Secure Shell (SSH) is a cryptographic protocol that securely transports data over an unsecured network. As the name suggests, its main purpose is to establish a secure connection to a remote shell account.
The SSH client is the software, which the user runs on his local computer to connect to the remote server. Once connected, the ssh-client enables the user to enter commands and perform tasks through the shell of the remote computer.
The Need for an Encrypting Shell Client
When you're working with client/server computing, a general terminal or a terminal
emulator is used to send and receive data from the host. Because of this, the terminal
or the PC that is running a terminal emulator
needs to somehow be connected to the server or mainframe computer.
There are several different methods that can be used to connect the two, but in the past, the most commonly used method of connecting a terminal to a server over a network was telnet.
Unencrypted transmission via telnet however presents a serious security risk today, because anybody on the network with some knowledge and tools can monitor the data exchange - even down to details like usernames, passwords.
Because of the ever-increasing demand for security, the SSH (Secure Shell) protocol was developed. SSH uses advanced encryption technology to encrypt every single piece of communication between the client (called SSH client) and the server. Should an unauthorized third party be able to intercept traffic somewhere along the communication path, they will see nothing but completely useless data.
Technical SSH BasicsThe basics of the SSH protocol are laid out in RFC 4253. The document describes SSH as a secure transport protocol that is provided by a server on tcp port 22 that provides strong encryption, cryptographic host authentication, and integrity protection.
Or, as RFC 4253 states in its intro:
The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network.It defines ways to create a shared encryption key in the possible presence of a listener, host and user authentication methods (i.e. ways in which users and server can prove that they are who they claim to be), and possible data compression to more effectively transmit data.
This document describes the SSH transport layer protocol, which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression.
Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated.
This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol.
An especially challenging part of encrypting such communication, is the need to negotiation a shared secret (an encryptino key) over a channel that might already be monitored. SSH answers this challenge through the initial key exchange phase of the connection using the older Diffie-Hellman kex method. Never versions now also support ED25519 elliptic curve kex. It is a specific implementation of the Edwards-curve Digital Signature Algorithm (EdDSA), which itself is a variant of Schnorr's signature system with Twisted Edwards curves (math heavy details can be found in the upcoming IETF standard for ED25519).
Symmetrical EncryptionSymmetrical encryption is a type of encryption where a key can be used to encrypt messages to the other party, and also to decrypt the messages received from the other participant. What makes the encryption symmetric the fact that the same key is used for encryption and decryption.
Symmetric encryption usually requires little computing power and is hence used to encrypt larger blocks of data. With SSH, it is used to encrypt the whole data stream.
Asymmetrical (Public/Private Key) Encryption
Asymmetrical encryption differs from symmetrical encryption in the fact that two different keys are used. One (any) of those two is used to encrypt the data and then the other is used to decrypt it. The benefit of this technique is that one party can give the other party a key to encrypt messages to you, but anyone knowing that key will still not be able to decrypt the message again. Such a key is called the public key. The other key, which is not shared and which is then used to decrypt the data block is called the private key.
This also works in the other direction. Data that was encrypted using the private key can only be decrypted using the public key. With SSH this fact can be used to prove identity. If a message is decryptable using the public key, it proves that whoever encrypted the message, is in possession of the private key.
Key ExchangeAn especially challenging part of encrypting such communication, is the need to negotiate a shared secret (an encryption key) over a channel that might already be monitored by a third party.
Think of the problem as such: You need to agree with someone else on a password, but you can only talk to each other about it in the presence of an enemy.
SSH answers this challenge through the initial key exchange phase of the connection using the older Diffie-Hellman kex method. Newer versions now also support ED25519 elliptic curve kex. It is a specific implementation of the Edwards-curve Digital Signature Algorithm (EdDSA), which itself is a variant of Schnorr's signature system with Twisted Edwards curves (math heavy details can be found in the upcoming IETF standard for ED25519.
Static port-forwarding (or tunneling) refers to situations where the desitination host and port are known in advance.
Programs and protocols which do not use data encrpytion (e.g. ftp or rsh) can connect to the tunnel's port on the client computer and the ssh client will transmit their data through the encrypted ssh connection to/from a final destination that is already known at the time when the ssh-connection is made.
For example, a user can set up a port-forwarding on the client software, listening on the client port 5514 and forwarding traffic to the address of an older device with a fixed IP address on the remote network that only supports the unencrypted rsh protocol.
As outlined above, static port-forwarding feature requires the client to set up the tunnel source port and destination before making the connection.
This problem is addressed by secure shell's dynamic port forwarding. With dynamic port forwarding, the client sets up a listening port (as with normal port fowarding), where a software that connects to the port can tell the client which host and port it wants to connect to. This is done in the same way that client software can request connections from a SOCK5 proxy.
The ssh client will then forward the connection request to the secure shell server which makes the connection to the destination host. This way, the ssh client could let an unencrypted rsh software access arbitrary rsh servers on the remote network through the encrypted data channel.
Other SSH Client Features and RequirementsIn other words, there are many benefits to using a SSH client. On top of the encryption of the data transfer and secure key exchange, the SSH protocol also offers verification that you are connected to the correct computer.
This may seem surprising, but it makes perfect sense. Keep in mind that if somebody were able to control any part of the communication path, they could actually reroute the traffic to another computer. This could then play the role of the computer which you actually wanted to connect to (this is called a man-in-the-middle attack), and could either display fake data or obtain information from the client computer. A feature called known_hosts can prevent this.
A SSH client should also support a variety of authentication methods. These include username/password, public/private key, and various custom formats. The latter might include a system where the server could obtain information that only the authorized users know, e.g. by using a SecurID card or by sending an access code to the user's mobile phone.
To be able to connect to various different servers, the ssh client it has to support latest key exchange and encryption protocols, because what seemed unbreakable five years ago, is considered less so today. Most server continually switch to more advanced encryption methods, ssh clients need to support these as well.
Other typical must have features for a ssh client would be:
SSH Connection via Proxy
ZOC Terminal, a Modern SSH Client
However, SSH only covers the actual transmission of data between the
client and server. But the secure shell client
is usually a terminal emulator,
i.e. a software that allows a remote computer to receive keyboard input
from, and send formatted text (color, cursor placement, etc.)
to the user's computer.
Obviously, the client still needs to be able to perform the functions of a terminal emulator but also extra functions like printing, logging, script-automation and so on.
Together with ssh features like latest encryption and public key authentication, port forwarding, tunneling, smart card authentication, etc. this makes ZOC the ideal SSH client.
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