macaroons.md

As part of the dcrlnd 0.3-alpha release, we have addressed issue 20, which is RPC authentication. Until this was implemented, all RPC calls to dcrlnd were unauthenticated. To fix this, we've utilized macaroons, which are similar to cookies but more capable. This brief overview explains, at a basic level, how they work, how we use them for dcrlnd authentication, and our future plans.

What are macaroons?

You can think of a macaroon as a cookie, in a way. Cookies are small bits of data that your browser stores and sends to a particular website when it makes a request to that website. If you're logged into a website, that cookie can store a session ID, which the site can look up in its own database to check who you are and give you the appropriate content.

A macaroon is similar: it's a small bit of data that a client (like dcrlncli) can send to a service (like dcrlnd) to assert that it's allowed to perform an action. The service looks up the macaroon ID and verifies that the macaroon was initially signed with the service's root key. However, unlike a cookie, you can delegate a macaroon, or create a version of it that has more limited capabilities, and then send it to someone else to use.

Just like a cookie, a macaroon should be sent over a secure channel (such as a TLS-encrypted connection), which is why we've also begun enforcing TLS for RPC requests in this release. Before SSL was enforced on websites such as Facebook and Google, listening to HTTP sessions on wireless networks was one way to hijack the session and log in as that user, gaining access to the user's account. Macaroons are similar in that intercepting a macaroon in transit allows the interceptor to use the macaroon to gain all the privileges of the legitimate user.

Macaroon delegation

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 README in the macaroons package 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 access the service (in our case, dcrlnd) to do anything permitted by the macaroon. There is a specific type of restriction, called a "third party caveat," that requires an external service to verify the request; however, dcrlnd doesn't currently implement those.

• If you add a caveat to a macaroon and share the resulting macaroon, the person receiving it cannot remove the caveat.

This is used in dcrlnd in an interesting way. By default, when dcrlnd starts, it creates three files which contain macaroons: a file called admin.macaroon, which contains a macaroon with no caveats, a file called readonly.macaroon, which is the same macaroon but with an additional caveat, that permits only methods that don't change the state of dcrlnd, and invoice.macaroon, which only has access to invoice related methods.

How macaroons are used by dcrlnd and dcrlncli.

On startup, dcrlnd checks to see if the admin.macaroon, readonly.macaroon and invoice.macaroon files exist. If they don't exist, dcrlnd updates its database with a new macaroon ID, generates the three files admin.macaroon, readonly.macaroon and invoice.macaroon, all with the same ID. The readonly.macaroon file has an additional caveat which restricts the caller to using only read-only methods and the invoice.macaroon also has an additional caveat which restricts the caller to using only invoice related methods. This means a few important things:

• You can delete the admin.macaroon and be left with only the readonly.macaroon, which can sometimes be useful (for example, if you want your dcrlnd instance to run in autopilot mode and don't want to accidentally change its state).

• If you delete the data directory which contains the macaroons.db file, this invalidates the admin.macaroon, readonly.macaroon and invoice.macaroon files. Invalid macaroon files give you errors like cannot get macaroon: root key with id 0 doesn't exist or verification failed: signature mismatch after caveat verification.

You can also run dcrlnd with the --no-macaroons option, which skips the creation of the macaroon files and all macaroon checks within the RPC server. This means you can still pass a macaroon to the RPC server with a client, but it won't be checked for validity.

Since dcrlnd requires macaroons by default in order to call RPC methods, dcrlncli now reads a macaroon and provides it in the RPC call. Unless the path is changed by the --macaroonpath option, dcrlncli tries to read the macaroon from the network directory of dcrlnd's currently active network (e.g. for simnet ~/.dcrlnd/data/chain/decred/simnet/admin.macaroon) by default and will error if that file doesn't exist unless provided the --no-macaroons option. Keep this in mind when running dcrlnd with --no-macaroons, as dcrlncli will error out unless called the same way or dcrlnd has generated a macaroon on a previous run without this option.

dcrlncli also adds a caveat which makes it valid for only 60 seconds by default to help prevent replay in case the macaroon is somehow intercepted in transmission. This is unlikely with TLS, but can happen e.g. when using a PKI and network setup which allows inspection of encrypted traffic, and an attacker gets access to the traffic logs after interception. The default 60 second timeout can be changed with the --macaroontimeout option; this can be increased for making RPC calls between systems whose clocks are more than 60s apart.

