Updated Blockcrypt algorithm to version 1

[christoffercarlsson]

Hey @sunknudsen,

This is a work-in-progress PR that updates the Blockcrypt algorithm to version 1 as we've been discussing:

• Using ChaCha20-Poly1305 as a single authenticated encryption scheme instead of AES-256-CBC (for headers) and AES-256-GCM (for data).
• Blocks are now fixed size (which is determined during encryption), where the encrypted headers and data are laid out as a continuous byte sequence.

Will update this PR as work on version 1 progress over time.

Looking forward to hearing your thoughts on this!

Thanks,
Chris

[sunknudsen]

Hey @christoffercarlsson, when you have a moment, can you please review above changes? Looking forward to your feedback.

I also added first draft of whitepaper.