One document matched: draft-ietf-oauth-v2-http-mac-02.xml
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<rfc category='std' ipr='trust200902' docName='draft-ietf-oauth-v2-http-mac-02'>
<front>
<title abbrev="OAuth 2.0 MAC Tokens">OAuth 2.0 Message Authentication Code (MAC) Tokens</title>
<author fullname="Justin Richer" initials="J" role="editor"
surname="Richer">
<organization>The MITRE Corporation</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email>jricher@mitre.org</email>
<uri></uri>
</address>
</author>
<author role="editor" fullname="William Mills" initials="W." surname="Mills">
<organization>Yahoo! Inc.</organization>
<address>
<postal>
<street/>
<city/>
<code/>
<region/>
<country/>
</postal>
<phone/>
<email>wmills@yahoo-inc.com </email>
</address>
</author>
<author role="editor" initials="H." surname="Tschofenig" fullname="Hannes Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<postal>
<street>Linnoitustie 6</street>
<city>Espoo</city>
<code>02600</code>
<country>Finland</country>
</postal>
<phone>+358 (50) 4871445</phone>
<email>Hannes.Tschofenig@gmx.net</email>
<uri>http://www.tschofenig.priv.at</uri>
</address>
</author>
<date year='2012'/>
<area>Security</area>
<workgroup>OAuth</workgroup>
<keyword>Internet-Draft</keyword>
<abstract>
<t>
This document specifies the HTTP MAC access authentication scheme, an HTTP
authentication method using a message authentication code (MAC) algorithm to provide
cryptographic verification of portions of HTTP requests. The document also defines an OAuth
2.0 binding for use as an access token type.
</t>
<t>NOTE: This document (and other OAuth 2.0 security documents, such as <xref target="I-D.tschofenig-oauth-hotk"/>) are still work in progress in the OAuth working group.
As such, the content of this document may change. For a discussion about security requirements please consult
<xref target="I-D.tschofenig-oauth-security"/>. Your input on the detailed security requirements is highly appreciated.
</t>
</abstract>
</front>
<middle>
<section title='Introduction'>
<t>
This specification defines the HTTP MAC access authentication scheme, providing a method
for making authenticated HTTP requests with partial cryptographic verification of the
request, covering the HTTP method, request URI, and host.
</t>
<t>
Similar to the HTTP Basic access authentication scheme <xref target='RFC2617' />, the MAC
scheme utilizes a set of client credentials which include an identifier and key. However,
in contrast with the Basic scheme, the key is never included in authenticated requests but
is used to calculate the request MAC value which is included instead.
</t>
<t>
The MAC scheme requires the establishment of a shared symmetric key between the client and
the server. This specification offers one such method for issuing a set of MAC credentials
to the client using OAuth 2.0 in the form of a MAC-type access token.
</t>
<t>
The primary design goal of this mechanism is to simplify and improve HTTP authentication
for services that are unwilling or unable to employ TLS for every request. In particular,
this mechanism leverage an initial TLS setup phase to establish a shared secret between the
client and the server. The shared secret is then used over an insecure channel to provide
protection against a passive network attacker.
</t>
<t>
In particular, when a server uses this mechanism, a passive network attacker will be
unable to "steal" the user's session token, as is possible today with cookies and other
bearer tokens. In addition, this mechanism helps secure the session token against leakage
when sent over a secure channel to the wrong server. For example, when the client uses some
form of dynamic configuration to determine where to send an authenticated request, or when
the client fails to properly validate the server's identity as part of its TLS handshake.
</t>
<t>
Unlike the HTTP Digest authentication scheme, this mechanism does not require interacting
with the server to prevent replay attacks. Instead, the client provides both a nonce and a
timestamp, which the server can use to prevent replay attacks using a bounded amount of
storage. Also unlike Digest, this mechanism is not intended to protect the user's
password itself because the client and server both have access to the key material in the
clear. Instead, servers should issue a short-lived derivative credential for this
mechanism during the initial TLS setup phase.
