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Hosting HTTPS With Flask

00:00 In the previous lesson, I showed you how to finish the code to become a Certificate Authority. In this lesson, I’m going to show you how to use the certificate generated through the CA to host an HTTPS site using Flask. To host a web server that uses HTTPS you need: a signed certificate, configuration for Flask to use the certificate, and configuration for your web browser to include your custom Certificate Authority in its list of Trusted Third Parties.

00:30 This is a new copy of the original Flask server, not the one using the Fernet keys. The only difference between this and the original is line 13. The Flask development server is being started with the ssl_context parameter.

00:45 This tells Flask to serve HTTPS. The ssl_context requires two arguments: the server public key and the server private key. The server public key is the signed certificate issued by the CA to Alice.

01:01 The server private key is Alice’s private key that she used to create the CSR for the CA. Remember that when a private key is created, it’s encrypted with a password.

01:12 That’s going to be important in a second. In the lower window, I’m going to start up the server.

01:19 And because the private key is being used, Flask asks for a PEM pass phrase. This is the password that was used to encrypt the server private key. Typing it in allows the server to start. It’s running on port 5684 just like before, in order to be consistent. In a third window, I’m going to hit that with curl.

01:44 Uh-oh, that doesn’t look very good. What’s the problem? Well, by default, curl doesn’t know who the CA is. Charlie’s CA service isn’t in curl’s default list of CAs.

01:57 That means you have to tell curl about Charlie. You can tell curl about a different CA by passing in the CA’s public key on the command line.

02:07 You do that with the --cacert (CA cert) argument.

02:13 Well, that still didn’t work, but it’s a different error message this time. This time, it’s complaining that it doesn’t like the hostname ''.

02:25 If you think back to the previous lesson, the CSR included two valid hostnames for the certificate: 'localhost' and 'alice.example.net'. '' isn’t one of those valid hostnames, so the certificate doesn’t recognize it.

02:44 Even though your computer thinks localhost and are the same thing, the certificate doesn’t. Most Certificate Authorities refuse to sign certificates for IP addresses, so you usually have to have a hostname. Third time’s the charm, this time with localhost. Still need the CA’s public key.

03:09 And there it is. shhhh, this is a secret. You’ve now successfully served your secret message over HTTPS. One thing to keep in mind is HTTPS only encrypts the channel.

03:22 It stops someone from sniffing the contents of the channel, but it doesn’t stop anyone from actually hitting the port. Anyone who’s willing to ignore a browser’s warning about an invalid certificate can still see the contents hosted on HTTPS.

03:38 You would need to combine the ideas from the Fernet code with this code to serve a secret message over HTTPS.

03:47 Congratulations! You’re now a CA capable of signing your own CSRs and hosting an HTTPS server with a self-signed certificate. In the last lesson, I’ll wrap up and show you some shortcuts that you could use to skip past all this code.

Avatar image for Stuart

Stuart on April 5, 2021

curl --cacert ca-public-key.pem https://localhost:5684/

I have followed the entire tutorial with the provided code on my machine up to this point, and everything has run just fine.

Unfortunately this final attempt to hit the server with curl is still returning exit code 60 (invalid certificate).

I have tried:

  • generating new keys (PEM files)
  • entering the full file path for ca-public-key.pem in the –cacert argument

I would appreciate any suggestions… I was really excited to get this final response from the server.

Avatar image for Bartosz Zaczyński

Bartosz Zaczyński RP Team on April 6, 2021

@Stuart The sample certificate included in the supporting material has already expired. However, generating a new one with the included script doesn’t work for me either. Unfortunately, I’m not sure what the reason is off the top of my head.

You could try creating a self-signed certificate with openssl as a quick workaround, though:

$ openssl req -x509 -newkey rsa:4096 -keyout server-private-key.pem -out server-public-key.pem -days 365

…and then ignore insecure certificates:

$ curl -k https://localhost:5684/
shhhh, this is secret

Anyway, we’ll probably need @Christopher’s help with this one.

Avatar image for Christopher Trudeau

Christopher Trudeau RP Team on April 7, 2021

Hi @Stuart,

@Bartosz and I have been plugging away at this, and I think we’ve figured it out. The root certificate generation code needs to change a little bit, I’ll show you how.

The version of curl I’m using is a bit older and uses TLS 1.2. The version Bartosz is using (and I suspect you are as well) uses the more up to date TLS 1.3. When I install a new version of curl, I get the same problem you are experiencing.

The key to fixing it is inside of the builder code. Currently, make_builder has the following:

    builder = (

To get a valid CA certificate you also need another parameter. Change your builder to include:

                path_length=None), critical=True)

This change should only be for generating the public CA. In my own code I added a is_ca parameter to make_builder and to PublicKey.generate. My make_builder now looks like this:

def make_builder(subject, issuer, is_ca=False):
    # Expire this certificate 30 days from now
    valid_from,valid_to = date_range(3000)

    # Chain-call certificate builder with necessary parameters
    builder = (

    if is_ca:
        builder = builder.add_extension(x509.BasicConstraints(ca=True,
            path_length=None), critical=True)

    return builder

Then you have to make changes to PublicKey and generate_keys.py to expose this new parameter. Or, you can cheat add the line by hand, commenting it in and out depending on your needs.

What does all this mean? I’m not 100% sure, but I suspect that either TLS 1.2 didn’t require this parameter, or was more forgiving when it was missing.

We’ll update the course with the new code at some point, but in the meantime, you can add the new .add_extension call and should be good to go.

Thanks for the comment. Hope you enjoyed the course, this bump not withstanding.

Happy coding!

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