I'm studying for the Network+ exam and wanted to understand the difference between one-way authentication and two-way authentication.

I've tried searching to no avail. It seems when I search for it the results return 2FA and single-factor authentication. Are single-factor authentication and one-way authentication the same thing? Ditto for 2FA.

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    Search on google for challenge-response protocol. This will help you understand what one-way and two-way authentication is, but very briefly: You have 2 parties that want to communicate, A and B. A needs to authenticate B (one-way), but if B needs to also authenticate that A is who they say they are (two-way). Multi-factor authentication (2FA, 3FA, etc) is a different thing. It is about HOW you authenticate someone (possession, knowledge, inherence). Jan 25, 2020 at 13:28
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    "Two-way authentication" is not the phrase most commonly used. You want to look for "mutual authentication".
    – schroeder
    Jan 25, 2020 at 13:32

2 Answers 2


"Two-Way Authentication" is more commonly called "Mutual Authentication".

At the network level, which is what you are studying, it means that the client and the server authenticate to each other. This is done through the exchange of certificates that are checked by a Certificate Authority (or the cert is known by the server).

It means that the client and server know that they are communicating with the expected party. This is established at the lower levels of the OSI stack before application-level authentication takes place (user authentication or process authentication). It's done at a lower level so that the users/processes have assurance before they attempt to authenticate.

At the network level:

  • client requests to connect to the server
  • server sends the server certificate and askes for the client certificate
  • client verifies the server certificate with the Certificate Authority (CA)
  • client sends client certificate
  • server verifies client certificate with CA
  • if all checks verify, then access is granted to the client to the resource

WiFi and ethernet have mutual authentication protocols, and sometimes the certificate is provided by the server. TLS also has a mutual authentication protocol.

Further reading:

Two-Factor Authentication is different. Factors are the different things one presents to authenticate with. Typically, the one factor is a password (something you know), then more factors are added to provide greater assurance that the user is who they say they are (or that the password is not compromised). Added factors could be one-time codes, hardware tokens, or other contextual factors. Most often, user accounts use multi-factor authentication and not devices although there is a growing use case to include contextual data to device authentication.


What is Mutual Authentication?

Mutual Auth IRL (In Real Life)

This is exactly what you do when you go to the bank. You "authenticate" the teller from the second you think about the bank.

  • You think "where is the bank" (location, check)
  • Does the bank look like a bank? (look around, looks like a bank, reputation, check)
  • Does the teller work for the bank? (She's behind the desk, so someone let her back there, uniform, name tag, and she is holding cash. Check)

Check, check, check. You're at a bank talking to an employee.

Now she authenticates you. Asks for - your ID, - your photo comes up on the screen, and - you know your ATM pin.

Check, check, check. She trusts you are you.

Mutual authentication. You both think you know who the other is.

Mutual Auth in Computers

In computers it's about the same. Both sides need to trust each other. And bad guys could be executing a man-in-the-middle attack.


One place we use mutual authentication is SSH. You want to SSH to a server, you want to know it's really the server you think it is.

Let's say you're going to copy up some secret information. How do you know for sure you're connecting to the right server? What if somebody is trying to trick you into uploading the secret info to their server? SSH servers do prove to the client connecting to them that they have a secret key. Read about it here: https://support.alertlogic.com/hc/en-us/articles/360002999732-How-does-SSL-mutual-authentication-work-

web sites

EVERY TIME YOU LOGIN ON A SERVER you are mutually authenticating.

Your browser trusts the remote server and puts the lock in the address bar. Then you type in a username and password, and now the server trusts you.


Or HTTPS - Sometimes still called SSL, but TLS is the accurate name for the protocol.

There are same protocol exchanges that mutually authenticate. You can mutually authenticate over https. You have a "client" certificate that you use in the key exchange with the server. The server verifies your client certificate is signed by a certificate authority that the server trusts to sign client certs. Just like your browser checks that the server's cert is signed by a certificate authority that the browser trusts.

Look at mutual TLS authentication in action

NodeJS is an interesting implementation. It's both a pain in the hind at first, and sorta easy code to read once you figure it out. Have a look at this writeup and the nodejs docs.



Read More

Mutual Authentication: https://en.wikipedia.org/wiki/Mutual_authentication

Definitions of authentication:

an act, process, or method of showing something (such as an identity, a piece of art, or a financial transaction) to be real, true, or genuine : the act or process of authenticating something

Source: “Authentication.” The Merriam-Webster.com Dictionary, Merriam-Webster Inc., https://www.merriam-webster.com/dictionary/authentication. Accessed 25 January 2020.

authentication is the process of verifying that identity. It might involve validating personal identity documents, verifying the authenticity of a website with a digital certificate,1 determining the age of an artifact by carbon dating, or ensuring that a product or document is not counterfeit.

Source: https://en.wikipedia.org/wiki/Authentication

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    "EVERY TIME YOU LOGIN ON A SERVER you are mutually authenticating." -- no. It's still only one-way. TLS only compares the cert with the domain. You could still be logging into a phishing site. You stretched your IRL banking analogy too far.
    – schroeder
    Jan 25, 2020 at 13:25
  • You are missing an understanding of server-side certs in TLS mutual authentication. Just having a cert does not mean you have passed authentication. I could serve https://totally-fake-bank.com and get a valid certificate for it and present a copy of a legitimate bank's site. In the nodejs blog page you posted, there is an additional understanding that it omitted but crucial: the client-side has hard-coded the server domain. That domain and it's cert is then checked against the CA. That's when it becomes an authenitcation (verifying of ID).
    – schroeder
    Jan 25, 2020 at 13:44
  • I disagree. You being fooled by a phishing site is irrelevant. You thought you properly authenticated the site before you put your password in. Otherwise, you wouldn’t put the credential into the site. All authentication has different levels of risk. Some are less risky than others. Some only protect you if you are very vigilant, and even then still carry risk.
    – Jonathan
    Jan 25, 2020 at 15:26
  • That's not authentication.
    – schroeder
    Jan 25, 2020 at 15:51
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    You are co-mingling general ideas of identity and authentication with the technical definition of mutual authentication as asked in the question. Please note that the context of the question is the Network+ exam. To tell a student that "logging into a server is mutual authentication" is irresponsible since there are technical specifications for mutual auth in networking which wouldn't be required if your statement was broadly true.
    – schroeder
    Jan 26, 2020 at 7:41

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