Calculate ComputedId value outside of shibboleth idp

[Cantor, Scott]

> Ok, I guess I'll put my dunce cap on today and say I'm flunking that
> exercise :(. We've got three values:
> 
> X = entityid of requestor
> Y = configured attribute
> Z = configured salt
> 
> Why is sha1(X!Y!Z) a better hash than sha1 of (XYZ)? Mixing in the
> entityid of the requestor makes it different for every SP. Mixing in the
> attribute makes it unique but reproducible for every user. Mixing in the
> secret salt makes it unguessable. What exactly do the exclamation marks
> do?

Consider the following:

SP: https://sp.example.com/foo
User: bar

SP: https://sp.example.com/fo
User: obar

Security is hard. (tm)

-- Scott