Provided by: liblwp-authen-oauth2-perl_0.14-1_all 
NAME
LWP::Authen::OAuth2::Overview - Overview of accessing OAuth2 APIs with LWP::Authen::OAuth2
Introduction
This attempts to be the document that I wished existed when I first tried to access an API
that used OAuth 2 for authentication. It explains what OAuth 2 is, how it works, what you
need to know to use it, and how LWP::Authen::OAuth2 tries to make that easier. It
hopefully also explains this in a way which will help you read documentation written by
other people who assume that you have this knowledge.
Feel free to read as much or little of this document as makes sense for you. It is not
actually designed to be read in a single sitting.
Since part of the purpose of this document is to familiarize you with the jargon that
you're likely to encounter, all terms commonly used in discussions of OAuth 2 with a
specific meaning are highlighted. Terms will hopefully be clear from context, but all
highlighted terms are explained in the "Terminology" section.
The Purpose of OAuth 2
OAuth 2 makes it easy for large service providers to write many APIs that users can
securely authorize third party consumers to use on their behalf. Everything good (and
bad!) about the specification comes from this fact.
It therefore specifies an authorization handshake through which permissions are set up,
and then a message signing procedure through which you can then access the API. Well,
actually it specifies many variations of the authorization handshake, and multiple
possible signing procedures, because large organizations run into a lot of use cases and
try to cover them all. But conceptually they are all fundamentally similar, and so have
been lumped together in one monster spec.
The Purpose of LWP::Authen::OAuth2
LWP::Authen::OAuth2 exists to help Perl programmers who want to be a consumer of an API
protected by OAuth 2 to construct and make all of the necessary requests to the service
provider that you need to make. You will still need to set up your relationship with the
service provider, build your user interaction, manage private data (hooks are provided to
make that straightforward), and figure out how to use the API.
If that does not sound like it will make your life easier, then this module is not
intended for you.
If you are not a consumer, this module is definitely not intended for you. (Though this
document may still be helpful.)
The Basic OAuth 2 Handshake
OAuth 2 allows a user to tell a service provider that a consumer should be allowed to
access the user's data through an API. This permissioning happens through the following
handshake.
The consumer sends the user to an authorization_url managed by the service provider. The
service provider tells the user that the consumer wants access to that account and asks if
this is OK. The user confirms that it is, and is sent back to the consumer with proof of
the conversation. The consumer presents that proof to the service provider along with
proof that it actually is the consumer, and is granted tokens that will act like keys to
the user's account. After that the consumer can use said tokens to access the API which
is protected by OAuth 2.
All variations of OAuth 2 follow this basic pattern. A large number of the details can
and do vary widely. For example JavaScript applications that want to make AJAX calls use
a different kind of proof. Applications installed on devices without web browsers will
pass information to/from the user in different ways. And each service provider is free to
do many, many things differently. The specification tries to document commonalities in
what different companies are doing, but does not mandate that they all do the same thing.
(This sort of complexity is inevitable from a specification that tries to make the lives
of large service providers easy, and the lives of consumers possible.)
Becoming a Consumer
If you want to access an OAuth 2 protected API, you need to become a consumer. Here are
the necessary steps, in the order that things happen in.
Register with the service provider
You cannot access a service provider without them knowing who you are. After you go
through their process, at a minimum you will get a public client_id, a private
client_secret, and have agreed on one or more redirect_uris that the user can use to
deliver an authorization code back to you. (That is not the only kind of proof that
the user can be given for the consumer, but it is probably the only one that makes
sense for a Perl consumer.)
The redirect_uri is often a "https:///..." URL under your control. You also are
likely to have had to tell the service provider about what type of software you're
writing (webserver, command line, etc). This determines your client type. They may
call this a scenario, or flow, or something else.
You will also need information about the service provider. Specifically you will need
to know their Authorization Endpoint and Token Endpoint. They hopefully also have
useful documentation about things like their APIs.
LWP::Authen::OAuth2 is not directly involved in this step.
If a LWP::Authen::OAuth2::ServiceProvider::Foo class exists, it should already have
the service provider specific information, and probably has summarized documentation
that may make this smoother. If you're really lucky, there will be a CPAN module (or
modules) for the API (or APIs) that you want to use. If those do not exist, please
consider creating them.
