The OAuth Core Protocol
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eran@hueniverse.comhttp://hueniverse.comromeda@gmail.comhttp://romeda.org/
This document specifies the OAuth core protocol. OAuth provides a method for clients
to access server resources on behalf of another party (such a different client or an end
user). It also provides a redirection-based user agent process for end users to authorize
access to clients by substituting their credentials (typically, a username and password
pair) with a different set of delegation-specific credentials.
The OAuth protocol provides a method for servers to allow third-party access to protected
resources, without forcing their end users to share their credentials. This pattern is
common among services that allow third-party developers to extend the service functionality,
by building applications using an open API.
For example, a web user (resource owner) can grant a printing service (client) access to
its private photos stored at a photo sharing service (server), without sharing its
credentials with the printing service. Instead, the user authenticates directly with
the photo sharing service and issue the printing service delegation-specific credentials.
OAuth introduces a third role to the traditional client-server authentication model: the
resource owner. In the OAuth model, the client requests access to resources hosted by the
server but not controlled by the client, but by the resource owner. In addition, OAuth
allows the server to verify not only the resource owner's credentials, but also those of
the client making the request.
In order for the client to access resources, it first has to obtain permission from the
resource owner. This permission is expressed in the form of a token and matching
shared-secret. The purpose of the token is to substitute the need for the resource owner to
share its server credentials (usually a username and password pair) with the client. Unlike
server credentials, tokens can be issued with a restricted scope and limited lifetime.
This specification consists of two parts. The first part defines a method for making
authenticated HTTP requests using two sets of credentials, one identifying the client
making the request, and a second identifying the resource owner on whose behalf the
request is being made.
The second part defines a redirection-based user agent process for end users to authorize
client access to their resources, by authenticating directly with the server and
provisioning tokens to the client for use with the authentication method.
[[ This draft is mostly an editorial revision of the
community specification. It is intended as starting point for future standardization
efforts within the IETF. See for list of changes. Please
discuss this draft on the
oauth@ietf.org mailing list. ]]
An HTTP client (per ) capable of making
OAuth-authenticated requests.
An HTTP server (per ) capable of accepting
OAuth-authenticated requests.
An access-restricted resource (per ) which can be obtained
from the server using an OAuth-authenticated request.
An entity capable of accessing and controlling protected resources by using credentials
to authenticate with the server.
An unique identifier issued by the server and used by the client to associate
authenticated requests with the resource owner whose authorization is requested or
has been obtained by the client. Tokens have a matching shared-secret that is used
by the client to establish its ownership of the token, and its authority to represent
the resource owner.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in
.
The HTTP authentication methods defined by , enable clients
to make authenticated HTTP requests. Clients using these methods gain access to protected
resource by using their server credentials (typically a username and password pair), which
allows the server to verify their authenticity. Using these methods for delegation requires
the client to pretend it was the resource owner.
OAuth provides a method designed to include two sets of credentials with each request, one
to identify the client, and another to identify the resource owner. Before a client can
make authenticated requests on behalf of the resource owner, it must obtain a token
authorized by the resource owner. provides one such
method in which the client can obtain a token authorized by the resource owner.
The client credentials take the form of a unique identifier, and an associated share-secret
or RSA key pair. Prior to making authenticated requests, the client establishes a set of
credentials with the server. The process and requirements for provisioning these are
outside the scope of this specification. Implementers are urged to consider the security
ramification of using client credentials, some of which are described in
.
Making authenticated requests requires prior knowledge of the server's configuration.
OAuth provides multiple methods for including protocol parameters in requests
(), as well as multiple methods for the client to
prove its rightful ownership of the credentials used ().
The way in which clients discovery the required configuration is outside the scope of
this specification.
An OAuth-authenticated request includes several protocol parameters. Each parameter name
begins with the oauth_ prefix, and the parameter names
and values are case sensitive. Protocol parameters MUST NOT appear more than once per
request. The parameters are:
The identifier portion of the client credentials (equivalent to a username). The
parameter name reflects a deprecated term (Consumer Key) used in previous revisions
of the specification, and has been retained to maintain backward compatibility.
The token value used to associate the request with the resource owner. If the request
is not associated with a resource owner (no token), clients MAY omit the parameter.
The name of the signature method used by the client to sign the request,
as defined in .
The signature value as defined in .
The timestamp value as defined in .
The nonce value as defined in .
The protocol version. If omitted, the protocol version defaults to
1.0.
