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How to create & sign SSL/TLS certificates

In the previous lecture, we’ve talked about how digital certificates help with authentication and provide a safe and reliable key exchange process in TLS. Today we will learn exactly how to generate a certificate and have it signed by a Certificate Authority (CA). For the purpose of this demo, we won't submit our Certificate Signing Request (CSR) to a real CA. Instead, we will play both roles: the certificate authority and the certificate applicant. So in the first step, we will generate a private key and its self-signed certificate for the CA. They will be used to sign the CSR later. In the second step, we will generate a private key and its paired CSR for the web server that we want to use TLS. Then finally we will use the CA's private key to sign the web server's CSR and get back the signed certificate.

# 1. Generate CA's private key and self-signed certificate

# 2. Generate web server's private key and certificate signing request (CSR)

# 3. Use CA's private key to sign web server's CSR and get back the signed certificate

In order to do all of these things, we need to have openssl installed. If you're on a mac, it's probably already there. You can run openssl version to see which version it's running. In my case, it's LibreSSL version 2.8.3. Let's open the browser and go to libressl.org. Here we have a link to the manual of openssl.

Generate CA's private key and certificate

The first command we're gonna used is req, which stands for request. This command is used to create and process certificate request. It can also be used to create a self-signed certificate for the CA, which is exactly what we want in the first step. This -x509 option is used to tell openssl to output a self-signed certificate instead of a certificate request. In case you don't know, X509 is just a standard format of the public key certificate. You can click on a lock button of any HTTPS website to see its certificate in X509 format. Alright, now let's get back to the terminal and run:

openssl req -x509 -newkey rsa:4096 -days 365 -keyout ca-key.pem -out ca-cert.pem

The -newkey rsa:4096 option basically tells openssl to create both a new private key with RSA 4096-bit key and its certificate at the same time. As we're using -x509 option, it will output a certificate instead of a request. The next option is -days 365, which specifies the number of days that the certificate is valid for. Then we use -keyout option to tell openssl to write the created private key to ca-key.pem file and finally the -out option to tell it to write the certificate to ca-cert.pem file. When we press Enter, openssl will start generating the private key. Once the key is generated, we will be asked to provide a pass phrase, which will be used to encrypt the private key before writing it to the PEM file. Why is it encrypted? Because if somehow the private key file is hacked, the hacker cannot use it to do anything without knowing the pass phrase to decrypt it first. Next, openssl will ask us for some identity information to generate the certificate. First the country code, the state or province name, the city name, the organization name, the unit name, the common name, or domain name, the email address. That's it! The certificate and private key files are successfully generated. If we cat the private key file, we can see it says "ENCRYPTED PRIVATE KEY":

ca-key.pem

-----BEGIN ENCRYPTED PRIVATE KEY-----

The certificate, on the other hand, is not encrypted, but only base64-encoded, because it just contains the public key, the identity information and the signature that should be visible to everyone. We can use the x509 command to display all the information encoded in this certificate. This command can also be used to sign certificate requests, which we will do in a few minute. Now let's run

openssl x509 -in ca-cert.pem -noout -text

Here we use the -in option to pass in the CA’s certificate file, the -noout option to tell it to not output the original encoded value. We want to display it in a readable text format, so let's use -text option and press Enter. Now we can see all information of the certificate, such as the version, the serial number. The issuer and the subject are the same in this case because this is a self-signed certificate. Then the RSA public key and signature. I'm gonna copy this command and save it to our gen.sh script. With this script, I want to automate te process of generating a set of keys and certificates.

