[Openswan Users] Pre-Shared Key Length

[Timmy]

Dear all Users,
I have a successful OpenSwan 2.6.38 installation with Pre-Shared Key
implementation.

Now, my consideration is:
What is the purpose of this Pre-Shared Key?  Is it for the purpose of
authentication or encryption?

What should be the suitable length of the Pre-Shared Key?  Should it be
as long as a SSH key?

Hope you can share your idea.

Thanks in advance.

Timmy