Temporarily storing piped input in C command line utility

I have been working on creating a coreutils style C program that does a Cesar shift cipher on a given input. Here is the link to the repository. Right now, the program only reads a specified file and shifts each character before printing it. I am wanting to add the ability to pipe input through my program like you would see in the wc command.

$ echo "pants" | ccipher --key 7
whuaz

When an encryption key is specified, 1-26, handling piped input would be trivial. Each character would be gathered with getchar(), shifted by the key and putchar() would print it to stdout.

In my program though, when a key isn't specified it loops through the input with keys 1-26 and prints to stdout.

$ cat input
pants
$ ccipher input
qbout
rcpvu
sdqwv
terxw
ufsyx
vgtzy
whuaz
xivba
yjwcb
zkxdc
alyed
bmzfe
cnagf
dobhg
epcih
fqdji
grekj
hsflk
itgml
juhnm
kvion
lwjpo
mxkqp
nylrq
ozmsr

This is easy with a file. In a file, the c program loops through the input file and calls rewind() after each iteration of the loop.

void allKeys(FILE** file) {
  long key;
  char c;
  for (key = 1; key < 26; key++) {
    while ((c = fgetc(*file)) != EOF) {
      rotateChar((char)c, key);
    }
    rewind(*file);
  }
  fclose(*file);
}

The problem that I am having is something that the wc utility wouldn't have to deal with. That is storing input from stdin for the subsequent times the input needs to be ran through.

I have came across a couple solutions that people said to use when storing input temporarily. The first was having a dynamic buffer that is resized whenever it is filled up. This seems great, but with the drawback being how the program could be cat'd a file larger than the amount of available ram and take up all of the system resources.

The second solution that I came across was temp files. With a temp file, the program would store the input in a temp file and be able to rewind it like any other file.

My goal with this stack overflow question was to see if there are any other and potentially better ways to solve this problem. If not, which one of the solutions I have listed should I use?