System Security : C code for Hill Cipher (should be executed in linux with gcc).

//Aim:To implement Hill Cipher in C //Program: #include<stdio.h> int check(int x) { if(x%3==0) return 0; int a=x/3; int b=3*(a+1); int c=b-x; return c; } void main() { int l,i,j; int temp1; int k[3][3]; int p[3][1]; int c[3][1]; char ch; printf("\nThe cipher has a key of length 9. ie. a 3*3 matrix.\nEnter the 9 character key. "); for(i=0;i<3;++i) { for(j=0;j<3;++j) { scanf("%c",&ch); if(65<=ch && ch<=91) k[i][j]=(int)ch%65; else k[i][j]=(int)ch%97; } } for(i=0;i<3;++i) { for(j=0;j<3;++j) { printf("%d ",k[i][j]); } printf("\n"); } printf("\nEnter the length of string to be encoded(without spaces). "); scanf("%d",&l); temp1=check(l); if(temp1>0) printf("You have to enter %d bogus characters.",temp1); char pi[l+temp1]; printf("\nEnter the string. "); for(i=-1;i<l+temp1;++i) { scanf("%c",&pi[i]); } int temp2=l; int n=(l+temp1)/3; int temp3; int flag=0; int count; printf("\n\nThe encoded cipher is : "); while(n>0) { count=0; for(i=flag;i<flag+3;++i) { if(65<=pi[i] && pi[i]<=91) temp3=(int)pi[i]%65; else temp3=(int)pi[i]%97; p[count][0]=temp3; count=count+1; } int k1; for(i=0;i<3;++i) c[i][0]=0; for(i=0;i<3;++i) { for(j=0;j<1;++j) { for(k1=0;k1<3;++k1) c[i][j]+=k[i][k1]*p[k1][j]; } } for(i=0;i<3;++i) { c[i][0]=c[i][0]%26; printf("%c ",(char)(c[i][0]+65)); } n=n-1; flag=flag+3; } } /*OUTPUT [adi@localhost Desktop]$ gcc hill.c [adi@localhost Desktop]$ ./a.out The cipher has a key of length 9. ie. a 3*3 matrix. Enter the 9 character key. GYBNQKURP 6 24 1 13 16 10 20 17 15 Enter the length of string to be encoded(without spaces). 6 Enter the string. actCAt The encoded cipher is : P O H F I N [adi@localhost Desktop]$ gcc hill.c [adi@localhost Desktop]$ ./a.out The cipher has a key of length 9. ie. a 3*3 matrix. Enter the 9 character key. GYBNQKURP 6 24 1 13 16 10 20 17 15 Enter the length of string to be encoded(without spaces). 7 You have to enter 2 bogus characters. Enter the string. actcatact The encoded cipher is : P O H F I N P O H */

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