SSH Tutorial for Freshmen – How Does SSH Work

How Does SSH Work

What’s SSH

SSH, or Safe Shell, is a distant administration protocol that permits customers to manage and modify their distant servers over the Web. The service was created as a safe alternative for the unencrypted Telnet and makes use of cryptographic strategies to make sure that all communication to and from the distant server occurs in an encrypted method. It supplies a mechanism for authenticating a distant person, transferring inputs from the purchaser to the host, and relaying the output again to the purchaser.

The Determine Under reveals a typical SSH Window. Any Linux or macOS person can SSH into their distant server instantly from the terminal window. Home windows customers can reap the benefits of SSH purchasers like Putty.  You possibly can execute shell instructions in the identical method as you’ll should you have been bodily working the distant laptop.

Example SSH connection showing how SSH works
This SSH tutorial will cowl the fundamentals of how does ssh work, together with the underlying applied sciences utilized by the protocol to supply a secured methodology of distant entry. It’ll cowl the totally different layers and varieties of encryption used, together with the aim of every layer.

How Does SSH Work

In case you’re utilizing Linux or Mac, then utilizing SSH could be very easy. In case you use Home windows, you will want to make the most of an SSH purchaser to open SSH connections. The preferred SSH purchaser is PuTTY, which you’ll be able to study extra about right here.

For Mac and Linux customers, head over to your terminal program after which observe the process under:

The SSH command consists of three distinct elements:

ssh {person}@{host}

The SSH key command instructs your system that you simply wish to open an encrypted Safe Shell Connection. {person} represents the account you wish to entry. For instance, you might wish to entry the root person, which is principally synonymous for system administrator with full rights to change something on the system. {host} refers back to the laptop you wish to entry. This may be an IP Tackle (e.g. or a website identify (e.g.

Once you hit enter, you may be prompted to enter the password for the requested account. Once you sort it in, nothing will seem on the display screen, however your password is, in truth being transmitted. When you’re accomplished typing, hit enter as soon as once more. In case your password is right, you may be greeted with a distant terminal window.

If you wish to find out about some extra SSH instructions, discover them out right here.

Understanding Totally different Encryption Methods

The numerous benefit supplied by SSH over its predecessors is the usage of encryption to make sure safe switch of data between the host and the purchaser. Host refers back to the distant server you are attempting to entry, whereas the purchaser is the pc you’re utilizing to entry the host. There are three totally different encryption applied sciences utilized by SSH:

  1. Symmetrical encryption
  2. Asymmetrical encryption
  3. Hashing.

Symmetric Encryption

Symmetric encryption is a type of encryption the place a secret key is used for each encryption and decryption of a message by each the purchaser and the host. Successfully, anyone possessing the important thing can decrypt the message being transferred.

SSH tutorial - Symmetric Encryption

Symmetrical encryption is usually referred to as shared key or shared secret encryption. There may be often just one key that’s used, or generally a pair keys the place one key can simply be calculated utilizing the opposite key.

Symmetric keys are used to encrypt the whole communication throughout a SSH Session. Each the purchaser and the server derive the key key utilizing an agreed methodology, and the resultant key’s by no means disclosed to any third occasion. The method of making a symmetric key’s carried out by a key trade algorithm. What makes this algorithm notably safe is the truth that the secret is by no means transmitted between the purchaser and the host. As a substitute, the 2 computer systems share public items of information after which manipulate it to independently calculate the key key. Even when one other machine captures the publically shared knowledge, it received’t be capable of calculate the important thing as a result of the important thing trade algorithm isn’t identified.

It have to be famous, nevertheless, that the key token is particular to every SSH session, and is generated previous to purchaser authentication. As soon as the important thing has been generated, all packets transferring between the 2 machines have to be encrypted by the exclusive key. This contains the password typed into the console by the person, so credentials are all the time protected against community packet sniffers.

A wide range of symmetrical encryption ciphers exist, together with, however not restricted to, AES (Superior Encryption Commonplace), CAST128, Blowfish and many others. Earlier than establishing a secured connection, the purchaser and a number determine upon which cipher to make use of, by publishing an inventory of supported cyphers so as of choice. Probably the most most popular cypher from the purchasers supported cyphers that’s current on the host’s listing is used because the bidirectional cypher.

For instance, if two Ubuntu 14.04 LTS machines are speaking with one another over SSH, they may use aes128-ctr as their default cipher.

Uneven Encryption

Not like symmetrical encryption, asymmetrical encryption makes use of two separate keys for encryption and decryption. These two keys are generally known as the public key and the exclusive key. Collectively, each these keys kind a public-private key pair.

Asymmetric Encryption

The general public key, because the identify counsel is brazenly distributed and shared with all events. Whereas it’s carefully linked with the exclusive key when it comes to performance, the exclusive key can’t be mathematically computed from the general public key. The relation between the 2 keys is extremely complicated: a message that’s encrypted by a machine’s public key, can solely be decrypted by the identical machine’s exclusive key. This one-way relation signifies that the general public key can not decrypt its personal messages, nor can it decrypt something encrypted by the exclusive key.

The exclusive key should stay exclusive i.e. for the connection to be secured, no third occasion should ever realize it. The power of the whole connection lies in the truth that the exclusive key’s by no means revealed, as it’s the solely part able to decrypting messages that have been encrypted utilizing its personal public key. Due to this fact, any occasion with the aptitude to decrypt publically signed messages should possess the corresponding exclusive key.

