Network Working Group E. Hammer-Lahav, Ed.
Internet-Draft Yahoo!
Intended status: Standards Track B. Cook
Expires: September 24, 2009 March 23, 2009
The OAuth Core Protocol
draft-hammer-oauth-02
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Abstract
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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 5
3. Authenticated Requests . . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 6
3.2. Nonce and Timestamp . . . . . . . . . . . . . . . . . . . 7
3.3. Signature . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.1. Signature Base String . . . . . . . . . . . . . . . . 8
3.3.2. HMAC-SHA1 . . . . . . . . . . . . . . . . . . . . . . 12
3.3.3. RSA-SHA1 . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.4. PLAINTEXT . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Parameter Transmission . . . . . . . . . . . . . . . . . . 14
3.4.1. Authorization Header . . . . . . . . . . . . . . . . . 15
3.4.2. Form-Encoded Body . . . . . . . . . . . . . . . . . . 16
3.4.3. Request URI Query . . . . . . . . . . . . . . . . . . 16
3.5. Server Response . . . . . . . . . . . . . . . . . . . . . 16
3.6. Percent Encoding . . . . . . . . . . . . . . . . . . . . . 17
4. Redirection-Based Authorization . . . . . . . . . . . . . . . 18
4.1. Temporary Credentials . . . . . . . . . . . . . . . . . . 19
4.2. Resource Owner Authorization . . . . . . . . . . . . . . . 20
4.3. Token Credentials . . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6.1. Credentials Transmission . . . . . . . . . . . . . . . . . 22
6.2. RSA-SHA1 Signature Method . . . . . . . . . . . . . . . . 22
6.3. PLAINTEXT Signature Method . . . . . . . . . . . . . . . . 22
6.4. Confidentiality of Requests . . . . . . . . . . . . . . . 23
6.5. Spoofing by Counterfeit Servers . . . . . . . . . . . . . 23
6.6. Proxying and Caching of Authenticated Content . . . . . . 23
6.7. Plaintext Storage of Credentials . . . . . . . . . . . . . 23
6.8. Secrecy of the Client Credentials . . . . . . . . . . . . 24
6.9. Phishing Attacks . . . . . . . . . . . . . . . . . . . . . 24
6.10. Scoping of Access Requests . . . . . . . . . . . . . . . . 24
6.11. Entropy of Secrets . . . . . . . . . . . . . . . . . . . . 24
6.12. Denial of Service / Resource Exhaustion Attacks . . . . . 25
6.13. Cryptographic Attacks . . . . . . . . . . . . . . . . . . 26
6.14. Signature Base String Limitations . . . . . . . . . . . . 26
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . 26
Appendix A.1. Obtaining Temporary Credentials . . . . . . . . . 27
Appendix A.2. Requesting Resource Owner Authorization . . . . . 27
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Appendix A.3. Obtaining Token Credentials . . . . . . . . . . . 28
Appendix A.4. Accessing protected resources . . . . . . . . . . 28
Appendix A.4.1. Generating Signature Base String . . . . . . . . . 28
Appendix A.4.2. Calculating Signature Value . . . . . . . . . . . 29
Appendix A.4.3. Requesting protected resource . . . . . . . . . . 30
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . 30
Appendix C. Document History . . . . . . . . . . . . . . . . . 30
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.1. Normative References . . . . . . . . . . . . . . . . . . . 31
7.2. Informative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
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 [OAuth Core 1.0]
community specification. It is intended as starting point for future
standardization efforts within the IETF. See Appendix C for list of
changes. Please discuss this draft on the oauth@ietf.org [1] mailing
list. ]]
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1.1. Terminology
client
An HTTP client (per [RFC2616]) capable of making OAuth-
authenticated requests (Section 3).
server
An HTTP server (per [RFC2616]) capable of accepting OAuth-
authenticated requests (Section 3).
protected resource
An access-restricted resource (per [RFC2616]) which can be
obtained from the server using an OAuth-authenticated request
(Section 3).
resource owner
An entity capable of accessing and controlling protected
resources by using credentials to authenticate with the server.
token
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.