</t>
<section title='Example'>
<t>
The client attempts to access a protected resource without authentication, making the
following HTTP request to the resource server:
</t>
<figure>
<artwork>
<![CDATA[
GET /resource/1?b=1&a=2 HTTP/1.1
Host: example.com
]]>
</artwork>
</figure>
<t>
The resource server returns the following authentication challenge:
</t>
<figure>
<artwork>
<![CDATA[
HTTP/1.1 401 Unauthorized
WWW-Authenticate: MAC
]]>
</artwork>
</figure>
<t>
The client has previously obtained a set of MAC credentials for accessing resources on
the <spanx style='verb'>http://example.com/</spanx> server. The MAC credentials
issued to the client include the following attributes:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='MAC key identifier:'>h480djs93hd8</t>
<t hangText='MAC key:'>489dks293j39</t>
<t hangText='MAC algorithm:'>hmac-sha-1</t>
</list>
</t>
<t>
The client constructs the authentication header by calculating a timestamp (e.g. the
number of seconds since January 1, 1970 00:00:00 GMT) and generating a random string
used as a nonce:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='Timestamp:'>1336363200</t>
<t hangText='Nonce:'>dj83hs9s</t>
</list>
</t>
<t>
The client constructs the normalized request string (the new line separator character is
represented by <spanx style='verb'>\n</spanx> for display purposes only; the trailing
new line separator signify that no extension value is included with the request,
explained below):
</t>
<figure>
<artwork>
<![CDATA[
1336363200\n
dj83hs9s\n
GET\n
/resource/1?b=1&a=2\n
example.com\n
80\n
\n
]]>
</artwork>
</figure>
<t>
The request MAC is calculated using the specified MAC algorithm
<spanx style='verb'>hmac-sha-1</spanx> and the MAC key over the normalized request
string. The result is base64-encoded to produce the request MAC:
</t>
<figure>
<artwork>
<![CDATA[
bhCQXTVyfj5cmA9uKkPFx1zeOXM=
]]>
</artwork>
</figure>
<t>
The client includes the MAC key identifier, nonce, and request MAC with the request
using the <spanx style='verb'>Authorization</spanx> request header field:
</t>
<figure>
<artwork>
<![CDATA[
GET /resource/1?b=1&a=2 HTTP/1.1
Host: example.com
Authorization: MAC id="h480djs93hd8",
ts="1336363200",
nonce="dj83hs9s",
mac="bhCQXTVyfj5cmA9uKkPFx1zeOXM="
]]>
</artwork>
</figure>
<t>
The server validates the request by calculating the request MAC again based on the
request received and verifies the validity and scope of the MAC credentials. If valid,
the server responds with the requested resource representation.
</t>
</section>
<section title='Notational Conventions'>
<t>
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD',
'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this specification are to be
interpreted as described in <xref target='RFC2119' />.
</t>
<t>
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
<xref target='I-D.ietf-httpbis-p1-messaging' />. Additionally, the following rules
are included from [RFC2617]: auth-param.
</t>
</section>
</section>
<section title='Issuing MAC Credentials' anchor='issue'>
<t>
This specification provides one method for issuing MAC credentials using OAuth 2.0 as
described in <xref target='oauth2' />. This specification does not mandate servers to
support any particular method for issuing MAC credentials, and other methods MAY be defined
and used. Whenever MAC credentials are issued, the credentials MUST include the following
attributes:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='MAC key identifier'>
<vspace />
A string identifying the MAC key used to calculate the request MAC. The string is
usually opaque to the client. The server typically assigns a specific scope and
lifetime to each set of MAC credentials. The identifier MAY denote a unique value
used to retrieve the authorization information (e.g. from a database), or self-contain
the authorization information in a verifiable manner (i.e. a string consisting of some
data and a signature).
</t>
<t hangText='MAC key'>
<vspace />
A shared symmetric secret used as the MAC algorithm key. The server MUST NOT reissue
a previously issued MAC key and MAC key identifier combination.
</t>
<t hangText='MAC algorithm'>
<vspace />
A MAC algorithm used to calculate the request MAC. Value MUST be one
of <spanx style='verb'>hmac-sha-1</spanx>, <spanx style='verb'>hmac-sha-256</spanx>, or
a registered extension algorithm name as described in <xref target='registry' />.
Algorithm names are case-sensitive. If the MAC algorithm is not understood by the client,
the client MUST NOT use the MAC credentials and continue as if no MAC credentials were
issued.
</t>
</list>
</t>
<figure>
<preamble>
The MAC key identifier, MAC key, MAC algorithm strings MUST NOT include characters other
than:
</preamble>
<artwork>
<![CDATA[
%x20-21 / %x23-5B / %x5D-7E
; Any printable ASCII character except for <"> and <\>
]]>
</artwork>
</figure>
</section>
<section title='Making Requests' anchor='requests'>
<t>
To make authenticated requests, the client must be in the possession of a valid set of MAC
credentials accepted by the server. The client constructs the request by calculating a set
of attributes, and adding them to the HTTP request using the
<spanx style='verb'>Authorization</spanx> request header field as described in
<xref target='authz_header' />.
