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chmod 400 — Read-Only for the Owner: SSH Keys and PEM Files

Convert Linux file permissions between rwx checkboxes, octal numbers and symbolic notation with a ready chmod command.

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Result

This page opens the calculator preset to 400: a single ticked checkbox — owner read. The octal digits are 4-0-0 and the symbolic notation is -r--------: the strictest practical mode a usable file can have.

400 is best known from SSH: if your private key is readable by anyone else, ssh aborts with 'WARNING: UNPROTECTED PRIVATE KEY FILE!' and refuses to use it. That is why AWS documentation tells you to run chmod 400 key-pair.pem before the first ssh -i connection, and why id_rsa/id_ed25519 must be 400 or 600. The read-only bit also protects the key from accidental overwrites by you or your scripts.

Choose 400 over 600 when the file should never change: downloaded .pem key pairs, certificate bundles, archived credentials. If you later need to edit or delete-protect semantics differ, add the write bit back with the owner-write checkbox and the command updates to chmod 600 instantly.

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Accepted Formats & Rules

  • Nine rwx checkboxes for owner, group and others synced live with the 3-digit octal value.
  • Symbolic notation (-rwxr-xr-x) plus a per-class breakdown showing how each digit is composed from 4+2+1.
  • Ready-to-copy chmod command and safety warnings for world-writable modes.

Key Benefits

✓ Works both ways: tick checkboxes to get the octal number, or type an octal like 755 and instantly see which permissions it grants.
✓ Every bit is explained — you see exactly why 7 means read + write + execute and what the -rwxr-xr-x string encodes.
✓ The ready chmod command and warnings about risky modes like 777 help you avoid opening files to the whole world by accident.

Use Cases

  • • Work out the right chmod value for a web server folder without accidentally granting write access to the whole world.
  • • Decode a cryptic -rwxr-xr-x string from ls -l output into plain owner/group/others permissions.
  • • Fix the SSH 'unprotected private key file' error by setting the key to 400 with a ready-made command.