Library not found for class Digest:SAH2 -- digest/sha2

I get this error in Firefox when I (try to) implement password encryption as per Agile Web Development.

I put "require 'digest/sha2'" at the top of my user controller as it suggests, but when I try to edit then save a user record, I get the message in the subject of this post.

The Ruby language installation has the ../digest/sha2 subdirectory, so am I supposed to tell rails where it is, or do something else to make it available?

Thanks in advance for any help. --Fred