2. Notational Conventions
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 [RFC2119].
3. Authenticated Requests
The HTTP authentication methods defined by [RFC2617], 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
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authorized by the resource owner. Section 4 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 Section 6.8.
Making authenticated requests requires prior knowledge of the
server's configuration. OAuth provides multiple methods for
including protocol parameters in requests (Section 3.4), as well as
multiple methods for the client to prove its rightful ownership of
the credentials used (Section 3.3). The way in which clients
discovery the required configuration is outside the scope of this
specification.
3.1. Protocol Parameters
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:
oauth_consumer_key
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.
oauth_token
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.
oauth_signature_method
The name of the signature method used by the client to sign the
request, as defined in Section 3.3.
oauth_signature
The signature value as defined in Section 3.3.
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oauth_timestamp
The timestamp value as defined in Section 3.2.
oauth_nonce
The nonce value as defined in Section 3.2.
oauth_version
The protocol version. If omitted, the protocol version
defaults to "1.0".
Server-specific request parameters MUST NOT begin with the "oauth_"
prefix.
3.2. Nonce and Timestamp
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.
3.3. Signature
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.
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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.
3.3.1. Signature Base String
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.
3.3.1.1. Collect Request Parameters
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.
For example, the URI:
http://example.com/request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2q
contains the following raw-form parameters:
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+------+-------+
| Name | Value |
+------+-------+
| b5 | =%3D |
| a3 | a |
| c@ | |
| a2 | r b |
| c2 | |
| a3 | 2q |
+------+-------+
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:
o The OAuth HTTP Authorization header (Section 3.4.1). The "realm"
parameter MUST be excluded if present.
o 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
[W3C.REC-html40-19980424].
* The HTTP request entity-header includes the "Content-Type"
header set to "application/x-www-form-urlencoded".
o The query component of the HTTP request URI as defined by
[RFC3986] 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.
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3.3.1.2. Normalize Request Parameters
The parameter collected in Section 3.3.1.1 are normalized into a
single string as follows:
1. First, the name and value of each parameter are encoded
(Section 3.6).
2. 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.
3. 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.
4. 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:
+------+----------+
| Name | Value |
+------+----------+
| b5 | %3D%253D |
| a3 | a |
| c%40 | |
| a2 | r%20b |
| c2 | |
| a3 | 2q |
+------+----------+
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Sorted:
+------+----------+
| Name | Value |
+------+----------+
| a2 | r%20b |
| a3 | 2q |
| a3 | a |
| b5 | %3D%253D |
| c%40 | |
| c2 | |
+------+----------+
Concatenated Pairs:
+-------------+
| Name=Value |
+-------------+
| a2=r%20b |
| a3=2q |
| a3=a |
| b5=%3D%253D |
| c%40= |
| c2= |
+-------------+
And concatenated together into a single string:
a2=r%20b&a3=2q&a3=a&b5=%3D%253D&c%40=&c2=
3.3.1.3. Construct Base String URI
The signature base string incorporates the scheme, authority, and
path of the request URI as defined by [RFC3986] section 3. The
request URI query component is included through the normalized
parameters string (Section 3.3.1.2), 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:
1. The scheme and host MUST be in lowercase.
2. 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
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cases by specifying the expected host value.
3. 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.
4. If the URI includes an empty path, it MUST be included as "/".
For example:
+----------------------------------+-------------------------------+
| The request URI | Is included in base string as |
+----------------------------------+-------------------------------+
| HTTP://EXAMPLE.com:80/r/x?id=123 | http://example.com/r/x |
| https://example.net:8080?q=1#top | https://example.net:8080/ |
+----------------------------------+-------------------------------+
3.3.1.4. Concatenate Base String Elements
Finally, the signature base string is put together by concatenating
its elements together. The elements MUST be concatenated in the
following order:
1. The HTTP request method in uppercase. For example: "HEAD",
"GET", "POST", etc. If the request uses a custom HTTP method, it
MUST be encoded (Section 3.6).