If no such classes exist, you can still use the module. Just pass the necessary
service provider facts in your call to "LWP::Authen::OAuth2->new(...)" and an
appropriate LWP::Authen::OAuth2::ServiceProvider will be created for you on the fly.
Decide how to store sensitive information
All of the data shared between you and the service provider has to be stored on your
end. This includes tokens that will let you access private information for the user.
You need to be able to securely store and access these.
LWP::Authen::OAuth2 does not address this, beyond providing hooks that you are free to
use as you see fit.
Build interaction asking for user permission
You need to have some way of convincing the user that they want to give you
permission, ending in giving them an authorization_url which sends them off to the
service provider to authorize access. This interaction can range from a trivial
conversation with yourself if you are the only user you will be handling, to a
carefully thought through sales pitch if you are trying to get members of the public
to sign up.
LWP::Authen::OAuth2 helps you build that URL. The rest is up to you.
Build interaction receiving your authorization code
When the user finishes their interaction with the service provider, if the service
provider is sure that they know where to send the user (they know your client_id, your
redirect_uri makes sense to them) then they will be sent to the redirect_uri to pass
information back to you.
If you succeeded, you will receive a code in some way. For instance if your
redirect_uri is a URL, it will have a get parameter named "code".
You could get an "error" parameter back instead. See RFC 6749
<http://tools.ietf.org/html/rfc6749#section-4.1.2.1> for a list of the possible
errors. Note that there are possible optional fields with extra detail. I would not
advise optimism about their presence.
LWP::Authen::OAuth2 is not involved with this.
Request tokens
Once you have that code you are supposed to immediately trade it in for tokens.
LWP::Authen::OAuth2 provides the "request_tokens" method to do this for you. Should
you not actually get tokens, then the "request_tokens" method will trigger an error.
NOTE that the code cannot be expected to work more than once. Nor can you expect the
service provider to repeatedly hand out working codes for the same permission. (The
qualifier "working" matters.) Being told this will hopefully let you avoid a painful
debugging session that I did not enjoy.
Save and pass around tokens (maybe)
If you will need access to information in multiple locations (for instance on several
different web pages), then you are responsible for saving and retrieving those tokens
for future use. LWP::Authen::OAuth2 makes it easy to serialize/deserialize tokens,
and has hooks for when they change, but leaves this step up to you.
Access the API
LWP::Authen::OAuth2 takes care of signing your API requests. What requests you need
to actually make are between you and the service provider. With luck there will be
documentation to help you figure it out, and if you are really lucky that will be
reasonably accurate.
Refresh access tokens (maybe)
The access token that is used to sign requests will only work for a limited time. If
you were given a request token, that can be used to request another access token at
any time. Which raises the possibility that you make a request, it fails because the
access token expired, you refresh it, then need to retry your request.
LWP::Authen::OAuth2 will perform this refresh/retry logic for you automatically if
possible, and provides a hook for you to know to save the updated token data.
Some client types are not expected to use this pattern. You are only given an access
token and are expected to send the user through the handshake again when that expires.
The second time through the redirect on the service provider's side is immediate, so
the user experience should be seamless. However LWP::Authen::OAuth2 does not try to
automate that logic. But "$oauth2->should_refresh" can let you know when it is time
to send the user through, and "$oauth2->can_refresh_tokens" will let you know whether
automatic refreshing is available.
Note that even if it is available, retry success is not guaranteed. The user may
revoke your access, the service provider may decide you are a suspicious character,
there may have been a service outage, etc. LWP::Authen::OAuth2 will throw errors on
these error conditions, handling them is up to you.
Terminology
This section is intended to be used in one of two ways.
The first option is that you can start reading someone else's documentation and then refer
back to here every time you run across a term that you do not immediately understand.
The second option is that you can read this section straight through for a reasonably
detailed explanation of the OAuth 2 protocol, with all terms explained. In fact if you
choose this option, you will find it explained in more detail than you need to be a
successful consumer.