Server-specific request parameters MUST NOT begin with the
oauth_ prefix.
Unless otherwise specified by the server, the timestamp is expressed in the number of
seconds since January 1, 1970 00:00:00 GMT. The timestamp value MUST be a positive
integer and MUST be equal or greater than the timestamp used in previous requests with
the same client credentials and token credentials combination.
A nonce is a random string, uniquely generated to allows the server to verify that a
request has never been made before and helps prevent replay attacks when requests are
made over a non-secure channel. The nonce value MUST be unique across all requests with
the same timestamp, client credentials, and token combinations.
To avoid the need to retain an infinite number of nonce values for future checks, servers
MAY choose to restrict the time period after which a request with an old timestamp is
rejected. Server applying such restriction SHOULD provide a way for the client to sync
its clock with the server's clock.
OAuth-authenticated requests can have two sets of credentials included via the
oauth_consumer_key parameter and the
oauth_token parameter. In order for the server to
verify the authenticity of the request and prevent unauthorized access, the client needs
to prove it is the rightful owner of the credentials. This is accomplished using the
shared-secret (or RSA key) part of each set of credentials.
OAuth provides three methods for the client to prove its rightful ownership of the
credentials: HMAC-SHA1, RSA-SHA1,
and PLAINTEXT. These methods are generally referred to as
signature methods, even though PLAINTEXT does not involve a
signature. In addition, RSA-SHA1 utilizes an RSA key instead
of the shared-secrets associated with the client credentials.
OAuth does not mandate a particular signature method, as each implementation can have its
own unique requirements. Servers are free to implement and document their own custom
methods. Recommending any particular method is beyond the scope of this specification.
The client declares which signature method is used via the
oauth_signature_method parameter. It then generates a signature
(or a sting of an equivalent value), and includes it in the
oauth_signature parameter. The server verifies the signature
as specified for each method.
The signature process does not change the request or its parameter, with the exception of
the oauth_signature parameter.
The signature base string is a consistent, reproducible concatenation of several
request elements into a single string. The string is used as an input to the
HMAC-SHA1 and RSA-SHA1
signature methods, or potential future extension.
The signature base string does not cover the entire HTTP request. Most notably, it
does not include the entity-body in most requests, nor does it include most HTTP
entity-headers. The importance of the signature base string scope is that the
authenticity of the excluded components cannot be verified using the signature.
The signature base string includes a specific set of request parameters. In order for
the parameter to be included in the signature base string, they MUST be used in their
unencoded form.
NameValueb5=%3Da3ac@a2r bc2a32q
Note that the value of b5 is =%3D
and not ==. Both c@ and
c2 have empty values.
The request parameters, which include both protocol parameters and request-specific
parameters, are extracted and restored to their original unencoded form, from
the following sources:
The OAuth HTTP Authorization header. The
realm parameter MUST be excluded if present.
The HTTP request entity-body, but only if:
The entity-body is single-part.
The entity-body follows the encoding requirements of the
application/x-www-form-urlencoded content-type as
defined by .
The HTTP request entity-header includes the Content-Type
header set to application/x-www-form-urlencoded.
The query component of the HTTP request URI as defined by
section 3.
The oauth_signature parameter MUST be
excluded if present.
In many cases, clients have direct access to the parameters in their original,
unencoded form. In such cases, clients SHOULD use the unencoded values instead of
extracting them. This option is not available for servers when validating incoming
requests. Even though the parameters are encoded again in the process, they are
decoded because each of the three sources uses a different encoding algorithm.
The output of this step is a list of unencoded parameter name / value pairs.
The parameter collected in are normalized into a
single string as follows:
First, the name and value of each parameter are encoded.
The parameters are sorted by name, using lexicographical byte value ordering. If
two or more parameters share the same name, they are sorted by their value.
The name of each parameter is concatenated to its corresponding value using an
= character (ASCII code 61) as separator, even if the
value is empty.
The sorted name / value pairs are concatenated together into a single string by
using an & character (ASCII code 38) as separator.
For example, the list of parameters from the previous section would be normalized as
follows:
Encoded:
NameValueb5%3D%253Da3ac%40a2r%20bc2a32q
Sorted:
NameValuea2r%20ba32qa3ab5%3D%253Dc%40c2
Concatenated Pairs:
Name=Valuea2=r%20ba3=2qa3=ab5=%3D%253Dc%40=c2=
The signature base string incorporates the scheme, authority, and path of the request
URI as defined by section 3. The request URI query component
is included through the normalized parameters string,
and the fragment component is excluded.
