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HOW-TO use plainrsa auth, contributed by Simon Chang <simonychang@gmail.com>
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Before you begin, you should understand that the RSA authentication
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mechanism hinges upon the idea of a split cryptographic key: one used
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by the public, the other readable only to you. Any data that is
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encrypted by a public key can be decrypted only by the corresponding
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private key, so that the private key user can be assured that the
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content of the transmission has not been examined by unauthorized
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parties. Similarly, any data encrypted by the private key can be
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decrypted by the public key so that the public knows that this
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transmission came from this user and nobody else (this idea is called
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non-repudiation). Also, the longer the key length, the more difficult
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it would be for potential attacker to conduct brute-force discovery of
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the keys. So, what all this means for the security administrator is
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that the setup needs a pair of reasonably long keys for each host that
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wishes to authenticate in this manner.
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With this in mind, it should be relatively straightforward to set up
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RSA authentication. For the purpose of this document, we assume that
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we are setting up RSA authentication between two networked hosts
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called Boston and Chicago. Unless otherwise noted, all steps should
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be performed on both hosts with corresponding key names. Here are the
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1) Included in each default installation of ipsec-tools is a binary
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called plainrsa-gen. This executable is used to generate a pair of
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RSA keys for the host. There are only two parameters that you should
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be concerned about: -b, which sets the number of bits for the keys,
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and -f, which specifies the output file for plainrsa-gen to send the
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results. On an ordinary Pentium-II with 128 MB of RAM, it takes only
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seconds to generate keys that are 2048 bits long, and only slightly
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longer to generate 4096-bit keys. Either key length should be
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sufficient; any longer key length actually reduces performance and
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does not increase security significantly. You should therefore run it
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plainrsa-gen -b 2048 -f /var/tmp/boston.keys
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2) When the process completes, you should have a text file that
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includes both public and private keys. GUARD THIS FILE CAREFULLY,
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because once a private key is compromised it is no longer any good,
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and you must generate a new pair from scratch. Reading the file
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itself, you should see several very long lines of alphanumeric data.
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The only line you should be concerned with is the line towards the top
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of the output file that begins with "# pubkey=0sAQPAmBdT/" or
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something to that effect. This line is your public key, which should
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be made available to the other host that you are setting up. Copy
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this line to a separate file called "boston.pub" and change the
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beginning of the line so that it reads ": PUB 0sAQPAmBdT/".
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Alternatively, you can also grab the first line of the boston.keys
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file and uncomment the line so that it reads the same as above. Now
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rename the file you generated initially to "boston.priv".
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3) You should now have two files, boston.priv and boston.pub
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(chicago.priv and chicago.pub on Chicago). The first file contains
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your private key and the second file your public key. Next you should
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find a way to get the public key over to the other host involved.
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Boston should have (1) its own key pair, and (2) Chicago's public key
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ONLY. Do not copy Chicago's private key over to Boston, because (a)
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it is not necessary, and (b) you would now have two potential places
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for losing control of your private key.
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4) You should now configure the racoon.conf configuration file for
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each host to (a) turn on RSA authentication, and (b) designate each
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host's private key and the remote host(s)'s public key(s). Take all
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your keys and place it in one directory and use the global directive
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"path certificate" to specify the location of the keys. This step is
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especially important if you are running racoon with privilege
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separation, because if racoon cannot find the keys inside the
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directory you have just specified it will fail the authentication
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process. So, write the directive like the following:
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path certificate "/etc/racoon";
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Next, you need to specify the host's own private key and the public
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keys of all the remote peers involved. For your local private key and
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remote public key(s), you should use the following directives:
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certificate_type plain_rsa "/etc/racoon/boston.priv";
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peers_certfile plain_rsa "/etc/racoon/chicago.pub";
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Notice the option "plain_rsa" for both directives.
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Finally, under the "proposal" statement section, you should specify
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the "rsasig" option for "authentication_method".
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5) You have finished configuring the host for RSA authentication.
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Now use racoonctl to reload the configuration or simply restart the
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machine and you should be all set.
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In the event that the hosts fail to communicate, first go back to the
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instructions above and make sure that:
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1) You have placed all the keys in the directory that is specified by
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the "path certificate" directive. Keep in mind that privilege
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separation will force racoon to look into that directory and nowhere
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2) You have specified correctly the host's own private key and the
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remote peer's public key.
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3) You have specified the "rsasig" method for authentication in the
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If you run into any further problems, you should try to use "racoon
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-v" to debug the setup, and send a copy of the debug messages to the
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mailing list so that we can help you determine what the problem is.
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Last modified: $Date: 2006/12/10 05:51:14 $
added
removed
src/racoon/doc/README.plainrsa
