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Note: This README file is a slightly adapted version of README from rescrv/libmacaroons.

Macaroons are Better Than Cookies!

This library is libmacaroons-compatible Erlang implementation of macaroons. Macaroons are flexible authorization tokens that work great in distributed systems. Like cookies, macaroons are bearer tokens that enable applications to ascertain whether their holders' actions are authorized. But macaroons are better than cookies!

Why Macaroons?

Macaroons are great for authorization because they're similar enough to cookies to be immediately usable by developers, but they include several features not present in cookies or other token-based authorization schemes. In particular:

  • Delegation with Contextual Caveats (i.e., confinement of the usage context):
    Macaroons support delegation. Give your macaroon to another user, and they can act on your behalf, with the same authority. Cookies permit delegation as well, but the remaining features of macaroons make it much more safe and practical to pass around macaroons than cookies. In particular, macaroons can limit when, where, and by whom the delegated authority can be exercised (e.g., within one minute, from a machine that holds a certain key, or by a certain logged-in user), by using attenuation and third-party caveats.

  • Attenuation: Macaroons enable users to add caveats to the macaroon that attenuate how, when, and where it may be used. Unlike cookies, macaroons permit their holder to attenuate them before delegating. Whereas cookies and authorization tokens enable an application to get access to all of your data and to perform actions on your behalf with your full privileges, macaroons enable you to restrict what they can do. Those questionable startups that "just want the address book, we swear it," become a whole lot more secure when the target application supports macaroons, because macaroons enable you to add caveats that restrict what the application can do.

  • Proof-Carrying: Macaroons are efficient, because they carry their own proof of authorization---cryptographically secured, of course. A macaroon's caveats are constructed using chained HMAC functions, which makes it really easy to add a caveat, but impossible to remove a caveat. When you attenuate a macaroon and give it to another application, there is no way to strip the caveats from the macaroon. It's easy for the entity that created a macaroon to verify the embedded proof, but others cannot.

  • Third-Party Caveats: Macaroons allow caveats to specify predicates that are enforced by third parties. A macaroon with a third-party caveat will only be authorized when the third party certifies that the caveat is satisfied. This enables loosely coupled distributed systems to work together to authorize requests. For example, a data store can provide macaroons that are authorized if and only if the application's authentication service says that the user is authenticated. The user obtains a proof that it is authenticated from the authentication service, and presents this proof alongside the original macaroon to the storage service. The storage service can verify that the user is indeed authenticated, without knowing anything about the authentication service's implementation---in a standard implementation, the storage service can authorize the request without even communicating with the authentication service.

  • Simple Verification: Macaroons eliminate complexity in the authorization code of your application. Instead of hand-coding complex conditionals in each routine that deals with authorization, and hoping that this logic is globally consistent, you construct a general verifier for macaroons. This verifier knows how to soundly check the proofs embedded within macaroons to see if they do indeed authorize access.

  • Decoupled Authorization Logic: Macaroons separate the policy of your application (who can access what, when), from the mechanism (the code that actually upholds this policy). Because of the way the verifier is constructed, it is agnostic to the actual underlying policies it is enforcing. It simply observes the policy (in the form of an embedded proof) and certifies that the proof is correct. The policy itself is specified when macaroons are created, attenuated, and shared. You can easily audit this code within your application, and ensure that it is upheld everywhere.

The rest of this document walks through the specifics of macaroons and see just how easy authorization can be. So pour a fresh cup of espresso to enjoy alongside your macaroons and read on!

Installing Macaroons

Building macaroons has the following requirements:

  • libsodium installed on the system
  • C compiler
  • Erlang compiled with enabled dirty NIFs
  • rebar

macaroons uses enacl to provide libsodium cryptographic functions in Erlang.

Including macaroons is trivial, provided your Erlang application uses rebar for dependency management. Simply add the library to the list of dependencies in rebar.conf:

{deps, [
    {macaroons, "1.0.2", {git, "https://github.com/kzemek/macaroons.git", {tag, "1.0.2"}}}
]}.

Creating Your First Macaroon

Imagine that you ran a bank, and were looking to use macaroons as the basis of your authorization logic. Assuming you already installed libmacaroons, you can create a macaroon like this:

1> Secret = "this is our super secret key; only we should know it".
2> Public = "we used our secret key".
3> Location = "http://mybank/".
4> M = macaroon:create(Location, Secret, Public).

We've created our first macaroon!

You can see here that it took three pieces of information to create a macaroon. We start with a secret. Here, we just have English text, but in reality we would want to use something more random and less predictable (see below for a suggestion on how to generate one). The public portion tells us which secret we used to create the macaroon, but doesn't give anyone else a clue as to the contents of the secret. Anyone in possession of the macaroon can see the public portion:

5> macaroon:identifier(M).
<<"we used our secret key">>

This public portion, known as the macaroon's identifier, can be anything that enables us to remember our secret. In this example, we know that the string 'we used our secret key' always refers to this secret. We could just as easily keep a database mapping public macaroon identities to private secrets, or encrypt the public portion using a key known only to us. The only requirement here is that our application be able to remember the secret given the public portion, and that no one else is able to guess the secret.

Our macaroon includes some additional information that enables us to add caveats and figure out where the macaroon should be used. The macaroon's location gives a hint to the user about where the macaroon is accepted for authorization. This hint is a free-form string maintained to help applications figure out where to use macaroons; the libmacaroons library (and by extension, the Erlang wrapper) does not ascribe any meaning to this location.

Each macaroon also has a signature that is the key used to add caveats and verify the macaroon. The signature is computed by the macaroons library, and is unique to each macaroon. Applications should never need to directly work with the signature of the macaroon. Any entity in possession of the macaroon's signature should be assumed to possess the macaroon itself.

Both of these pieces of information are publicly accessible:

6> macaroon:location(M).
<<"http://mybank/">>
7> macaroon:signature(M).
<<"e3d9e02908526c4c0039ae15114115d97fdd68bf2ba379b342aaf0f617d0552f">>

We can share this macaroon with others by serializing it. The serialized form is pure-ASCII, and is safe for inclusion in secure email, a standard HTTPS cookie, or a URL. We can get the serialized form with:

8> macaroon:serialize(M).
{ok,<<"MDAxY2xvY2F0aW9uIGh0dHA6Ly9teWJhbmsvCjAwMjZpZGVudGlmaWVyIHdlIHVzZWQgb3VyIHNlY3JldCBrZXkKMDAyZnNpZ25hdHVyZSDj"...>>}

Of course, this serialized form can be displayed in a more human-readable form for easy debugging:

9> io:format("~s", [macaroon:inspect(M)]).
location http://mybank/
identifier we used our secret key
signature e3d9e02908526c4c0039ae15114115d97fdd68bf2ba379b342aaf0f617d0552f
ok

Adding Caveats

At this point, we have an unconstrained macaroon that authorizes everything within our bank. In practice, such a macaroon is dangerous, because very few people should hold such power. Let's add a caveat to our macaroon that restricts it to just the account number 3735928559.

10> macaroon:add_first_party_caveat(M, "account = 3735928559").

This new macaroon includes the same identifier and location that our old macaroon from our initial macaroon, but includes the additional caveat that restricts the bank account. The signature of this new macaroon is different, and incorporates the new caveat we've just added. An entity in possession of this new macaroon cannot simply remove our new caveat to construct the old macaroon:

11> M2 = macaroon:add_first_party_caveat(M, "account = 3735928559").
12> io:format("~s", [macaroon:inspect(M2)]).
location http://mybank/
identifier we used our secret key
cid account = 3735928559
signature 1efe4763f290dbce0c1d08477367e11f4eee456a64933cf662d79772dbb82128
ok

Of course, we can add a few more caveats, and the macaroon's signature will change with each of them.

13> M3 = macaroon:add_first_party_caveat(M2, "time < 2020-01-01T00:00").
14> macaroon:signature(M3).
<<"b5f06c8c8ef92f6c82c6ff282cd1f8bd1849301d09a2db634ba182536a611c49">>
15> M4 = macaroon:add_first_party_caveat(M3, "email = alice@example.org").
16> macaroon:signature(M4).
<<"ddf553e46083e55b8d71ab822be3d8fcf21d6bf19c40d617bb9fb438934474b6">>
17> io:format("~s", [macaroon:inspect(M4)]).
location http://mybank/
identifier we used our secret key
cid account = 3735928559
cid time < 2020-01-01T00:00
cid email = alice@example.org
signature ddf553e46083e55b8d71ab822be3d8fcf21d6bf19c40d617bb9fb438934474b6
ok

The combination of all caveats in this macaroon authorize alice@example.org to access account 3735928559 until the end of 2019. Alice may present this macaroon to the bank any time she wishes to prove to the bank that she is authorized to access her account. Ideally, she'll transmit the serialized form of the macaroon to the bank:

18> {ok, Msg} = macaroon:serialize(M4).
{ok,<<"MDAxY2xvY2F0aW9uIGh0dHA6Ly9teWJhbmsvCjAwMjZpZGVudGlmaWVyIHdlIHVzZWQgb3VyIHNlY3JldCBrZXkKMDAxZGNpZCBhY2NvdW50"...>>}
19> % send msg to the bank

Verifying Macaroons

Our bank application's purpose is to protect users accounts from unauthorized access. For that reason, it cannot just accept anything that looks like a macaroon---that would defeat the point of using macaroons in the first place. So how can we ensure that only authorized users access the bank?

We can determine whether a request is authorized through a process called verification. First, we construct a verifier that can determine whether the caveats on macaroons are satisfied. We can then use our verifier to determine whether a given macaroon is authorized in the context of the request. For example, our bank account application knows the account number specified in the request, and can specify account = # when building the verifier. The verifier can then check that this matches the information within the macaroon, and authorize the macaroon if it does indeed match.

Let's walk through the verification process for Alice's macaroon that we constructed in the previous section. The first step, of course, is for the bank to deserialize the macaroon from the message. This converts the macaroon into a form we can work with.

1> {ok, M} = macaroon:deserialize(Msg).
2> io:format("~s", [macaroon:inspect(M)]).
location http://mybank/
identifier we used our secret key
cid account = 3735928559
cid time < 2020-01-01T00:00
cid email = alice@example.org
signature ddf553e46083e55b8d71ab822be3d8fcf21d6bf19c40d617bb9fb438934474b6
ok

We have the same macaroon that Alice believes authorizes her to access her own account, but we must verify this for ourselves. One (very flawed) way we could try to verify this macaroon would be to manually parse it and authorize the request if its caveats are true. But handling things this way completely sidesteps all the crypto-goodness that macaroons are built upon.

Another approach to verification would be to use libmacaroons's built-in verifier to process the macaroon. The verifier hides many of the details of the verification process, and provides a natural way to work with many kinds of caveats. The verifier itself is constructed once, and may be re-used to verify multiple macaroons.

3> V = macaroon_verifier:create().

Let's go ahead and try to verify the macaroon to see if the request is authorized. To verify the request, we need to provide the verifier with Alice's macaroon, and the secret that was used to construct it. In a real application, we would retrieve the secret using macaroon:identifier(M); here, we know the secret and provide it directly. A verifier can only ever successfully verify the macaroon when provided with the macaroon and its corresponding secret---no secret, no authorization.

Intuitively, our verifier should say that this macaroon is unauthorized because our verifier cannot prove that any of the caveats we've added are satisfied. We can see that it fails just as we would expect:

4> macaroon_verifier:verify(V, M, Secret).
{error,{unverified_caveat,<<"account = 3735928559">>}}

We can inform the verifier of the caveats used by our application using two different techniques. The first technique is to directly provide the verifier with the caveats that match the context of the request. For example, every account-level action in a typical banking application is performed by a specific user and targets a specific account. This information is fixed for each request, and is known at the time of the request. We can tell the verifier directly about these caveats like so:

5> V1 = macaroon_verifier:satisfy_exact(V, "account = 3735928559").
6> V2 = macaroon_verifier:satisfy_exact(V1, "email = alice@example.org").

Caveats like these are called exact caveats because there is exactly one way to satisfy them. Either the account number is 3735928559, or it isn't. At verification time, the verifier will check each caveat in the macaroon against the list of satisfied caveats provided to satisfyExact. When it finds a match, it knows that the caveat holds and it can move onto the next caveat in the macaroon.

Generally, you will specify multiple true statements as exact caveats, and let the verifier decide which are relevant to each macaroon at verification time. If you provide all exact caveats known to your application to the verifier, it becomes trivial to change policy decisions about authorization. The server performing authorization can treat the verifier as a black-box and does not need to change when changing the authorization policy. The actual policy is enforced when macaroons are minted and when caveats are embedded. In our banking example, we could provide some additional satisfied caveats to the verifier, to describe some (or all) of the properties that are known about the current request. In this manner, the verifier can be made more general, and be "future-proofed", so that it will still function correctly even if somehow
the authorization policy for Alice changes; for example, by adding the three following facts, the verifier will continue to work even if Alice decides to self-attenuate her macaroons to be only usable from her IP address and browser:

7> V3 = macaroon_verifier:satisfy_exact(V2, "IP = 127.0.0.1").
8> V4 = macaroon_verifier:satisfy_exact(V3, "browser = Chrome").
9> V5 = macaroon_verifier:satisfy_exact(V4, "action = deposit").

Although it's always possible to satisfy a caveat within a macaroon by providing it directly to the verifier, doing so can be quite tedious. Consider the caveat on access time embedded within Alice's macaroon. While an authorization routine could provide the exact caveat time < 2020-01-01T00:00, doing so would require inspecting the macaroon before building the verifier. Just like using MD5 to hash passwords, inspecting a macaroon's structure to build a verifier for it is considered to be very bad practice, and should be violently demonized in Hacker News discussions with vague, slightly inaccurate allusions to pbkdf2.

So how can we tell our verifier that the caveat on access time is satisfied? We could provide many exact caveats of the form time < YYYY-mm-ddTHH:MM, but this reeks of inefficiency. The second technique for satisfying caveats provides a more general solution.

Called general caveats, the second technique for informing the verifier that a caveat is satisfied allows for expressive caveats. Whereas exact caveats are checked by simple byte-wise equality, general caveats are checked using an application-provided callback that returns true if and only if the caveat is true within the context of the request. There's no limit on the contents of a general caveat, so long as the callback understands how to determine whether it is satisfied.

We can verify the time caveat on Alice's macaroon by writing a function that checks the current time against the time specified by the caveat (the example uses qdate library for date operations):

10> CheckTime =
10>     fun
10>         (<<"time < ", Date/binary>>) ->
10>             qdate:compare(erlang:now(), '<=', Date);
10>         (_) ->
10>             false
10>     end.

This callback processes all caveats that begin with time < , and returns true if the specified time has not yet passed. We can see that our caveat does indeed return true when the caveat holds, and false otherwise:

11> CheckTime(<<"time < 2020-01-01T00:00">>).
true
12> CheckTime(<<"time < 2014-01-01T00:00">>).
false
13> CheckTime(<<"account = 3735928559">>).
false

We can provide the CheckTime function directly to the verifier, so that it may check time-based predicates.

14> V6 = macaroon_verifier:satisfy_general(V5, CheckTime).

It's finally time to verify our macaroon! Now that we've informed the verifier of all the various caveats that our application could embed within a macaroon, we can expect that the verification step will succeed.

15> macaroon_verifier:verify(V6, M, Secret).
ok

More importantly, the verifier will also work for macaroons we've not yet seen, like one that only permits Alice to deposit into her account:

16> N = macaroon:add_first_party_caveat(M, "action = deposit").
17> macaroon_verifier:verify(V6, N, Secret).
ok

Equally important is the verifier's ability to reject improper macaroons because they are not authorized. An improper macaroon could have additional caveats not known to the verifier, or have false general caveats. Worse, an unauthorized macaroon could be an attempt by a determined attacker to break into our bank. The verifier we've built will reject all of these scenarios:

18> % Unknown caveat
18> N2 = macaroon:add_first_party_caveat(M, "OS = Windows XP").
19> macaroon_verifier:verify(V6, N2, Secret).
{error,{unverified_caveat,<<"OS = Windows XP">>}}
20>
20> % False caveat
20> N3 = macaroon:add_first_party_caveat(M, "time < 2014-01-01T00:00").
21> macaroon_verifier:verify(V6, N3, Secret).
{error,{unverified_caveat,<<"time < 2020-01-01T00:00">>}}
22>
22> % Bad secret
22> macaroon_verifier:verify(V6, M, "this is not the secret we were looking for").
{error,{bad_signature_for_macaroon,<<"we used our secret key">>}}
23>
23> % Incompetent hackers trying to change the signature
23> {ok, N4} = macaroon:deserialize(<<"MDAxY2xvY2F0aW9uIGh0dHA6Ly9teWJhbmsvCjAwMjZpZGVudGlmaWVyIHdlIHVzZWQgb3VyIHNl\nY3JldCBrZXkKMDAxZGNpZCBhY2NvdW50ID0gMzczNTkyODU1OQowMDIwY2lkIHRpbWUgPCAyMDIw\nLTAxLTAxVDAwOjAwCjAwMjJjaWQgZW1haWwgPSBhbGljZUBleGFtcGxlLm9yZwowMDJmc2lnbmF0\ndXJlID8f19FL+bkC9p/aoMmIecC7GxdOcLVyUnrv6lJMM7NSCg==\n">>).
24> io:format("~s", [macaroon:inspect(N4)]).
location http://mybank/
identifier we used our secret key
cid account = 3735928559
cid time < 2020-01-01T00:00
cid email = alice@example.org
signature 3f1fd7d14bf9b902f69fdaa0c98879c0bb1b174e70b572527aefea524c33b352
ok
25> macaroon:signature(M) =:= macaroon:signature(N4).
false
26> macaroon_verifier:verify(V6, N4, Secret).
{error,{bad_signature_for_macaroon,<<"we used our secret key">>}}

Using Third-Party Caveats

Like first-party caveats, third-party caveats restrict the context in which a macaroon is authorized, but with a different form of restriction. Where a first-party caveat is checked directly within the verifier, a third-party caveat is checked by the third-party, who provides a discharge macaroon to prove that the original third-party caveat is true. The discharge macaroon is recursively inspected by the verifier; if it verifies successfully, the discharge macaroon serves as a proof that the original third-party caveat is satisfied. Of course, nothing stops discharge macaroons from containing embedded first- or third-party caveats for the verifier to consider during verification.

Let's rework the above example to provide Alice with access to her account only after she authenticates with a service that is separate from the service processing her banking transactions.

As before, we'll start by constructing a new macaroon with the caveat that is limited to Alice's bank account.

1> Secret = "this is a different super-secret key; never use the same secret twice".
2> Public = "we used our other secret key".
3> Location = "http://mybank/".
4> M = macaroon:create(Location, Secret, Public).
5> M2 = macaroon:add_first_party_caveat(M, "account = 3735928559").
6> io:format("~s", [macaroon:inspect(M2)]).
location http://mybank/
identifier we used our other secret key
cid account = 3735928559
signature 1434e674ad84fdfdc9bc1aa00785325c8b6d57341fc7ce200ba4680c80786dda
ok

So far, so good. Now let's add a third party caveat to this macaroon that requires that Alice authenticate with http://auth.mybank/ before being authorized access to her account. To add a third-party caveat we'll need to specify a location hint, generate a secret, and somehow be able to identify this secret later. Like the location used when creating a macaroon, the location used when adding a third-party caveat is simply a hint to the application that is uninterpreted by libmacaroons.

The secret and identity are handled differently than during creation, however, because they need to be known to the third party. To do this, we'll provide the third-party with a randomly generated CaveatKey and the predicate we wish for it to check. In response, it will give us an identifier that we can embed within the macaroon. Later, when we need to construct a discharge macaroon, we can provide it with just this identifier, from which it can recall the key and predicate to check.

Here's what this looks like in code:

7> % you'll likely want to use a higher entropy source to generate this key
7> CaveatKey = "4; guaranteed random by a fair toss of the dice".
8> Predicate = "user = Alice".
9> % send_to_auth(CaveatKey, Predicate).
9> % Identifier = recv_from_auth().
9> Identifier = "this was how we remind auth of key/pred".
10> M3 = macaroon:add_third_party_caveat(M2, "http://auth.mybank/", CaveatKey, Identifier).
11> io:format("~s", [macaroon:inspect(M3)]).
location http://mybank/
identifier we used our other secret key
cid account = 3735928559
cid this was how we remind auth of key/pred
vid f678186d92de0589225da7827ed26509ed33eb83601b9e96b21cdee80f4a9834176b77f06f3fc3b59a1547231362797158ca61226681a7946fa2781bef9a95e746f52e91af74f9ae
cl http://auth.mybank/
signature 9a986629809a40171d4cc32c3814fbf9e0d82f0b1eebb781fc3675d204e6066a
ok

We now have a macaroon with a third-party caveat. The most interesting thing to note about this macaroon is that it includes no information that reveals the predicate it encodes. The third-party service knows the key and the predicate, and internally associates them with the identifier, but the identifier itself is arbitrary and betrays no information about the predicate to others. The service at http://auth.mybank/ can authenticate that the user is Alice, and provide proof that the caveat is satisfied, without revealing Alice's identity. Other services can verify M and its associated discharge macaroon, without knowing the predicates the third-parties verified.

The process for discharging third party caveats starts with the holder of the initial root macaroon, Alice. Alice looks at the macaroon for the list of third party caveat (location, identifier) pairs that must be addressed.

12> macaroon:third_party_caveats(M3).
[{<<"http://auth.mybank/">>,
  <<"this was how we remind auth of key/pred">>}]

In a real application, we'd look at these third party caveats, and contact each location to retrieve the requisite discharge macaroons. We would include the identifier for the caveat in the request itself, so that the server can recall the secret used to create the third-party caveat. The server can then generate and return a new macaroon that discharges the caveat:

13> D = macaroon:create("http://auth.mybank/", CaveatKey, Identifier).
14> D2 = macaroon:add_first_party_caveat(D, "time < 2020-01-01T00:00").
15> io:format("~s", [macaroon:inspect(D2)]).
location http://auth.mybank/
identifier this was how we remind auth of key/pred
cid time < 2020-01-01T00:00
signature 2ed1049876e9d5840950274b579b0770317df54d338d9d3039c7c67d0d91d63c
ok

This new macaroon enables the verifier to determine that the third party caveat is satisfied. Our target service added a time-limiting caveat to this macaroon that ensures that this discharge macaroon does not last forever. This ensures that Alice (or, at least someone authenticated as Alice) cannot use the discharge macaroon indefinitely and will eventually have to re-authenticate.

Once Alice has both the root macaroon and the discharge macaroon in her possession, she can make the request to the target service. Making a request with discharge macaroons is only slightly more complicated than making requests with a single macaroon. In addition to serializing and transmitting all involved macaroons, there is preparation step that binds the discharge macaroons to the root macaroon. This binding step ensures that the discharge macaroon is useful only when presented alongside the root macaroon. The root macaroon is used to bind the discharge macaroons like this:

16> DP = macaroon:prepare_for_request(M3, D2).

If we were to look at the signatures on these prepared discharge macaroons, we would see that the binding process has irreversibly altered their signature(s).

17> macaroon:signature(D2).
<<"2ed1049876e9d5840950274b579b0770317df54d338d9d3039c7c67d0d91d63c">>
18> macaroon:signature(DP).
<<"dd3690773cd08867dc544d7a84a26aae2581f56a21f77f886812865abdea9554">>

The root macaroon M3 and its discharge macaroons DP are ready for the request. Alice can serialize them all and send them to the bank to prove she is authorized to access her account. The bank can verify them using the same verifier we built before. We provide the discharge macaroons as a third argument to the verify call:

19> macaroon_verifier:verify(V6, M3, Secret, [DP]).
ok

Without the prepareForRequest call, the verification would fail:

20> macaroon_verifier:verify(V6, M3, Secret, [D2]).
{error,{bad_signature_for_macaroon,<<"this was how we remind auth of key/pred">>}}

Choosing Secrets

For clarity, we've generated human-readable secrets that we use as the root keys of all of our macaroons. In practice, this is terribly insecure and can lead to macaroons that can easily be forged because the secret is too predictable. To avoid this, we recommend generating secrets using a sufficient number of suitably random bytes. Because the bytes are a secret key, they should be drawn from a source with enough entropy to ensure that the key cannot be guessed before the macaroon falls out of use.

The macaroon module exposes a constant that is the ideal number of bytes these secret keys should contain. Any shorter is wasting an opportunity for security.

1> macaroon:suggested_secret_length().
32

Third-Party Caveats with Public Keys

Public key cryptography can enable much more efficient schemes for adding third-party caveats. In the above example where we added a third-party caveat, the caveat's identifier was generated by the third party and retrieved with in one round trip. We can eliminate the round trip when the third party has a well-known public key. We can encrypt the caveat's secret, and the predicate to be checked using this public key, and use the ciphertext directly as the caveat's identifier. This saves a round trip, and frees the third party from having to remember an association between identifiers and key/predicate pairs.