Why can't a Rails session cookie be Base64 decoded completely?

Question

One example of a Rails 2.3.8 session cookie is

BAh7BzoLZm9vYmFyaQc6D3Nlc3Npb25faWQiJWIzOTRhNGFkNDg1Mjk2NGM2NDU1Mzc4ZTM0YjkzZjE2--67046ba78aa6d656ec7c64e73aac156f5e503627

so I assume the second part (after the --)is a checksum, and if a Base64 decode is done:

$ script/console
Loading development environment (Rails 2.3.8)

 > Base64.decode64("BAh7BzoLZm9vYmFyaQc6D3Nlc3Npb25faWQiJWIzOTRhNGFkNDg1Mjk2NGM2NDU1Mzc4ZTM0YjkzZjE2")
 => "\004\b{\a:\vfoobari\a:\017session_id\"%b394a4ad4852964c6455378e34b93f16" 

 > puts Base64.decode64("BAh7BzoLZm9vYmFyaQc6D3Nlc3Npb25faWQiJWIzOTRhNGFkNDg1Mjk2NGM2NDU1Mzc4ZTM0YjkzZjE2")
{:
  foobari:session_id"%b394a4ad4852964c6455378e34b93f16

supposedly foobar should have a value of 2 and it won't show... and what is the session_id for if it is based on a cookie -- why does it need an id?

Answer 1

The code after the -- is a SHA, hashed with the session secrect defined in the application.

And here about the ID.

Edit:

Marshal.load(Base64.decode64("BAh7BzoLZm9vYmFyaQc6D3Nlc3Npb25faWQiJWIzOTRhNGFkNDg1Mjk2NGM2NDU1Mzc4ZTM0YjkzZjE2--67046ba78aa6d656ec7c64e73aac156f5e503627".split('--').first))

=> {:foobar=>2, :session_id=>"b394a4ad4852964c6455378e34b93f16"}