# 1. Generate CA's private key and self-signed certificate
openssl req -x509 -newkey rsa:4096 -days 365 -keyout ca-key.pem -out ca-cert.pem

echo "CA's self-signed certificate"
openssl x509 -in ca-cert.pem -noout -text

Before moving to the 2nd step, I’m gonna show you another way to provide the identity information without entering it interactively as before. To do this, we use the subj option, I'm gonna add it to openssl request command and copy identity information from the certificate.

openssl req -x509 -newkey rsa:4096 -days 365 -keyout ca-key.pem -out ca-cert.pem -subj "/C=FR/ST=Occitanie/L=Toulouse/O=Tech School/OU=Education/CN=*.techschool.guru/[email protected]"

Now let's add a command to remove all pem files at the top of this script, then run gen.sh in the terminal. We still being prompted for a pass phrase, but it doesn't ask for identity information anymore, because we already provided them in the subj option. Great!

Generate web server's private key and CSR

Now the next step is to generate a private key and CSR for out web server. It's almost the same as the command we used in the 1st step. Except that, this time we don't want to self-sign it, so we should remove this -x509 option. This -days option should be removed as well, since we don't create a certificate, but just a CSR. Then we change the name of the output key to server-key.pem. The output certificate request file should be server-req.pem, because we're creating a certificate signing request. Now we should change all the subject information to our web server's information. OK, let's run it.

openssl req -newkey rsa:4096 -keyout server-key.pem -out server-req.pem -subj "/C=FR/ST=Ile de France/L=Paris/O=PC Book/OU=Computer/CN=*.pcbook.com/[email protected]"

Enter a pass phrase to encrypt the web server's private key. Then the files are successfully generated.

This time, in the server-req.pem file, it says CERTIFICATE REQUEST, not CERTIFICATE as in the ca-cert.pem file. That's because it’s not a certificate as before, but a certificate signing request instead.

-----BEGIN CERTIFICATE REQUEST-----
MIIE2DCCAsACAQAwgZIxCzAJBgNVBAYTAkZSMRYwFAYDVQQIDA1JbGUgZGUgRnJh
bmNlMQ4wDAYDVQQHDAVQ...pWofr2eOeBQ4Q=
-----END CERTIFICATE REQUEST-----

# ...

# 2. Generate web server's private key and certificate signing request (CSR)
openssl req -newkey rsa:4096 -keyout server-key.pem -out server-req.pem -subj "/C=FR/ST=Ile de France/L=Paris/O=PC Book/OU=Computer/CN=*.pcbook.com/[email protected]"

# ...

So now let's move to step 3 and sign this request.

Sign the web server's certificate request

For that, we will use the same x509 command that we've used to display certificate before. Let's open the terminal and run this:

openssl x509 -req -in server-req.pem -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem

In this command, we use the -req option to tell openssl that we’re gonna pass in a certificate request. We use the -in option follow by the name of the request file: server-req.pem. Next we use the -CA option to pass in the certificate file of the CA: ca-cert.pem. And the -CAkey option to pass in the private key of the CA: ca-key.pem. Then 1 important option is -CAcreateserial. Basically the CA must ensure that each certificate it signs goes with a unique serial number. So with this option, a file containing the next serial number will be generated if it doesn't exist. Finally we use the -out option to specify the file to write the output certificate to. Now as you can see here, because the CA's private key is encrypted, OpenSSL is asking for the pass phrase to decrypt it before it can be used to sign the certificate. It’s a countermeasure in case the CA’s private key is hacked. OK, now we've got the signed certificate for our web server. Let's print it out in text format.

openssl x509 -in server-cert.pem -noout -text

It has its unique serial number 0xb141e873fd7b8567. We can also see a ca-cert.srl file, which contains the same serial number.

B141E873FD7B8567

Issuer section contains the information of the CA, which is Tech School in this case. By default, the certificate is valid for 30 days. We can change it by adding the -days option to the signing command.

openssl x509 -req -in server-req.pem -days 60 -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem

Now the validity duration has changed to 60 days.

If you remember the Youtube certificate that we've seen in the previous lecture, this certificate is used for many Google websites with different domain names. We can also do that for our web server by specifying the Subject Alternative Name extension when signing the certificate request. Here we can see the -extfile option that allows us to state the file containing the extensions. We can see the format of this config file in this page. Let's search for "subject alternative name".