2. An "&" character (ASCII code 38).
3. The base string URI from Section 3.3.1.3, after being encoded
(Section 3.6).
4. An "&" character (ASCII code 38).
5. The normalized request parameters string from Section 3.3.1.2,
after being encoded (Section 3.6).
3.3.2. HMAC-SHA1
The "HMAC-SHA1" signature method uses the HMAC-SHA1 signature
algorithm as defined in [RFC2104]:
digest = HMAC-SHA1 (key, text)
The HMAC-SHA1 function variables are used in following way:
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text
is set to the value of the signature base string from
Section 3.3.1.4.
key
is set to the concatenated values of:
1. The client shared-secret, after being encoded
(Section 3.6).
2. An "&" character (ASCII code 38), which MUST be included
even when either secret is empty.
3. The token shared-secret, after being encoded
(Section 3.6).
digest
is used to set the value of the "oauth_signature" protocol
parameter, after the result octet string is base64-encoded per
[RFC2045] section 6.8.
3.3.3. RSA-SHA1
The "RSA-SHA1" signature method uses the RSASSA-PKCS1-v1_5 signature
algorithm as defined in [RFC3447] 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 [RFC3447] section 8.2.1:
S = RSASSA-PKCS1-V1_5-SIGN (K, M)
Where:
K
is set to the client's RSA private key,
M
is set to the value of the signature base string from
Section 3.3.1.4, and
S
is the result signature used to set the value of the
"oauth_signature" protocol parameter, after the result octet
string is base64-encoded per [RFC2045] section 6.8.
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The server verifies the signature per [RFC3447] section 8.2.2:
RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S)
Where:
(n, e)
is set to the client's RSA public key,
M
is set to the value of the signature base string from
Section 3.3.1.4, and
S
is set to the octet string value of the "oauth_signature"
protocol parameter received from the client.
3.3.4. PLAINTEXT
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:
1. The client shared-secret, after being encoded (Section 3.6).
2. An "&" character (ASCII code 38), which MUST be included even
when either secret is empty.
3. The token shared-secret, after being encoded (Section 3.6).
3.4. Parameter Transmission
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:
1. The HTTP "Authorization" header as described in Section 3.4.1.
2. The HTTP request entity-body as described in Section 3.4.2.
3. The HTTP request URI query as described in Section 3.4.3.
In addition to these three methods, future extensions may provide
other methods for including protocol parameters in the request.
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3.4.1. Authorization Header
Protocol parameters can be transmitted using the HTTP "Authorization"
header as defined by [RFC2617] with the auth-scheme name set to
"OAuth" (case-insensitive).
For example:
Authorization: OAuth realm="http://server.example.com/",
oauth_consumer_key="0685bd9184jfhq22",
oauth_token="ad180jjd733klru7",
oauth_signature_method="HMAC-SHA1",
oauth_signature="wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%3D",
oauth_timestamp="137131200",
oauth_nonce="4572616e48616d6d65724c61686176",
oauth_version="1.0"
Protocol parameters SHALL be included in the "Authorization" header
as follows:
1. Parameter names and values are encoded per Parameter Encoding
(Section 3.6).
2. 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).
3. Parameters are separated by a "," character (ASCII code 44) and
OPTIONAL linear whitespace per [RFC2617].
4. The OPTIONAL "realm" parameter MAY be added and interpreted per
[RFC2617], 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 [RFC2617] such a response
MAY include additional HTTP "WWW-Authenticate" headers:
For example:
WWW-Authenticate: OAuth realm="http://server.example.com/"
The realm parameter defines a protection realm per [RFC2617], section
1.2.
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3.4.2. Form-Encoded Body
Protocol parameters can be transmitted in the HTTP request entity-
body, but only if the following REQUIRED conditions are met:
o The entity-body is single-part.
o The entity-body follows the encoding requirements of the
"application/x-www-form-urlencoded" content-type as defined by
[W3C.REC-html40-19980424].
o The HTTP request entity-header includes the "Content-Type" header
set to "application/x-www-form-urlencoded".