Stateless initialization

As mentioned above, by default lnd creates several macaroon files in its directory. These are unencrypted and in case of the admin.macaroon provide full access to the daemon. This can be seen as quite a big security risk if the lnd daemon runs in an environment that is not fully trusted.

The macaroon files are the only files with highly sensitive information that are not encrypted (unlike the wallet file and the macaroon database file that contains the root key, these are always encrypted, even if no password is used).

To avoid leaking the macaroon information, lnd supports the so called stateless initialization mode:

• The three startup commands create, unlock and changepassword of lncli all have a flag called --stateless_init that instructs the daemon not to create *.macaroon files.
• The two operations create and changepassword that actually create/update the macaroon database will return the admin macaroon in the RPC call. Assuming the daemon and the lncli are not used on the same machine, this will leave no unencrypted information on the machine where lnd runs on.
  • To be more precise: By default, when using the changepassword command, the macaroon root key in the macaroon DB is just re-encrypted with the new password. But the key remains the same and therefore the macaroons issued before the changepassword command still remain valid. If a user wants to invalidate all previously created macaroons, the --new_mac_root_key flag of the changepassword command should be used!
• An user of lncli will see the returned admin macaroon printed to the screen or saved to a file if the parameter --save_to=some_file.macaroon is used.
• Important: By default, lnd will create the macaroon files during the unlock phase, if the --stateless_init flag is not used. So to avoid leakage of the macaroon information, use the stateless initialization flag for all three startup commands of the wallet unlocker service!

Examples:

• Create a new wallet stateless (first run):
  • lncli create --stateless_init --save_to=/safe/location/admin.macaroon
• Unlock a wallet that has previously been initialized stateless:
  • lncli unlock --stateless_init
• Use the created macaroon:
  • lncli --macaroonpath=/safe/location/admin.macaroon getinfo

Using Macaroons with GRPC clients

When interacting with dcrlnd using the GRPC interface, the macaroons are encoded as a hex string over the wire and can be passed to dcrlnd by specifying the hex-encoded macaroon as GRPC metadata:

GET https://localhost:8080/v1/getinfo
Grpc-Metadata-macaroon: <macaroon>

Where <macaroon> is the hex encoded binary data from the macaroon file itself.

A very simple example using curl may look something like this:

curl --insecure --header "Grpc-Metadata-macaroon: $(xxd -ps -u -c 1000  $HOME/.lnd/data/chain/decred/simnet/admin.macaroon)" https://localhost:8080/v1/getinfo

Have a look at the Java GRPC example for programmatic usage details.

Creating macaroons with custom permissions

The macaroon bakery is described in more detail in the README in the macaroons package.

Future improvements to the dcrlnd macaroon implementation

The existing macaroon implementation in dcrlnd and dcrlncli lays the groundwork for future improvements in functionality and security. We will add features such as:

• Improved replay protection for securing RPC calls

• Macaroon database encryption

• Root key rotation and possibly macaroon invalidation/rotation

• Additional restrictions, such as limiting payments to use (or not use) specific routes, channels, nodes, etc.

• Accounting-based macaroons, which can make an instance of dcrlnd act almost like a bank for apps: for example, an app that pays to consume APIs whose budget is limited to the money it receives by providing an API/service

• Support for third-party caveats, which allows external plugins for authorization and authentication

With this new feature, we've started laying the groundwork for flexible authentication and authorization for RPC calls to dcrlnd. We look forward to expanding its functionality to make it easy to develop secure apps.