Create Self-Signed S/MIME Certificates

What if you need to send an e-mail containing sensitive information? Do you send anything and everything through e-mail without concern for prying eyes? Recent news stories about e-mail account hacks and interceptions by third-parties make me even more hesitant and unwilling to send anything of importance through standard plain-text e-mail. If you’ve ever been through the process of buying a home, the amount of sensitive information that is transferred between the various parties is astounding and, from my experience, it is primarily done through plain-text e-mail (gasp). So, what can you do?

While this post doesn’t address the larger systemic issues around private information transfer, it does provide a basic method for public-key encryption and signing of MIME data (e-mail) using the S/MIME (Secure/Multipurpose Internet Mail Extensions) standard. Most well-known e-mail clients support S/MIME and this post provides instructions for creating your own certificate authority (CA) to create self-signed S/MIME certificates.

For those using an iOS or iPadOS device and Apple Mail, after completing the guide below and creating your self-signed certificate, please read Using Self-Signed S/MIME Certificates in iOS Mail App. It guides you through the steps to add your self-signed certificates to the Apple Mail app for S/MIME usage. For those using Microsoft Outlook, please read Using Self-Signed S/MIME Certificates in Outlook to guide you through the process of adding the personal certificate authority to the Windows trust store and using your self-signed certificate in Microsoft Outlook. For those using Mozilla Thunderbird, please read Using Self-Signed S/MIME Certificates in Thunderbird after completing this guide.

UPDATED (March 2023): I tested the steps on a Windows 10 64-bit machine using both the Win64 OpenSSL v3.1.0 Light distribution (EXE) and the Win64 OpenSSL v1.1.1t Light distribution (EXE) from Win32/Win64 OpenSSL Installer for Windows – Shining Light Productions.

A (Very) Brief Primer on Public-key Encryption

Prepare to be confused!

Since certificates are based on public-key encryption (also known as public-key cryptography or asymmetric cryptography), please keep in mind that you will need to have the public key of the intended e-mail recipient in order to encrypt the e-mail. Conversely, if someone wants to send you an encrypted e-mail, that person needs your public key. As an example, if I want to send an e-mail to Bob, I will need Bob’s public key to encrypt the e-mail. When Bob receives the encrypted e-mail, Bob’s e-mail client uses his personal private key to decrypt the e-mail. If Bob wants to send me an e-mail, he will need my public key to encrypt the e-mail. This post steps through the creation of your own personal public/private key pair. The public key is what an e-mail sender will need to encrypt an e-mail sent to you. Your private key is kept only by you since that is used to decrypt any e-mails encrypted using your public key. If your private key is obtained by anyone else, then that person would be able to decrypt and read your e-mails.

Is There an Easier Way?

Yes. You can obtain a basic certificate for free from a number of companies such as Comodo.

Where’s the fun in that? By creating your own certificate, you do not rely on an external party and you get to learn a little bit more along the way.

You’ve made it this far, so let’s get started.

Step 1 – Install OpenSSL

We will use OpenSSL to create a certificate authority which will then sign the certificate that we create. The latest OpenSSL toolkit is found at the OpenSSL site. If a binary distribution is needed, e.g., pre-compiled installation files for Microsoft Windows, those can be found on the OpenSSL binaries page.

Once you’ve found the appropriate distribution for your operating system, please proceed with the installation instructions provided with that distribution.

I am using a Windows distribution so portions of this post may be specific to that operating system. Please also note that I have installed OpenSSL in the c:\openssl\ directory.

Step 2 – Create an OpenSSL Configuration File

NOTE: This step may not be necessary depending on the OpenSSL distribution used. In some distributions of OpenSSL, the basic configuration file already includes the required extensions referenced below. In other distributions, a basic configuration file isn’t provided at all. If the default configuration file has the following extensions and you duplicate them in your own custom configuration file, then errors will be thrown in Step 4 and Step 7. With the distribution I used (referenced above), there is a default configuration file containing the appropriate [req], [req_distinguished_name], and [v3_ca] sections.