This is done by constructing a base string URI representing the request without the query
or fragment components. The base string URI is constructed as follows:
The scheme and host MUST be in lowercase.
The host and port values MUST match the content of the HTTP request
Host header, if present. If the
Host header is not present, the client MUST use the
hostname and port used to make the request. Servers SHOULD remove potential
ambiguity in such cases by specifying the expected host value.
The port MUST be included if it is not the default port for the scheme, and MUST
be excluded if it is the default. Specifically, the port MUST be excluded when an
http request uses port 80 or when an
https request uses port 443. All other non-default
port numbers MUST be included.
If the URI includes an empty path, it MUST be included as /.
For example:
The request URIIs included in base string asHTTP://EXAMPLE.com:80/r/x?id=123http://example.com/r/xhttps://example.net:8080?q=1#tophttps://example.net:8080/
Finally, the signature base string is put together by concatenating its elements
together. The elements MUST be concatenated in the following order:
The HTTP request method in uppercase. For example: HEAD,
GET, POST, etc. If the
request uses a custom HTTP method, it MUST be encoded.
An & character (ASCII code 38).
The base string URI from , after being
encoded.
An & character (ASCII code 38).
The normalized request parameters string from ,
after being encoded.
The HMAC-SHA1 signature method uses the HMAC-SHA1 signature
algorithm as defined in :
The HMAC-SHA1 function variables are used in following way:
is set to the value of the signature base string from .
is set to the concatenated values of:
The client shared-secret, after being encoded.
An & character (ASCII code 38), which MUST be
included even when either secret is empty.
The token shared-secret, after being encoded.
is used to set the value of the oauth_signature
protocol parameter, after the result octet string is base64-encoded per
section 6.8.
The RSA-SHA1 signature method uses the RSASSA-PKCS1-v1_5
signature algorithm as defined in section 8.2 (also known as
PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5. To use this method, the
client MUST have established client credentials with the server which included its RSA
public key (in a manner which is beyond the scope of this specification).
The signature base string is signed using the client's RSA private key per
section 8.2.1:
Where:
is set to the client's RSA private key,
is set to the value of the signature base string from , and
is the result signature used to set the value of the oauth_signature
protocol parameter, after the result octet string is base64-encoded per
section 6.8.
The server verifies the signature per section 8.2.2:
Where:
is set to the client's RSA public key,
is set to the value of the signature base string from , and
is set to the octet string value of the oauth_signature
protocol parameter received from the client.
The PLAINTEXT method does not employ a signature algorithm
and does not provide any security as it transmits secrets in the clear. It SHOULD only
be used with a transport-layer mechanisms such as TLS or SSL. It does not use the
signature base string.
The oauth_signature protocol parameter is set to the
concatenated value of:
The client shared-secret, after being encoded.
An & character (ASCII code 38), which MUST be
included even when either secret is empty.
The token shared-secret, after being encoded.
When making an OAuth-authenticated request, protocol parameters SHALL be included in
the request using one and only one of the following locations, listed in order of
decreasing preference:
The HTTP Authorization header as described in
.
The HTTP request entity-body as described in .
The HTTP request URI query as described in .
In addition to these three methods, future extensions may provide other methods for
including protocol parameters in the request.
Protocol parameters can be transmitted using the HTTP
Authorization header as defined by
with the auth-scheme name set to OAuth (case-insensitive).
Protocol parameters SHALL be included in the Authorization
header as follows:
Parameter names and values are encoded per
Parameter Encoding.
Each parameter's name is immediately followed by an =
character (ASCII code 61), a " character (ASCII code 34),
the parameter value (MAY be empty), and another " character
(ASCII code 34).
Parameters are separated by a , character (ASCII code 44)
and OPTIONAL linear whitespace per .
The OPTIONAL realm parameter MAY be added and
interpreted per , section 1.2.
Servers MAY indicate their support for the OAuth
auth-scheme by returning the HTTP WWW-Authenticate
response header upon client requests for protected resources. As per
such a response MAY include additional HTTP
WWW-Authenticate headers:
The realm parameter defines a protection realm per ,
section 1.2.
Protocol parameters can be transmitted in the HTTP request entity-body, but only if the
following REQUIRED conditions are met:
The entity-body is single-part.
The entity-body follows the encoding requirements of the
application/x-www-form-urlencoded content-type as
defined by .
The HTTP request entity-header includes the Content-Type
header set to application/x-www-form-urlencoded.
The entity-body MAY include other request-specific parameters, in which case, the
protocol parameters SHOULD be appended following the request-specific parameters,
properly separated by an & character (ASCII code 38).
Protocol parameters can be transmitted by being added to the HTTP request URI as a
query parameter as defined by section 3.
The request URI MAY include other request-specific query parameters, in which case,
the protocol parameters SHOULD be appended following the request-specific parameters,
properly separated by an & character (ASCII code 38).
Servers receiving an authenticated request MUST:
Recalculate the request signature independently and compare it to the value
received from the client.
Ensure that the nonce / timestamp / token combination has not been used before, and
MAY reject requests with stale timestamps.
If a token is present, verify the scope and status of the client authorization by
using the token, and MAY choose to restrict token usage to the client to which it
was issued.
Ensure that the protocol version used is 1.0.
If the request fails verification, the server SHOULD respond with the appropriate HTTP
response status code. The server MAY include further details about why the request was
rejected in the response body. The following status codes SHOULD be used:
400 (Bad Request)
Unsupported parameters
Unsupported signature method
Missing parameters
Duplicated protocol parameters
401 (Unauthorized)
Invalid client credentials
Invalid or expired token
Invalid signature
Invalid or used nonce
OAuth uses the following percent-encoding rules:
Text values are first encoded as UTF-8 octets per if they are
not already. This does not include binary values which are not intended for human
consumption.
The values are then escaped using the percent-encoding
(%XX) mechanism as follows:
Characters in the unreserved character set as defined by
section 2.3 (ALPHA, DIGIT, "-", ".", "_", "~") MUST NOT be encoded.
All other characters MUST be encoded.
The two hexadecimal characters use to represent encoded characters MUST be
upper case.
OAuth uses a set of token credentials to represent the authorization granted
to the client by the resource owner. Typically, token credentials are issued by the server
at the resource owner's request, after authenticating the resource owner's identity using
its server credentials (usually a username and password pair).
There are many ways in which a resource owner can facilitate the provisioning of token
credentials. This section defines one such way, using HTTP redirections and the resource
owner's user agent. This redirection-based authorization method includes three steps:
The client obtains a set of temporary credentials from the server.
The resource owner authorizes the server to issue token credentials to the client
using the temporary credentials.
The client uses the temporary credentials to request a set of token credentials from
the server, which will enable it to access the resource owner's protected resources.
The temporary credentials discarded.
The temporary credentials MUST be revoked after being used once to obtain the token credentials.
It is RECOMMENDED that the temporary credentials have a limited lifetime. Servers SHOULD
enable resource owners to revoke token credentials after they have been issued to clients.
In order for the client to perform these steps, the server needs to advertise the URIs of
these three endpoints, as well as the HTTP method (GET, POST, etc.) used to make each requests.
To assist in communicating these endpoint, each is given a name:
The endpoint used by the client to obtain temporary credentials as described in
.
The endpoint to which the resource owner is redirected to grant authorization as
described in .
The endpoint used by the client to request a set of token credentials using the
temporary credentials as described in .
The three URIs MAY include a query component as defined by
section 3, but if present, the query MUST NOT contain any parameters beginning with the
oauth_ prefix.
The method in which the server advertises its three endpoint is beyond the scope of this
specification.
The client obtains a set of temporary credentials from the server by making an
authenticated request to the Temporary Credential Request
endpoint URI. The client MUST use the HTTP method advertised by the server. The HTTP POST
method is RECOMMENDED.
When making the request, the client authenticates using only the client credentials. The
client MUST omit the oauth_token protocol parameter from the
request and use an empty string as the token secret value.
The server MUST verify the request and if valid,
respond back to the client with a set of temporary credentials. The temporary credentials
are included in the HTTP response body using the
application/x-www-form-urlencoded content type as defined by
.
The response contains the following parameters:
The temporary credentials identifier.
The temporary credentials shared-secret.
Note that even though the parameter names include the term 'token', these credentials are
not token credentials, but are used in the next two steps in a similar manner to token
credentials.
Before the client requests a set of token credentials from the server, it MUST send
the user to the server to authorize the request. The client constructs a request URI by
adding the following parameters to the Resource Owner Authorization endpoint URI:
REQUIRED. The temporary credentials identifier obtained in
in the oauth_token parameter. Servers MAY declare this
parameter as OPTIONAL, in which case they MUST provide a way for the resource owner
to indicate the identifier through other means.
