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December, 2011:

Simple Intrusion Detection

Sometimes I want to have a simple way to determine if a file has been changed or has been compromised without the configuration required of a full feature IDS such as AIDE or Tripwire.  This technique uses CFV, a free and open-source program written in Python and “has been verified to work on linux, freebsd, openbsd, netbsd, solaris, macosx, and windows.”  This program can generate a variety of checksum formats including the SHA1 used in this example.

To create the signature file for a directory’s and sub-directories’ files use the following commands in a Linux or Mac OS X command window:

cd /usr/local/bin
cfv -C -rr -f bin.sha1 -t sha1
gpg -sab bin.sha1

To verify the file signatures use the following commands:

cd /usr/local/bin
gpg --verify bin.sha1.asc
cfv -M -f bin.sha1

The gpg command verifies the integrity of the signature file (bin.sha1).  The cfv command then verifies all the files originally tested when creating the signature file.