There are several things that we can use as the alternative name, such as email, DNS, or IP. I will create a new file server-ext.cnf with this content:

subjectAltName=DNS:*.pcbook.com,DNS:*.pcbook.org,IP:0.0.0.0

Here I set DNS to multiple domain names: *.pcbook.com and *.pcbook.org. I also set IP to 0.0.0.0 which will be used when we develop on localhost. Now in the certificate signing command, let's add the -extfile option and pass in the name of the extension config file:

openssl x509 -req -in server-req.pem -days 60 -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem -extfile server-ext.cnf

Now the result certificate file has a new extensions section with all the subject alternative names that we've chosen:

Certificate:
    ...
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=FR, ST=Occitanie, L=Toulouse, O=Tech School, OU=Education, CN=*.techschool.guru/[email protected]
        Validity
            Not Before: Apr 10 18:17:05 2020 GMT
            Not After : Jun  9 18:17:05 2020 GMT
        Subject: C=FR, ST=Ile de France, L=Paris, O=PC Book, OU=Computer, CN=*.pcbook.com/[email protected]
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (4096 bit)
                Modulus:
                    00:cb:e2:2b:c3:68:...
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Alternative Name: 
                DNS:*.pcbook.com, DNS:*.pcbook.org, IP Address:0.0.0.0
    Signature Algorithm: sha1WithRSAEncryption
         5e:67:4d:f7:91:89:fc:...

So looks like our automate script is ready except for the fact that we have to enter a lot of password to protect the private keys.

# ...

# 3. Use CA's private key to sign web server's CSR and get back the signed certificate
openssl x509 -req -in server-req.pem -days 60 -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem -extfile server-ext.cnf

echo "Server's self-signed certificate"
openssl x509 -in ca-cert.pem -noout -text

In case we just want to use this for development and testing, we can tell openssl to not encrypt the private key, so that it won't ask us for the pass phrase.

We do that by adding the -nodes option to the openssl req command like this:

# 1. Generate CA's private key and self-signed certificate
openssl req -x509 -newkey rsa:4096 -days 365 -nodes -keyout ca-key.pem -out ca-cert.pem -subj "/C=FR/ST=Occitanie/L=Toulouse/O=Tech School/OU=Education/CN=*.techschool.guru/[email protected]"

echo "CA's self-signed certificate"
openssl x509 -in ca-cert.pem -noout -text

# 2. Generate web server's private key and certificate signing request (CSR)
openssl req -newkey rsa:4096 -nodes -keyout server-key.pem -out server-req.pem -subj "/C=FR/ST=Ile de France/L=Paris/O=PC Book/OU=Computer/CN=*.pcbook.com/[email protected]"

# 3. Use CA's private key to sign web server's CSR and get back the signed certificate
openssl x509 -req -in server-req.pem -days 60 -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem -extfile server-ext.cnf

echo "Server's self-signed certificate"
openssl x509 -in ca-cert.pem -noout -text

Now if I run gen.sh again, it will not ask for passwords anymore. And if we look at the private key files, it will be PRIVATE KEY, and not ENCRYPTED PRIVATE KEY as before.

-----BEGIN PRIVATE KEY-----
MIIJQwIBADANBgkqhkiG9w0BAQEFAASCCS0wggkpAgEAAoICAQDL4ivDaIzDM3my
VDzT2Mw5R9bicXS...AxAt2Ldmc4=
-----END PRIVATE KEY-----

Verify a certificate

One last thing before we finish, I will show you how to verify if a certificate is valid or not. We can do that with the openssl verify command:

openssl verify -CAfile ca-cert.pem server-cert.pem

We just pass in the trusted CA's certificate and the certificate that we want to verify. If it returns OK then the certificate is valid.

And that's it for today's lecture. I hope it's useful for you. Thanks for reading, and I’ll see you guys in the next one!