OPTIONAL. The client MAY specify an absolute URI for the server to redirect the resource owner
back to the client when authorization has been obtained or denied.
The client redirects the resource owner to the constructed URI using an HTTP redirection
response, or by other means available to it via the resource owner's user agent. The
request MUST use the HTTP GET method.
The way in which the server handles the authorization request is beyond the scope of this
specification. However, the server MUST first verify the identity of the resource owner.
When asking the resource owner to authorize the requested access, the server SHOULD
present to the resource owner information about the client requesting access based on the
association of the temporary credentials with the client identity. When displaying any
such information, the server SHOULD indicate if the information has been verified.
After receiving an authorization decision from the resource owner, the server redirects
the resource owner to the callback URI if one was provided in the
oauth_callback parameter. The server constructs the request URI
by adding the following parameter to the callback URI query component:
The temporary credentials identifier the resource owner authorized or denied access
to.
If the callback URI already includes a query component, the server MUST append the
oauth_token parameter to the end of the existing query.
The client obtains a set of token credentials from the server by making an
authenticated request to the Token Request endpoint URI.
The client MUST use the HTTP method advertised by the server. The HTTP POST method is
RECOMMENDED.
When making the request, the client authenticates using the client credentials as well as
the temporary credentials. The temporary credentials are used as a substitution for token
credentials in the authenticated request.
The server MUST verify the validity of the request,
ensure that the resource owner has authorized the provisioning of token credentials to
the client, and that the temporary credentials have not expired or used before. If the
request is valid and authorized, the token credentials are included in the HTTP response
body using the application/x-www-form-urlencoded content type
as defined by .
The response contains the following parameters:
The token identifier.
The token shared-secret.
The token credentials issued by the server MUST reflect the exact scope, duration, and
other attributes approved by the resource owner.
Once the client receives the token credentials, it can proceed to access protected
resources on behalf of the resource owner by making
authenticated request using the client credentials and the
token credentials received.
This memo includes no request to IANA.
As stated in , the greatest sources of risks are usually found not
in the core protocol itself but in policies and procedures surrounding its use. Implementers
are strongly encouraged to assess how this protocol addresses their security requirements.
The OAuth specification does not describe any mechanism for protecting tokens and
shared-secrets from eavesdroppers when they are transmitted from the server to the
client during the authorization phase. Servers should ensure that these transmissions are
protected using transport-layer mechanisms such as TLS or SSL.
When used with RSA-SHA1 signatures, the
OAuth protocol does not use the token shared-secret, or any provisioned client
shared-secret. This means the protocol relies completely on the secrecy of the private
key used by the client to sign requests.
When used with the PLAINTEXT method, the protocol makes no
attempts to protect credentials from eavesdroppers or man-in-the-middle attacks. The
PLAINTEXT method is only intended to be used in conjunction
with a transport-layer security mechanism such as TLS or SSL which does provide such
protection.
While OAuth provides a mechanism for verifying the integrity of requests, it provides no
guarantee of request confidentiality. Unless further precautions are taken, eavesdroppers
will have full access to request content. Servers should carefully consider the kinds of
data likely to be sent as part of such requests, and should employ transport-layer
security mechanisms to protect sensitive resources.
OAuth makes no attempt to verify the authenticity of the server. A hostile party could
take advantage of this by intercepting the client's requests and returning misleading or
otherwise incorrect responses. Service providers should consider such attacks when
developing services based on OAuth, and should require transport-layer security for any
requests where the authenticity of the server or of request responses is an issue.
The HTTP Authorization scheme is optional. However,
relies on the Authorization and
WWW-Authenticate headers to distinguish authenticated content
so that it can be protected. Proxies and caches, in particular, may fail to adequately
protect requests not using these headers.
For example, private authenticated content may be stored in (and thus retrievable from)
publicly-accessible caches. Servers not using the
HTTP Authorization header should take care to use other
mechanisms, such as the Cache-Control header, to ensure that
authenticated content is protected.
The client shared-secret and token shared-secret function the same way passwords do in
traditional authentication systems. In order to compute the signatures used in methods
other than RSA-SHA1, the server must have access to these
secrets in plaintext form. This is in contrast, for example, to modern operating systems,
which store only a one-way hash of user credentials.
