SSH

see Symmetrical encryption

Symmetric keys are used by SSH in order to encrypt the entire connection. Contrary to what some users assume, public/private asymmetrical key pairs that can be created are only used for authentication, not encrypting the connection. The symmetrical encryption allows even password authentication to be protected against snooping.

The client and server both contribute toward establishing this key, and the resulting secret is never known to outside parties. The secret key is created through a process known as a key exchange algorithm. This exchange results in the server and client both arriving at the same key independently by sharing certain pieces of public data and manipulating them with certain secret data. This process is explained in greater detail later on.

The symmetrical encryption key created by this procedure is session-based and constitutes the actual encryption for the data sent between server and client. Once this is established, the rest of the data must be encrypted with this shared secret. This is done prior to authenticating a client.

SSH can be configured to use a variety of different symmetrical cipher systems, including Advanced Encryption Standard (AES), Blowfish, 3DES, CAST128, and Arcfour. The server and client can both decide on a list of their supported ciphers, ordered by preference. The first option from the client’s list that is available on the server is used as the cipher algorithm in both directions.

On Ubuntu 20.04, both the client and the server are defaulted like the following:

You probably already have a basic understanding of how SSH works. The SSH protocol employs a client-server model to authenticate two parties and encrypt the data between them.

The server component listens on a designated port for connections. It is responsible for negotiating the secure connection, authenticating the connecting party, and spawning the correct environment if the credentials are accepted.

The client is responsible for beginning the initial transmission control protocol (TCP) handshake with the server, negotiating the secure connection, verifying that the server’s identity matches previously recorded information, and providing credentials to authenticate.

An SSH session is established in two separate stages. The first is to agree upon and establish encryption to protect future communication. The second stage is to authenticate the user and discover whether access to the server should be granted.

When a TCP connection is made by a client, the server responds with the protocol versions it supports. If the client can match one of the acceptable protocol versions, the connection continues. The server also provides its public host key, which the client can use to check whether this was the intended host.

At this point, both parties negotiate a session key using a version of something called the diffie-hellman algo.

The session key will be used to encrypt the entire session. The public and private key pairs used for this part of the procedure are completely separate from the SSH keys used to authenticate a client to the server.

The shared secret encryption that is used for the rest of the connection is called binary packet protocol.

The generated secret is a symmetric key, meaning that the same key used to encrypt a message can be used to decrypt it on the other side. The purpose of this is to wrap all further communication in an encrypted tunnel that cannot be deciphered by outsiders.

After the session encryption is established, the user authentication stage begins.

The next step involves authenticating the user and deciding on access. There are a few methods that can be used for authentication, based on what the server accepts.

The general method is password authentication, which is when the server prompts the client for the password of the account they are attempting to log in with. The password is sent through the negotiated encryption, so it is secure from outside parties.

Even though the password will be encrypted, this method is not generally recommended due to the limitations on the complexity of the password. Automated scripts can break passwords of normal lengths very easily compared to other authentication methods.

The most popular and recommended alternative is the use of SSH key pairs. SSH key pairs are asymmetric keys, meaning that the two associated keys serve different functions.

The public key is used to encrypt data that can only be decrypted with the private key. The public key can be freely shared, because, although it can encrypt for the private key, there is no method of deriving the private key from the public key.

Authentication using SSH key pairs begins after the symmetric encryption has been established as described in the previous section. The procedure happens as follows:

In sum, the asymmetry of the keys allows the server to encrypt messages to the client using the public key. The client can then prove that it holds the private key by decrypting the message correctly. The two types of encryption that are used (symmetric shared secret and asymmetric public/private keys) are each able to leverage their specific strengths in this model.

Comments on page :
Let me point out an error regarding asymmetric encryption: “This is a one-way ability, meaning that the public key has no ability to decrypt the messages it writes, nor can it decrypt anything the private key may send it.”
The first part is correct: You cannot use any of those two keys to decrypt a message encrypted with the same key.
The second part is wrong: You can use either of those keys to decrypt a message encrypted with the other one.