Why Unix File Permissions Like 600, 644, and 755 Follow a Security Principle

Unix-based operating systems use a 3x3 grid of permissions — owner, group, and others, each with read, write, and execute rights — to control file access. Numeric codes like 600, 644, and 755 compress these permissions into octal digits, where read equals 4, write equals 2, and execute equals 1. A private SSH key set to 600 means only the owner can read or write it, while a script at 755 allows anyone to read and run it but not modify it. These conventions reflect the principle of least privilege, which limits access to only what is strictly necessary for a given file's purpose. Tools like OpenSSH actively enforce these rules, refusing to load a private key if its permissions are too open, treating even the possibility of unauthorized access as a security risk.