15.9. OpenSSL

Written by: Tom Rhodes.

One feature that many users overlook is the OpenSSL toolkit included in FreeBSD. OpenSSL provides an encryption transport layer on top of the normal communications layer; thus allowing it to be intertwined with many network applications and services.

Some uses of OpenSSL may include encrypted authentication of mail clients, web based transactions such as credit card payments and more. Many ports such as www/apache13-ssl, and mail/sylpheed-claws will offer compilation support for building with OpenSSL.

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In most cases the Ports Collection will attempt to build the security/openssl port unless the WITH_OPENSSL_BASE make variable is explicitly set to «yes».

The version of OpenSSL included in FreeBSD supports Secure Sockets Layer v2/v3 (SSLv2/SSLv3), Transport Layer Security v1 (TLSv1) network security protocols and can be used as a general cryptographic library.

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While OpenSSL supports the IDEA algorithm, it is disabled by default due to United States patents. To use it, the license should be reviewed and, if the restrictions are acceptable, the MAKE_IDEA variable must be set in make.conf.

One of the most common uses of OpenSSL is to provide certificates for use with software applications. These certificates ensure that the credentials of the company or individual are valid and not fraudulent. If the certificate in question has not been verified by one of the several «Certificate Authorities», or CAs, a warning is usually produced. A Certificate Authority is a company, such as VeriSign, which will sign certificates in order to validate credentials of individuals or companies. This process has a cost associated with it and is definitely not a requirement for using certificates; however, it can put some of the more paranoid users at ease.

15.9.1. Generating Certificates

To generate a certificate, the following command is available:

# openssl req -new -nodes -out req.pem -keyout cert.pem
Generating a 1024 bit RSA private key
................++++++
.......................................++++++
writing new private key to 'cert.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]:PA
Locality Name (eg, city) []:Pittsburgh
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Company
Organizational Unit Name (eg, section) []:Systems Administrator
Common Name (eg, YOUR name) []:localhost.example.org
Email Address []:trhodes@FreeBSD.org

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:SOME PASSWORD
An optional company name []:Another Name

Notice the response directly after the «Common Name» prompt shows a domain name. This prompt requires a server name to be entered for verification purposes; placing anything but a domain name would yield a useless certificate. Other options, for instance expire time, alternate encryption algorithms, etc. are available. A complete list may be obtained by viewing the openssl(1) manual page.

Two files should now exist in the directory in which the aforementioned command was issued. The certificate request, req.pem, may be sent to a certificate authority who will validate the credentials that you entered, sign the request and return the certificate to you. The second file created will be named cert.pem and is the private key for the certificate and should be protected at all costs; if this falls in the hands of others it can be used to impersonate you (or your server).

In cases where a signature from a CA is not required, a self signed certificate can be created. First, generate the RSA key:

# openssl dsaparam -rand -genkey -out myRSA.key 1024

Next, generate the CA key:

# openssl gendsa -des3 -out myca.key myRSA.key

Use this key to create the certificate:

# openssl req -new -x509 -days 365 -key myca.key -out new.crt

Two new files should appear in the directory: a certificate authority signature file, myca.key and the certificate itself, new.crt. These should be placed in a directory, preferably under /etc, which is readable only by root. Permissions of 0700 should be fine for this and they can be set with the chmod utility.

15.9.2. Using Certificates, an Example

So what can these files do? A good use would be to encrypt connections to the Sendmail MTA. This would dissolve the use of clear text authentication for users who send mail via the local MTA.

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This is not the best use in the world as some MUAs will present the user with an error if they have not installed the certificate locally. Refer to the documentation included with the software for more information on certificate installation.

The following lines should be placed inside the local .mc file:

dnl SSL Options
define(`confCACERT_PATH',`/etc/certs')dnl
define(`confCACERT',`/etc/certs/new.crt')dnl
define(`confSERVER_CERT',`/etc/certs/new.crt')dnl
define(`confSERVER_KEY',`/etc/certs/myca.key')dnl
define(`confTLS_SRV_OPTIONS', `V')dnl

Where /etc/certs/ is the directory to be used for storing the certificate and key files locally. The last few requirements are a rebuild of the local .cf file. This is easily achieved by typing make install within the /etc/mail directory. Follow that up with make restart which should start the Sendmail daemon.

If all went well there will be no error messages in the /var/log/maillog file and Sendmail will show up in the process list.

For a simple test, simply connect to the mail server using the telnet(1) utility:

# telnet example.com 25
Trying 192.0.34.166...
Connected to example.com.
Escape character is '^]'.
220 example.com ESMTP Sendmail 8.12.10/8.12.10; Tue, 31 Aug 2004 03:41:22 -0400 (EDT)
ehlo example.com
250-example.com Hello example.com [192.0.34.166], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP
quit
221 2.0.0 example.com closing connection
Connection closed by foreign host.

If the «STARTTLS» line appears in the output then everything is working correctly.

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