Network Security Hacks

Automate Cryptographic Signature Verification

Use scripting and key servers to automate the chore of checking software authenticity.

One of the most important things you can do for the security of your system is to be familiar with the software you are installing. You probably will not have the time, knowledge, or resources to actually go through the source code for all of the software that you are installing. However, verifying that the software you are compiling and installing is what the authors intended it to be can go a long way toward preventing the widespread distribution of Trojan horses. Recently, several pivotal pieces of software (such as tcpdump, LibPCap, Sendmail, and OpenSSH) have had Trojaned versions distributed. Since this is an increasingly popular vector for attack, verifying your software is critically important.

Why is this even an issue? Unfortunately, it takes a little bit of effort to verify software before installing it. Either through laziness or ignorance, many system administrators overlook this critical step. This is a classic example of “false” laziness, as it will likely lead to more work for the sysadmin in the long run. This problem is difficult to solve because it relies on the programmers and distributors to get their acts together. Then there’s the laziness aspect: many times, software packages don’t even come with a signature to use for verifying the legitimacy of what you’ve downloaded. Often, signatures are available right along with the source code, but in order to verify the code, you must then hunt through the site for the public key that was used to create the signature. After finding the public key, you have to download it, verify that the key is genuine, add it to your keyring, and finally check the signature of the code.

Here is what this would look like when checking the signature for Version 1.3.28 of the Apache web server using GnuPG (http://www.gnupg.org):

# gpg -import KEYS
# gpg -verify apache_1.3.28.tar.gz.asc apache_1.3.28.tar.gz
gpg: Signature made Wed Jul 16 13:42:54 2003 PDT using DSA key ID 08C975E5
gpg: Good signature from "Jim Jagielski <[e-mail]>"
gpg:                 aka "Jim Jagielski <[e-mail]>"
gpg:                 aka "Jim Jagielski <[e-mail]>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Fingerprint: 8B39 757B 1D8A 994D F243  3ED5 8B3A 601F 08C9 75E5

As you can see, it’s not terribly difficult to do, but this step is often overlooked when you are in a hurry. This is where this hack comes to the rescue. We’ll use a little bit of shell scripting and what are known as key servers to reduce the number of steps to perform this process.

Key servers are a part of a public-key cryptography infrastructure that allows you to retrieve keys from a trusted third party. A nice feature of GnuPG is its ability to query key servers for a key ID and to download the result into a local keyring. To figure out which key ID to ask for, we rely on the fact that the error message generated by GnuPG tells us which key ID it was unable to find locally when trying to verify the signature.

In the previous example, if the key that GnuPG was looking for had not been imported prior to verifying the signature, it would have generated an error like this:

gpg: Signature made Wed Jul 16 13:42:54 2003 PDT using DSA key ID 08C975E5
gpg: Can't check signature: public key not found

The following script takes advantage of that error:

#!/bin/sh                                                                                                             
VENDOR_KEYRING=vendors.gpg
KEYSERVER=search.keyserver.net
KEYID="0x`gpg --verify $1 $2 2>&1 | grep 'key ID' | awk '{print $NF}'`"
gpg --no-default-keyring --keyring $VENDOR_KEYRING --recv-key \
  --keyserver $KEYSERVER $KEYID
gpg --keyring $VENDOR_KEYRING --verify $1 $2

The first line of the script specifies the keyring in which the result from the key server query will be stored. You could use pubring.gpg (which is the default keyring for GnuGP), but using a separate file will make managing vendor public keys easier. The second line of the script specifies which key server to query (the script uses http://search.keyserver.net; another good one is http://pgp.mit.edu). The third line attempts (and fails) to verify the signature without first consulting the key server. It then uses the key ID it saw in the error, and prepends an 0x in order to query the key server on the next line. Finally, GnuPG attempts to verify the signature, and specifies the keyring in which the query result was stored.

This script has shortened the verification process by eliminating the need to search for and import the public key that was used to generate the signature. Going back to the example of verifying the Apache 1.3.28 source code, you can see how much more convenient it is to verify the package’s authenticity:

# checksig apache_1.3.28.tar.gz.asc apache_1.3.28.tar.gz
gpg: requesting key 08C975E5 from HKP keyserver search.keyserver.net
gpg: key 08C975E5: public key imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg: Warning: using insecure memory!
gpg: please see http://www.gnupg.org/faq.html for more information
gpg: Signature made Wed Jul 16 13:42:54 2003 PDT using DSA key ID 08C975E5
gpg: Good signature from "Jim Jagielski <[e-mail]>"
gpg:                 aka "Jim Jagielski <[e-mail]>"
gpg:                 aka "Jim Jagielski <[e-mail]>"
gpg: checking the trustdb
gpg: no ultimately trusted keys found
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Fingerprint: 8B39 757B 1D8A 994D F243  3ED5 8B3A 601F 08C9 75E5

With this small and quick script, both the number steps needed to verify a source package and the amount of time needed have been reduced. As with any good shell script, it should help you to be lazy in a good way: by doing more work properly, but with less effort on your part.