Why does 'a' prematurely end this loop? - c

Question

Pointers rather than straight answers if you will please.
This loop does some manipulation on chars and outputs a ciphertext c based on a key k and some plaintext p.
When 'a' or 'A' comes up in the plaintext, the program will output that letter as expected but then end the loop prematurely.
p suddenly becomes just one character long, this character being 1.
while (i < strlen(p))
{
char stdp = p[i];
char stdk = k[j];
if (isalpha(stdp))
{
if (islower(stdp))
{
p[i] = stdp - 'a';
Aa = 0;
}
else
{
p[i] = stdp - 'A';
Aa = 1;
}
if (islower(k[j]))
{
k[j] = stdk - 'a';
}
else
{
k[j] = stdk - 'A';
}
}
if (isalpha(stdp))
{
c[i] = ((p[i] + k[j]) % 26);
}
else
{
c[i] = p[i];
}
if (isalpha(stdp))
{
if (Aa == 1)
{
c[i] = c[i] + 'A';
}
else if (Aa == 0)
{
c[i] = c[i] + 'a';
}
}
if (isalpha(stdp))
{
if (j + 1 == kk)
{
j = (j + 1) % kk;
strcpy(k, argv[1]);
}
else
{
j++;
}
}
i++;
}

Answer 1

I have now solved the issues I was facing with the program terminating early.
Most of the issues seem to have been solved by stopping the constant refs to strlen(p) and just assigning that to a static variable. I have tried to tidy up the code as suggested.
int main(int argc, char *argv[])
{
if (argc != 2 || argv[1] == NULL)//no more than 1 arg and not empty
{
printf("Command requires one argument to run.\n");
return 1;
}
char *k = malloc(100);
strcpy(k, argv[1]);
int kInt = strlen(k);
for (int i = 0, n = kInt; i < n ; i++)// iterating though the key to check it is alphabetic
{
if (isalpha(k[i]) == false)
{
printf("Make sure key is alphabetic!\n");
return 1;
}
}
printf ("plaintext: ");
char *p = malloc(100);
strcpy(p, get_string());
int pInt = strlen(p);
int i = 0;
int j = 0;
int Aa = 0;
char *c = malloc(100);
while (j < kInt)
{
if (islower(k[j]))
{
k[j] = k[j] - 'a';
}
else
{
k[j] = k[j] - 'A';
}
j++;
}
j = 0;
while (i < pInt)
{
if (isalpha(p[i]))
{
if (islower(p[i]))
{
p[i] = p[i] - 'a';
Aa = 0;
}
else
{
p[i] = p[i] - 'A';
Aa = 1;
}
c[i] = ((p[i] + k[j]) % 26);
if (Aa == 1)
{
c[i] = c[i] + 'A';
}
else if (Aa == 0)
{
c[i] = c[i] + 'a';
}
if (j + 1 == kInt)
{
j = (j+1) % kInt;
}
else
{
j++;
}
}
else if (isalpha(p[i]) != true)
{
c[i] = p[i];
}
i++;
}
printf("ciphertext: %s\n", c);
free(c);
free(k);
free(p);
}