Caesar cipher encryption-decryption

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#include<stdio.h>

#include<conio.h>

void main()

{

char pt[100],ct[100],dt[100];

int i,key;

clrscr();

printf("ENter the plain text and press ']' to terminate\n");

for(i=0; i<100; i++)

{

            scanf("%c",&pt[i]);

            if(pt[i]==']')

            {

                        pt[i]='\0';

                        break;

            }

}

printf("ENter the key\n");

scanf("%d",&key);

for(i=0; pt[i]!='\0'; i++)

{

            if(pt[i]>64 && pt[i]<91)

                        ct[i]=(pt[i]-65+key)%26+65;

            if(pt[i]>96 && pt[i]<123)

                        ct[i]=(pt[i]-97+key)%26+97;

            if(pt[i]==32)

                        ct[i]=pt[i];

}

printf("Plain text :\n");

for(i=0; pt[i]!='\0'; i++)

printf("%c",pt[i]);

printf("\nCeasar cipher:\n");

for(i=0; ct[i]!='\0'; i++)

printf("%c",ct[i]);

printf("\n");

for(key=0; key<26; key++)

{

            printf("For key %d ",key);

            for(i=0; ct[i]!='\0'; i++)

            {

                        if(ct[i]>64 && ct[i]<91)

                        {

                                    dt[i]=(ct[i]-39-key)%26+65;

                                    printf("%c",dt[i]);

                        }

                        if(ct[i]>96 && ct[i]<123)

                        {

                                    dt[i]=(ct[i]-71-key)%26+97;

                                    printf("%c",dt[i]);

                        }

                        if(ct[i]==32)

                        {

                                    dt[i]=ct[i];

                                    printf("%c",dt[i]);

                        }

            }

                        printf("\n");

}

getch();

}

output: