add MsgV2 to protospec.md

protospec.md

@@ -1,52 +1,136 @@
-# Feed V2
+# Msg V2
+
+## Terminology
+
+- **Msg** = published data that is signed and shareable
+- **Msg ID** = hash(msg.metadata)
+- **Tangle** = any single-root DAG of msgs that can be replicated by peers
+- **Tangle Root** = the origin msg of a tangle
+- **Tangle Tips** = tangle msgs that are not yet referenced by any other msg in the tangle
+- **Tangle ID** = Msg ID of the tangle's root msg
+- **Identity tangle** = tangle with msgs that add (or remove?) public keys used for signing msgs
+- **Group** = (mutable) set of public keys, implemented by an identity tangle
+- **Group ID** = ID of an identity tangle (Msg Id of the identity tangle's root msg)
+- **Feed** = tangle with msgs authored by any pubkey in a group
+- **Feed root** = a msg that is deterministically predictable and empty, so to allow others to pre-know its hash
+- **Feed ID** = ID of a feed (Msg ID of the feed's root msg)
 
 JSON
 
 ```typescript
 interface Msg {
-  data: any | null, // any object, or null
+  data: any | null // any object, or null
   metadata: {
-    dataHash: ContentHash, // blake3 hash of the `content` object serialized
-    dataSize: number, // byte size (unsigned integer) of the `content` object serialized
-    group: string | null, // blake3 hash of a group tangle root msg, or null
-    groupTips: Array<string> | null, // list of blake3 hashes of group tangle tips, or null
+    dataHash: ContentHash | null // blake3 hash of the `content` object serialized
+    dataSize: number // byte size (unsigned integer) of the `content` object serialized
+    group: string | null // blake3 hash of a group tangle root msg, or null
+    groupTips: Array<string> | null // list of blake3 hashes of group tangle tips, or null
     tangles: {
       // for each tangle this msg belongs to, identified by the tangle's root
       [rootMsgHash: string]: {
-        depth: number, // maximum distance (positive integer) from this msg to the root
-        prev: Array<MsgHash>, // list of msg hashes of existing msgs, unique set and ordered alphabetically
-      },
-    },
-    type: string, // alphanumeric string, at least 3 chars, max 100 chars
-    v: 2, // hard-coded at 2, indicates the version of the feed format
-  },
-  pubkey: Pubkey, // base58 encoded string for the author's public key
-  sig: Signature, // Signs the `metadata` object
+        depth: number // maximum distance (positive integer) from this msg to the root
+        prev: Array<MsgHash> // list of msg hashes of existing msgs, unique set and ordered alphabetically
+      }
+    }
+    type: string // alphanumeric string, at least 3 chars, max 100 chars
+    v: 2 // hard-coded at 2, indicates the version of the feed format
+  }
+  pubkey: Pubkey // base58 encoded string for the author's public key
+  sig: Signature // Signs the `metadata` object
 }
 ```
 
+## Identity tangle msgs
+
+Msgs in an identity tangle are special because they have empty `group` and `groupTips` fields.
+
+```typescript
+interface Msg {
+  data: {
+    add: string // pubkey being added to the group
+    nonce?: string // nonce required only on the identity tangle's root
+  }
+  metadata: {
+    dataHash: ContentHash
+    dataSize: number
+    group: null // MUST be null
+    groupTips: null // MUST be null
+    tangles: {
+      [identityTangleId: string]: {
+        depth: number // maximum distance (positive integer) from this msg to the root
+        prev: Array<MsgHash> // list of msg hashes of existing msgs, unique set and ordered alphabetically
+      }
+    }
+    type: 'group' // MUST be 'group'
+    v: 2
+  }
+  pubkey: Pubkey
+  sig: Signature
+}
+```
+
+## Feed root
+
+The root msg for a feed is special, its `metadata` is predictable and can be constructed by any peer. It is a data-less msg with the following shape:
+
+```typescript
+interface Msg {
+  data: null // MUST be null
+  metadata: {
+    dataHash: null // MUST be null
+    dataSize: 0 // MUST be 0
+    group: string // MUST be a group ID
+    groupTips: null // MUST be null
+    tangles: {} // MUST be empty object
+    type: string
+    v: 2
+  }
+  pubkey: Pubkey
+  sig: Signature
+}
+```
+
+Thus, given a `group` and a `type`, any peer can construct the `metadata` part of the feed root msg, and thus can derive the "msg ID" for the root based on that `metadata`.
+
+Given the root msg ID, any peer can thus refer to the feed tangle, because the root msg ID is the tangle ID for the feed tangle.
+
+Note also that _any peer_ can construct the root msg and sign it! Which renders the signatures for feed roots meaningless and ignorable.
+
+## Prev links
+
+A msg can refer to 0 or more prev msgs. The prev links are used to build the tangle.
+
+The `prev` array for a tangle should list:
+
+- All current "tips" (msgs that are not yet listed inside any `prev`) of this tangle
+- All msgs that are at the previous "lipmaa" depth relative to this `depth`
+
+## JSON serialization
+
+Whenever we need to serialize any JSON in the context of creating a Feed V1 message, we follow the "JSON Canonicalization Scheme" (JSC) defined by [RFC 8785](https://tools.ietf.org/html/rfc8785).
+
 # Feed V1
 
 JSON
 
 ```typescript
 interface Msg {
-  content: any | null, // any object, or null
+  content: any | null // any object, or null
   metadata: {
-    hash: ContentHash, // blake3 hash of the `content` object serialized
-    size: number, // byte size (unsigned integer) of the `content` object serialized
+    hash: ContentHash // blake3 hash of the `content` object serialized
+    size: number // byte size (unsigned integer) of the `content` object serialized
     tangles: {
       // for each tangle this msg belongs to, identified by the tangle's root
       [rootMsgHash: string]: {
-        depth: number, // maximum distance (positive integer) from this msg to the root
-        prev: Array<MsgHash>, // list of msg hashes of existing msgs, unique set and ordered alphabetically
-      },
-    },
-    type: string, // alphanumeric string, at least 3 chars, max 100 chars
-    v: 1, // hard-coded at 1, indicates the version of the feed format
-    who: Pubkey, // base58 encoded string for the author's public key
-  },
-  sig: Signature, // Signs the `metadata` object
+        depth: number // maximum distance (positive integer) from this msg to the root
+        prev: Array<MsgHash> // list of msg hashes of existing msgs, unique set and ordered alphabetically
+      }
+    }
+    type: string // alphanumeric string, at least 3 chars, max 100 chars
+    v: 1 // hard-coded at 1, indicates the version of the feed format
+    who: Pubkey // base58 encoded string for the author's public key
+  }
+  sig: Signature // Signs the `metadata` object
 }
 ```