Caesar cipher with string as a key

Question

I need some help in caesar cipher, here's my keygen and encrypt function

char *CSK_keygen(char key[]) {
    int i, j, k, n;
    
    strcat(key, letters);
    n = strlen(key);
    for(i=0; i<n; i++)
        for(j = i+1; j<n;)
            if(key[i] == key[j]) {
                for(k=j; k<n; k++)
                    key[k]=key[k+1];
                n--;
            }
            else
                j++;
    return key;
}

void CSK_encrypt(char msg[])
{
    char key[26], enc[25];
    int i, j, k;
    
    printf("Key: ");
    scanf("%s", &key);
    
    CSK_keygen(key);
    for(i=0; i<strlen(msg); i++)
        for(j=0; j<strlen(letters); j++)
            if(msg[i] == letters[j]) {
                k = j%strlen(key);
                enc[i] = key[k];
                break;
            }
            
    printf("Enc: ");
    for (i=0; i<strlen(msg); i++)
    printf("%c", enc[i]);
    printf("\n");
}

And here is the decrypt function which is not working properly, maybe the formula is not correct?

void CSK_decrypt(char msg[])
{
    char key[26], enc[25];
    int i, j, k;
    
    printf("Key: ");
    scanf("%s", &key);
    
    CSK_keygen(key);
    for(i=0; i<strlen(msg); i++)
        for(j=0; j<strlen(letters); j++)
            if(msg[i] == letters[j]) {
                k = j%strlen(key) - k % 26;
                enc[i] = key[k];
                break;
            }
            
    printf("Denc: ");
    for (i=0; i<strlen(msg); i++)
    printf("%c", enc[i]);
    printf("\n");
}

Can someone pleae tellme what is wrong in the formula and what should I do?