Posted by Marcus_Redivo on Thu 3 Nov 2005 at 12:30
This document covers a very specific, limited purpose, but one that meets a common need: preventing browser, mail, and other clients from complaining about the certificates installed on your server. Not covered is dealing with a commercial root certificate authority (CA). Instead, we will become our own root CA, and sign our own certificates.
(These procedures were developed using OpenSSL 0.9.6.)Quick Start
Those who want to start creating certificates right away without reading this whole document should skip to the summary at the end.
Note: a self-signed cert can be created with the simple command mod-ssl-makecert, part of the Debian package libapache-mod-ssl.Background
Why be our own root CA? So that we can take advantage of SSL encryption without spending unnecessary money on having our certificates signed.
A drawback is that browsers will still complain about our site not being trusted until our root certificate is imported. However, once this is done, we are no different from the commercial root CAs.
Clients will only import our root certificate if they trust us. This is where the commercial CAs come in: they purport to do extensive research into the people and organizations for whom they sign certificates. By importing (actually, by the browser vendors incorporating) their trusted root certificates, we are saying that we trust them when they guarantee that someone else is who they say they are. We can trust additional root CAs (like ourselves) by importing their CA certificates.
Note: If you are in the business of running a commercial secure site, obtaining a commercially signed certificate is the only realistic choice.
You will need an installed copy of OpenSSL for this, which is available from http://www.openssl.org/ Chances are it is already installed on your machine. This document will not cover the installation procedure.> apt-get install openssl
Creating a Root Certificate
First, we will create a directory where we can work. It does not matter where this is; I am arbitrarily going to create it in my home directory.mkdir CA cd CA mkdir newcerts private
The CA directory will contain:
- Our Certificate Authority (CA) certificate
- The database of the certificates that we have signed
- The keys, requests, and certificates we generate
It will also be our working directory when creating or signing certificates.
The CA/newcerts directory will contain:
- A copy of each certificate we sign
The CA/private directory will contain:
- Our CA private key
This key is important - Do not lose this key. Without it, you will not be able to sign or renew any certificates. Do not disclose this key to anyone. If it is compromised, others will be able to impersonate you.
Our next step is to create a database for the certificates we will sign:echo '01' > serial touch index.txt
Rather than use the configuration file that comes with OpenSSL, we are going to create a minimal configuration of our own in this directory. Start your editor (vi, pico, ...) and create a basic openssl.cnf:# # OpenSSL configuration file. # # Establish working directory. dir = .
Creating a Certificate Signing Request (CSR)
With OpenSSL, a large part of what goes into a certificate depends on the contents of the configuration file, rather than the command line. This is a good thing, because there is a lot to specify.
The configuration file is divided into sections, which are selectively read and processed according to openssl command line arguments. Sections can include one or more other sections by referring to them, which helps to make the configuration file more modular. A name in square brackets (e.g. " req ") starts each section.
We now need to add the section that controls how certificates are created, and a section to define the type of certificate to create.
The first thing we need to specify is the Distinguished Name. This is the text that identifies the owner of the certificate when it is viewed. It is not directly referenced in the configuration file, but is included into the section processed when certificate requests are created. The command is "openssl req", so the section is titled req.
Add the following to openssl.cnf:[ req ] default_bits = 1024 # Size of keys default_keyfile = key.pem # name of generated keys default_md = md5 # message digest algorithm string_mask = nombstr # permitted characters distinguished_name = req_distinguished_name [ req_distinguished_name ] # Variable name Prompt string #---------------------- ---------------------------------- 0.organizationName = Organization Name (company) organizationalUnitName = Organizational Unit Name (department, division) emailAddress = Email Address emailAddress_max = 40 localityName = Locality Name (city, district) stateOrProvinceName = State or Province Name (full name) countryName = Country Name (2 letter code) countryName_min = 2 countryName_max = 2 commonName = Common Name (hostname, IP, or your name) commonName_max = 64 # Default values for the above, for consistency and less typing. # Variable name Value #------------------------------ ------------------------------ 0.organizationName_default = The Sample Company localityName_default = Metropolis stateOrProvinceName_default = New York countryName_default = US [ v3_ca ] basicConstraints = CA:TRUE subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always,issuer:always
In order to protect ourselves from unauthorized use of our CA certificate, it is passphrase protected. Each time you use the CA certificate to sign a request, you will be prompted for the passphrase. Now would be a good time to pick a secure passphrase and put it in a safe place.
