sync key representation

The sync key is represented to the user as:

X-XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

Deriving encryption and HMAC keys from the Sync Key

The hmac used is an SHA-256 HMAC.

(A deep link to Utils.makeHMACKey() would be helpful here.)

Return to "Services/Sync/SimplifiedCrypto" page.