However if you use it in the second way, please be advised that this does not try to be a
complete and exact explanation of the specification. In particular the specification
requires specific error handling from the service provider that I have glossed over, and
allows for extra types of requests that I also glossed over. (Particularly the bit about
how any service provider at any time can add any new method that they want so long as they
invent a new grant_type for it.)
consumer
The consumer is the one who needs to be authorized by OAuth 2 to be able to "consume"
an API. If you're reading this document, that's likely to be you.
client
The software on the consumer's side which actually will access the API. From a
consumer's point of view, a consumer and the client are usually the same thing. But,
in fact, a single consumer may actually write multiple clients. And if one is a web
application while another is a command line program, the differences can matter to how
OAuth 2 will work.
Where I have a choice in this document I say consumer rather than client because that
term is less likely overloaded in most organizations.
user
The user is the entity (person or company) who wishes to let the consumer access their
account.
Resource Owner
What the OAuth 2 specification calls the user, to focus attention on the fact that
they own the data which will get accessed.
I chose to say user instead of Resource Owner because that is my best guess as to what
the consumer is most likely to already call them.
service provider
The service provider is the one which hosts the account, restricts access and offers
the API. For example, Google.
Resource Server
In the OAuth 2 specification, this is the service run by the service provider which
hosts provides an API to the user's data. The name has deliberate symmetry with
Resource Owner.
Authorization Server
In the OAuth 2 specification, this is the service run by the service provider which is
responsible for granting access to the Resource Server.
The consumer does not need to care about this distinction, but it exposes an important
fact about how the service provider is likely to be structured internally. You
typically will have one team that is responsible for granting access, tracking down
clients that seem abusive, and so on. And then many teams are free to create useful
stuff and write APIs around them, with authorization offloaded to the first team.
As a consumer, you will make API requests to the Resource Server signed with proof of
auhorization from the Authorization Server, the Resource Server will confirm
authorization with the Authorization Server, and then the Resource Server will do
whatever it was asked to do.
Organizing internal responsibilities in this manner makes it easier for many
independent teams in a large company to write public APIs.
client type
The service provider internally tags each client with a client type which tells it
something about what environment it is in, and how it interacts with the user. Are
are the basic types listed in RFC 6749
<http://tools.ietf.org/html/rfc6749#section-2.1>:
web application
Runs on a web server. Is expected to keep secrets. Likely to be appropriate for
a Perl client.
user-agent-based application
JavaScript application running in a browser that wants to make AJAX calls. Can't
keep secrets. Does not make sense for A Perl client.
native application
Application installed on a user's machine. Can't keep secrets. Possibly
appropriate for a Perl client.
Of course all of this is up to the service provider. For example at the time of this
writing, Google documents no less than six client types at
<https://developers.google.com/accounts/docs/OAuth2>, none of which have been given
the above names. (They also call them "Scenarios" rather than client type.) They
rename the top two, split native application into two based on whether your
application controls a browser, and add two new ones.
flow
Your flow is the sequence and methods of interactions that set up authorization. The
flow depends on your service provider and client type. For example the service
provider might redirect the user to a URL controlled by a web application, while
instead for a native application the user is told to cut and paste a code somewhere.
Despite flow being more common terminology in OAuth 2, client type is more self-
explanatory, so I've generally gone with that instead.
client_id
The client_id is a public ID that tells the service provider about the client that is
accessing it. That is, it says both who the consumer is, and what the client type is.
Being public, the client_id can be shared with the user. The details of how this is
assigned are between the consumer and the service provider.
client_secret
The client_secret is a somewhat private piece of information that the consumer can
pass to the service provider to prove that the request really comes from the consumer.
How much this is trusted, and how it is used, will depend on the client type and
service provider.
redirect_uri
The service provider needs a way to tell the user how to pass information back to the
consumer in a secure way. That is provided by the redirect_uri which can be anything
from a "https://..." URL that the consumer controls to an instruction that lets the
service provider know that it should tell the user to cut and paste some information.
It is up to the service provider what values of are acceptable for the redirect_uri,
and whether it is a piece of information that is remembered or passed in during the
authorization process.
state
The state is an optional piece of information that can be created by the consumer then
added to all requests as an extra piece of protection against forgery. (You are
supposed to create a random piece of information for each request, then check that you
get it back.) In the OAuth 2 specification it is optional, but recommended.
Depending on the combination of your service provider and client type, it may be
required.
scope
The scope describes what permissions are to be granted. To get multiple permissions,
you need to join the permissions requested with spaces. Everything else is up to the
service provider.
Inside of the service provider, what likely happens is that the team which runs a
given Resource Server tells the team running the Authorization Server what permissions
to their API should be called. And then the Authorization Server can limit a given
consumer to just the APIs that the user authorized them for.
Authorization Endpoint
The Authorization Endpoint is the URL provided by the service provider for the purpose
of sending requests to authorize the consumer to access the user's account. This is
part of the Authorization Server.
response_type
The response_type tells the service provider what kind of information it is supposed
to pass back. I am not aware of a case where a Perl client could usefully use any
value other than "code". However there are flows where other things happen. For
example the flow for the user-agent-based application client type uses a response_type
of token.
While the field is not very useful for Perl clients, it is required in the
specification. So you have to pass it.
authorization_url
This is the URL on the service provider's website that the user goes to in order to
let the service provider know what authorization is being requested.
It is constructed as the Authorization Endpoint with get parameters added for the
response_type, client_id, and optionally state. The specification mentions both
redirect_uri and scope but does not actually mandate that they be accepted or
required. However they may be. And, of course, a given service provider can add more
parameters at will, and require (or not) different things by client type.
An example URL for Google complete with optional extensions is
<https://accounts.google.com/o/oauth2/auth?scope=https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fuserinfo.email+https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fuserinfo.profile&state=%2Fprofile&redirect_uri=https%3A%2F%2Foauth2-login-demo.appspot.com%2Fcode&response_type=code&client_id=812741506391.apps.googleusercontent.com&approval_prompt=force>
In LWP::Authen::OAuth2 the "authorization_url" method constructs this URL. If your
request needs to include the state, scope, or any service provider specific parameter,
you need to pass those as parameters. The others are usefully defaulted from the
service provider and object.
(authorization) code
If the response_type is set to "code" (which should be the case), then on success the
service provider will generate a one use authorization code to give to the user to
take back to the consumer. Depending on the flow this could happen with no effort on
the part of the user. For example the user can be redirected to the redirect_uri with
the code passed as a get parameter. The web server would then pick these up, finish
the handshake, and then redirect the user elsewhere.
In all interactions where it is passed it is simply called the code. But it is
described in one interaction as an authorization_code.
Token Endpoint
The Token Endpoint is the URL provided by the service provider for the purpose of
sending requests from the consumer to get tokens allowing access to the user's
account.
grant_type
The grant_type is the type of grant you expected to get based on the response_type
requested in the authorization_url. For a response_type of "code" (which is almost
certainly what will be used with any consumer written in Perl), the grant_type has to
be "authorization_code". If they were being consistent, then that would be code like
it is everywhere else, but that's what the spec says.
We will later encounter the grant_type "refresh_token". The specification includes
potential requests that can be in a flow that might prove useful. However you are
only likely to encounter that if you are subclassing
LWP::Authen::OAuth2::ServiceProvider. In that case you will hopefully discover the
applicability and details of those grant_types from the service provider's
documentation.
Access Token Request
Once the consumer has a code the consumer can submit an Access Token Request by
sending a POST request to the Token Endpoint with the grant_type, code, client_id,
client_secret, redirect_uri and (if in the authorization code) the state. Your
service provider can also require you to authenticate in any further way that they
please. You will get back a JSON response.
An example request might look like this:
POST /o/oauth2/token HTTP/1.1
Host: accounts.google.com
Content-Type: application/x-www-form-urlencoded
code=4/P7q7W91a-oMsCeLvIaQm6bTrgtp7&
client_id=8819981768.apps.googleusercontent.com&
client_secret={client_secret}&
redirect_uri=https://oauth2-login-demo.appspot.com/code&
grant_type=authorization_code
and the response if you're lucky will look something like:
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"1/fFAGRNJru1FTz70BzhT3Zg",
"expires_in":3920,
"token_type":"Bearer",
"refresh_token":"1/xEoDL4iW3cxlI7yDbSRFYNG01kVKM2C-259HOF2aQbI"
}
or if you're unlucky, maybe like this:
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"error":"invalid_grant"
}
Success is up to the service provider which can decide not to give you tokens for any
reason that they want, including that you asked twice, they think the user might be
compromised, they don't like the client, or the phase of the Moon. (I am not aware of
any service provider that makes failure depend on the phase of the Moon, but the
others are not made up.)
