The mention of the shadow file is in the README:
> S-Cache is an SUID-root executable that works by storing the secret in a file that is owned by root and only readable by root. Its security is thus comparable to the security of shadow passwords.
> S-Cache is an SUID-root executable that works by storing the secret in a file that is owned by root and only readable by root. Its security is thus comparable to the security of shadow passwords.