All the preparation is now in place for creating our self-signed root certificate. For this, we want to override some of the defaults we just put into the configuration, so we will specify our overrides on the command line.
Our overrides to the "openssl req" command are:
Create a new self-signed certificate: "-new -x509".
Create a CA certificate: "-extensions v3_ca ".
Make it valid for more than 30 days: -"days 3650 ".
Write output to specific locations: "-keyout, -out ".
Use our configuration file: "-config ./openssl.cnf ".
(A note on the term of validity of root certificates: When a root certificate expires, all of the certificates signed with it are no longer valid. To correct this situation, a new root certificate must be created and distributed. Also, all certificates signed with the expired one must be revoked, and re-signed with the new one. As this can be a lot of work, you want to make your root certificate valid for as long as you think you will need it. In this example, we are making it valid for ten years.)
Run the command as shown. In this case, the PEM pass phrase it asks for is a new one, which you must enter twice:# openssl req -new -x509 -extensions v3_ca -keyout private/cakey.pem \ -out cacert.pem -days 3650 -config ./openssl.cnf Using configuration from ./openssl.cnf Generating a 1024 bit RSA private key .......++++++ ..........................++++++ writing new private key to 'private/cakey.pem' Enter PEM pass phrase:demo Verifying password - Enter PEM pass phrase:demo ----- 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. ----- Organization Name (company) [The Sample Company]: Organizational Unit Name (department, division) :CA Division Email Address :email@example.com Locality Name (city, district) [Metropolis]: State or Province Name (full name) [New York]: Country Name (2 letter code) [US]: Common Name (hostname, IP, or your name) :TSC Root CA
This process produces two files as output:
- A private key in private/cakey.pem .
- A root CA certificate in cacert.pem.
cacert.pem is the file you want to distribute to your clients.
The private key (cakey.pem) looks like this:-----BEGIN RSA PRIVATE KEY----- Proc-Type: 4,ENCRYPTED DEK-Info: DES-EDE3-CBC,0947F49BB28FE5F4 jlQvt9WdR9Vpg3WQT5+C3HU17bUOwvhp/r0+viMcBUCRW85UqI2BJJKTi1IwQQ4c tyTrhYJYOP+A6JXt5BzDzZy/B7tjEMDBosPiwH2m4MaP+6wTbi1qR1pFDL3fXYDr ZsuN08dkbw9ML6LOX5Rl6bIBL3i5hnGiqm338Fl52gNstThv0C/OZhXT3B4qsJn8 qZb3mC6U2nRaP/NpZPcEx4lv2vH7OzHTu1TZ7t0asSpgpuH58dfHPw775kZDep2F LXA3Oeavg0TLFHkaFBUx2xaeEG6Txpt9I74aAsw1T6UbTSjqgtsK0PHdjPNfPGlY 5U3Do1pnU9hfoem/4RAOe0cCovP/xf6YPBraSFPs4XFfnWwgEtL09ReFqO9T0aSp 5ajLyBOYOBKQ3PCSu1HQDw/OzphInhKxdYg81WBBEfELzSdMFQZgmfGrt5DyyWmq TADwWtGVvO3pEhO1STmCaNqZQSpSwEGPGo5RFkyFvyvyozWX2SZg4g1o1X40qSg9 0FMHTEB5HQebEkKBoRQMCJN/uyKXTLjNB7ibtVbZmfjsi9oNd3NJNVQQH+o9I/rP wtFsjs+t7SKrsFB2cxZQdDlFzD6EBA+5ytebGEI1lJHcOUEa6P+LTphlwh/o1QuN IKX2YKHA4ePrBzdgZ+xZuSLn/Qtjg/eZv6i73VXoHk8EdxfOk5xkJ+DnsNmyx0vq W53+O05j5xsxzDJfWr1lqBlFF/OkIYCPcyK1iLs4GOwe/V0udDNwr2Uw90tefr3q X1OZ9Dix+U0u6xXTZTETJ5dF3hV6GF7hP3Tmj9/UQdBwBzr+D8YWzQ== -----END RSA PRIVATE KEY-----
Of course, you don't want to show this to anyone! Needless to say, the one shown here is now useless as a private key.