The "request_tokens" method of LWP::Authen::OAuth2 will make this request for you,
read the JSON and create the token or tokens. If you passed in a "save_tokens"
callback in constructing your object, that will be called for you to store the tokens.
On future API calls you can retrieve that to skip the handshake if possible.
token_type
The token_type is a case insensitive description of the type of token that you could
be given. In theory there is a finite list of types that you could encounter. In
practice service providers can add more at any time, either intentionally or
unintentionally by failing to correctly implement the one that they claimed to have
created.
See LWP::Authen::OAuth2::AccessToken for advice on how to add support for a new or
incorrectly implemented token_type.
expires_in
The number of seconds until you will need a new token because the old one should have
expired. LWP::Authen::OAuth2 provides the "should_refresh" method to let you know
when you need that new token. (It actually starts returning true slightly early to
avoid problems if clocks are not synchronized, or you begin a series of operations.)
access_token
An access_token is a temporary token that gives the consumer access to the user's data
in the service provider's system. In the above response the "access_token" is the
value of the token, "expires_in" is the number of seconds it is good for in theory
(practice tends to be close but not always exact), and "token_type" specifies how it
is supposed to be used.
Once the authorization handshake is completed, if the access_token has a supported
token_type. then LWP::Authen::OAuth2 will automatically sign any requests for you.
Bearer token
If the token_type is "bearer" (case insensitive), then you should have a bearer token
as described by RFC 6750 <http://tools.ietf.org/html/rfc6750>. For as long as the
token is good, any request signed with it is authorized. Signing is as simple as
sending an https request with a header of:
Authorization: Bearer 1/fFAGRNJru1FTz70BzhT3Zg
You can also sign by passing "access_token=..." as a post or get parameter, though the
specification recommends against using a get parameter. If you are using
LWP::Authen::OAuth2, then it is signed with the header.
refresh_token
The above example also included a refresh_token. If you were given one, you can use
it later to ask for a refreshed access_token. Whether you get one is up to your
service provider, who is likely to decide that based on your client_type.
Refresh Access Token
If you have a refresh_token, you can at any time send a Refresh Access Token request.
This is a POST to the Token Endpoint with the refresh_token, client_id and
client_secret arguments. You also have to send a grant_type of "refresh_token".
Thus in the above case we'd send
POST /o/oauth2/token HTTP/1.1
Host: accounts.google.com
Content-Type: application/x-www-form-urlencoded
refresh_token=1/xEoDL4iW3cxlI7yDbSRFYNG01kVKM2C-259HOF2aQbI&
client_id=8819981768.apps.googleusercontent.com&
client_secret={client_secret}&
grant_type=refresh_token
and if lucky could get a response like
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"ya29.AHES6ZSiArSow0zeKokajrri5gMBpGc6Sq",
"expires_in":3600,
"token_type":"Bearer",
}
and if unlucky could get an error as before.
In LWP::Authen::OAuth2 this request is made for you transparently behind the scenes if
possible. If you're curious when, look in the source for the "refresh_access_token"
method. There are also optional callbacks that you can pass to let you save the
tokens, or hijack the refresh method for your own purposes. (Such as making sure that
only one process tries to refresh tokens even though many are accessing it.)
But note that not all flows offer a refresh_token. If you're on one of those flows
then you need to send the user back to the service provider for authorization renewal.
From the user's point of view this is likely to be painless because it will be done
with transparent redirects. But the consumer needs to be aware of it.
AUTHOR
Ben Tilly, "<btilly at gmail.com>"
ACKNOWLEDGEMENTS
Thanks to Rent.com <http://www.rent.com> for their generous support in letting me develop
and release this module. My thanks also to Keith Cascio "<cascio@helminthist.net>" for
very helpful feedback on early drafts.