encrypted blobs

forking from %GBLCZLI..., %inoekoQ...

i have implemented encrypted blobs for private messages in #patchfoo, in a branch: %C09cEab.... this depends on a patch to ssb-mentions: &yqKS/1K...

example message content:

{
  "type": "post",
  "text": "[empty](&6lF87HG0l6vGnto3M7/mTNFShOFmp/zAH05Ett1+eqk=.sha256#47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=)",
  "mentions": [
    {
      "link": "&6lF87HG0l6vGnto3M7/mTNFShOFmp/zAH05Ett1+eqk=.sha256",
      "name": "empty",
      "key": "47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=",
      "size": 34,
      "type": "text/plain"
    }
  ]
}

what do other client implementers and uxers think of this?
cc @Vendan @mmckegg @mix @mikey @ev @andrestaltz

on second thought, i'm not sure if it is a good idea to overload the mention object to be for both the encrypted blob and its cleartext. it complicates mapping from a link in markdown to the metadata in the mentions object, since you have to reconstruct the encrypted blob link. it's also unclear to me how to best represent both the size of the cleartext and the size of the cyphertext (both of which are useful) in that object. maybe the mention object could be split into two, one for the cyphertext and one for the cleartext:

"mentions": [
  {
    "link": BlobId#BoxKey,
    "name": string, // name of mention
    "size": number, // size of cleartext
    "type": MimeType // mime-type of cleartext
  },
  {
    "link": BlobId,
    "size": number // size of cyphertext
  }
]