1. Overview

In this tutorial, we’ll learn how to extract information from an X.509 public-key certificate using the x509 subcommand of the openssl tool.

2. What Is an X.509 Public Key Certificate?

In the TLS and SSL cryptographic protocols, a public key certificate is an electronic certificate that a website presents to the end-user. Through the certificate, a website can prove its legitimacy to its visitors. Visitors can then confidently interact with the website.

X.509 is one of the standards for defining public-key certificates. It’s the most commonly used one on the internet.

3. What Does an X.509 Certificate Contain?

Let’s look at a sample X.509 certificate:

        Serial Number:
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = US, O = Google Trust Services LLC, CN = GTS CA 1C3
            Not Before: Jul 12 01:35:31 2021 GMT
            Not After : Oct  4 01:35:30 2021 GMT
        Subject: CN = *.google.com
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature
            X509v3 Extended Key Usage:
                TLS Web Server Authentication
            X509v3 Subject Alternative Name:
                DNS:*.google.com, DNS:*.appengine.google.com, DNS:*.bdn.dev, 
    Signature Algorithm: sha256WithRSAEncryption

The above certificate snippet is for google.com. It contains a variety of information.

Firstly, every certificate contains a Serial Number. It’s a value given by the issuer when it signs the certificate.

The issuer of the certificate is defined under the field Issuer. For this particular certificate, the issuer is under the Google Trust Services LLC organization that’s residing in the US. Additionally, the issuer has a common name of GTS CA 1C3.

Next, the Validity field defines the period during which a certificate is effective. Particularly, a certificate is only valid during the period defined by the Not Before and Not After.

On the Subject field, we can see that this certificate has a common name of *.google.com. This is a wildcard common name that allows all the subdomains of google.com to identify themselves using the same certificate. Furthermore, the Subject Public Key Info field specifies the public key for this certificate. Beneath the same field, the certificate also defines the algorithm type of the public key as well as the necessary parameters.

In the X509v3 extensions field, we can find several extended properties that are on version 3 of the X.509 certificate standard. For example, the X509v3 Subject Alternative Name field defines other domains that are authenticating using the same certificates. In other words, this certificate would also be valid for the *.cloud.google.com, *.appengine.google.com, and so on.

Finally, we can see a Signature Algorithm field followed by a block of hexadecimal. This field specifies the algorithm that is used when the signature is made. In this case, it’s the sha256WithRSAEncryption. Additionally, the block of hexadecimal is the signature signed by the issuer. The signature serves to make sure the information on the certificate has not been trampled after it has been verified by the certificate authority.

4. The openssl Tool

The openssl tool is a cryptography library that implements the SSL/TLS network protocols. It contains different subcommands for any SSL/TLS communications needs.

For instance, the s_client subcommand is an implementation of an SSL/TLS client. Besides that, the x509 subcommand offers a variety of functionality for working with X.509 certificates.

4.1. Installation

Most Linux distributions include the openssl command. If it’s not available, we can install it using a package manager.

For Debian-based Linux, we can use the apt-get package manager:

$ sudo apt-get update
$ sudo apt-get install -y openssl

For RHEL-based Linux, we can use the yum package manager:

$ sudo yum update
$ sudo yum install -y openssl

And for Alpine Linux, we can use the apk package manager:

$ sudo apk add openssl

We can run the openssl subcommand version to confirm successful installation:

$ openssl version
OpenSSL 1.1.1k  25 Mar 2021

5. Fetching the X.509 Public Key Certificate File

Let’s say we want to fetch the public key certificate file of google.com. We can do that using the s_client and x509 subcommands of openssl:

$ openssl s_client -connect google.com:443 -showcerts </dev/null | openssl x509 -outform pem > googlecert.pem

Connecting to port 443 of host google.com using s_client initiates the TLS handshake.

The -showcerts option indicates that we want to print the certificate to the standard output. Redirecting /dev/null to the standard input terminates the connection after we’ve obtained the certificate.

We then pipe the certificate to the x509 subcommand along with the -outform option to encode it into the PEM format. Finally, we save the certificate file as googlecert.pem.

6. Decoding the Entire Certificate

The certificates in PEM format are base64 encoded. For instance, here’s the googlecert.pem file we got earlier:

$ cat googlecert.pem

To decode the entire certificate into plain text, we use the x509 command with the -text option:

$ openssl x509 -in googlecert.pem -noout -text
        Version: 3 (0x2)
        Serial Number:
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = US, O = Google Trust Services LLC, CN = GTS CA 1C3
            Not Before: Jul 12 01:35:31 2021 GMT
            Not After : Oct  4 01:35:30 2021 GMT

Firstly, the -in option specify the certificate file to be decoded. Then, the -noout option prevents any output from the command. Without the -noout option, the command will by default return the base64 encoded certificate.

Finally, we specify the -text option to print the entire certificate in plain text form.

7. Extracting Specific Information from the Certificate

Using the x509 subcommand, we can extract different fields from the certificate.

7.1. Extracting the Subject

The -subject option in the x509 subcommand allows us to extract the subject of the certificate.

Let’s extract the subject information from the googlecert.pem file using x509:

$ openssl x509 -in googlecert.pem -noout -subject
subject=CN = *.google.com

7.2. Extracting the Issuer

We can extract the issuer information from a certificate using the -issuer option.

For example, to extract the issuer information from the googlecert.pem file:

$ openssl x509 -in googlecert.pem -noout -issuer
issuer=C = US, O = Google Trust Services LLC, CN = GTS CA 1C3

7.3. Extracting the Extension Fields

To obtain the extension fields in X.509 public-key certificates, we can use the -ext option.

For instance, specifying the -ext option followed by subjectAltName will give us the subject alternative name in the certificate:

$ openssl x509 -in googlecert.pem -noout -ext subjectAltName
X509v3 Subject Alternative Name:
    DNS:*.google.com, DNS:*.appengine.google.com, DNS:*.bdn.dev, DNS:*.cloud.google.com, DNS:*.crowdsource.google.com, DNS:*.datacompute.google.com, ...(truncated)

Similarly, we can use the same option to obtain the key usage field using the keyUsage argument:

$ openssl x509 -in googlecert.pem -noout -ext keyUsage
X509v3 Key Usage: critical
    Digital Signature

7.4. Formatting the Name Output

For output such as issuer and subject, we can additionally specify the display format using -nameopt option.

For example, we can separate the issuer information into multiple lines using -nameopt option followed by sep_multiline operator:

$ openssl x509 -in googlecert.pem -noout -issuer -nameopt sep_multiline
    O=Google Trust Services LLC
    CN=GTS CA 1C3

We could also display the long field name instead of the abbreviation by passing in the lname operator to -nameopt:

$ openssl x509 -in googlecert.pem -noout -issuer -nameopt lname
issuer=countryName=US, organizationName=Google Trust Services LLC, commonName=GTS CA 1C3

Finally, the -nameopt option can be specified multiple time to combine different operators’ behavior. For instance, we could combine the lname and sep_multiline operators:

$ openssl x509 -in googlecert.pem -noout -issuer -nameopt lname -nameopt sep_multiline
    organizationName=Google Trust Services LLC
    commonName=GTS CA 1C3

The man page for the openssl command provides a full list of formatting arguments.

7.5. Extracting the Start Date and the Expiry Date

Every certificate specifies a start date and an expiry date. These 2 dates are important since a certificate is only valid during this period.

To obtain both dates, we can specify the -dates option:

$ openssl x509 -in googlecert.pem -noout -dates
notBefore=Jul 12 01:35:31 2021 GMT
notAfter=Oct  4 01:35:30 2021 GMT

Alternatively, we can print only the start date using -startdate option:

$ openssl x509 -in googlecert.pem -noout -startdate
notBefore=Jul 12 01:35:31 2021 GMT

Similarly, using the -enddate option, we can obtain the expiry date of the certificate:

$ openssl x509 -in googlecert.pem -noout -enddate
notAfter=Oct  4 01:35:30 2021 GMT

7.6. Extracting Other Information

Each certificate has a fingerprint which is used for uniquely identifying a particular certificate.

To extract the fingerprint, we can run the x509 subcommand with the -fingerprint option:

$ openssl x509 -in googlecert.pem -noout -fingerprint
SHA1 Fingerprint=5E:0B:46:9E:55:07:70:5A:C3:40:12:66:06:89:9A:92:E8:C2:15:E4

Besides that, we can obtain the serial number of a certificate using the -serial option. For instance:

$ openssl x509 -in googlecert.pem -noout -serial

Finally, we can extract the public key of a certificate using the -pubkey option:

$ openssl x509 -in googlecert.pem -noout -pubkey 
-----END PUBLIC KEY-----

7.7. Checking if a Certificate Is About to Expire

Using the -checkend option of the x509 subcommand, we can quickly check if a certificate is about to expire.

The option takes an additional argument n which has a unit of seconds. Generally:

$ openssl x509 -in <certificate-filename> -noout -checkend n

The command above will check if the certificate is expiring in the next n seconds. If it is, the command will result in a 1 return status code. The command returns a 0 status code if the certificate given is not expiring within the next n seconds. Furthermore, the command will return a message indicating the expiry status of the certificate.

For example, to determine if a certificate is expiring within the next 60 seconds:

$ openssl x509 -in googlecert.pem -noout -checkend 60
Certificate will not expire
$ echo $?

The command above returns a status code of 0 because the certificate is not expiring in the next 60 seconds.

Let’s now check if the same certificate will expire in the next 20 weeks:

$ openssl x509 -in googlecert.pem -noout -checkend 12096000
Certificate will expire
$ echo $?

Since the certificate will expire within the next 20 weeks, we get a return status code of 1.

8. Conclusion

In this article, we’ve learned what an X.509 certificate is, what data it contains and how to fetch it from a website that we are working on.

We also learned about the openssl tool and how we can use its x509 subcommand to decode the certificate and extract various pieces of information such as the subject, the issuer, the validity period of the certificate, extension fields, etc. from it. We also found out how we can determine if a certificate is about to expire.

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