1. Error Ssl Context Is Not Usable Without Certificate And Private Key West
2. Error Ssl Context Is Not Usable Without Certificate And Private Keys
3. Error Ssl Context Is Not Usable Without Certificate And Private Key Bank
4. Error Ssl Context Is Not Usable Without Certificate And Private Keyboard

The even more uncommon case of no certificates at all. SSL/TLS can also be used without certificates at all, i.e. Not even at the server side. In this case authentication is done with other methods, like a secret key pre-shared between client and server (PSK). These methods are rarely used and browsers don't support these. The following warnings are presented by web browsers when you access a site that has a security certificate installed (for SSL/TLS data encryption) that cannot be verified by the browser. Internet Explorer: 'The security certificate presented by this website was not issued by a trusted certificate. Or just that the private key does not correspond to the supplied public key. If you like I can have look at your certs if you send them to support (@) markbrilman (.) nl. But I do need both the private key and the public key. I can imagine it’s not option to send them. With kind regards, Mark. Reply this message.

2017-06-03T18:41:15Z

While you work on your Flask application, you normally run the development web server, which provides a basic, yet functional WSGI complaint HTTP server. But eventually you will want to deploy your application for production use, and at that time, one of the many things you will need to decide is if you should require clients to use encrypted connections for added security.

People ask me all the time about this, in particular how to expose a Flask server on HTTPS. In this article I'm going to present several options for adding encryption to a Flask application, going from an extremely simple one that you can implement in just five seconds, to a robust solution that should give you an A+ rating like my site gets from this exhaustive SSL analysis service.

How Does HTTPS Work?

The encryption and security functionality for HTTP is implemented through the Transport Layer Security (TLS) protocol. Basically put, TLS defines a standard way to make any network communication channel secure. Since I'm not a security expert, I don't think I can do a great job if I try to give you a detailed description of the TLS protocol, so I will just give you some of the details that are of interest for our purpose of setting up a secure and encrypted Flask server.

The general idea is that when the client establishes a connection with the server and requests an encrypted connection, the server responds with its SSL Certificate. The certificate acts as identification for the server, as it includes the server name and domain. To ensure that the information provided by the server is correct, the certificate is cryptographically signed by a certificate authority, or CA. If the client knows and trusts the CA, it can confirm that the certificate signature indeed comes from this entity, and with this the client can be certain that the server it connected to is legitimate.

Error Ssl Context Is Not Usable Without Certificate And Private Key West

After the client verifies the certificate, it creates an encryption key to use for the communication with the server. To make sure that this key is sent securely to the server, it encrypts it using a public key that is included with the server certificate. The server is in possession of the private key that goes with that public key in the certificate, so it is the only party that is able to decrypt the package. From the point when the server receives the encryption key all traffic is encrypted with this key that only the client and server know.

From this summary you can probably guess that to implement TLS encryption we need two items: a server certificate, which includes a public key and is signed by a CA, and a private key that goes with the public key included in the certificate.

The Simplest Way To Do It

Flask, and more specifically Werkzeug, support the use of on-the-fly certificates, which are useful to quickly serve an application over HTTPS without having to mess with certificates. All you need to do, is add ssl_context='adhoc' to your app.run() call. As an example, below you can see the 'Hello, World' Flask application from the official documentation, with TLS encryption added:

This option is also available through the Flask CLI if you are using a Flask 1.x release:

To use ad hoc certificates with Flask, you need to install an additional dependency in your virtual environment:

When you run the script (or start with flask run if you prefer), you will notice that Flask indicates that it is running an https:// server:

Simple, right? The problem is that browsers do not like this type of certificate, so they show a big and scary warning that you need to dismiss before you can access the application. Once you allow the browser to connect, you will have an encrypted connection, just like what you get from a server with a valid certificate, which make these ad hoc certificates convenient for quick & dirty tests, but not for any real use.

