macaroons: add technical documentation, fix comments

docs/macaroons.md

@@ -36,7 +36,8 @@ legitimate user.
 A macaroon is delegated by adding restrictions (called caveats) and an
 authentication code similar to a signature (technically an HMAC) to it. The
 technical method of doing this is outside the scope of this overview
-documentation, but the macaroon paper linked above describes it quite well. The
+documentation, but the [README in the macaroons package](../macaroons/README.md)
+or the macaroon paper linked above describe it in more detail. The
 user must remember several things:
 
 * Sharing a macaroon allows anyone in possession of that macaroon to use it to

lnd.go

@@ -791,8 +791,9 @@ func genCertPair(certFile, keyFile string) error {
 	return nil
 }
 
-// genMacaroons generates a pair of macaroon files; one admin-level and one
-// read-only. These can also be used to generate more granular macaroons.
+// genMacaroons generates three macaroon files; one admin-level, one
+// for invoice access and one read-only. These can also be used
+// to generate more granular macaroons.
 func genMacaroons(ctx context.Context, svc *macaroons.Service,
 	admFile, roFile, invoiceFile string) error {
 

macaroons/README.md

@@ -0,0 +1,80 @@
+# macaroons
+
+This is a more detailed, technical description of how macaroons work and how authentication 
+and authorization is implemented in `lnd`.
+
+For a more high-level overview see [macaroons.md in the docs](../docs/macaroons.md).
+
+## Root key
+
+At startup, if the option `--no-macaroons` is **not** used, a Bolt DB key/value store
+named `data/macaroons.db` is created with a bucket named `macrootkeys`.
+In this DB the following two key/value pairs are stored:
+
+* Key `0`: the encrypted root key (32 bytes).
+  * If the root key does not exist yet, 32 bytes of pseudo-random data is generated and used.
+* Key `enckey`: the parameters used to derive a secret encryption key from a passphrase.
+  * The following parameters are stored: `<salt><digest><N><R><P>`
+    * `salt`: 32 byte of random data used as salt for the `scrypt` key derivation.
+    * `digest`: sha256 hashed key derived from the `scrypt` operation. Is used to verify if the
+      password is correct.
+    * `N`, `P`, `R`: Parameters used for the `scrypt` operation.
+  * The root key is symmetrically encrypted with the derived secret key, using the
+    `secretbox` method of the library [btcsuite/golangcrypto](https://github.com/btcsuite/golangcrypto).
+  * If the option `--noencryptwallet` is used, then the default passphrase `hello` is used
+    to encrypt the root key.
+
+## Generated macaroons
+
+With the root key set up, `lnd` continues with creating three macaroon files:
+
+* `invoice.macaroon`: Grants read and write access to all invoice related gRPC
+  commands (like generating an address or adding an invoice). Can be used for a
+  web shop application for example. Paying an invoice is not possible, even if
+  the name might suggest it. The permission `offchain` is needed to pay an
+  invoice which is currently only granted in the admin macaroon.
+* `readonly.macaroon`: Grants read-only access to all gRPC commands. Could be
+  given to  a monitoring application for example.
+* `admin.macaroon`: Grants full read and write access to all gRPC commands.
+  This is used by the `lncli` client.
+
+These three macaroons all have the location field set to `lnd` and have no conditions/first party caveats
+or third party caveats set.
+
+The access restrictions are implemented with a list of entity/action pairs that is mapped
+to the gRPC functions by the `rpcserver.go`. 
+For example, the permissions for the `invoice.macaroon` looks like this:
+
+```go
+	// invoicePermissions is a slice of all the entities that allows a user
+	// to only access calls that are related to invoices, so: streaming
+	// RPCs, generating, and listening invoices.
+	invoicePermissions = []bakery.Op{
+		{
+			Entity: "invoices",
+			Action: "read",
+		},
+		{
+			Entity: "invoices",
+			Action: "write",
+		},
+		{
+			Entity: "address",
+			Action: "read",
+		},
+		{
+			Entity: "address",
+			Action: "write",
+		},
+	}
+```
+
+## Constraints / First party caveats
+
+There are currently two constraints implemented that can be used by `lncli` to restrict the
+macaroon it uses to communicate with the gRPC interface. These can be found in `constraints.go`:
+
+* `TimeoutConstraint`: Set a timeout in seconds after which the macaroon is no longer valid.
+  This constraint can be set by adding the parameter `--macaroontimeout xy` to the `lncli` command.
+* `IPLockConstraint`: Locks the macaroon to a specific IP address.
+  This constraint can be set by adding the parameter `--macaroonip a.b.c.d` to the `lncli` command.

rpcserver.go

@@ -110,7 +110,7 @@ var (
 
 	// invoicePermissions is a slice of all the entities that allows a user
 	// to only access calls that are related to invoices, so: streaming
-	// RPC's, generating, and listening invoices.
+	// RPCs, generating, and listening invoices.
 	invoicePermissions = []bakery.Op{
 		{
 			Entity: "invoices",