If an attacker were to gain access to these secrets - or worse, to the server's database
of all such secrets - he or she would be able to perform any action on behalf of any
resource owner. Accordingly, it is critical that servers protect these secrets from
unauthorized access.
In many cases, the client application will be under the control of potentially untrusted
parties. For example, if the client is a freely available desktop application, an
attacker may be able to download a copy for analysis. In such cases, attackers will be
able to recover the client credentials.
Accordingly, servers should not use the client credentials alone to verify the identity
of the client. Where possible, other factors such as IP address should be used as well.
Wide deployment of OAuth and similar protocols may cause resource owners to become inured
to the practice of being redirected to websites where they are asked to enter their
passwords. If resource owners are not careful to verify the authenticity of these
websites before entering their credentials, it will be possible for attackers to exploit
this practice to steal resource owners' passwords.
Servers should attempt to educate resource owners about the risks phishing attacks pose,
and should provide mechanisms that make it easy for resource owners to confirm the
authenticity of their sites.
By itself, OAuth does not provide any method for scoping the access rights granted to a
client. However, most applications do require greater granularity of access rights. For
example, servers may wish to make it possible to grant access to some protected resources
but not others, or to grant only limited access (such as read-only access) to those
protected resources.
When implementing OAuth, servers should consider the types of access resource owners may
wish to grant clients, and should provide mechanisms to do so. Servers should also take
care to ensure that resource owners understand the access they are granting, as well as
any risks that may be involved.
Unless a transport-layer security protocol is used, eavesdroppers will have full access
to OAuth requests and signatures, and will thus be able to mount offline brute-force
attacks to recover the credentials used. Servers should be careful to assign
shared-secrets which are long enough, and random enough, to resist such attacks for at
least the length of time that the shared-secrets are valid.
For example, if shared-secrets are valid for two weeks, servers should ensure that it is
not possible to mount a brute force attack that recovers the shared-secret in less than
two weeks. Of course, servers are urged to err on the side of caution, and use the longest
secrets reasonable.
It is equally important that the pseudo-random number generator (PRNG) used to generate
these secrets be of sufficiently high quality. Many PRNG implementations generate number
sequences that may appear to be random, but which nevertheless exhibit patterns or other
weaknesses which make cryptanalysis or brute force attacks easier. Implementers should be
careful to use cryptographically secure PRNGs to avoid these problems.
The OAuth protocol has a number of features which may make resource exhaustion attacks
against servers possible. For example, if a server includes a nontrivial amount of
entropy in token shared-secrets as recommended above, then an attacker may be able to
exhaust the server's entropy pool very quickly by repeatedly obtaining temporary credentials
from the server.
Similarly, OAuth requires servers to track used nonces. If an attacker is able to use
many nonces quickly, the resources required to track them may exhaust available capacity.
And again, OAuth can require servers to perform potentially expensive computations in
order to verify the signature on incoming requests. An attacker may exploit this to
perform a denial of service attack by sending a large number of invalid requests to the
server.
Resource Exhaustion attacks are by no means specific to OAuth. However, OAuth implementers
should be careful to consider the additional avenues of attack that OAuth exposes, and
design their implementations accordingly. For example, entropy starvation typically
results in either a complete denial of service while the system waits for new entropy or
else in weak (easily guessable) secrets. When implementing OAuth, servers should consider
which of these presents a more serious risk for their application and design accordingly.
SHA-1, the hash algorithm used in HMAC-SHA1 signatures, has
been shown to have a number of cryptographic
weaknesses that significantly reduce its resistance to collision attacks. Practically
speaking, these weaknesses are difficult to exploit, and by themselves do not pose a
significant risk to users of OAuth. They may, however, make more efficient attacks
possible, and NIST has announced that it will phase
out use of SHA-1 by 2010. Servers should take this into account when considering whether
SHA-1 provides an adequate level of security for their applications.
The signature base string has been designed to support the signature methods defined in
this specification. When designing additional signature methods, the signature base
string should be evaluated to ensure compatibility with the algorithms used.
Since the signature base string does not cover the entire HTTP request, such as most
request entity-body, most entity-headers, and the order in which parameters are sent,
servers should employ additional mechanisms to protect such elements.
In this example, photos.example.net is a photo sharing website (server), and
printer.example.com is a photo printing service (client). Jane (resource owner) would like
printer.example.com to print a private photo stored at photos.example.net.
When Jane signs-into photos.example.net using her username and password, she can access the
photo by requesting the URI http://photos.example.net/photo?file=vacation.jpg
(which also supports the optional size parameter).
