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MD5 is a one-way hashing algorithm for creating digest "signatures" or checksums of strings (usually entire files). Ruby’s standard library includes MD5 as part of the Digest:: set of extension classes.

Creating MD5 checksums is a simple matter of requiring the digest/md5 library and using either the Digest::MD5.digest or Digest::MD5.hexdigest class methods to return the digest of a given string — we will use hexdigests in this article as they are printable:

  #!/usr/bin/ruby -w
  require 'digest/md5'
  digest = Digest::MD5.hexdigest("Hello World\n")
  puts digest
  __END__

  e59ff97941044f85df5297e1c302d260

MD5 digests are 128 bit (16 byte) signatures and are the most common method of providing checksums for files available on the net. To create a checksum of an entire file you need only pass in the file as a string. The following will print out the filename and md5 digest of all the files passed to it on the command line:

  #!/usr/bin/ruby -w
  require 'digest/md5'
  ARGV.each do |f|
    digest = Digest::MD5.hexdigest(File.read(f))
    puts &
1000
amp;quot;#{f}: #{digest}"
  end
  __END__

Sometimes you want to do more than just calculate the checksum of a single string — maybe you have a large file and want to calculate the digest in small, memory friendly chunks; or maybe you are calculating a digest from a stream of input. In such cases you can create a Digest::MD5 object and use the #update (alias: #<<), digest, and #hexdigest methods.

For example purposes, we will create a digest by reading and adding one line at a time from a test file as well as calculating the digest all at once. I will use the source for this article as the test-file:

  #!/usr/bin/ruby -w
  require 'digest/md5'
  filename = 'MD5.rdoc'

  all_digest = Digest::MD5.hexdigest(File.read(filename))

  incr_digest = Digest::MD5.new()
  file = File.open(filename, 'r')
  file.each_line do |line|
    incr_digest << line
  end

  puts incr_digest.hexdigest
  puts all_digest
  __END__

Saving this as md.rb and running it produced the following output:

  ~$ ruby md.rb
  a075a1debea63e5d7073d9eed19ce031
  a075a1debea63e5d7073d9eed19ce031

In addition to providing checksums for files you make available, or checking files and packages you’ve downloaded, another use is fingerprinting sensitive directories on your system. Creating a database of MD5 digests for sensitive files or directories means you can periodically cross-check your sensitive data against the database to see if anything has been changed without your knowledge. This can provide you with a very simple to implement addition to your intrusion detection tools.