then re apply the distance to the the message in order to encrypt it as per the key.

i have double checked the math and the distance is calculated properly yet upon re applying it something is going wrong and i cant figure out what.

string message = get_string("Plaintext :");
int meslen = strlen(message);
for(int i=0;i<meslen;i++){
if(isupper(message[i])){
message[i]+=32;
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]= message[i]-alphdiff[a];

}
else{}
}
message[i]-=32;
}
else if(islower(message[i])){
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]=message[i]-alphdiff[a];

}
else{}
}

}
else{message[i]=message[i];}

}
printf("%s",message);
}

rare fogBOT Oct 10, 2022, 6:05 PM

#

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novel cradle Oct 10, 2022, 6:05 PM

#

string message = get_string("Plaintext :");
int meslen = strlen(message);
for(int i=0;i<meslen;i++){
if(isupper(message[i])){
message[i]+=32;
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]= message[i]-alphdiff[a];

}
else{}
}
message[i]-=32;
}
else if(islower(message[i])){
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]=message[i]-alphdiff[a];

}
else{}
}

}
else{message[i]=message[i];}

}
printf("%s",message);
}

novel cradle Oct 10, 2022, 6:05 PM

#

novel cradle string message = get_string("Plaintext :"); int meslen = strlen(message); for(in...

!f

novel cradle Oct 10, 2022, 6:06 PM

#

novel cradle string message = get_string("Plaintext :"); int meslen = strlen(message); for(in...

!format

#

noo

#

string message = get_string("Plaintext :");
int meslen = strlen(message);
for (int i = 0; i < meslen; i++) {
  if (isupper(message[i])) {
    message[i] += 32;
    for (int a = 0; a < alphlen; a++) {
      if (message[i] == alphabet[a]) {
        message[i] = message[i] - alphdiff[a];

      } else {}
    }
    message[i] -= 32;
  } else if (islower(message[i])) {
    for (int a = 0; a < alphlen; a++) {
      if (message[i] == alphabet[a]) {
        message[i] = message[i] - alphdiff[a];

      } else {}
    }

  } else {
    message[i] = message[i];
  }

}
printf("%s", message);
}

#

i assume alphlen is 26 ?

slate wagon Oct 10, 2022, 6:09 PM

#

yeah

#

uhh can ia sk u how to format like that im new here

novel cradle Oct 10, 2022, 6:09 PM

#

int x

#

and alphdiff is what ?

slate wagon Oct 10, 2022, 6:11 PM

#

alphdiff is alphabet[i] - key[i]

#

running in a loop to get 26 variations

novel cradle Oct 10, 2022, 6:11 PM

#

post the full code

slate wagon Oct 10, 2022, 6:11 PM

#

sure

#

do i wrap it in the '''c'''

novel cradle Oct 10, 2022, 6:12 PM

#

`

#

backticks

#

not '

slate wagon Oct 10, 2022, 6:13 PM

#

cc c'''#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main(int argc, string argv[])
{
int keylen=strlen(argv[1]);
string key = argv[1];
int alphdiff[26] ;
if(keylen!=26){
printf("key must contain 26 Letters");
return 1;
}
else if(argc!=2){printf("Usage: ./substitution key");
return 1;

}//all works till here
else{
for(int i=0; i<keylen;i++){
if(isupper(key[i])){
key[i]+=32;
}

}
string alphabet= "abcdefghijklmnopqrstuvwxyz";
int alphlen= strlen(alphabet);
for(int i=0;i<keylen;i++){
alphdiff[i]=alphabet[i]-key[i];// havent tested

}
for(int i=0;i<alphlen;i++){
    printf("%i  %c  %c", alphdiff[i], alphabet[i],key[i]);
    printf("\n");
}

string message = get_string("Plaintext :");
int meslen = strlen(message);
for(int i=0;i<meslen;i++){
if(isupper(message[i])){
message[i]+=32;
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]= message[i]-alphdiff[a];

}
else{}
}
message[i]-=32;
}
else if(islower(message[i])){
for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
message[i]=message[i]-alphdiff[a];

}
else{}
}

}
else{message[i]=message[i];

#

uhm ...

novel cradle Oct 10, 2022, 6:14 PM

#

3x ` followed by a c
your code
3x `

slate wagon Oct 10, 2022, 6:29 PM

#

include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main(int argc, string argv[])
{
int keylen=strlen(argv[1]);
string key = argv[1];
int alphdiff[26] ;
if(keylen!=26){
    printf("key must contain 26 Letters");
    return 1;
}
else if(argc!=2){printf("Usage: ./substitution key");
return 1;


}//all works till here
else{
    for(int i=0; i<keylen;i++){
        if(isupper(key[i])){
 key[i]+=32;
        }


    }

}
string  alphabet= "abcdefghijklmnopqrstuvwxyz";
int alphlen= strlen(alphabet);
    for(int i=0;i<keylen;i++){
        alphdiff[i]=alphabet[i]-key[i];// havent tested


    }
    for(int i=0;i<alphlen;i++){
        printf("%i  %c  %c", alphdiff[i], alphabet[i],key[i]);
        printf("\n");
    }
string message = get_string("Plaintext :");
int meslen = strlen(message);
for(int i=0;i<meslen;i++){
    if(isupper(message[i])){
        message[i]+=32;
        for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
    message[i]= message[i]-alphdiff[a];


