Implemnetation of vignere cipher

vigenere.c

#include "vigenere.h"

void vigenereEnc(const char * text, const char * key, char * ciphertext){
    unsigned int i, size = strlen(key);

    for(i = 0; text[i] != '\0'; i++){
        if( (text[i] >= 'a') && (text[i] <= 'z') ){
            int rang = (text[i] + key[i % size] - 2 * 'a') % 26;
            ciphertext[i] = 'a' + rang;
        }
        else if( (text[i] >= 'A') && (text[i] <= 'Z') ){
            int rang = (text[i] + key[i % size] - 'a' - 'A') % 26;
            ciphertext[i] = 'A' + rang;
        }
        else
            ciphertext[i] = text[i];
    }
    ciphertext[i] = '\0';
}

void vigenereDec(const char * ciphertext, const char * key, char * text){
    unsigned int i, size = strlen(key);
    char keytemp[size + 1];
    for(i = 0; key[i] != 0; i++){
        int rang =  (26 - (key[i] - 'a')) % 26;
        keytemp[i] = rang + 'a';
    }
    keytemp[i] = '\0';
    vigenereEnc(ciphertext, keytemp, text);
}

vigenere.h

void vigenereEnc(const char * text, const char * key, char * ciphertext);
void vigenereDec(const char * ciphertext, const char * key, char * text);