The certificate (cacert.pem) looks like this:-----BEGIN CERTIFICATE----- MIIDrTCCAxagAwIBAgIBADANBgkqhkiG9w0BAQQFADCBnDEbMBkGA1UEChMSVGhl IFNhbXBsZSBDb21wYW55MRQwEgYDVQQLEwtDQSBEaXZpc2lvbjEcMBoGCSqGSIb3 DQEJARYNY2FAc2FtcGxlLmNvbTETMBEGA1UEBxMKTWV0cm9wb2xpczERMA8GA1UE CBMITmV3IFlvcmsxCzAJBgNVBAYTAlVTMRQwEgYDVQQDEwtUU0MgUm9vdCBDQTAe Fw0wMTEyMDgwNDI3MDVaFw0wMjEyMDgwNDI3MDVaMIGcMRswGQYDVQQKExJUaGUg U2FtcGxlIENvbXBhbnkxFDASBgNVBAsTC0NBIERpdmlzaW9uMRwwGgYJKoZIhvcN AQkBFg1jYUBzYW1wbGUuY29tMRMwEQYDVQQHEwpNZXRyb3BvbGlzMREwDwYDVQQI EwhOZXcgWW9yazELMAkGA1UEBhMCVVMxFDASBgNVBAMTC1RTQyBSb290IENBMIGf MA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDaiAwfKB6ZBtnTRTIo6ddomt0S9ec0 NcuvtJogt0s9dXpHowh98FCDjnLtCi8du6LDTZluhlOtTFARPlV/LVnpsbyMCXMs G2qpdjJop+XIBdvoCz2HpGXjUmym8WLqt+coWwJqUSwiEba74JG93v7TU+Xcvc00 5MWnxmKZzD/R3QIDAQABo4H8MIH5MAwGA1UdEwQFMAMBAf8wHQYDVR0OBBYEFG/v yytrBtEquMX2dreysix/MlPMMIHJBgNVHSMEgcEwgb6AFG/vyytrBtEquMX2drey six/MlPMoYGipIGfMIGcMRswGQYDVQQKExJUaGUgU2FtcGxlIENvbXBhbnkxFDAS BgNVBAsTC0NBIERpdmlzaW9uMRwwGgYJKoZIhvcNAQkBFg1jYUBzYW1wbGUuY29t MRMwEQYDVQQHEwpNZXRyb3BvbGlzMREwDwYDVQQIEwhOZXcgWW9yazELMAkGA1UE BhMCVVMxFDASBgNVBAMTC1RTQyBSb290IENBggEAMA0GCSqGSIb3DQEBBAUAA4GB ABclymJfsPOUazNQO8aIaxwVbXWS+8AFEkMMRx6O68ICAMubQBvs8Buz3ALXhqYe FS5G13pW2ZnAlSdTkSTKkE5wGZ1RYSfyiEKXb+uOKhDN9LnajDzaMPkNDU2NDXDz SqHk9ZiE1boQaMzjNLu+KabTLpmL9uXvFA/i+gdenFHv -----END CERTIFICATE-----
We can query the contents of this certificate with openssl to learn to whom belongs, what it is valid for, etc.:openssl x509 -in cacert.pem -noout -text openssl x509 -in cacert.pem -noout -dates openssl x509 -in cacert.pem -noout -purpose
Signing a Certificate
Now that we have a root certificate, we can create any number of certificates for installation into our SSL applications such as HTTPS, SPOP, or SIMAP. The procedure involves creating a private key and certificate request, and then signing the request to generate the certificate.