For example (line breaks are for display purposes only):
oauth_consumer_key=0685bd9184jfhq22&oauth_token=ad180jjd733klr
u7&oauth_signature_method=HMAC-SHA1&oauth_signature=wOJIO9A2W5
mFwDgiDvZbTSMK%2FPY%3D&oauth_timestamp=137131200&oauth_nonce=4
572616e48616d6d65724c61686176&oauth_version=1.0
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).
3.4.3. Request URI Query
Protocol parameters can be transmitted by being added to the HTTP
request URI as a query parameter as defined by [RFC3986] section 3.
For example (line breaks are for display purposes only):
GET /example/path?oauth_consumer_key=0685bd9184jfhq22&
oauth_token=ad180jjd733klru7&oauth_signature_method=HM
AC-SHA1&oauth_signature=wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%
3D&oauth_timestamp=137131200&oauth_nonce=4572616e48616
d6d65724c61686176&oauth_version=1.0 HTTP/1.1
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).
3.5. Server Response
Servers receiving an authenticated request MUST:
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o Recalculate the request signature independently and compare it to
the value received from the client.
o Ensure that the nonce / timestamp / token combination has not been
used before, and MAY reject requests with stale timestamps.
o 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.
o 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:
o 400 (Bad Request)
* Unsupported parameters
* Unsupported signature method
* Missing parameters
* Duplicated protocol parameters
o 401 (Unauthorized)
* Invalid client credentials
* Invalid or expired token
* Invalid signature
* Invalid or used nonce
3.6. Percent Encoding
OAuth uses the following percent-encoding rules:
1. Text values are first encoded as UTF-8 octets per [RFC3629] if
they are not already. This does not include binary values which
are not intended for human consumption.
2. The values are then escaped using the [RFC3986] percent-encoding
(%XX) mechanism as follows:
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* Characters in the unreserved character set as defined by
[RFC3986] 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.
4. Redirection-Based Authorization
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:
1. The client obtains a set of temporary credentials from the
server.
2. The resource owner authorizes the server to issue token
credentials to the client using the temporary credentials.
3. 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:
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Temporary Credential Request
The endpoint used by the client to obtain temporary credentials
as described in Section 4.1.
Resource Owner Authorization
The endpoint to which the resource owner is redirected to grant
authorization as described in Section 4.2.
Token Request
The endpoint used by the client to request a set of token
credentials using the temporary credentials as described in
Section 4.3.
The three URIs MAY include a query component as defined by [RFC3986]
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.
4.1. Temporary Credentials
The client obtains a set of temporary credentials from the server by
making an authenticated request (Section 3) 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 (Section 3.5) 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
[W3C.REC-html40-19980424].
The response contains the following parameters:
oauth_token
The temporary credentials identifier.
oauth_token_secret
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
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two steps in a similar manner to token credentials.
For example:
oauth_token=ab3cd9j4ks73hf7g&oauth_token_secret=xyz4992k83j47x0b
4.2. Resource Owner Authorization
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:
oauth_token
REQUIRED. The temporary credentials identifier obtained in
Section 4.1 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.
oauth_callback
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.
Servers MAY specify additional parameters.
In this example (line breaks are for display purposes only):
https://server.example.com/authorize?
oauth_token=ab3cd9j4ks73hf7g&
oauth_callback=http%3A%2F%2Fclient.example.net%2Fcb%3Fstate%3D1
the temporary credentials identifier is "ab3cd9j4ks73hf7g" and the
callback URI is "http://client.example.net/cb?state=1".
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
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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:
oauth_token
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.
For example (line breaks are for display purposes only):
http://client.example.net/cb?state=1&oauth_token=ab3cd9j4ks73hf7g
4.3. Token Credentials
The client obtains a set of token credentials from the server by
making an authenticated request (Section 3) 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 (Section 3.5) 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 [W3C.REC-html40-19980424].
The response contains the following parameters:
oauth_token
The token identifier.
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oauth_token_secret
The token shared-secret.
For example:
oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk
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 (Section 3) using the client credentials and
the token credentials received.
