TITLE: Online Finite Field Calculator
QUESTION [6 upvotes]: I need to find an online Finite Field calculator with the following operations:
  Inverse
  SqrRoot
  Mult
  Div
I have found one a couple of days ago but lost the url, and cannot find it now. Any help would be highly appreciated.

REPLY [12 votes]: SageMathCloud (https://cloud.sagemath.com) can do absolutely everything you need online, for free.    Tell your students to:

Go to https://cloud.sagemath.com and create an account.
Click "New Project" under projects.
Open the project and click "+New" and click on "Sage Worksheet"
In the worksheet that comes up they can do pretty much anything related to finite field calculations.  Here is an image that shows how to do everything you requested in your question with a general finite field:


More comments:
k.<a> = GF(25)   # create the finite field of order 25 with generator a.  
                 # type a.minpoly() to find the poly that a satisfies

(3+a)*(5-2*a)    # do all standard arithmetic as usual

The implementation in Sage is extremely efficient for small-cardinality fields, and is built on top of Givaro and Pari.