Jane does not want to share her username and password with printer.example.com, but would
like it to access the photo and print it.
The server documentation advertises support for the HMAC-SHA1
and PLAINTEXT methods, with PLAINTEXT
restricted to secure (HTTPS) requests. It also advertises the following endpoint URIs:
https://photos.example.net/initiate, using HTTP POST
http://photos.example.net/authorize, using HTTP GET
https://photos.example.net/token, using HTTP POST
The printer.example.com has already established client credentials with photos.example.net:
dpf43f3p2l4k3l03kd94hf93k423kf44
When printer.example.com attempts to print the request photo, it receives an HTTP response
with a 401 (Unauthorized) status code, and a challenge to use OAuth:
The client sends the following HTTPS POST request to the server:
The server validates the request and replies with a set of temporary credentials in the body
of the HTTP response:
The client redirects Jane's browser to the server's Resource Owner Authorization endpoint
URI to obtain Jane's approval for accessing her private photos.
The server asks Jane to sign-in using her username and password and if successful, asks
her if she approves granting printer.example.com access to her private photos. Jane approves
the request and is redirects her back to the client's callback URI:
After being informed by the callback request that Jane approved authorized access,
printer.example.com requests a set of token credentials using its temporary credentials:
The server validates the request and replies with a set of token credentials in the body
of the HTTP response:
The printer is now ready to request the private photo. Since the photo URI does not use
HTTPS, the HMAC-SHA1 method is required.
To generate the signature, it first needs to generate the signature base string. The
request contains the following parameters (oauth_signature
excluded) which need to be ordered and concatenated into a normalized string:
dpf43f3p2l4k3l03nnch734d00sl2jdkHMAC-SHA11191242096kllo9940pd9333jh1.0vacation.jpgoriginal
The following inputs are used to generate the signature base string:
The HTTP request method: GET
The request URI: http://photos.example.net/photos
The encoded normalized request parameters string:
file=vacation.jpg&oauth_consumer_key=dpf43f3p2l4k3l03&oauth_nonce=kllo9940pd9333jh&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1191242096&oauth_token=nnch734d00sl2jdk&oauth_version=1.0&size=original
The signature base string is (line breaks are for display purposes only):
HMAC-SHA1 produces the following digest value as a base64-encoded
string (using the signature base string as text and
kd94hf93k423kf44&pfkkdhi9sl3r4s00 as key):
All together, the client request for the photo is:
The photos.example.net sever validates the request and responds with the requested
photo.
This specification is directly based on the community
specification which was the product of the OAuth community. OAuth was modeled after
existing proprietary protocols and best practices that have been independently implemented
by various web sites. This specification was orignially authored by: Mark Atwood,
Richard M. Conlan, Blaine Cook, Leah Culver, Kellan Elliott-McCrea, Larry Halff,
Eran Hammer-Lahav, Ben Laurie, Chris Messina, John Panzer, Sam Quigley, David Recordon,
Eran Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy Smith
The authors takes all responsibility for errors and omissions.
[[ To be removed by the RFC editor before publication as an RFC. ]]
-02
Corrected mistake in parameter sorting order (c%40 comes before c2).
Added requirement to normalize empty paths as '/'.
-01
Complete rewrite of the entire specification from scratch. Separated the spec structure
into two parts and flipped their order.
Corrected errors in instructions to encode the signature base sting by some methods. The
signature value is encoded using the transport rules, not the spec method for encoding.
Replaced the entire terminology.
-00
Initial draft based on the community specification
with the following changes.
Various changes required to accommodate the strict format requirements of the IETF,
such as moving sections around (Security, Contributors, Introduction, etc.), cleaning
references, adding IETF specific text, etc.
Moved the Parameter Encoding sub-section from section 5 (Parameters) to section 9.1
(Signature Base String) to make it clear it only applies to the signature base string.
Nonce language adjusted to indicate it is unique per token/timestamp/consumer
combination.
Added security language regarding lack of token secrets in RSA-SHA1.
Fixed the bug in the Normalize Request Parameters section. Removed the 'GET' limitation
from the third bullet (query parameters).
Removed restriction of only signing application/x-www-form-urlencoded in POST requests,
allowing the entity-body to be used with all HTTP request methods.
OAuth Core 1.0Finding SHA-1 Characteristics: General Results and ApplicationsNIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1, August, 2004.