These are recommended plugins. If you find a better one let me know.

Drawing - Gliffy
Prototyping - Balsamiq 

Evaluating

Spreadsheet Functionality in Tables

Must work in mobile. Integrate with existing Confluence Table inline.

Spreadsheets for Confluence - not able to meet criteria.

Encryption

Looks promising, no public key, but simple enough - https://marketplace.atlassian.com/plugins/com.prcpo.confluence.plugins.secure-content-block

Form Capture

http://confluence.atlassian.com/display/CONFHOST/Capturing+data+with+forms - seems to have a good run down of the available options.

http://blogs.atlassian.com/2009/05/confluence_wufo/ - expensive but looks to be very powerful and mature. Video demo on home page. Requires an additional server.

Workflow

Still looking, have not tried anything yet.