|Maximum RPM: Taking the RPM Package Manager to the Limit|
|Prev||Chapter 7. Using RPM to Verify Package Files||Next|
After all the preliminaries with PGP, it's time to get down to business. First, we need to get the package builder's public key and add it to the public keyring file used by RPM. You'll need to do this once for each package builder whose packages you'll want to check. This is what you'll need to do:
# pgp -ka RPM-PGP-KEY ./pubring.pgp 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: 1996/06/01 22:50 GMT Looking for new keys... pub 1024/CBA29BF9 1996/02/20 Red Hat Software, Inc. <email@example.com> Checking signatures... Keyfile contains: 1 new key(s) One or more of the new keys are not fully certified. Do you want to certify any of these keys yourself (y/N)? n
Here we've added Red Hat's public key, since we're going to check some package files produced by them. The file RPM-PGP-KEY contains the key. At the end, PGP asks us if we want to certify the new key. We've answered "no" since it isn't necessary to certify keys to verify package files.
Next, we'll verify a package file:
# rpm -K rpm-2.3-1.i386.rpm rpm-2.3-1.i386.rpm: size pgp md5 OK #
While the output might seem somewhat anti-climactic, we can now be nearly 100% certain this package:
was produced by Red Hat.
is unchanged from their original copy.
The output from this command shows that there are actually three distinct features of the package file that are checked by the -K option:
The size message indicates that the size of the packaged files has not changed.
The pgp message indicates that the digital signature contained in the package file is a valid signature of the package file contents, and was produced by the organization that originally signed the package.
The md5 message indicates that a checksum contained in the package file and calculated when the package was built, matches a checksum calculated by RPM during verification. Because the two checksums match, it is unlikely that the package has been modified.
The OK means that each of these tests were successful. If any had failed, the name would have been printed in parentheses. A bit later in the chapter, we'll see what happens when there are verification problems.
Adding v to a verification command will produce more interesting output:
# rpm -Kv rpm-2.3-1.i386.rpm rpm-2.3-1.i386.rpm: Header+Archive size OK: 278686 bytes Good signature from user "Red Hat Software, Inc. <firstname.lastname@example.org>". Signature made 1996/12/24 18:37 GMT using 1024-bit key, key ID CBA29BF9 WARNING: Because this public key is not certified with a trusted signature, it is not known with high confidence that this public key actually belongs to: "Red Hat Software, Inc. <email@example.com>". MD5 sum OK: 8873682c5e036a307dee87d990e75349 #
With a bit of digging, we can see that each of the three tests was performed, and each passed. The reason for that dire-sounding warning is that PGP is meant to operate without a central authority managing key distribution. PGP certifies keys based on webs of trust. For example, if an acquaintance of yours creates a public key, you can certify it by attaching your digital signature to it. Then anyone that knows and trusts you can also trust your acquaintance's public key.
In this case, the key came directly from a mass-produced Red Hat Linux CDROM. If someone was trying to masquerade as Red Hat then they have certainly gone through a lot of trouble to do so. In this case, the lack of a certified public key is not a major problem, given the fact that the CDROM came directly from the Red Hat offices. 
As mentioned earlier, not every package you'll run across is going to be signed. If this is the case, here's what you'll see from RPM:
# rpm -K bother-3.5-1.i386.rpm bother-3.5-1.i386.rpm: size md5 OK #
Note the lack of a pgp message. The size and md5 messages indicate that the package still has size and checksum information that verified properly. In fact, all recently-produced package files will have these verification measures built in automatically.
If you happen to run across an older unsigned package, you'll know it right away:
# rpm -K apmd-2.4-1.i386.rpm apmd-2.4-1.i386.rpm: No signature available #
Older package files had only a PGP-based signature; if that was missing, there was nothing left to verify.
If you happen to forget to add the right public key to RPM's keyring, you'll see the following response:
# rpm -K rpm-2.3-1.i386.rpm rpm-2.3-1.i386.rpm: size (PGP) md5 OK (MISSING KEYS) #
Here the PGP in parentheses indicates that there's a problem with the signature, and the message at the end of the line (MISSING KEYS) shows what the problem is. Basically, RPM asked PGP to verify the package against a key that PGP didn't have, and PGP complained.
Eventually it's going to happen — you go to verify a package, and it fails. We'll look at an example of a package that fails verification a bit later. Before we do that, let's make a package that won't verify, to demonstrate how sensitive RPM's verification is.
First, we made a copy of a signed package, rpm-2.3-1.i386.rpm, to be specific. We called the copy rpm-2.3-1.i386-bogus.rpm. Next, using Emacs (in hexl-mode, for all you Emacs buffs), we changed the first letter of the name of the system that built the original package. The file rpm-2.3-1.i386-bogus.rpm is now truly bogus: it has been changed from the original file.
