Implementation of mono-alphabetic substitution based encryption

substitution.c

#include <stdbool.h> 
#include "substitution.h"

void genkey(char * key){
    int i;
    strncpy(key, "abcdefghijklmnopqrstuvwxyz", 27);
    srand(time(NULL));
    for(i = 0; i < 100; i++){
        int k, l;
        k = rand()%26; l = rand()%26;
        char tmp = key[k];
        key[k] = key[l];
        key[l] = tmp;
    }
}

void encrypt(const char * text, const char * key, char * ciphertext){
    if(! check(key)){
        ciphertext[0] = '\0';
        return;
    }
    int i;
    for(i = 0; text[i] != 0; i++){

        if( (text[i] >= 'a') && ( text[i] <= 'z') ){
            short rang = text[i] - 'a';
            ciphertext[i] = key[rang];
        }
        else if( (text[i] >= 'A') && ( text[i] <= 'Z') ){
            short rang = text[i] - 'A';
            ciphertext[i] = toupper(key[rang]);
        }
        else
            ciphertext[i] = text[i];
    }
    ciphertext[i] = '\0';
}

void decrypt(const char * ciphertext, const char * key, char * text){
    if(! check(key)){
        text[0] = '\0';
        return;
    }
    int i;
    for(i=0; ciphertext[i] != '\0'; i++){

        if( (ciphertext[i] >= 'a') && ( ciphertext[i] <= 'z') ){
            char * pos = strchr(key, ciphertext[i]);
            short rang = pos - key;
            text[i] = 'a' + rang;
        }
        else if( (ciphertext[i] >= 'A') && ( ciphertext[i] <= 'Z') ){
            char * pos = strchr(key, tolower(ciphertext[i]));
            short rang = pos - key;
            text[i] = 'A' + rang;
        }
        else
            text[i] = ciphertext[i];
    }
    text[i] = '\0';
}

substitution.h

#include <stdbool.h> 

void genkey(char * key);
void encrypt(const char * text, const char * key, char * ciphertext);
void decrypt(const char * ciphertext, const char * key, char * text);
bool check(const char * key);