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
As stated in [RFC2617], 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.
6.1. Credentials Transmission
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.
6.2. RSA-SHA1 Signature Method
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.
6.3. PLAINTEXT Signature Method
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
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conjunction with a transport-layer security mechanism such as TLS or
SSL which does provide such protection.
6.4. Confidentiality of Requests
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.
6.5. Spoofing by Counterfeit Servers
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.
6.6. Proxying and Caching of Authenticated Content
The HTTP Authorization scheme (Section 3.4.1) is optional. However,
[RFC2616] 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 (Section 3.4.1) should take care to use
other mechanisms, such as the "Cache-Control" header, to ensure that
authenticated content is protected.
6.7. Plaintext Storage of Credentials
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
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to perform any action on behalf of any resource owner. Accordingly,
it is critical that servers protect these secrets from unauthorized
access.
6.8. Secrecy of the Client Credentials
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.
6.9. Phishing Attacks
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.
6.10. Scoping of Access Requests
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.
6.11. Entropy of Secrets
Unless a transport-layer security protocol is used, eavesdroppers
will have full access to OAuth requests and signatures, and will thus
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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.
6.12. Denial of Service / Resource Exhaustion Attacks
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.
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6.13. Cryptographic Attacks
SHA-1, the hash algorithm used in "HMAC-SHA1" signatures, has been
shown [SHA1-CHARACTERISTICS] 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 [SHA-COMMENTS] 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.
6.14. Signature Base String Limitations
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.
Appendix A. Examples
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:
Temporary Credential Request
https://photos.example.net/initiate, using HTTP POST
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Resource Owner Authorization URI:
http://photos.example.net/authorize, using HTTP GET
Token Request URI:
https://photos.example.net/token, using HTTP POST
The printer.example.com has already established client credentials
with photos.example.net:
Client Identifier
"dpf43f3p2l4k3l03"
Client Shared-Secret:
"kd94hf93k423kf44"
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:
WWW-Authenticate: OAuth realm="http://photos.example.net/"
Appendix A.1. Obtaining Temporary Credentials
The client sends the following HTTPS POST request to the server:
POST /initiate HTTP/1.1
Host: photos.example.net
Authorization: OAuth realm="http://photos.example.com/",
oauth_consumer_key="dpf43f3p2l4k3l03",
oauth_signature_method="PLAINTEXT",
oauth_signature="kd94hf93k423kf44%26",
oauth_timestamp="1191242090",
oauth_nonce="hsu94j3884jdopsl",
oauth_version="1.0"
The server validates the request and replies with a set of temporary
credentials in the body of the HTTP response:
oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03
Appendix A.2. Requesting Resource Owner Authorization
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.
http://photos.example.net/authorize?oauth_token=hh5s93j4hdidpola&
oauth_callback=http%3A%2F%2Fprinter.example.com%2Frequest_token_ready
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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:
http://printer.example.com/request_token_ready?
oauth_token=hh5s93j4hdidpola
Appendix A.3. Obtaining Token Credentials
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:
POST /token HTTP/1.1
Host: photos.example.net
Authorization: OAuth realm="http://photos.example.com/",
oauth_consumer_key="dpf43f3p2l4k3l03",
oauth_token="hh5s93j4hdidpola",
oauth_signature_method="PLAINTEXT",
oauth_signature="kd94hf93k423kf44%26hdhd0244k9j7ao03",
oauth_timestamp="1191242092",
oauth_nonce="dji430splmx33448",
oauth_version="1.0"
The server validates the request and replies with a set of token
credentials in the body of the HTTP response:
oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00
Appendix A.4. Accessing protected resources
The printer is now ready to request the private photo. Since the
photo URI does not use HTTPS, the "HMAC-SHA1" method is required.