Self-Signed Certificates

A so called self-signed certificate is one where the signature is generated using the private key that is associated with that same certificate. I mentioned above that the client needs to 'know and trust' the CA that signed a certificate, because that trust relationship is what allows the client to validate a server certificate. Web browsers and other HTTP clients come pre-configured with a list of known and trusted CAs, but obviously if you use a self-signed certificate the CA is not going to be known and validation will fail. That is exactly what happened with the ad hoc certificate we used in the previous section. If the web browser is unable to validate a server certificate, it will let you proceed and visit the site in question, but it will make sure you understand that you are doing it at your own risk.

But what is the risk, really? With the Flask server from the previous section you obviously trust yourself, so there is no risk to you. The problem is when users are presented with this warning when connecting to a site they do not directly know or control. In those cases, it is impossible for the user to know if the server is authentic or not, because anyone can generate certificates for any domain, as you will see below.

While self-signed certificates can be useful sometimes, the ad hoc certificates from Flask are not that great, because each time the server runs, a different certificate is generated on the fly through pyOpenSSL. When you are working with a self-signed certificate, it is better to have the same certificate used every time you launch your server, because that allows you to configure your browser to trust it, and that eliminates the security warnings.

You can generate self-signed certificates easily from the command line. All you need is to have openssl installed:

This command writes a new certificate in cert.pem with its corresponding private key in key.pem, with a validity period of 365 days. When you run this command, you will be asked a few questions. Below you can see in red how I answered them to generate a certificate for localhost:

We can now use this new self-signed certificate in our Flask application by setting the ssl_context argument in app.run() to a tuple with the filenames of the certificate and private key files:

Alternatively, you can add the --cert and --key options to the flask run command if you are using Flask 1.x or newer:

The browser will continue to complain about this certificate, but if you inspect it, you will see the information that you entered when you created it:

Using Production Web Servers

Of course we all know that the Flask development server is only good for development and testing. So how do we install an SSL certificate on a production server?

If you are using gunicorn, you can do this with command line arguments:

If you use nginx as a reverse proxy, then you can configure the certificate with nginx, and then nginx can 'terminate' the encrypted connection, meaning that it will accept encrypted connections from the outside, but then use regular unencrypted connections to talk to your Flask backend. This is a very useful set up, as it frees your application from having to deal with certificates and encryption. The configuration items for nginx are as follows:

Another important item you need to consider is how are clients that connect through regular HTTP going to be handled. The best solution, in my opinion, is to respond to unencrypted requests with a redirect to the same URL but on HTTPS. For a Flask application, you can achieve that using the Flask-SSLify extension. With nginx, you can include another server block in your configuration:

If you are using a different web server, check their documentation and you will likely find similar ways to create the configurations shown above.

Using 'Real' Certificates

We have now explored all the options we have for self-signed certificates, but in all those cases, the limitation remains that web browsers are not going to trust those certificates unless you tell them to, so the best option for server certificates for a production site is to obtain them from one of these CAs that are well known and automatically trusted by all the web browsers.

When you request a certificate from a CA, this entity is going to verify that you are in control of your server and domain, but how this verification is done depends on the CA. If the server passes this verification then the CA will issue a certificate for it with its own signature and give it to you to install. The certificate is going to be good for a period of time that is usually not longer than a year. Most CAs charge money for these certificates, but there are a couple that offer them for free. The most popular free CA is called Let's Encrypt.

Getting a certificate from Let's Encrypt is fairly easy, since the whole process is automated. Assuming you are using an Ubuntu based server, you have to begin by installing their open source certbot tool on your server:

And now you are ready to request the certificate using this utility. There are a few ways that certbot uses to verify your site. The 'webroot' method is, in general, the easiest to implement. With this method, certbot adds some files in a directory that your web server exposes as static files, and then tries to access these files over HTTP, using the domain you are trying to generate a certificate for. If this test is successful, certbot knows that the server in which it is running it is associated with the correct domain, and with that it is satisfied and issues the certificate. The command to request a certificate with this method is as follows:

In this example, we are trying to generate a certificate for a example.com domain, which uses the directory in /var/www/example as a static file root. Unfortunately a Flask based website does not have a static file root directory, all static files from the application are accessed with the /static prefix, at least when you use the default configuration, so more planning is necessary.