</t>
<section title='The "Authorization" Request Header' anchor='authz_header'>
<t>
The <spanx style='verb'>Authorization</spanx> request header field uses the framework
defined by <xref target='RFC2617' /> as follows:
</t>
<figure>
<artwork>
<![CDATA[
credentials = "MAC" 1*SP #params
params = id / ts / nonce / ext / mac
id = "id" "=" string-value
ts = "ts" "=" ( <"> timestamp <"> ) / timestamp
nonce = "nonce" "=" string-value
ext = "ext" "=" string-value
mac = "mac" "=" string-value
timestamp = 1*DIGIT
string-value = ( <"> plain-string <"> ) / plain-string
plain-string = 1*( %x20-21 / %x23-5B / %x5D-7E )
]]>
</artwork>
</figure>
<t>
The header attributes are set as follows:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='id'>
<vspace />
REQUIRED. The MAC key identifier.
</t>
<t hangText='ts'>
<vspace />
REQUIRED. The request timestamp. The value MUST be a positive integer set by the
client when making each request to the number of seconds elapsed from a fixed point
in time (e.g. January 1, 1970 00:00:00 GMT). The value MUST NOT include leading zeros
(e.g. <spanx style='verb'>000273154346</spanx>).
</t>
<t hangText='nonce'>
<vspace />
REQUIRED. A unique string generated by the client. The value MUST be unique across
all requests with the same timestamp and MAC key identifier combination.
</t>
<t hangText='ext'>
<vspace />
OPTIONAL. A string used to include additional information which is covered by the
request MAC. The content and format of the string is beyond the scope of this
specification.
</t>
<t hangText='mac'>
<vspace />
REQUIRED. The HTTP request MAC as described in <xref target='mac' />.
</t>
</list>
</t>
<t>
Attributes MUST NOT appear more than once. Attribute values are limited to a subset of
ASCII, which does not require escaping, as defined by the plain-string ABNF.
</t>
</section>
<section title='Request MAC' anchor='mac'>
<t>
The client uses the MAC algorithm and the MAC key to calculate the request MAC. This
specification defines two algorithms: <spanx style='verb'>hmac-sha-1</spanx> and
<spanx style='verb'>hmac-sha-256</spanx>, and provides an extension registry for
additional algorithms.
</t>
<section title='Normalized Request String' anchor='base_string'>
<t>
The normalized request string is a consistent, reproducible concatenation of several of
the HTTP request elements into a single string. By normalizing the request into a
reproducible string, the client and server can both calculate the request MAC over the
exact same value.
</t>
<t>
The string is constructed by concatenating together, in order, the following HTTP
request elements, each followed by a new line character (%x0A):
</t>
<t>
<list style='numbers'>
<t>
The timestamp value calculated for the request.
</t>
<t>
The nonce value generated for the request.
</t>
<t>
The HTTP request method in upper case. For example:
<spanx style='verb'>HEAD</spanx>, <spanx style='verb'>GET</spanx>,
<spanx style='verb'>POST</spanx>, etc.
</t>
<t>
The HTTP request-URI as defined by <xref target='RFC2616' /> section 5.1.2.
</t>
<t>
The hostname included in the HTTP request using the
<spanx style='verb'>Host</spanx> request header field in lower case.
</t>
<t>
The port as included in the HTTP request using the
<spanx style='verb'>Host</spanx> request header field. If the header field does not
include a port, the default value for the scheme MUST be used (e.g. 80 for HTTP and
443 for HTTPS).
</t>
<t>
The value of the <spanx style='verb'>ext</spanx>
<spanx style='verb'>Authorization</spanx> request header field attribute if one was
included in the request, otherwise, an empty string.
</t>
</list>
</t>
<t>
Each element is followed by a new line character (%x0A) including the last element and
even when an element value is an empty string.
</t>
<figure>
<preamble>
For example, the HTTP request:
</preamble>
<artwork>
<![CDATA[
POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1
Host: example.com
Hello World!
