Protect Alice's ~/.ssh/id_rsa.
Allow only the owner to read and write it; grant no group or other permissions.
Which mode is appropriate?
SSH private keys should not be readable by other users. Mode 600 gives the owner read/write and no group or other permissions.
Detailed explanation
chmod 644 ~/.ssh/id_rsaIncorrect. 644 allows group and other users to read the private key.
Incorrect. 644 allows group and other users to read the private key.
chmod 755 ~/.ssh/id_rsaIncorrect. 755 grants broad read and execute permissions.
Incorrect. 755 grants broad read and execute permissions.
chmod 666 ~/.ssh/id_rsaIncorrect. 666 allows every user to read and write the key.
Incorrect. 666 allows every user to read and write the key.
chmod 600 ~/.ssh/id_rsaCorrect. chmod 600 ~/.ssh/id_rsa protects the key from group and others.
Correct. chmod 600 ~/.ssh/id_rsa protects the key from group and others.
Try it yourself
An example you can run in a temporary verification environment.
stat -c '%a %n' ~/.ssh/id_rsaExpected result
鍵が存在すれば数値モード。変更は行わないKey points
- Mode 600
- Owner only
- Private-key protection
Notes
- Environment: OpenSSH 9.x／GNU coreutils 9.x
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Read the scope first
Check whether the command acts on the current shell, a new process, an existing process, or a file.
Verify the observable result
Use the supplied command and compare the output with the expected result.