There are 4 public key algorithms for authentication: The server proves its identity to the client by signing the key resulting from the key exchange. We also have to make sure that they share this secret with each other and not an NSA analyst. The key exchange ensures that the server and the client shares a secret no one else knows. This will take a while so continue while it’s running. Ssh-keygen -T /etc/ssh/moduli.safe -f /etc/ssh/moduli.all If it does not exist, create it: ssh-keygen -G /etc/ssh/moduli.all -b 4096 Pb 2000' /etc/ssh/moduli > '$/moduli' # make sure there is something left Its security is based on the hardness of the discrete logarithm problem. The server and the client will end up with a shared secret number at the end without a passive eavesdropper learning anything about this number.Īfter we have a shared secret we have to derive a cryptographic key from this using a key derivation function.Ĭollision attacks on this hash function have been proven to allow downgrade attacks.ĭH works with a multiplicative group of integers modulo a prime. There are basically two ways to do key exchange: Diffie-Hellman and Elliptic Curve Diffie-Hellman.īoth provide forward secrecy which the NSA hates because they can’t use passive collection and key recovery later. If a man in the middle were to change the lists, then the server and the client would calculate different keys. This hurts interoperability but everyone uses OpenSSH anyway.įortunately, downgrade attacks are not possible because the supported algorithm lists are included in the key derivation. Some of the supported algorithms are not so great and should be disabled completely.

The server and the client choose a set of algorithms supported by both, then proceed with the key exchange. SSH supports different key exchange algorithms, ciphers and message authentication codes. Reading the documents, I have the feeling that the NSA can 1) decrypt weak crypto and 2) steal keys. It should work with 6.5 but I have only tested 6.7 and connections to Github. Warning: You will need a recent OpenSSH version. TL DR: Scan this post for fixed width fonts, these will be the config file snippets and commands you have to use. My goal with this post here is to make NSA analysts sad. This post will still be here when you finish. If you have not, then read the latest batch of Snowden documents now. You may have heard that the NSA can decrypt SSH at least some of the time.

Github openssh