Let us consider that I have a file /home/base/alpha.txt with permission 777 i.e. -rwxrwxrwx in user@host.
Now, from system host1 as user1 i.e. user1@host1, I try to pull the file mentioned above using the following command:
scp user@host:/home/base/alpha.txt .
As per file permissions 777, everyone has read-write-execute permission i.e. anyone should be able to read, write and execute the mentioned file. Why does Unix ask for the user@host password then?
The file permissions apply on the host. The file still needs to be made visible externally; you’re using SSH (via scp) to do so, and it’s SSH which is asking for your password. This happens before anything involving the target file at all; the SSH server authenticates the connecting user, and only after that’s completed (successfully) is scp involved on the target host and the target file accessed.
If you want passwordless SSH access, you can set up keys to do so.
Incidentally, since you’re logging in as user on host, the relevant permissions for alpha.txt are only those which apply to user. If user is the owner of alpha.txt, alpha.txt only needs to be readable by its owner.
user’s relationship to alpha.txt is, which is why I wrote that the relevant permissions are those which apply to user (not “owner”, the specific user you’re accessing the file as). If user is alpha.txt’s owner, then the owner permissions apply; if user is a member of alpha.txt’s owning group, then the group permissions apply; otherwise, the other permissions apply.