Our configuration file needs some more definitions for creating non-CA certificates. Add the following at the end of the file:[ v3_req ] basicConstraints = CA:FALSE subjectKeyIdentifier = hash
To avoid having to repeatedly put this on the command line, insert the following line to the req section after the distinguished_name line as shown:distinguished_name = req_distinguished_name req_extensions = v3_req
Now we are ready to create our first certificate request. In this example, we are going to create a certificate for a secure POP server at mail.sample.com. Everything looks the same as when we created the CA certificate, but three of the ensuing prompts get different responses.Organizational Unit: a reminder of what the certificate is for Email Address: the postmaster Common Name: the server hostname
The Common Name must be (or the IP address must resolve to) the server name your clients use to contact your host. If this does not match, every time they connect your clients will get a message asking them if they want to use this server. In effect, the client software is saying:"Warning! You asked for mail.sample.com; the responding machine's certificate is for smtp.sample.com. Are you sure you want to continue?"openssl req -new -nodes -out req.pem -config ./openssl.cnf Organizational Unit Name (department, division) :Mail Server Email Address :firstname.lastname@example.org Common Name (hostname, IP, or your name) :mail.sample.com
This process produces two files as output:
- A private key in key.pem
- A certificate signing request in req.pem
These files should be kept. When the certificate you are about to create expires, the request can be used again to create a new certificate with a new expiry date. The private key is of course necessary for SSL encryption. When you save these files, meaningful names will help; for example, mailserver.key.pem and mailserver.req.pem..
The certificate signing request looks like this:-----BEGIN CERTIFICATE REQUEST----- MIICJDCCAY0CAQAwgagxGzAZBgNVBAoTElRoZSBTYW1wbGUgQ29tcGFueTEUMBIG A1UECxMLTWFpbCBTZXJ2ZXIxJDAiBgkqhkiG9w0BCQEWFXBvc3RtYXN0ZXJAc2Ft cGxlLmNvbTETMBEGA1UEBxMKTWV0cm9wb2xpczERMA8GA1UECBMITmV3IFlvcmsx CzAJBgNVBAYTAlVTMRgwFgYDVQQDEw9tYWlsLnNhbXBsZS5jb20wgZ8wDQYJKoZI hvcNAQEBBQADgY0AMIGJAoGBAPJhc++WxcBaoDbJpzFbDg42NcOz/ELVFMU4FlPa yUzUO+xXkdFRMPKo54d4Pf1w575Jhlu9lE+kJ8QN2st6JFySbc9QjPwVwl9D2+I3 SSf2kVTu+2Ur5izCPbVAfU0rPZxxK8ELoOkA1uwwjFz6EFuVvnHwlguonWKDtmYW u7KTAgMBAAGgOzA5BgkqhkiG9w0BCQ4xLDAqMAkGA1UdEwQCMAAwHQYDVR0OBBYE FLWaQsUVIQzWr58HtDinH1JfeCheMA0GCSqGSIb3DQEBBAUAA4GBAAbe0jrGEQ3i tyVfy5Lg4/f69rKvDGs+uhZJ9ZRx7Dl92Qq2osE7XrLB1bANmcoEv/ORLZOjWZEY NjMvuz60O7R8GKBrvb/YhAwWhIIt2LJqPkpAEWS0kY0AkoQcfZ7h6oC35+eJ7okg Uu3WuE57RgcNt7/ftr0sG1jUyRwMLvhv -----END CERTIFICATE REQUEST-----
We can view the contents to make sure our request is correct:openssl req -in req.pem -text -verify -noout
Installing the Certificate and Key
Now we need to add the configuration file section that deals with being a Certificate Authority. This section will identify the paths to the various pieces, such as the database, the CA certificate, and the private key. It also provides some basic default values. Insert the following into openssl.cnf just before the req section:[ ca ] default_ca = CA_default [ CA_default ] serial = $dir/serial database = $dir/index.txt new_certs_dir = $dir/newcerts certificate = $dir/cacert.pem private_key = $dir/private/cakey.pem default_days = 365 default_md = md5 preserve = no email_in_dn = no nameopt = default_ca certopt = default_ca policy = policy_match [ policy_match ] countryName = match stateOrProvinceName = match organizationName = match organizationalUnitName = optional commonName = supplied emailAddress = optional
To sign the request we made in the previous step, execute the following and respond to the prompts. Note that you are asked for the PEM passphrase selected earlier:openssl ca -out cert.pem -config ./openssl.cnf -infiles req.pem Using configuration from ./openssl.cnf Enter PEM pass phrase:demo Check that the request matches the signature Signature ok The Subjects Distinguished Name is as follows organizationName :PRINTABLE:'The Sample Company' organizationalUnitName:PRINTABLE:'Mail Server' emailAddress :IA5STRING:'email@example.com' localityName :PRINTABLE:'Metropolis' stateOrProvinceName :PRINTABLE:'New York' countryName :PRINTABLE:'US' commonName :PRINTABLE:'mail.sample.com' Certificate is to be certified until Dec 8 04:37:38 2002 GMT (365 days) Sign the certificate? [y/n]:y 1 out of 1 certificate requests certified, commit? [y/n]y Write out database with 1 new entries Data Base Updated
This process updates the CA database, and produces two files as output:
- A certificate in cert.pem
- A copy of the certificate in newcerts/.pem
Again, you can inspect the certificate:openssl x509 -in cert.pem -noout -text -purpose | more
The certificate has both the encoded version and a human-readable version in the same file. You can strip off the human-readable portion as follows:mv cert.pem tmp.pem openssl x509 -in tmp.pem -out cert.pem
Distributing the CA Certificate
This depends on the application. Some want the key and the certificate in the same file, and others want them separately. Combining them is easily done with:cat key.pem cert.pem >key-cert.pem
After this step, you have three installable components to choose from:
- A private key in key.pem.