}
else{}
        }
        message[i]-=32;
    }
    else if(islower(message[i])){
        for(int a=0; a<alphlen;a++){
if(message[i]==alphabet[a]){
    message[i]=message[i]-alphdiff[a];

}
else{}
}

    }
    else{message[i]=message[i];}




}
printf("%s",message);
}```

novel cradle Oct 10, 2022, 6:29 PM

#

is it supposed to subsitute each letter of message with key ?

slate wagon Oct 10, 2022, 6:29 PM

#

exactly

novel cradle Oct 10, 2022, 6:30 PM

#

encrypted[i] = key[tolower(message[i]) - 'a']

#

e.g. if message = "ABC" and key = "zyx"

i = 0

encrypted[i] = key[tolower(message[i]) - 'a'] 
encrypted[0] = key[tolower(message[0]) - 'a']
encrypted[0] = key[tolower('A') - 'a']
encrypted[0] = key['a' - 'a']
encrypted[0] = key[0]
encrypted[0] = 'z'

...
encrypted[1] = 'y'
encrypted[2] = 'x'

slate wagon Oct 10, 2022, 6:36 PM

#

i dont follow what its doing man

novel cradle Oct 10, 2022, 6:36 PM

#

what part dont you get

slate wagon Oct 10, 2022, 6:38 PM

#

i dont see how this takes the users input of a key and transforms the second input i.e the message they type when prompted into a new encrypted message, because u have no reference to the inputted text

#

you are not using the distance between characters in intigers?

novel cradle Oct 10, 2022, 6:39 PM

#

message is the user input
key is well yea the key

#

each letter of message is replaced with the letter in key

slate wagon Oct 10, 2022, 6:40 PM

#

and placed into a new array encrypted

#

?

novel cradle Oct 10, 2022, 6:40 PM

#

yea

#

you can also place it in message[i] instead of encrypted[i]

slate wagon Oct 10, 2022, 6:40 PM

#

do you see where the error is in my code?

#

or is it a downright mess

#

lol

novel cradle Oct 10, 2022, 6:41 PM

#

its a mess yea

slate wagon Oct 10, 2022, 6:41 PM

#

haha ok

#

can i ask you what the - 'a' is

#

or is that just for me

novel cradle Oct 10, 2022, 6:42 PM

#

its so we can index at 0 where the key string starts

#

'a' - 'a' = 0
'b' - 'a' = 1
'c' - 'a' = 2
...
'z' - 'a' = 25

slate wagon Oct 10, 2022, 6:46 PM

#

can i try to explain my logic to you if you have time ? i dont wanna be a bother so feel free to say no im sure with more experience ill understand better but i rlly dont see why my program isnt running as expected

novel cradle Oct 10, 2022, 6:46 PM

#

you can explain it yea

slate wagon Oct 10, 2022, 6:46 PM

#

right

novel cradle Oct 10, 2022, 6:46 PM

#

but i wont debug or read it troll

slate wagon Oct 10, 2022, 6:46 PM

#

no problem

#

ill explain in english

#

i take the key and set it into a string and i also create a string of the regular alphabet to start

#

i run a for loop where every character of the alphabet is deducted by every character of the key and store it into a new array alphdiff

#

then i run through a nester for where for each character of the message input i check wether it is the same as the alphabet character and when it is i re add the diffirence stored in alph diff

#

/cann u see any problem with it?

novel cradle Oct 10, 2022, 6:48 PM

#

novel cradle ```c 'a' - 'a' = 0 'b' - 'a' = 1 'c' - 'a' = 2 ... 'z' - 'a' = 25 ```

(this is esentially alphdiff )
letter in message MINUS the first letter in the alphabet is the offset or the difference between those two

slate wagon Oct 10, 2022, 6:49 PM

#

but you had to manually set that right

#

?

#

i wanted to avoid that

#

took it as like a challenge to do it automatic

#

or am i missing something

novel cradle Oct 10, 2022, 6:50 PM

#

i dont set it manually

#

the difference is calced on the fly

#

message[i] - 'a'

slate wagon Oct 10, 2022, 6:51 PM

#

so conceptually were doing the same thing

novel cradle Oct 10, 2022, 6:51 PM

#

ye

slate wagon Oct 10, 2022, 6:52 PM

#

i have to assume then my syntax is fucking it up

#

so im gonna try to understand your syntax betetr

#

you take key at index msg as your and set it as encrypted at index 0

#

is that right?

novel cradle Oct 10, 2022, 6:54 PM

#

at index alphadiff

#

message[i] would be 65 if message[i] = 'a'

slate wagon Oct 10, 2022, 6:55 PM

#

oh i c you are bypassing that whole bit by using the -a- to set it straight into alphadiff?

novel cradle Oct 10, 2022, 6:55 PM

#

our key is only 26 letters long id index out of bounds

#

if i subtract it by 'a' i get a 0

slate wagon Oct 10, 2022, 6:56 PM

#

ahhhaaaaaaaaaaaaaaa

#

im so sry i didnt realise the - 'a' was a direct subtraction i thought it was some unknown to me way of setting parameters of an array

#

thank you so much man its clear to me now

#

thank you for your patience ill catch you later

rare fogBOT Oct 10, 2022, 7:03 PM

#

@slate wagon Has your question been resolved? If so, run !solved :)

rare fogBOT Oct 11, 2022, 7:15 AM

#

This question thread is being automatically closed. If your question is not answered feel free to bump the post or re-ask. Take a look at !howto ask for tips on improving your question.

#encryption behaviour unexpected