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Stronger SSH Keys

Have a look in your ~/.ssh folder and browse through your keys. How many are using a recent encryption method or a long bit-length? RSA keys are far and away the most popular, and if you're using it with a bit-length that's 2048 or lower, you may be in danger.

Let's review the common options:

  • DSA: No longer supported and unsafe.
  • RSA: Key bit lengths greater than 3072 are still safe, but get lengthy. 1024 keys are officially unsafe. The clock is ticking on RSA of any key length as it has no protection from quantum computing.
  • ECDSA: The cryptographic safety of these keys depend on your specific hardware, and older/weaker machines may have insufficient entropy. The mathematics in use are not completely trusted by the security community.
  • Ed25519: An EdDSA implementation using the Twisted Edwards curve! These keys are strong, have protection from quantum techniques, and generate a tiny public key that's only 68 characters long. They can be generated quickly en mass.

Ed25519 is the best recommendation in 2019.

Generate an Ed25519 key

ssh-keygen -a 100 -t ed25519 -f ~/.ssh/id_ed25519

Switches:

  • -a: Key Derivation Function rounds. Higher numbers result in slower passphrase verification, increasing the resistance to brute-force password cracking should the private-key be stolen.
  • -t: Specifies the type of key to create, in our case the Ed25519.
  • -f: Specify the filename of the generated key file. If you want it to be discovered automatically by the SSH agent, it must be stored in the default .ssh directory within your home directory. There are other strategies for organizing specific machine keys not covered in this guide.

When asked to enter a passphrase, do so. An ssh key without a passphrase is vulnerable if stolen.

Keys used in cron-jobs

Some keys may be used for automated scripts and are not appropriate to passphrase. In this case, extra security can be gained by setting authorized_keys entries to restrict the commands the key is permitted to run, down to the command-line arguments.