Server host public key (or certificate) HPub. The client version, server version, clients SSH_MSG_KEXINIT message, The exchange hash is generated by taking the hash (either SHA256, SHA384, or SHA512, depending on the key exchange algorithm) of the following fields. Hash, the size of the data to be signed is substantially reduced and results Therefore the client is connected to the correct server (as long as theĬlient can trust the corresponding public key, more on this later).īy signing the exchange hash, instead of signing the input to the exchange The client to verify the server has ownership of the host private key and The signature/verification loop of the exchange hash and signature allows Since the exchange hash includes the shared secret, it proves the other side The exchange hash and its signature serves several purposes:
Next, the server generates something referred to as the exchange hash H and signs it generating HS, see Figure (3) for more details. With the client’s public key and its own keypair, the server can generate the shared secret K. The server listens for the SSH_MSG_KEX_ECDH_INIT message, and upon receipt, generates its own ephemeral keypair. This property is called forward secrecy.įigure 1: Generation of the key exchange initialization message. It’s very difficult to steal something that simply no longer exists. This makes a class of attack where an attacker passively records encrypted traffic with the hope of stealing a private key sometime in the future extremely difficult. It's worthwhile to emphasise that this keypair is ephemeral: it will only be used during the key exchange and disposed of afterwards. Since Teleport only supports Elliptic Curve Diffie-Hellman (ECDH), the key exchange begins by the client generating an ephemeral keypair (private and associated public key) and sending the server it's public key in a SSH_MSG_KEX_ECDH_INIT message. Elliptic Curve Diffie-Hellman Initializationīecause both sides use the same algorithm to select cryptographic primitives out of the supported list, after the key exchange initialization, the key exchange can begin immediately. The table below lists of cryptographic primitives that Teleport supports.Ībove: Teleport default cryptographic primitives. The cryptographic primitives are to establish the building blocks that will be used to perform the key exchange and then bulk data encryption. The key exchange is kicked off by both sides sending a SSH_MSG_KEX_INIT message to each other with a list of cryptographic primitives they support with the order reflecting their preference. SSH key exchange (sometimes called KEX) is used by the client and server to exchange information in public that leads to a secret shared by the client and server that an observer can not discover or derive from public information. You can read more about SSH1 limitations here.
Terribly exciting happens in this part of the handshake, but it should be noted that most relatively modern clients and servers only support SSH 2.0 due to several flaws in the design of SSH1 most notably: SSH begins by both sides sending a version string to each other. Teleport is an open source access place offering security-hardened SSH access with RBAC and security auditing features. Although this post references SSH handshake based on Teleport, the process is compliant with SSH protocol and is compatible with OpenSSH SSH handshake process.
This post will cover these five steps of the handshake process in detail. Elliptic Curve Diffie-Hellman Initialization.SSH handshake is a process in the SSH protocol responsible for negotiating initial trust factors for establishing a secure channel between an SSH client and SSH server for an SSH connection. Secure Shell (SSH) is a widely used Transport Layer Protocol to secure connections between clients and servers.
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