- A certificate in cert.pem.
- A combined private key and certificate in key-cert.pem .
Copy the appropriate files into the locations specified by the instructions for your application and system. Restart the applications, and you are in operation with your new certificate.ApacheStunnel
Apache has separate configuration directives for the key and the certificate, so we keep each in its own file. These files should be kept outside of the DocumentRoot subtree, so a reasonable directory structure might be:File Comment /home/httpd/html Apache DocumentRoot /home/httpd/ssl SSL-related files /home/httpd/ssl/cert.pem Site certificate /home/httpd/ssl/key.pem Site private key
Within the directive for the site (which of course should be on port 443), include the directives that point to these files:ServerName mail.sample.com DocumentRoot /home/httpd/html ... other directives for this site ... SSLEngine on SSLLog /var/log/ssl_engine_log SSLCertificateFile /home/httpd/ssl/cert.pem SSLCertificateKeyFile /home/httpd/ssl/key.pem
stunnel is used as an SSL wrapper for normal non-secure services such as IMAP and POP. It accepts as arguments (among other things) the service to execute, and the location of the certificate and private key.
The key and the certificate are provided in the same file. These can go anywhere, but a good location might be /etc/ssl/certs. Specify it on the stunnel command line as follows:stunnel -p /etc/ssl/certs/key-cert.pem
This, finally, is the step that stops the clients from complaining about untrusted certificates. Send cacert.pem to anyone who is going to use your secure servers, so they can install it in their browsers, mail clients, et cetera as a root certificate.
Getting a Commercially Signed Certificate
Your certificate chain can break due to certificate expiry in two ways:
- The certificates you signed with your root certificate have expired.
- Your root certificate itself has expired.
In the second case, you have some work to do. A new root CA certificate must be created and distributed, and then your existing certificates must be recreated or re-signed.
In the first case, you have two options. You can either generate new certificate signing requests and sign them as described above, or (if you kept them) you can re-sign the original requests. In either case, the old certificates must be revoked, and then the new certificates signed and installed into your secure applications as described earlier.
You cannot issue two certificates with the same Common Name, which is why the expired certificates must be revoked. The certificate is in the newcerts directory; you can determine its filename by browsing index.txt and searching for the Common Name (CN) on it. The filename is the index plus the extension ".pem", for example "02.pem". To revoke a certificate:openssl ca -revoke newcerts/02.pem -config ./openssl.cnf Using configuration from ./openssl.cnf Enter PEM pass phrase: demo Revoking Certificate 02. Data Base Updated
Now that the certificate has been revoked, you can re-sign the original request, or create and sign a new one as described above.
Publishing Your CA Certificate
The process is basically the same as the one just demonstrated, but the CA does most of it. You need to generate a Certificate Signing Request as shown above, and then submit it for signing. You will receive a signed certificate for installation.
This certificate will automatically be trusted by your client's browser, as the browser has the commercial CA's certificate built in. There is no need to distribute anything.