Appendix A.4.1. Generating Signature Base String
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:
oauth_consumer_key
"dpf43f3p2l4k3l03"
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oauth_token
"nnch734d00sl2jdk"
oauth_signature_method
"HMAC-SHA1"
oauth_timestamp
"1191242096"
oauth_nonce
"kllo9940pd9333jh"
oauth_version
"1.0"
file
"vacation.jpg"
size
"original"
The following inputs are used to generate the signature base string:
1. The HTTP request method: "GET"
2. The request URI: "http://photos.example.net/photos"
3. The encoded normalized request parameters string: "file=vacation.
jpg&oauth_consumer_key=dpf43f3p2l4k3l03&oauth_nonce=kllo9940pd933
3jh&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1191242096&o
auth_token=nnch734d00sl2jdk&oauth_version=1.0&size=original"
The signature base string is (line breaks are for display purposes
only):
GET&http%3A%2F%2Fphotos.example.net%2Fphotos&file%3Dvacation.jpg%26
oauth_consumer_key%3Ddpf43f3p2l4k3l03%26oauth_nonce%3Dkllo9940pd933
3jh%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D119124
2096%26oauth_token%3Dnnch734d00sl2jdk%26oauth_version%3D1.0%26size%
3Doriginal
Appendix A.4.2. Calculating Signature Value
HMAC-SHA1 produces the following "digest" value as a base64-encoded
string (using the signature base string as "text" and
"kd94hf93k423kf44&pfkkdhi9sl3r4s00" as "key"):
tR3+Ty81lMeYAr/Fid0kMTYa/WM=
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Appendix A.4.3. Requesting protected resource
All together, the client request for the photo is:
GET /photos?file=vacation.jpg&size=original HTTP/1.1
Host: photos.example.com
Authorization: OAuth realm="http://photos.example.net/",
oauth_consumer_key="dpf43f3p2l4k3l03",
oauth_token="nnch734d00sl2jdk",
oauth_signature_method="HMAC-SHA1",
oauth_signature="tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D",
oauth_timestamp="1191242096",
oauth_nonce="kllo9940pd9333jh",
oauth_version="1.0"
The photos.example.net sever validates the request and responds with
the requested photo.
Appendix B. Acknowledgments
This specification is directly based on the [OAuth Core 1.0]
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.
Appendix C. Document History
[[ To be removed by the RFC editor before publication as an RFC. ]]
-02
o Corrected mistake in parameter sorting order (c%40 comes before
c2).
o Added requirement to normalize empty paths as '/'.
-01
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o Complete rewrite of the entire specification from scratch.
Separated the spec structure into two parts and flipped their
order.
o 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.
o Replaced the entire terminology.
-00
o Initial draft based on the [OAuth Core 1.0] community
specification with the following changes.
o 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.
o 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.
o Nonce language adjusted to indicate it is unique per token/
timestamp/consumer combination.
o Added security language regarding lack of token secrets in RSA-
SHA1.
o Fixed the bug in the Normalize Request Parameters section.
Removed the 'GET' limitation from the third bullet (query
parameters).
o 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.
7. References
7.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
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Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[W3C.REC-html40-19980424]
Jacobs, I., Raggett, D., and A. Hors, "HTML 4.0
Specification", World Wide Web Consortium
Recommendation REC-html40-19980424, April 1998,
<http://www.w3.org/TR/1998/REC-html40-19980424>.
7.2. Informative References
[OAuth Core 1.0]
OAuth, OCW., "OAuth Core 1.0".
[]
National Institute of Standards and Technology, NIST.,
"NIST Brief Comments on Recent Cryptanalytic Attacks on
Secure Hashing Functions and the Continued Security
Provided by SHA-1, August, 2004.".
[SHA1-CHARACTERISTICS]
De Canniere, C. and C. Rechberger, "Finding SHA-1
Characteristics: General Results and Applications".
URIs
Hammer-Lahav & Cook Expires September 24, 2009 [Page 32]
Internet-Draft OAuth March 2009
[1] <https://www.ietf.org/mailman/listinfo/oauth>
Authors' Addresses
Eran Hammer-Lahav (editor)
Yahoo!
Email: eran@hueniverse.com
URI: http://hueniverse.com
Blaine Cook
Email: romeda@gmail.com
URI: http://romeda.org/
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