What Is the authorized_keys File in Linux?

The authorized_keys file in a Linux operating system user profile serves as a fundamental security component for the Secure Shell (SSH) protocol. Its primary purpose is to store the public keys of clients who are granted permission to log into that specific user account using SSH key-based authentication. By replacing or supplementing traditional password logins with asymmetric cryptography, this file enables secure, automated, and passwordless remote access.

How the authorized_keys File Works

SSH authentication relies on asymmetric public-key cryptography, which uses a mathematically linked key pair: a private key and a public key.

• The private key remains strictly with the client machine and must never be shared.
• The public key is copied to the remote Linux server and appended to the target user's authorized_keys file.

When a client attempts to connect to the Linux host via SSH, the SSH daemon (sshd) checks the target account's authorized_keys file. The server issues a cryptographic challenge based on the public key. If the client can prove ownership of the corresponding private key by decrypting or signing the challenge, the server grants access immediately without prompting for the user's Linux password.

File Location and Format

The authorized_keys file resides inside a hidden directory within the user's home directory:

/home/<username>/.ssh/authorized_keys

For the root user, the typical location is /root/.ssh/authorized_keys.

The file is a standard plaintext document where each line represents a single authorized public key. A standard line includes:

1. Key Type: Specifies the cryptographic algorithm (e.g., ssh-ed25519 or ssh-rsa).
2. Key Data: The base64-encoded string representing the actual public key.
3. Comment: An optional identifier, usually noting the creation date, email address, or hostname of the key owner.

Optional options can also precede the key type on the same line to restrict capabilities, such as locking down the connection to a specific IP address (from="192.168.1.50") or restricting execution to a single predefined command.

Critical Permission Requirements

Because the authorized_keys file directly dictates who can access an account, the SSH daemon enforces strict file system permission checks. If permissions are too open, the server will reject the authentication attempt for security reasons:

• The .ssh directory must be owned by the user and set to permissions 700 (rwx------).
• The authorized_keys file must be owned by the user and set to permissions 600 (rw-------) or 644 (rw-r--r--).

Primary Benefits

• Enhanced Security: Public-key authentication is virtually immune to brute-force password guessing attacks.
• Automation: Scripts, configuration management tools (like Ansible), and CI/CD pipelines rely on this file to interact with servers without human interaction.
• Granular Access Management: System administrators can grant access to multiple users by adding their respective public keys, and immediately revoke access simply by deleting a specific line from the file.