Although the change was a small one, it still showed up when the package file was queried. Here's a listing from the original package:
# rpm -qip rpm-2.3-1.i386.rpm Name : rpm Distribution: Red Hat Linux Vanderbilt Version : 2.3 Vendor: Red Hat Software Release : 1 Build Date: Tue Dec 24 09:07:59 1996 Install date: (none) Build Host: porky.redhat.com Group : Utilities/System Source RPM: rpm-2.3-1.src.rpm Size : 631157 Summary : Red Hat Package Manager Description : RPM is a powerful package manager, which can be used to build, install, query, verify, update, and uninstall individual software packages. A package consists of an archive of files, and package information, including name, version, and description. #
And here's the same listing from the bogus package file:
# rpm -qip rpm-2.3-1.i386-bogus.rpm Name : rpm Distribution: Red Hat Linux Vanderbilt Version : 2.3 Vendor: Red Hat Software Release : 1 Build Date: Tue Dec 24 09:07:59 1996 Install date: (none) Build Host: qorky.redhat.com Group : Utilities/System Source RPM: rpm-2.3-1.src.rpm Size : 631157 Summary : Red Hat Package Manager Description : RPM is a powerful package manager, which can be used to build, install, query, verify, update, and uninstall individual software packages. A package consists of an archive of files, and package information, including name, version, and description. #
Notice that the build host name changed from porky.redhat.com to qorky.redhat.com. Using the cmp utility to compare the two files, we find that the difference occurs at byte 1201, which changed from "p" (octal 160), to "q" (octal 161):
# cmp -cl rpm-2.3-1.i386.rpm rpm-2.3-1.i386-bogus.rpm 1201 160 p 161 q #
People versed in octal numbers will note that only one bit has been changed in the entire file. That's the smallest possible change you can make! Let's see how our bogus friend fares:
# rpm -K rpm-2.3-1.i386-bogus.rpm rpm-2.3-1.i386-bogus.rpm: size PGP MD5 NOT OK #
Given that the command's output ends with NOT OK in big capital letters, it's obvious there's a problem. Since the word size was printed in lowercase, the bogus package's size was OK, which makes sense — we only changed the value of one bit without adding or subtracting anything else.
However, the PGP signature, printed in uppercase, didn't verify. Again, this makes sense, too. The package that was signed by Red Hat has been changed. The fact that the package's MD5 checksum also failed to verify provides further evidence that the bogus package is just that: bogus.
Perhaps you want to be able to verify packages but, for one reason or another, you cannot use PGP. Maybe you don't have a trustworthy source of the necessary public keys, or maybe it's illegal to possess encryption (like PGP) software in your country. Is it still possible to verify packages?
Certainly — in fact, we've already done it, in the Section called When You Are Missing the Correct Public Key. You lose the ability to verify the package's origins, as well as some level of confidence in the package's integrity, but the size and MD5 checksums still give some measure of assurance as to the package's state.
Of course, when PGP can't be used, the output from a verification always looks like something's wrong:
# rpm -K rpm-2.3-1.i386.rpm rpm-2.3-1.i386.rpm: size (PGP) md5 OK (MISSING KEYS) #
The --nopgp option directs RPM to ignore PGP entirely. If we use the --nopgp option on our example above, we find that things look a whole lot better:
# rpm -K --nopgp rpm-2.3-1.i386.rpm rpm-2.3-1.i386.rpm: size md5 OK #
Nine times out of ten, you'll probably never have to use it, but if you're the curious type, the -vv option will give you insights into how RPM verifies packages. Here's an example:
# rpm -Kvv rpm-2.3-1.i386.rpm D: New Header signature D: magic: 8e ad e8 01 D: got : 8e ad e8 01 D: Signature size: 236 D: Signature pad : 4 D: sigsize : 240 D: Header + Archive: 278686 D: expected size : 278686 rpm-2.3-1.i386.rpm: Header+Archive size OK: 278686 bytes Good signature from user "Red Hat Software, Inc. <firstname.lastname@example.org>". Signature made 1996/12/24 18:37 GMT using 1024-bit key, key ID CBA29BF9 WARNING: Because this public key is not certified with a trusted signature, it is not known with high confidence that this public key actually belongs to: "Red Hat Software, Inc. <email@example.com>". MD5 sum OK: 8873682c5e036a307dee87d990e75349 #
The lines starting with D: represent extra output produced by the -vv option. This output is normally used by software developers in the course of adding new features to RPM and is subject to change, but there's no law against looking at it.
Briefly, the output shows that RPM has detected a new-style signature block, containing size, MD5 checksum, and PGP signature information. The size of the signature, the size of the package file's header and archive sections, and the expected size of those sections are all displayed.
The --rcfile option is used to specify a file containing default settings for RPM. Normally, this option is not needed. By default, RPM uses /etc/rpmrc and a file named .rpmrc located in your login directory.
This option would be used if there was a need to switch between several sets of RPM defaults. Software developers and package builders will normally be the only people using the --rcfile option. For more information on rpmrc files, see Appendix B.
Red Hat Software's public key is also available from their website, at http://www.redhat.com/redhat/contact.html . The RPM sources also contain the key, and are available from their FTP site at ftp://ftp.redhat.com/pub/redhat/code/rpm.