Understanding KEX Algorithms in sshd\_config: Pseudo Override Take Effect?
Secure Shell (SSH) is a widely-used protocol for secure remote login from one computer to another. It provides strong password authentication and encrypted communications over insecure channels. The SSH daemon configuration file, sshd\_config, contains various options that allow you to customize the security and functionality of the SSH service. One such option is the KEX algorithm, which is used for key exchange during the SSH handshake process.
What is a KEX Algorithm?
KEX (Key Exchange) algorithms are used during the SSH handshake process to securely exchange cryptographic keys between the client and the server. These algorithms help to ensure the confidentiality and integrity of the SSH session. The KEX algorithm is negotiated during the initial connection setup, and both the client and the server must support the same algorithm for the connection to be established.
KEX Algorithms in sshd\_config
The sshd\_config file contains a number of options related to KEX algorithms. By default, the SSH daemon supports a variety of KEX algorithms, including diffie-hellman-group1-sha1, diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1, and diffie-hellman-group-exchange-sha256. You can specify the KEX algorithms that the SSH daemon should use by including the following line in the sshd\_config file:
KexAlgorithms [email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
This line specifies the KEX algorithms that the SSH daemon should use, in order of preference. The algorithms are separated by commas, and the order of the algorithms indicates the preferred order of use.
Pseudo Override of KEX Algorithms
The term "pseudo override" in the question likely refers to the ability to specify a KEX algorithm that is not explicitly supported by the SSH daemon. For example, the question mentions the KEX algorithm "[email protected]", which is not a standard KEX algorithm supported by the SSH daemon.
It is possible to specify a non-standard KEX algorithm in the sshd\_config file by including it in the KexAlgorithms line. However, this does not guarantee that the SSH daemon will be able to use the algorithm for key exchange. The SSH daemon will only use a KEX algorithm if both the client and the server support it. If the client does not support the specified KEX algorithm, the SSH connection will fail.
KEX algorithms are an important part of the SSH handshake process, and the sshd\_config file allows you to customize the KEX algorithms used by the SSH daemon. However, it is important to note that the SSH daemon will only use a KEX algorithm if both the client and the server support it. Specifying a non-standard KEX algorithm in the sshd\_config file does not guarantee that it will be used for key exchange.
References
- SSH Daemon Configuration File (man sshd\_config)
- Understanding SSH Key Exchange Algorithms (https://www.digitalocean.com/community/tutorials/understanding-the-ssh-key-exchange-algorithms)
- SSH Key Exchange Algorithms (https://en.wikipedia.org/wiki/Secure_Shell#Key_exchange)