Secure Shell

SSH (or Secure SHell, in Spanish: secure command interpreter) is the name of a protocol and of the program that It implements it whose main function is remote access to a server through a secure channel in which all information is encrypted. In addition to connecting to other devices, SSH allows you to copy data securely (both single files and simulate encrypted FTP sessions), manage RSA keys so you don't type passwords when connecting to devices, and pass data from any other application through a secure channel. tunneled via SSH and can also redirect traffic from (X Window System) to be able to run graphical programs remotely. The assigned TCP port is 22.

It is a protocol that facilitates secure communications between two systems using a client/server architecture and allows users to connect to a host remotely. Unlike other remote communication protocols such as FTP or Telnet, SSH encrypts the connection session, making it impossible for anyone to obtain unencrypted passwords. SSH is designed to replace the older and less secure methods of remotely logging into another system via a command shell, such as telnet or rsh. A related program, scp, replaces other programs designed to copy files between hosts such as rcp. Since these old applications do not encrypt passwords between the client and the server.

Operation

You need three essential points: user, port and server. The SSH client will contact the server to initiate the connection. That server is listening on port 22 or whatever is assigned to it. Later the server will send the public key and they begin to organize the parameters and open a secure channel. The client logs in to connect to that server.

Security

SSH works in a similar way to telnet. The main difference is that SSH uses encryption techniques that make the information that travels through the communication medium go unreadable, preventing third parties from discovering the username and password of the connection or what is written during the entire session.; although it is possible to attack this type of system through REPLAY attacks and thus manipulate the information between destinations.

The SSH protocol provides the following types of protection:

• After the initial connection, the client can verify that it is connecting to the same server that was previously connected to.
• The client transmits its authentication information to the server using a robust 128-bit encryption.
• All data sent and received during the session are transferred through 128-bit encryption, which makes them extremely difficult to decipher and read.
• The customer has the possibility to forward X11 applications from the server. This technique, called X11 forward, provides a safe means to use graphical applications on a network.

Since the SSH protocol encrypts everything you send and receive, it can be used to secure insecure protocols. The SSH server can become a conduit for making insecure protocols secure by using a technique called port forwarding.

There are different cryptographic structures that can be applied when using the SSH protocol on demand. There are basically three alternatives:

• Symmetric
• Asymmetric
• Hashing

Symmetric cryptography is performed by means of a secret key, which is shared only between the server and the user. Its role is to encrypt or decrypt the message that is transferred in this process, however, the Secure Shell only offers to read the content by presenting that key.

Asymmetric cryptography uses two keys, one for the client and one for the server, to encrypt the data transferred. The keys are called public and private, thus forming the necessary combination to generate the SSH and its security protocol. In this model, the public key is distributed in an open and shared way. However, from it it is not possible to discover what the private key is. This happens thanks to a process that works as follows: messages encrypted by public keys can only be decrypted by the private key of the same machine.

Hashing is a one-way method of cryptography used in SSH. This practice consists of creating a hash, by means of an algorithm, to guarantee that the message will be protected in a specific form of cryptography and authentication codes.

Most common attack

The most common attack is a brute force attack, these work by trying every possible combination that the user could use as a password and then testing to see if it is the correct password. To see if the password is correct or not, check the server response for errors. As password length increases, the amount of time used to find the correct password also increases rapidly. That means short passwords are pretty easy to crack. To make this attack more effective, the support of dictionaries is used, these are tools with the list of possible passwords to use against the target system until the correct password for the user is obtained.

History

At first there were only r-commands, which were based on the rlogin program, which works in a similar way to telnet.

The first version of the protocol and the program were free and were created by a Finn named Tatu Ylönen, but his license changed and the company SSH Communications Security ended up appearing, offering it free for home and academic use, but requiring payment to other companies. In 1997 (two years after the first version was created) it was proposed as a draft to the IETF.

At the beginning of 1999 a version began to be written that would become the free implementation par excellence, that of OpenBSD, called OpenSSH.

Versions

There are 2 versions of SSH, SSH version 1 makes use of many proprietary encryption algorithms (however, some of these patents have expired) and is vulnerable to a security hole that potentially allows an attacker to insert data into the communication stream. The OpenSSH suite under Red Hat Enterprise Linux defaults to version 2 of SSH, which has an improved key exchange algorithm that is not vulnerable to the security hole in version 1. However, the OpenSSH suite also supports connections. from version 1.