C++ - 220 241 chars, 3 lines

#include<cstdlib>
#include<cstdio>
#define a scanf("%s"
char i,s[99],t[99];int main(){a,t);a,s);if(t[0]>68){for(;s[i];++i)s[i]=(s[i]+(t[i]=rand()%26+65))%26+65;puts(t);}else for(a,t);s[i];++i){s[i]=65+t[i]-s[i];if(s[i]<65)s[i]+=26;}puts(s);}

Edit 1- The MSVS standard library seems to include a lot of unnecessary files which meant that ios had all the includes that I needed but this didn't work with other compilers. Changed ios for the actual files that the functions needed appear in cstdlib and cstdio. Thanks to Ilmari Karonen for pointing this out.

C++ - 220 241 chars, 4 lines

#include<cstdlib>
#include<cstdio>
#define a scanf("%s"
char i,s[99],t[99];int main(){a,t);a,s);if(t[0]>68){for(;s[i];++i)s[i]=(s[i]+(t[i]=rand()%26+65))%26+65;puts(t);}else for(a,t);s[i];++i){s[i]=65+t[i]-s[i];if(s[i]<65)s[i]+=26;}puts(s);}

Edit 1- The MSVS standard library seems to include a lot of unnecessary files which meant that ios had all the includes that I needed but this didn't work with other compilers. Changed ios for the actual files that the functions needed appear in cstdlib and cstdio. Thanks to Ilmari Karonen for pointing this out.

C++ - 220 chars, 3 lines

#include<ios>
#define a scanf("%s"
char i,s[99],t[99];int main(){a,t);a,s);if(t[0]>68){for(;s[i];++i)s[i]=(s[i]+(t[i]=rand()%26+65))%26+65;puts(t);}else for(a,t);s[i];++i){s[i]=65+t[i]-s[i];if(s[i]<65)s[i]+=26;}puts(s);}

C++ - 220 241 chars, 3 lines

#include<cstdlib>
#include<cstdio>
#define a scanf("%s"
char i,s[99],t[99];int main(){a,t);a,s);if(t[0]>68){for(;s[i];++i)s[i]=(s[i]+(t[i]=rand()%26+65))%26+65;puts(t);}else for(a,t);s[i];++i){s[i]=65+t[i]-s[i];if(s[i]<65)s[i]+=26;}puts(s);}

Edit 1- The MSVS standard library seems to include a lot of unnecessary files which meant that ios had all the includes that I needed but this didn't work with other compilers. Changed ios for the actual files that the functions needed appear in cstdlib and cstdio. Thanks to Ilmari Karonen for pointing this out.