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Getting Ready to Sign

OK, we've convinced you that signing packages is a good idea. Now we've got to make sure PGP and RPM are up to the task. As you might imagine, there are two parts to this process: one for PGP, and one for RPM. Let's get PGP ready first.

Preparing PGP: Creating a Key Pair

There is really very little to be done to PGP, assuming it's been installed properly. The only thing required is to generate a key pair. As mentioned in our mini-primer on PGP, the key pair consists of a secret key and a public key. In terms of signing packages, you will use your secret key to do the actual signing. Anyone interested in checking your signature will need your public key.

Creating a key pair is quite simple. All that's required is to issue a pgp -kg command, enter some information, and create some random bits. Here's an example key generating session:
# pgp -kg
Pretty Good Privacy(tm) 2.6.3a - Public-key encryption for the masses.
(c) 1990-96 Philip Zimmermann, Phil's Pretty Good Software. 1996-03-04
Uses the RSAREF(tm) Toolkit, which is copyright RSA Data Security, Inc.
Distributed by the Massachusetts Institute of Technology.
Export of this software may be restricted by the U.S. government.
Current time: 1996/10/31 00:42 GMT

Pick your RSA key size:
    1)   512 bits- Low commercial grade, fast but less secure
    2)   768 bits- High commercial grade, medium speed, good security
    3)  1024 bits- "Military" grade, slow, highest security 
Choose 1, 2, or 3, or enter desired number of bits: 3
Generating an RSA key with a 1024-bit modulus.

You need a user ID for your public key.  The desired form for this
user ID is your name, followed by your E-mail address enclosed in
<angle brackets>, if you have an E-mail address.
For example:  John Q. Smith <12345.6789@compuserve.com>
Enter a user ID for your public key: 

Example Key for RPM Book
You need a pass phrase to protect your RSA secret key.
Your pass phrase can be any sentence or phrase and may have many
words, spaces, punctuation, or any other printable characters. 
            Enter pass phrase: <passphrase> (Not echoed)
            Enter same pass phrase again: <passphrase> (Still not echoed)

Note that key generation is a lengthy process.

We need to generate 952 random bits.  This is done by measuring the
time intervals between your keystrokes.  Please enter some random text
on your keyboard until you hear the beep: 
(Many random characters were entered)
   0 * -Enough, thank you.
............................................
................................**** ...**** 
Pass phrase is good.  Just a moment....
Key signature certificate added.
Key generation completed.
# 
          

Let's review each of the times PGP required information. The first thing PGP needed to know was the key size we wanted. Depending on your level of paranoia, simply choose an appropriate key size. In our example, we chose the "They're out to get me" key size of 1024 bits.

Next, we needed to choose a user ID for the key. The user ID should be descriptive and should also include sufficient information for someone to contact you. We entered Example Key for RPM Book, which goes against our suggestion, but is sufficient for the purposes of our example.

After entering a user ID, we needed to add a pass phrase. The pass phrase is used to protect your secret key, so it should be something difficult for someone else to guess. It should also be memorable for you, because if you forget your pass phrase, you won't be able to use your secret key! I entered a couple of words and numbers, put together in such a way that no one could ever guess I typed rpm2kool4words

Oops…

The pass phrase is entered twice, to ensure that no typing mistakes were made. PGP also performs some cursory checks on the pass phrase, ensuring that the phrase is at least somewhat secure.

Finally comes the strangest part of the key-generation process, creating random bits. This is done by measuring the time between keystrokes. The secret here is to not hold down a key so that it auto-repeats and to not wait several seconds between keystrokes. Simply start typing anything (even nonsense text) until PGP tells you you've typed enough.

After generating enough random bits, PGP takes a minute or so to create the key pair. Assuming everything completed successfully, you'll see an ending message similar to the one above. You'll also find, in a subdirectory of your login directory called .pgp, the following files:
# ls -al ~/.pgp
total 6
drwxr-xr-x   2 root     root         1024 Oct 30 19:44 .
drwxr-xr-x   5 root     root         1024 Oct 30 19:44 ..
-rw-------   1 root     root          176 Oct 30 19:44 pubring.bak
-rw-------   1 root     root          331 Oct 30 19:44 pubring.pgp
-rw-------   1 root     root          408 Oct 30 19:44 randseed.bin
-rw-------   1 root     root          509 Oct 30 19:44 secring.pgp
# 
          

For those interested in learning exactly what each file is, feel free to consult any of the fine books on PGP. For the purposes of signing packages, all we need to know is where these files are located.

That's it! Now it's time to configure RPM to use your newly generated key.

Preparing RPM

RPM's configuration process is quite straightforward. It consists of adding a few rpmrc entries in a file of your choice. For more information on rpmrc files in general, please see Appendix B.

The entries that need to be added to an rpmrc file are:

  • signature

  • pgp_name

  • pgp_path

Let's check out the entries.

signature

The signature entry is used to select the type of signature that RPM is to use. At the time this book was written, the only legal value is pgp. So you would enter:
signature: pgp
            

pgp_name

The pgp_name entry gives RPM the user ID of the key it is to sign packages with. In our key generation example, the user ID of the key we created was Example Key for RPM Book, so this is what our entry should look like:
pgp_name: Example Key for RPM Book
            

pgp_path

The pgp_path entry is used to define the path to the directory where the keys are kept. This entry is not needed if the environment variable PGPPATH has been defined. In our example, we didn't move them from PGP's default location, which is in the subdirectory .pgp, off the user's login directory. Since we generated the key as root, our path is /root/.pgp. Therefore, our entry would look like this:
pgp_path: /root/.pgp
            

And that's it. Now it's time to sign some packages.


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