SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Authentication by Cryptographic Key

In our running example, the user pat is authenticated by the SSH server via login password. Passwords, however, have serious drawbacks:

• In order for a password to be secure, it should be long and random, but such passwords are hard to memorize.

• A password sent across the network, even protected by an SSH secure channel, can be captured when it arrives on the remote host if that host has been compromised.

• Most operating systems support only a single password per account. For shared accounts (e.g., a superuser account), this presents difficulties:

  • Password changes are inconvenient because the new password must be communicated to all people with access to the account.

  • Tracking usage of the account becomes difficult because the operating system doesn’t distinguish between the different users of the account.

To address these problems, SSH supports public-key authentication: instead of relying on the password scheme of the host operating system, SSH may use cryptographic keys . [3.2.2] Keys are more secure than passwords in general and address all the weaknesses mentioned earlier.

2.4.1 A Brief Introduction to Keys

A key is a digital identity. It’s a unique string of binary data that means “This is me, honestly, I swear.” And with a little cryptographic magic, your SSH client can prove to a server that its key is genuine, and you are really you.

An SSH identity uses a pair of keys, one private and one public. The private key is a closely guarded secret only ...