What certbot does to the static root directory is add a .well-known subdirectory, and then store some files in it. Then it uses a HTTP client to retrieve those files as http://example.com/.well-known/.... If it can retrieve the files, then that is confirmation that your server is in full control of the domain name. For Flask and other applications that don't have a static file root directory, it is necessary to define one.

If you are using nginx as reverse proxy, you can take advantage of the powerful mappings that you can create in the configuration to give certbot a private directory where it can write its verification files. In the following example, I extended the HTTP server block shown in the previous section to send all Let's Encrypt related requests (which always begin with /.well-known/...) to a specific directory of your choice:

Then you can give this director to certbot:

If certbot is able to verify the domain, it will write the certificate file as /etc/letsencrypt/live/example.com/fullchain.pem and the private key as /etc/letsencrypt/live/example.com/privkey.pem, and these are going to be valid for a period of 90 days.

To use this newly acquired certificate, you can enter the two filenames mentioned above in place of the self-signed files we used before, and this should work with any of the configurations described above. And of course you will also need to make your application available through the domain name that you registered, as that is the only way the browser will accept the certificate as valid.

Certbot is also used when you need to renew the certificates. To do that, you simply issue the following command:

If there are any certificates in your system that are close to expire, the above command renews them, leaving new certificates in the same locations. You will likely need to restart your web server if you want the renewed certificates to be picked up.

Achieving an SSL A+ Grade

If you use a certificate from Let's Encrypt or another known CA for your production site and you are running a recent and maintained operating system on this server, you are likely very close to have a top-rated server in terms of SSL security. You can head over to the Qualys SSL Labs site and get a report to see where you stand.

Chances are you will still have some minor things to do. The report will indicate what areas you need to improve, but in general, I expect you'll be told that the options the server exposes for the encrypted communication are too wide, or too weak, leaving you open to known vulnerabilities.

One of the areas in which it is easy to make an improvement is in how the coefficients that are used during the encryption key exchange are generated, which usually have defaults that are fairly weak. In particular, the Diffie-Hellman coefficients take a considerable amount of time to be generated, so servers by default use smaller numbers to save time. But we can pre-generate strong coefficients and store them in a file, which then nginx can use. Using the openssl tool, you can run the following command:

You can change the 2048 above for a 4096 if you want even stronger coefficients. This command is going to take some time to run, specially if your server does not have a lot of CPU power, but when it's done, you will have a dhparam.pem file with strong coefficients that you can plug into the ssl server block in nginx:

Next, you will probably need to configure which ciphers the server allows for the encrypted communication. This is the list that I have on my server:

In this list, disabled ciphers are prefixed with a !. The SSL report will tell you if there are any ciphers that are not recommended. You will have to check from time to time to find out if new vulnerabilities have been discovered that require modifications to this list.

Below you can find my current nginx SSL configuration, which includes the above settings, plus a few more that I added to address warnings from the SSL report:

You can see the results that I obtained for my site at the top of this article. If you are after 100% marks in all categories, you will have to add additional restrictions to your configuration, but this is going to limit the number of clients that can connect to your site. In general, older browsers and HTTP clients use ciphers that are not considered to be the strongest, but if you disable those, then these clients will not be able to connect. So you will basically need to compromise, and also routinely review the security reports and make updates as things change over time.

Unfortunately for the level of sophistication on these last SSL improvements you will need to use a professional grade web server, so if you don't want to go with nginx, you will need to find one that supports these settings, and the list is pretty small. I know Apache does, but besides that, I don't know any other.

Conclusion

So there you go, this is how you can implement top-of-the-line SSL security for your Flask application. Is there anything that I left out? Do you do things differently? Let me know below in the comments!

Hello, and thank you for visiting my blog! If you enjoyed this article, please consider supporting my work on this blog on Patreon!

