System Security : C code for Additive cipher / Shift cipher / Caeser cipher ( can be executed in Linux )

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plain text is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caeser, who used it in his private correspondence.
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Given below is the C code for the same.

//Aim: To implement additive cipher in C //Program: #include<stdio.h> void main() { char c[20]; int a1[20]; int a2[20]; int n=0; int k=0; printf("\nPlease Enter the key value: "); scanf("%d",&k); printf("\nHow many letters in the text? : "); scanf("%d",&n); int i; printf("\nPlease enter a word to be encoded:"); for(i=-1;i<n;++i) { scanf("%c",&c[i]); } int temp=0; int j=0; int count=0; for(i=0;i<=n;++i) { l1:temp=(int)c[i]; if(c[i]==' ') { i=i+1; count=count+1; goto l1; } if(65<=temp && temp<=91) a1[j]=temp%65; else if(97<=temp<=123) a1[j]=temp%97; a2[j]=(a1[j]+k)%26; ++j; } n=n-count; printf("\nThe encoded cipher is: "); char c2[20]; for(i=0;i<n;++i) { c2[i]=(char)(a2[i]+97); printf("%c",c2[i]); } printf("\n\nDecoding it receiver side "); int a3[20]; for(i=0;i<n;++i) a3[i]=(int)(c2[i]-97); for(i=0;i<n;++i) { if(a3[i]>=k) a3[i]=a3[i]-k; else a3[i]=a3[i]-15+26; } printf("\nThe text is: "); for(i=0;i<n;++i) { printf("%c",(char)(a3[i]+97)); } } /*OUTPUT Please Enter the key value: 15 How many letters in the text? : 5 Please enter a word to be encoded:hello The encoded cipher is: wtaad Decoding it receiver side The text is: hello Please Enter the key value: 15 How many letters in the text? : 12 Please enter a word to be encoded:good morning The encoded cipher is: vddsbdgcxcv Decoding it receiver side The text is: goodmorning */

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