Hack 45 Create Your Own Certificate Authority

Sign your own certificates to use in securing your network.

SSL certificates are usually thought of as being used for secure communications over the HTTP protocol. However, they are also useful in providing both a means for authentication and a means for initiating key exchange for a myriad of other services where encryption is desired, such as POP and IMAP [Hack #47], SMTP [Hack #48], IPSec (see Chapter 6), and, of course, SSL tunnels [Hack #76] . To make the best use of SSL, you will need to properly manage your own certificates.

If an SSL client needs to verify the authenticity of an SSL server, the cert used by the server needs to be signed by a Certificate Authority (CA) that is already trusted by the client. Well-known Certificate Authorities (such as Thawte and VeriSign) exist to serve as an authoritative, trusted third party for authentication. They are in the business of signing SSL certificates that are used on sites dealing with sensitive information (such as account numbers or passwords). If a site's SSL certificate is signed by a trusted authority, then presumably it is possible to verify the identity of a server supplying that cert's credentials. However, for anything other than e-commerce applications, a self-signed certificate is usually sufficient for gaining all of the security advantages that SSL provides. But even a self-signed cert must be signed by an authority that the client recognizes.

OpenSSL, a free SSL implementation, is perfectly capable of generating everything you need to run your own Certificate Authority. The CA.pl utility makes the process very simple.

In these examples, you'll need to type anything in boldface, and enter passwords wherever appropriate (they don't echo to the screen). To establish your new Certificate Authority, first change to the misc/ directory under wherever OpenSSL is installed (/System/Library/OpenSSL/ on OpenBSD; /usr/ssl/ or /usr/local/ssl/ on most Linux systems). Then use these commands:

$./CA.pl -newca CA certificate filename (or enter to create) Making CA certificate ... Generating a 1024 bit RSA private key ..........++++++ .....................++++++ writing new private key to './demoCA/private/cakey.pem' Enter PEM pass phrase: Verifying - Enter PEM pass phrase: ----- 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) []:US State or Province Name (full name) []:Colorado Locality Name (eg, city) []:Denver Organization Name (eg, company) []:NonExistant Enterprises Organizational Unit Name (eg, section) []:IT Services Common Name (eg, fully qualified host name) []:ca.nonexistantdomain.com Email Address []:[e-mail]

Note that you don't necessarily need root permissions, but you will need write permissions on the current directory.

Congratulations! You're the proud owner of your very own Certificate Authority. Take a look around:

$ ls -l demoCA/ total 16 -rw-r--r--  1 andrew  andrew  1399  3 Dec 19:52 cacert.pem drwxr-xr-x  2 andrew  andrew    68  3 Dec 19:49 certs drwxr-xr-x  2 andrew  andrew    68  3 Dec 19:49 crl -rw-r--r--  1 andrew  andrew     0  3 Dec 19:49 index.txt drwxr-xr-x  2 andrew  andrew    68  3 Dec 19:49 newcerts drwxr-xr-x  3 andrew  andrew   102  3 Dec 19:49 private -rw-r--r--  1 andrew  andrew     3  3 Dec 19:49 serial

The public key for your new Certificate Authority is contained in cacert.pem, and the private key is in private/cakey.pem. You can now use this private key to sign other SSL certs.

By default, CA.pl will create keys that are good for only one year. To change this behavior, edit CA.pl and change the line that reads:

$DAYS="-days 365";

Alternatively, you can forego CA.pl altogether and generate the public and private keys manually with a command like this:

$ openssl req -new -x509 -keyout cakey.pem -out \   cakey.pem -days 3650

This will create a key pair that is good for the next 10 years, which can of course be changed by using a different argument to the -days switch. Additionally, you should change the private key's permissions to 600, to ensure that it is protected from being read by anyone.

So far, we have only created the Certificate Authority. To actually create keys that you can use with your services, you need to create a certificate-signing request and a key. Again, this can be done easily with CA.pl. First, a certificate-signing request is created:

$ ./CA.pl -newreq-nodes Generating a 1024 bit RSA private key ...++++++ ...............................................++++++ writing new private key to 'newreq.pem' ----- 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]:US State or Province Name (full name) [Some-State]:Colorado Locality Name (eg, city) []:Denver Organization Name (eg, company) [Internet Widgits Pty Ltd]:NonExistant Enterprises Organizational Unit Name (eg, section) []:IT Services Common Name (eg, YOUR name) []:mail.nonexistantdomain.com Email Address []:[e-mail] Please enter the following 'extra' attributes to be sent with your certificate request A challenge password []: An optional company name []:NonExistant Enterprises Request (and private key) is in newreq.pem

If you wish to encrypt the private key, you can use the -newreq switch in place of -newreq-nodes. However, if you encrypt the private key, you will have to enter the password for it each time the service that uses it is started. If you decide not to use an encrypted private key, be extremely cautious with your private key, as anyone who can obtain a copy of it can impersonate your server.

Now, to actually sign the request and generate the signed certificate:

$ ./CA.pl -sign Using configuration from /System/Library/OpenSSL/openssl.cnf Enter pass phrase for ./demoCA/private/cakey.pem: Check that the request matches the signature Signature ok Certificate Details:         Serial Number: 1 (0x1)         Validity             Not Before: Dec  3 09:05:08 2003 GMT             Not After : Dec  3 09:05:08 2004 GMT         Subject:             countryName               = US             stateOrProvinceName       = Colorado             localityName              = Denver             organizationName          = NonExistant Enterprises             organizationalUnitName    = IT Services             commonName                = mail.nonexistantdomain.com             emailAddress              = [e-mail]         X509v3 extensions:             X509v3 Basic Constraints:              CA:FALSE             Netscape Comment:              OpenSSL Generated Certificate             X509v3 Subject Key Identifier:              94:0F:E9:F5:22:40:2C:71:D0:A7:5C:65:02:3E:BC:D8:DB:10:BD:88             X509v3 Authority Key Identifier:              keyid:7E:AF:2D:A4:39:37:F5:36:AE:71:2E:09:0E:49:23:70:61:28:5F:4A             DirName:/C=US/ST=Colorado/L=Denver/O=NonExistant Enterprises/OU=IT Services/ CN=Certificate Administration/emailAddress=[e-mail]             serial:00 Certificate is to be certified until Dec  7 09:05:08 2004 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 Signed certificate is in newcert.pem

Now you can set up keys in this manner for each server that needs to provide an SSL-encrypted service. It is easier to do this if you designate a single workstation to maintain the certificate authority and all the files associated with it. Don't forget to distribute your CA cert to programs that need to trust it [Hack #46] .