The configuration described here may be inadequate for this purpose, as there is much more that can go into a request. Different certificate authorities require different features in the certificate signing request, none of which we have gone into here. This additional material is beyond the current scope of this document.
You can post the certificate on your web site for download. If you do this, you should also post a Certificate Revocation List (CRL), and a means of displaying a certificate given its serial number. This is outside the current scope of this document.
Apache will serve your certificate in a form recognizable to browsers if you specify its MIME type. For example, you can use the filename extension ".crt" for downloadable certificates, and put the following into the general section of your Apache configuration:AddType application/x-x509-ca-cert .crt
Now you can post the certificate for download with a link like Our Root Certificate
and when the link is followed the visitor's browser would offer to install the certificate.
The CRL can be created as follows:openssl ca -gencrl -crldays 31 -config ./openssl.cnf -out rootca.crl
You now have enough information to create and sign certificates on your own behalf. While this is a fairly long document, the procedure can be summarized easily.One-Time Setup
Set up, and create a root CA certificate. Commands:# mkdir CA # cd CA # mkdir newcerts private # echo '01' >serial # touch index.txt # (IMPORTANT: Install and edit the configuration file shown below.) # openssl req -new -x509 -extensions v3_ca -keyout private/cakey.pem \ -out cacert.pem -days 365 -config ./openssl.cnf
- cacert.pem - CA certificate
- private/cakey.pem - CA private key
Distribute cacert.pem to your clients.Per Certificate
Create certificate signing requests and sign them, supplying appropriate values for the Common Name and the Organizational Unit.
Commands :openssl req -new -nodes -out req.pem -config ./openssl.cnf openssl ca -out cert.pem -config ./openssl.cnf -infiles req.pem cat key.pem cert.pem >key-cert.pem
- key.pem - Private key
- req.pem - Certificate signing request
- cert.pem - Certificate
- key-cert.pem - Combined private key and certificate
Install key.pem and cert.pem, or just key-cert.pem as appropriate for your server application.Per Certificate - Renewal
Revoke the expired certificate, and re-sign the original request.
Commands :openssl ca -revoke newcerts/.pem -config ./openssl.cnf openssl ca -out cert.pem -config ./openssl.cnf -infiles req.pem
Install the renewed certificates in the same manner as the original ones.
Configuration File# # OpenSSL configuration file. # # Establish working directory. dir = . [ ca ] default_ca = CA_default [ CA_default ] serial = $dir/serial database = $dir/index.txt new_certs_dir = $dir/newcerts certificate = $dir/cacert.pem private_key = $dir/private/cakey.pem default_days = 365 default_md = md5 preserve = no email_in_dn = no nameopt = default_ca certopt = default_ca policy = policy_match [ policy_match ] countryName = match stateOrProvinceName = match organizationName = match organizationalUnitName = optional commonName = supplied emailAddress = optional [ req ] default_bits = 1024 # Size of keys default_keyfile = key.pem # name of generated keys default_md = md5 # message digest algorithm string_mask = nombstr # permitted characters distinguished_name = req_distinguished_name req_extensions = v3_req [ req_distinguished_name ] # Variable name Prompt string #---------------------- ---------------------------------- 0.organizationName = Organization Name (company) organizationalUnitName = Organizational Unit Name (department, division) emailAddress = Email Address emailAddress_max = 40 localityName = Locality Name (city, district) stateOrProvinceName = State or Province Name (full name) countryName = Country Name (2 letter code) countryName_min = 2 countryName_max = 2 commonName = Common Name (hostname, IP, or your name) commonName_max = 64 # Default values for the above, for consistency and less typing. # Variable name Value #------------------------------ ------------------------------ 0.organizationName_default = The Sample Company localityName_default = Metropolis stateOrProvinceName_default = New York countryName_default = US [ v3_ca ] basicConstraints = CA:TRUE subjectKeyIdentifier = hash authorityKeyIdentifier = keyid:always,issuer:always [ v3_req ] basicConstraints = CA:FALSE subjectKeyIdentifier = hash
It has been brought to my attention that this article has been plagiarized by the poster who claimed it as his own work. That poster has now has his accoutn suspended.
The original author of the article, Marcus Redivo, has kindly allowed the text to remain on this site. The original article as written by Marcus can be found upon his website here: