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Chapter 1. Understanding macaroons

Macaroons are a standard way of handling delegated authorisation that brings several benefits. Delegated authorisation is where the person who is allowed to do something (for example, read a specific file in dCache) allows someone else to do that operation. There are lots of use-cases where delegated authorisation is useful, some of which are described below.

Macaroons are bearer tokens. This means that the client presents the macaroon along with the request. Unlike with X.509 and Kerberos authentication, there is no back-and-forth communication between the client and dCache when accepting a macaroon. This makes using a macaroon very easy, but also increases the risks if a macaroon being stolen.

Macaroons and caveats

A macaroon is a string of characters without any spaces. The following is an example of a macaroon, where the single long line has been split into multiple lines to make the macaroon more readable.

MDAxY2xvY2F0aW9uIE9wdGlvbmFsLmVtcHR5CjAwMThpZGVudGlmaWVyIGh \
sQ0kremlRCjAwMTVjaWQgaWlkOnBGTTA1MnJTCjAwMjFjaWQgaWQ6MjAwMj \
sxMDAxLDIwMDIsMDtwYXVsCjAwMjhjaWQgYmVmb3JlOjIwMTktMDQtMTdUM \
Dk6NTE6MjIuODQwWgowMDE5Y2lkIGhvbWU6L1VzZXJzL3BhdWwKMDAyZnNp \
Z25hdHVyZSCT6Lea6oBIEpiF2KOsZ1FQvLeoXve_a3q38TZTBWhM1Qo

A macaroon contains a location and an identifier. Neither of these values is protected from being modified; however, dCache ignores the location information and modifying the identifier simply prevents the macaroon from working.

Other than this, a macaroons contains a list of caveats.

Caveats either carry context information or limit how the macaroon may be used. Caveats are strings and, in dCache, caveats have the form <key>:<value>. where <key> describes what kind of limitation this caveat represends and <value> is information about the restriction. For example, the caveat before:2019-04-17T09:51:22.840Z limits when that macaroon may be used. Once this time has elapsed, the macaroon will be rejected.

Macaroon caveats have a property that makes them very useful.

Given a macaroon (like the one above), it is easy to create a new macaroon that includes the existing caveats and some additional caveats. If done correctly, that new macaroon is valid and will be accepted by dCache.

The opposite does not work: given a macaroon, it is “impossible” (cryptographically hard) to create a new macaroon with any of the given macaroon’s caveat removed.

In simple terms, you can add caveats but cannot remove them.

Caveats supported by dCache

dCache supports the following caveats: root, home, path, before, ip, id, iid and activity.

Some of these caveats are included in the macaroon automatically. Creating a new macaroon where such caveat is mentioned a second time results in an invalid macaroon.

The other caveats may be included multiple times, each caveat modifying what may be done with the macaroon.

activity

before

home

id

iid

ip

path

root

Requesting a macaroon

To request a macaroon, make a POST request to the WebDAV door with the Content-Type request header set to application/macaroon-request. The request must be authenticated, but any authentication mechanism should work: X.509 client certificate, Kerberos, username+password, OpenID-Connect and SciToken.

The following example shows a simple macaroon request where X.509 client authentication is used:

curl -E /tmp/x509up_u1000 -X POST \
|    -H 'Content-Type: application/macaroon-request' \
|    https://dcache.example.org/
|{
|    "macaroon": "MDA[...]Qo",
|    "uri": {
|        "targetWithMacaroon": "https://dcache.example.org/?authz=MDA[...]Qo",
|        "baseWithMacaroon": "https://dcache.example.org/?authz=MDA[..]o",
|        "target": "https://dcache.example.org/",
|        "base": "https://dcache.example.org/"
|    }
|}

The response to this request is a JSON Object. The macaroon property value is the macaroon. The four uri properties (targetWithMacaroon, baseWithMacaroon, target and base) are for convenience only.

This returns the most powerful macaroon: a macaroon with the least number of caveats.

Once you have received the macaroon, you can create a new macaroon that has additional caveats to make the new macaroon less powerful.

A convenient way of obtaining such a macaroon is to request dCache includes the extra caveats in the macaroon. This has the advantage that the macaroon only leaves dCache with the additional caveats, providing a slight security benefit.

Send a JSON object with the POST request to request dCache issues a macaroon that has additional caveats. The general form for these caveat-requesting JSON is:

{
    "caveats": [
        "caveat-1",
	"caveat-2",
	"...",
	"caveat-n"
    ]
}

For example, to request a single additional caveat activity:DOWNLOAD,LIST, the JSON object is:

{
    "caveats": [
        "activity:DOWNLOAD,LIST"
    ]
}

As a curl command, this request looks like:

curl -E /tmp/x509up_u1000 -X POST \
|        -d '{"caveats": ["activity:DOWNLOAD,LIST"]}' \
|        -H 'Content-Type: application/macaroon-request' \
|        https://prometheus.desy.de/
|{
|    "macaroon": "MDA[...]bgK",
|    "uri": {
|        "targetWithMacaroon": "https://prometheus.desy.de/?authz=MDA[...]bgK",
|        "baseWithMacaroon": "https://prometheus.desy.de/?authz=MDA[...]bgK",
|        "target": "https://prometheus.desy.de/",
|        "base": "https://prometheus.desy.de/"
|    }
|}

Requesting path caveats

The path caveat may be specified by specifying a path in the POST request.

The following operation requests a macaroon with the path:/data/2019 caveat:

curl -E /tmp/x509up_u1000 -X POST \
|        -d '{"caveats": ["activity:DOWNLOAD,LIST", "path:/data/2019]}' \
|        -H 'Content-Type: application/macaroon-request' \
|        https://prometheus.desy.de/

The following operation requests a macaroon with the same path:/data/2019 caveat, but specified in the request URL:

curl -E /tmp/x509up_u1000 -X POST \
|        -d '{"caveats": ["activity:DOWNLOAD,LIST"]}' \
|        -H 'Content-Type: application/macaroon-request' \
|        https://prometheus.desy.de/data/2019

Requesting a macaroon with limited validity

Including the before caveat in a macaroon limits for how long a macaroon may be used. Once the time has elapsed, the macaroon is useless.

The value is an instant in time, which is often not really what is desired. Instead, it is often desired to have a macaroon that is valid for a fixed duration (e.g., the next five minutes). This then requires calculating the expiry time in order to build the before caveat. An additional problem is that the clocks on the client and server might not agree exactly.

Both these problems are resolved by the validity property in the request JSON object. The value is an ISO 8601 duration; for example the value PT5M represents five minutes.

For example, the following JSON object requests a macaroon that allows read-only access to dCache for one hour.

{
    "caveats": [
        "activity:DOWNLOAD,LIST"
    ],
    "validity": "PT1H"
}

Adding caveats

Anyone with a macaroon can create a new macaroon with additional caveats. As each successive caveat either leaves the set of allowed operations the same or reduces it, the new macaroon either has the same authorisation, or weaker authorisation.

Adding caveats autonomously

Various libraries exist for handling macaroons. These libraries may be used to create a new macaroon with more caveats.

Adding caveats with dCache

Another way to create a more restricted macaroon is to use the macaroon to request a new macaroon. Simply use the macaroon to authenticate when issuing the HTTP POST request. All the current macaroon’s caveats are copied into the new macaroons and any requested caveats are added subsequently.

Using a macaroon

dCache supports macaroon based requests with the WebDAV door and the frontend door.

For both doors, there are two ways of using a macaroon: in the Authorization request header and in the URL.

In the Authorization header

Include the macaroon prefixed by the word Bearer as the Authorization HTTP request header: Authorization: Bearer MDA[...]bgK.

The following example shows a curl request authorised by including the macaroon in the Authorization request header. The macaroon is stored in the variable MACAROON to make this curl command (and any subsequent ones) easier to read.

MACAROON=MDA[...]bgK
curl -H "Authorization: Bearer $MACAROON" \
|        https://dcache.example.org/

In the URL

In some cases, you cannot control the HTTP requests, so cannot include the macaroon in the Authorization request header. This is perhaps most common for web-browsers, but can occur with frameworks.

To support these clients, dCache accepts the macaroon in the URL, as an authz query parameter.

In the following example, curl sends the macaroon as part of the URL.

MACAROON=MDA[...]bgK
curl https://dcache.example.org/?authz=$MACAROON

Example macaroon use-cases

This section describes some scenarios where macaroons may prove useful. This is not an exhaustive list, but meant more to whet your appetite.

Portal use-case

Direct sharing

Third-party transfer

Enfore catalogue permissions