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Signing Packages

There are three different ways to sign a package:

  1. Signing a package at build-time.

  2. Replacing the signature on an already-existing package.

  3. Adding a signature to an already-existing package.

Lets take a look at each one, starting with build-time signing.

--sign — Sign a Package At Build-Time

The --sign option is used to sign a package as it is being built. When this option is added to an RPM build command, RPM will ask for your PGP pass phrase. If the pass phrase is correct, the build will proceed. If not, the build stops immediately.

Here's an example of --sign in action:
# rpm -ba --sign blather-7.9.spec
            Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
* Package: blather
…
Binary Packaging: blather-7.9-1
Finding dependencies...
…
Generating signature: 1002
Wrote: /usr/src/redhat/RPMS/i386/blather-7.9-1.i386.rpm
…
Source Packaging: blather-7.9-1
…
Generating signature: 1002
Wrote: /usr/src/redhat/SRPMS/blather-7.9-1.src.rpm
# 
          

Once the pass phrase is entered, there's very little that is different from a normal build. The only obvious difference is the Generating signature message in both the binary and source packaging sections. The number following the message indicates that the signature added was created using PGP. [1]

Notice, that since RPM only signs the source and binary package files, only the -bb, and -ba options make any sense when used with --sign. This is due to the fact that only the -bb and -ba options create package files.

If we issue a quick signature check using RPM's --checksig option, we can see that there is, in fact, a PGP signature present:
# rpm --checksig blather-7.9-1.i386.rpm
blather-7.9-1.i386.rpm: size pgp md5 OK
#
          

It's clear to see that, in addition to the usual size and MD5 signatures, the package has a PGP signature.

Multiple Builds? No Problem!

You might be wondering how the --sign option would work if more than one package is to be built. Do you have to enter the pass phrase for every single package you build? The answer is no, as long as you build the packages with a single RPM command. Here's an example:
# rpm -ba --sign b*.spec
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
* Package: blather
…
Binary Packaging: blather-7.9-1
…
Generating signature: 1002
Wrote: /usr/src/redhat/RPMS/i386/blather-7.9-1.i386.rpm
…
Source Packaging: blather-7.9-1
…
Generating signature: 1002
Wrote: /usr/src/redhat/SRPMS/blather-7.9-1.src.rpm
…
* Package: bother
…
Binary Packaging: bother-3.5-1
…
Generating signature: 1002
Wrote: /usr/src/redhat/RPMS/i386/bother-3.5-1.i386.rpm
…
Source Packaging: bother-3.5-1
…
Generating signature: 1002
Wrote: /usr/src/redhat/SRPMS/bother-3.5-1.src.rpm
# 
            

Using the --sign option makes it as easy to sign one package as it is to sign one hundred. But what happens if you need to change your public key? Will you need to rebuild every single one of your packages just to update the signature?

--resign — Replace a Package's Signature(s)

As we mentioned at the end of the previous section, from time to time it may be necessary to change your public key. Certainly this would be necessary if your key's security was compromised, but other, more mundane situations might require this.

Fortunately, RPM has an option that permits you to replace the signature on an already-built package, with a new one. The option is called --resign, and here's an example of its use:
# rpm --resign blather-7.9-1.i386.rpm
            Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
#
          

While the output is not as exciting as a package build, the --resign option can be a life-saver if you need to change a package's signature, and you don't want to rebuild.

As you might have guessed, the --resign option works properly on multiple package files:
# rpm --resign b*.rpm
            Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
bother-3.5-1.i386.rpm:
#
          

There Are Limits, However…

Unfortunately, older package files cannot be re-signed. The package file must be in version 3 format, at least. If you attempt to resign a package that is too old, here's what you'll see:
# rpm --resign blah.rpm
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blah.rpm:
blah.rpm: Can't re-sign v2.0 RPM
#
            
Not sure what version your package files are at? Just use the file command to check:
# file blather-7.9-1.i386.rpm
blather-7.9-1.i386.rpm: RPM v3 bin i386 blather-7.9-1
#
            

The "v3" in file's output indicates the package file format.

--addsign — Add a Signature To a Package

The --addsign option, as the name suggests, is used to add another signature to the package. It's pretty easy to see why someone would want to have a package that had been signed by the package builders. But what reason would there be for adding a signature to a package?

One reason to have more than one signature on a package would be to provide a means of documenting the path of ownership from the package builder to the end-user.

As an example, the division of a company creates a package and signs it with the division's key. The company's headquarters then checks the package's signature and adds the corporate signature to the package, in essence stating that the signed package received by them is authentic.

Continuing the example, the doubly-signed package makes its way to a retailer. The retailer checks the package's signatures and, when they check out, adds their signature to the package.

The package now makes its way to a company that wishes to deploy the package. After checking every signature on the package, they know that it is an authentic copy, unchanged since it was first created. Depending on the deploying company's internal controls, they may choose to add their own signature, thereby reassuring their employees that the package has received their corporate "blessing".

After this lengthy example, the actual output from the --addsign option is a bit anti-climactic:
# rpm --addsign blather-7.9-1.i386.rpm
            Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
#
          
If we check the signatures of this package, we'll be able to see the multiple signatures:
# rpm --checksig blather-7.9-1.i386.rpm
blather-7.9-1.i386.rpm: size pgp pgp md5 OK
#
          

The two pgp's in --checksig's output clearly shows that the package has been signed twice.

A Few Caveats

As with the --resign option, the --addsign option cannot do its magic on pre-V3 package files:
# rpm --addsign blah.rpm
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blah.rpm:
blah.rpm: Can't re-sign v2.0 RPM
#
            

OK, the error message may not be 100% accurate, but you get the idea.

Another thing to be aware of is that the --addsign option does not check for multiple identical signatures. Although it doesn't make much sense to do so, RPM will happily let you add the same signature as many times as you'd like:
# rpm --addsig blather-7.9-1.i386.rpm
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
# rpm --addsig blather-7.9-1.i386.rpm
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
# rpm --addsig blather-7.9-1.i386.rpm
              Enter pass phrase: <passphrase> (Not echoed)
 Pass phrase is good.
blather-7.9-1.i386.rpm:
# rpm --checksig blather-7.9-1.i386.rpm
blather-7.9-1.i386.rpm: size pgp pgp pgp pgp md5 OK
#
            

As we can see from --checksig's output, the package now has four identical signatures. Maybe this is the digital equivalent of pressing down extra hard while writing your name…

Notes

[1]

The list of possible signature types can be found in the RPM sources, specifically signature.h in RPM's lib subdirectory.


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