NOTE: With the distribution I used (referenced above), there is a default configuration file containing the appropriate [req], [req_distinguished_name], and [v3_ca] sections so I do not need those sections in my smime.cnf. However, I do use smime.cnf and, specifically, the [smime] section to set the appropriate extensions in Step 7. If you are using the same distribution, the default configuration file may be found at c:\Program Files\Common Files\SSL\openssl.cnf.

Now that OpenSSL is installed, a configuration file is needed. If openssl.exe is executed at this point without the configuration file in place, the message WARNING: can’t open config file: /usr/local/ssl/openssl.cnf may be received.

Create a new file named smime.cnf containing the following configuration. The contents of the file follow the x509 certificate extension configuration format. For more information about the format and content, please review x509 v3 configuration page. The [req] and [req_distinguished_name] sections are generally part of any standard OpenSSL configuration file. Some distributions include a default configuration file that includes some version of these sections. I included them specifically in this configuration file because I was receiving an error message stating unable to find ‘distinguished_name’ in config and this resolved the error.

NOTE: The [v3_ca] and [smime] sections are the important for this exercise because they set the appropriate extensions for an S/MIME certificate authority and personal certificates.

[req]
distinguished_name = req_distinguished_name

[req_distinguished_name]
countryName = Country Name (2 letter code)
countryName_default = AU
countryName_min = 2
countryName_max = 2
stateOrProvinceName = State or Province Name (full name)
stateOrProvinceName_default = Some-State
localityName = Locality Name (eg, city)
0.organizationName = Organization Name (eg, company)
0.organizationName_default = Internet Widgits Pty Ltd
organizationalUnitName = Organizational Unit Name (eg, section)
commonName = Common Name (e.g. server FQDN or YOUR name)
commonName_max = 64
emailAddress = Email Address
emailAddress_max = 64

[v3_ca]
basicConstraints = critical, CA:TRUE
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer

[smime]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = emailProtection
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer
subjectAltName = email:copy

Next, we will need to set the OPENSSL_CONF environment variable to reference the new configuration file. Setting this environment variable will eliminate the warning message mentioned earlier. This part is Windows specific. Recall that I have installed OpenSSL in the c:\openssl\ directory, named the configuration file smime.cnf, and saved it in the c:\openssl\ directory.

Open a command prompt window and be sure to Run as administrator if you are on Windows. Execute the following command:

set OPENSSL_CONF=c:\openssl\smime.cnf

When openssl.exe is executed (from the c:\openssl\bin\ directory), there is no warning message and the OpenSSL> prompt is displayed. Type exit and you’ll be returned to the c:\openssl\bin\> prompt.

For those using Windows, if you ever need to remove the OPENSSL_CONF environment variable, then use the following command to launch the Environment Variables dialog box to create, edit, or delete user or system variables.

rundll32 sysdm.cpl,EditEnvironmentVariables

Step 3 – Generate an RSA Private Key for the Certificate Authority

NOTE: The following steps and OpenSSL commands should be executed from the command prompt (on Windows) and not in OpenSSL interactive mode.

In this post, we are creating a new certificate authority to sign personal certificates. Execute the following command to generate the RSA private key for the new certificate authority:

openssl genrsa -aes256 -out ca.key 4096

The options specify to use the aes256 encryption cipher and output the results to a file named ca.key with a size of 4096 bits. Please be aware that the corresponding public key is derived from this private key. No extra step or command is required to generate the public key.

The following message will be displayed using OpenSSL 1.1x versions. Follow the prompts to create a pass phrase for this key. Remember this pass phrase for subsequent steps.

Generating RSA private key, 4096 bit long modulus (2 primes)
........................................................++++
...............................................++++
e is 65537 (0x010001)
Enter pass phrase for ca.key:
Verifying - Enter pass phrase for ca.key:

The message is modified slightly using OpenSSL 3.x versions, but the intent is the same.

Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:

Step 4 – Create Self-Signed Certificate for the Certificate Authority

NOTE: As mentioned in Step 2, if the distribution already has a proper configuration file then the smime.cnf file created in Step 2 is unnecessary and the last argument should be excluded from the command, i.e., remove “-extensions v3_ca”.

Execute the following command to generate the new self-signed certificate for the certificate authority:

openssl req -new -x509 -days 3650 -key ca.key -out ca.crt -extensions v3_ca

The -x509 option outputs a self-signed certificate instead of a certificate request. The -days 3650 option specifies that the generated certificate is certified for 10 years (ignoring leap years). The -key option specifies the private key to use. We will use the private key (ca.key) that was created in Step 3 and output the self-signed certificate to a file named ca.crt.

Follow the displayed prompts. You will need to use the pass phrase from Step 3. I have left most fields blank by simply entering a . character. I have provided example entries below between the brackets following the prompts. Please change the values to meet your own particular needs. Do not include the brackets in your entries.

Enter pass phrase for ca.key:
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:[.]
State or Province Name (full name) [Some-State]:[.]
Locality Name (eg, city) []:[.]
Organization Name (eg, company) [Internet Widgits Pty Ltd]:[TEST COMPANY]
Organizational Unit Name (eg, section) []:[.]
Common Name (e.g. server FQDN or YOUR name) []:[TEST COMPANY CERTIFICATE AUTHORITY]
Email Address []:[.]

The certificate authority has been created. Now, we will begin creating the personal certificate for a particular e-mail address.

Step 5 – Generate an RSA Private Key for the Personal E-Mail Certificate

Similar to Step 3, we will need to create a new private key. This private key is for your personal certificate instead of the certificate authority. Again, please be aware that the corresponding public key is derived from this private key. No extra step or command is required to generate the public key.

Execute the following command:

openssl genrsa -aes256 -out smime_test_user.key 4096

When prompted, enter a pass phrase that is different from the one used in the certificate authority private key. The following is the output using OpenSSL 1.1x versions.

Generating RSA private key, 4096 bit long modulus (2 primes)
........++++
.................++++
e is 65537 (0x010001)
Enter pass phrase for smime_test_user.key:
Verifying - Enter pass phrase for smime_test_user.key:

Again, the message is modified slightly using OpenSSL 3.x versions, but the intent is the same.

Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:

Step 6 – Create the Certificate Signing Request

Now that we have a personal private key, we will need to create a certificate signing request. This command looks similar to Step 4 where we created a self-signed certificate for the certificate authority. In this step, however, the options are slightly different because we are creating a certificate signing request instead of a self-signed certificate. We are creating a certificate signing request because we will use the certificate authority to sign the certificate.

Execute the following command:

openssl req -new -key smime_test_user.key -out smime_test_user.csr

When prompted, enter the pass phrase used to create the private key in Step 5. Again, I have left most fields blank by simply entering a . character. I have provided example entries below between the brackets following the prompts. The example uses a fake person named Test User with an e-mail address of test_user@dalesandro.net. As always, please change the values to meet your own particular needs. Do not include the brackets in your entries.

Please note that the Common Name used in this step should be different from the one used in Step 4. I also didn’t set a challenge password or company name in the final two entries.

Enter pass phrase for smime_test_user.key:
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:[.]
State or Province Name (full name) [Some-State]:[.]
Locality Name (eg, city) []:[.]
Organization Name (eg, company) [Internet Widgits Pty Ltd]:[TEST COMPANY]
Organizational Unit Name (eg, section) []:[.]
Common Name (e.g. server FQDN or YOUR name) []:[Test User]
Email Address []:[test_user@dalesandro.net]
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

Step 7 – Sign the Certificate Using the Certificate Authority

NOTE: As mentioned in Step 2, if the distribution already has a proper configuration file then the smime.cnf file created in Step 2 is unnecessary and the last two arguments should be excluded from the command, i.e., remove “-extfile c:\openssl\smime.cnf -extensions smime”. Please confirm that the extensions listed in the [smime] section in the above configuration file exist in the default configuration before removing the last two arguments.

At this point, we are finally creating the personal self-signed certificate. We will use the configuration file we created in Step 2 to set the necessary extensions and we will use the certificate authority to sign the new personal certificate.

Execute the following command (incrementing set_serial with each signing request):

openssl x509 -req -days 3650 -in smime_test_user.csr -CA ca.crt -CAkey ca.key -set_serial 1 -out smime_test_user.crt -addtrust emailProtection -addreject clientAuth -addreject serverAuth -trustout -extfile c:\openssl\smime.cnf -extensions smime

When prompted, enter the pass phrase for the certificate authority private key from Step 3.

