SSH

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Secure Shell (SSH) is a method to securely communicate between computers. This page will discuss SSH under the context of the OpenSSH server.

Cryptography

SSH supports different key exchange algorithms, ciphers, and message authentication codes (MACs). The server and client negotiates and chooses a set of these algorithms that are supported by both and then proceeds with the key exchange.

Key Exchange

SSH handles key exchange using one of two ways: Diffie-Hellman and Elliptic Curve Diffie-Hellman. A shared secret is generated between the client and server for every session which will be used as the key, thereby providing forward secrecy.

Diffie-Hellman works with a multiplicative group of integers modulo a prime. Its security is based on the hardness of the discrete logarithm problem.

Elliptic Curve Diffie-Hellman works with elliptic curves over finite fields. Its security is based on the hardness of the elliptic curve discrete logarithm problem.

Authentication

With a shared secret derived from Diffie-Hellman, the client and server need to prove to each other they are who they are.

The server proves its identity to the client by signing the shared secret (the key) using a public key algorithm.

The client proves its identity to the server by using:

  1. Password authentication
  2. Public Key Authentication (same as the server's)

Securing SSH

Disable Weak Key Exchange Protocols

SSH supports a few protocols containing different factors:

  1. ECDH curve choice
  2. Bit size of the DH modulus
  3. The hash function.

To provide good security:

  • Avoid NIST curves - they leak data and cannot be trusted.
  • Bit sizes smaller than 1024 does not provide sufficient security
  • Do not use SHA1

This means, your sshd_config should contain: 

KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256

And your ssh_config should contain: 

Host *
    KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256

Prime numbers smaller than 2000 bits should also be removed from the /etc/ssh/moduli file (the 5th column).

Disable Weak Server Authentication

There are 4 public key algorithms for server authentication.

  1. DSA with SHA1
  2. ECDSA with SHA256, SHA384 or SHA512 depending on key size
  3. Ed25519 with SHA512
  4. RSA with SHA1

For security:

  1. Do not use SSH v1
  2. Avoid DSA which requires exactly 1024 bits.
  3. Avoid NIST (ECDSA uses it)
  4. DSA and ECDSA requires randomness for its signatures. If random numbers are bad, it is possible to recover the secret key.

Using SHA1 here is not an issue if the value being signed is using something stronger (eg SHA2).

Your sshd_config should contain: 

Protocol 2
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key


See Also