Not like the final notion, asymmetrical encryption isn’t used to encrypt the whole SSH session. As a substitute, it is just used throughout the important thing trade algorithm of symmetric encryption. Earlier than initiating a secured connection, each events generate momentary public-private key pairs, and share their respective exclusive keys to supply the shared secret key.

As soon as a secured symmetric communication has been established, the server makes use of the purchasers public key to generate and problem and transmit it to the purchaser for authentication. If the purchaser can efficiently decrypt the message, it signifies that it holds the exclusive key required for the connection. The SSH session then begins.


One-way hashing is one other type of cryptography utilized in Safe Shell Connections. One-way-hash capabilities differ from the above two types of encryption within the sense that they’re by no means meant to be decrypted. They generate a novel worth of a set size for every enter that reveals no clear pattern which might exploited. This makes them virtually unimaginable to reverse.


It’s straightforward to generate a cryptographic hash from a given enter, however unimaginable to generate the enter from the hash. Which means that if a purchaser holds the right enter, they’ll generate the crypto-graphic hash and examine its worth to confirm whether or not they possess the right enter.

SSH makes use of hashes to confirm the authenticity of messages. That is accomplished utilizing HMACs, or Hash-based Message Authentication Codes. This ensures that the command acquired isn’t tampered with in any means.

Whereas the symmetrical encryption algorithm is being chosen, an appropriate message authentication algorithm can be chosen. This works in the same option to how the cipher is chosen, as defined within the symmetric encryption part.

Every message that’s transmitted should include a MAC, which is calculated utilizing the symmetric key, packet sequence quantity, and the message contents. It’s despatched outdoors the symmetrically encrypted knowledge because the concluding part of the communication packet.

How Does SSH Work with These Encryption Methods

The best way SSH works is by making use of a client-server mannequin to permit for authentication of two distant programs and encryption of the info that passes between them.

SSH operates on TCP port 22 by default (although this may be modified if wanted). The host (server) listens on port 22 (or every other SSH assigned port) for incoming connections. It organizes the safe connection by authenticating the purchaser and opening the right shell setting if the verification is profitable.

SSH Client and Server

The purchaser should start the SSH connection by initiating the TCP handshake with the server, guaranteeing a secured symmetric connection, verifying whether or not the id displayed by the server match earlier information (sometimes recorded in an RSA key retailer file), and presenting the required person credentials to authenticate the connection.

There are two levels to establishing a connection: first each the programs should agree upon encryption requirements to guard future communications, and second, the person should authenticate themselves. If the credentials match, then the person is granted entry.

Session Encryption Negotiation

When a purchaser tries to hook up with the server through TCP, the server presents the encryption protocols and respective variations that it helps. If the purchaser has the same matching pair of protocol and model, an settlement is reached and the connection is began with the accepted protocol. The server additionally makes use of an uneven public key which the purchaser can use to confirm the authenticity of the host.

As soon as that is established, the 2 events use what is named a Diffie-Hellman Key Trade Algorithm to create a symmetrical key. This algorithm permits each the purchaser and the server to reach at a shared encryption key which will likely be used henceforth to encrypt the whole communication session.

Right here is how the algorithm works at a really primary degree:

  1. Each the purchaser and the server agree on a really massive prime quantity, which in fact doesn’t have any consider widespread. This prime quantity worth is also called the seed worth.
  2. Subsequent, the 2 events agree on a typical encryption mechanism to generate one other set of values by manipulating the seed values in a particular algorithmic method. These mechanisms, also called encryption turbines, carry out massive operations on the seed. An instance of such a generator is AES (Superior Encryption Commonplace).
  3. Each the events independently generate one other prime quantity. That is used as a secret exclusive key for the interplay.
  4. This newly generated exclusive key, with the shared quantity and encryption algorithm (e.g. AES), is used to compute a public key which is distributed to the opposite laptop.
  5. The events then use their private exclusive key, the opposite machine’s shared public key and the unique prime quantity to create a ultimate shared key. This key’s independently computed by each computer systems however will create the identical encryption key on each side.
  6. Now that each side have a shared key, they’ll symmetrically encrypt the whole SSH session. The identical key can be utilized to encrypt and decrypt messages (learn: part on symmetrical encryption).

Now that the secured symmetrically encrypted session has been established, the person have to be authenticated.

Authenticating the Consumer

The ultimate stage earlier than the person is granted entry to the server is authenticating his/her credentials. For this, most SSH customers use a password. The person is requested to enter the username, adopted by the password. These credentials securely go by the symmetrically encrypted tunnel, so there isn’t a likelihood of them being captured by a 3rd occasion.

Though passwords are encrypted, it’s nonetheless not really helpful to make use of passwords for safe connections. It is because many bots can merely brute drive straightforward or default passwords and acquire entry to your account. As a substitute, the really helpful different is SSH Key Pairs.

These are a set of uneven keys used to authenticate the person with out the necessity of inputting any password.


Gaining an in-depth understanding of the underlying how SSH works may help customers perceive the safety features of this know-how. Most individuals think about this course of to be extraordinarily complicated and un-understandable, however it’s a lot easier than most individuals suppose. In case you’re questioning how lengthy it takes for a pc to calculate a hash and authenticate a person, nicely, it occurs in lower than a second. Actually, the utmost period of time is spent in transferring knowledge throughout the Web.

Hopefully, this SSH tutorial has helped you see the way in which totally different applied sciences will be clubbed collectively to create a sturdy system during which every mechanism has a vital function to play. Additionally, now you already know why Telnet turned a factor of the previous as quickly as SSH got here up.

For extra Linux tutorials, you should definitely take a look at our VPS tutorials part.


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