Signature ok
subject=O = TEST COMPANY, CN = Test User, emailAddress = test_user@dalesandro.net
Getting CA Private Key
Enter pass phrase for ca.key:

NOTE: Repeat Steps 5 through 7 to create certificates for additional e-mail addresses. In Step 7, increment the set_serial argument (or assign a new unique number) for each additional certificate.

Step 8 – Package the Certificate into the PKCS12 Format

After all of that work, I imagine you’ll want to use your new self-signed digital certificate to send e-mail. Many e-mail clients will need the certificate packaged in a standard format. This step bundles the necessary files into the PKCS12 format.

Execute the following command:

openssl pkcs12 -export -in smime_test_user.crt -inkey smime_test_user.key -out smime_test_user.p12

When prompted, enter the pass phrase associated with your personal private key created in Step 5. You will also create another pass phrase which is used when importing the P12 file into an e-mail client.

Enter pass phrase for smime_test_user.key:
Enter Export Password:
Verifying - Enter Export Password:

Summary

You now have your very own self-signed S/MIME certificate which can be used to send signed e-mails. This also allows others to send you encrypted e-mails by using your public key. Once your recipients provide you with their public keys, then you’ll be able to send encrypted e-mails to them as well.

Further Reading

• Using Self-Signed S/MIME Certificates in iOS Mail App
• Using Self-Signed S/MIME Certificates in Outlook
• Using Self-Signed S/MIME Certificates in Thunderbird

Common Issues

Missing Key Usages and Extensions

Based on the comments that this post receives, the most common issue is associated with Step 7 where the required key usages are not included in the certificate due to an improper configuration file. If these extensions are not included in the certificate, then the mail client will not use the certificate for digital signatures or encryption. To verify that the certificate includes the required extensions, execute the following command.

openssl x509 -in smime_test_user.crt -purpose -noout -text

This command outputs a list of certificate purposes and extensions as well as the public key itself. Verify that the certificate includes the following (highlighted in red):

• S/MIME signing = Yes
• S/MIME encryption = Yes
• X509v3 Key Usage = Digital Signature, Non Repudiation, Key Encipherment
• X509v3 Extended Key Usage = E-mail Protection
• Trusted Uses = E-mail Protection

Certificate purposes:
SSL client : No
SSL client CA : No
SSL server : No
SSL server CA : No
Netscape SSL server : No
Netscape SSL server CA : No
S/MIME signing : Yes
S/MIME signing CA : No
S/MIME encryption : Yes
S/MIME encryption CA : No
CRL signing : No
CRL signing CA : No
Any Purpose : Yes
Any Purpose CA : Yes
OCSP helper : Yes
OCSP helper CA : No
Time Stamp signing : No
Time Stamp signing CA : No
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 1 (0x1)
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: O = TEST COMPANY, CN = TEST COMPANY CERTIFICATE AUTHORITY
        Validity
            Not Before: May 17 19:20:33 2022 GMT
            Not After : May 14 19:20:33 2032 GMT
        Subject: O = TEST COMPANY, CN = Test User, emailAddress = test_user@dalesandro.net
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                RSA Public-Key: (4096 bit)
                Modulus:
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints:
                CA:FALSE
            X509v3 Key Usage:
                Digital Signature, Non Repudiation, Key Encipherment
            X509v3 Extended Key Usage:
                E-mail Protection
            X509v3 Subject Key Identifier:
            X509v3 Authority Key Identifier:
                keyid:

            X509v3 Subject Alternative Name:
                email:test_user@dalesandro.net
    Signature Algorithm: sha256WithRSAEncryption
Trusted Uses:
  E-mail Protection
Rejected Uses:
  TLS Web Client Authentication, TLS Web Server Authentication

Certificate Authority Trust

Since we are creating self-signed certificates instead of using a well-known certificate authority, no operating system or mail client will recognize nor trust your certificate authority by default. Your certificate authority will need to be added to the operating system or mail client trust store before any personal certificates signed by that certificate authority are recognized and trusted as valid. By paying well-known providers for a certificate, you avoid this administrative hassle. This is the price you pay to do it yourself.