79 comments

• #1Ezequiel said 2017-06-05T14:53:45Z

• #2netchose said 2017-06-05T18:37:06Z

• #3Joe Labi said 2017-06-09T13:57:27Z

• #4Benjamin said 2017-06-09T21:22:52Z

• #5Miguel Grinberg said 2017-06-10T16:19:25Z

• #6Florian said 2017-06-29T14:15:52Z

• #7Miguel Grinberg said 2017-06-29T16:30:59Z

• #8mz said 2017-08-11T04:56:26Z

• #9Rob Mathers said 2017-10-05T17:20:14Z

• #10cmb said 2017-10-06T05:56:18Z

• #11zhihong Pei said 2017-11-25T13:56:31Z

• #12Miguel Grinberg said 2017-11-25T18:08:41Z

• #13Doron said 2018-03-04T11:53:53Z

• #14Vipin Reyaroth said 2018-04-13T01:58:36Z

• #15igor said 2018-05-10T13:17:24Z

• #16Miguel Grinberg said 2018-05-13T03:16:43Z

• #17faceless stranger said 2018-07-01T23:08:47Z

• #18Olav Vahtras said 2018-08-16T06:04:20Z

• #19Miguel Grinberg said 2018-08-16T07:56:15Z

• #20Doaa Altarawy said 2018-09-13T18:30:33Z

• #21Miguel Grinberg said 2018-09-13T22:37:29Z

• #22Vladyslav said 2018-10-30T14:53:14Z

• #23Aleksandr Mikheev said 2018-11-03T18:08:56Z

• #24Alexis ROLLAND said 2018-12-01T10:38:55Z

• #25Miguel Grinberg said 2018-12-02T09:01:37Z

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Symptoms or Error

When configuring the certificate for the Citrix Secure Gateway Server, the following error message appears:

'The server certificate specified is unusable.'

Solution

Solution 1

Ensure the Private key for the Certificate is available.

Solution 2

Ensure the permissions are correct for MachineKeys folders. The MachineKeys folder is located at the All Users ProfileApplication DataMicrosoftCryptoRSA folder. The following settings are the default permissions for the MachineKeys folder:

• Administrator (Full Control) This folder only
• Everyone (Special) This folder, subfolders, and files
• SYSTEM (Full Control) This folder, subfolders, and files

The Everyone group, select the following Special permissions:

• List Folder/Read Data
• Read Attributes
• Read Extended Attributes
• Create Files/Write Data
• Create Folders/Append Data
• Write Attributes
• Write Extended Attributes
• Read Permissions

For more information regarding the default permissions on MachineKeys folders, refer to this Microsoft article -Default permissions for the MachineKeys folders.

Problem Cause

Cause 1

The Server Certificates contained in the Local Computers Personal Store are queried.

The Private Key is not available.

See Page 17 and 95 of CTX112429 – Secure Gateway for Windows Administrator's Guide.
Page 17 - Improved certificate selection. The Secure Gateway Configuration wizard prevents the selection of a certificate that does not have a private key.
Page 95 - When you view the certificate, ensure that it contains a key icon and the caption “You have private key that corresponds to this certificate” at the bottom of the General tab. The lack of an associated private key can result in the CSG0188 error.

Cause 2

The permissions for MachineKeys folders (All Users ProfileApplication DataMicrosoftCryptoRSA) are misconfigured.

Additional Resources

Error Ssl Context Is Not Usable Without Certificate And Private Keys

Extracted from Page 85 of CTX112429 – Secure Gateway for Windows Administrator's Guide:

Error Ssl Context Is Not Usable Without Certificate And Private Key Bank

Certificate Requirements

Error Ssl Context Is Not Usable Without Certificate And Private Keyboard

Load balancing relies on the use of a virtual IP address. The virtual IP address is bound to an FQDN and all clients request connections from the virtual IP address rather than the individual servers running the Secure Gateway behind it. A single IP address, the virtual IP, acts as an entry point to your servers running the Secure Gateway, simplifying the way clients access Web content, published applications, and services on computers running Citrix Presentation Server. If you are using a load balancing solution, all servers running the Secure Gateway can be accessed using a common FQDN; for example, csgwy.company.com. In conclusion, you need a single server certificate, issued to the FQDN (mapped to the virtual IP or DNS name) of the load balancing server. The certificate must be installed on every server running the Secure Gateway in the server array that is being load balanced.