SSH

SSH is the foundational protocol for securely accessing and operating remote computers over untrusted networks. It is also the infrastructure behind remote development, server operations, and automated connections.

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RFC 4251 OpenSSH Manual ssh(1) - OpenBSD manual pages

Permissions 0644 for id_rsa are too open.

When SSH on macOS reports Permissions 0644 for id_rsa are too open, tighten the private key to 0600.

0644 means other users can read the private key, so OpenSSH refuses to load it.
Run chmod 600 ~/.ssh/id_rsa. If the directory is also too open, run chmod 700 ~/.ssh.