]]>
</artwork>
<postamble>
using timestamp <spanx style='verb'>264095:7d8f3e4a</spanx>, nonce
<spanx style='verb'>7d8f3e4a</spanx>, and extension string
<spanx style='verb'>a,b,c</spanx> is normalized into the following string (the new
line separator character is represented by <spanx style='verb'>\n</spanx> for display
purposes only):
</postamble>
</figure>
<figure>
<artwork>
<![CDATA[
264095\n
7d8f3e4a\n
POST\n
/request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q\n
example.com\n
80\n
a,b,c\n
]]>
</artwork>
</figure>
</section>
<section title='hmac-sha-1'>
<t>
<spanx style='verb'>hmac-sha-1</spanx> uses the HMAC-SHA1 algorithm as defined in
<xref target='RFC2104' />:
</t>
<figure>
<artwork>
<![CDATA[
mac = HMAC-SHA1 (key, text)
]]>
</artwork>
</figure>
<t>
Where:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='text'>
<vspace />
is set to the value of the normalized request string as described in
<xref target='base_string' />,
</t>
<t hangText='key'>
<vspace />
is set to the MAC key provided by the server, and
</t>
<t hangText='mac'>
<vspace />
is used to set the value of the <spanx style='verb'>mac</spanx> attribute,
after the result octet string is base64-encoded per <xref target='RFC2045' />
section 6.8.
</t>
</list>
</t>
</section>
<section title='hmac-sha-256'>
<t>
<spanx style='verb'>hmac-sha-256</spanx> uses the HMAC algorithm as defined in
<xref target='RFC2104' /> together with the SHA-256 hash function defined in
<xref target='NIST-FIPS-180-3' />:
</t>
<figure>
<artwork>
<![CDATA[
mac = HMAC-SHA256 (key, text)
]]>
</artwork>
</figure>
<t>
Where:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='text'>
<vspace />
is set to the value of the normalize request string as described in
<xref target='base_string' />,
</t>
<t hangText='key'>
<vspace />
is set to the MAC key provided by the server, and
</t>
<t hangText='mac'>
<vspace />
is used to set the value of the <spanx style='verb'>mac</spanx> attribute,
after the result octet string is base64-encoded per <xref target='RFC2045' />
section 6.8.
</t>
</list>
</t>
</section>
</section>
</section>
<section title='Verifying Requests'>
<t>
A server receiving an authenticated request validates it by performing the following
REQUIRED steps:
</t>
<t>
<list style='numbers'>
<t>
Recalculate the request MAC as described in <xref target='mac' /> and compare the
request MAC to the value received from the client via the
<spanx style='verb'>mac</spanx> attribute.
</t>
<t>
Ensure that the combination of timestamp, nonce, and MAC key identifier received from
the client has not been received before in a previous request. The server MAY reject
requests with stale timestamps as described in <xref target='ts' />.
</t>
<t>
Verify the scope and validity of the MAC credentials.
</t>
</list>
</t>
<t>
If the request fails verification, the server SHOULD respond using the 401 (Unauthorized)
HTTP status code and include the <spanx style='verb'>WWW-Authenticate</spanx> response
header field as described in <xref target='authn-header' />.
</t>
<section title='Timestamp Verification' anchor='ts'>
<t>
The timestamp, nonce, and MAC key identifier combination provide a unique identifier
which enables the server to prevent replay attacks. Without replay protection, an
attacker can use a compromised (but otherwise valid and authenticated) request more
than once, gaining long term access to a protected resource.
</t>
<t>
Including a timestamp with the nonce removes the need to retain an infinite number of
nonce values for future checks, by enabling the server to restrict the time period after
which a request with an old timestamp is rejected. If such a restriction is enforced, the
server MUST:
</t>
<t>
<list style='symbols'>
<t>
At the time the first request is received from the client for each MAC key
identifier, calculate the difference (in seconds) between the request timestamp and
the server's clock. The difference - the request time delta - MUST be kept as long as
the MAC key credentials are valid.
</t>
<t>
For each subsequent client request, apply the request time delta to request timestamp
to calculate the adjusted request time - the time when the request MAC has been
generated by the client, adjusted to the server's clock.
</t>
<t>
Verify that the adjusted request time is within the allowed time period defined by
the server. The server SHOULD allow for a sufficiently large window to accommodate
network delays (between the time the request has been generated by the client to the
time it is received by the server and processed).
</t>
</list>
</t>
</section>
<section title='The "WWW-Authenticate" Response Header Field' anchor='authn-header'>
<t>
If the protected resource request does not include authentication credentials, contains
an invalid MAC key identifier, or is malformed, the server SHOULD include the HTTP
<spanx style='verb'>WWW-Authenticate</spanx> response header field.
</t>
<figure>
<preamble>
For example:
</preamble>
<artwork>
<![CDATA[
HTTP/1.1 401 Unauthorized
WWW-Authenticate: MAC
]]>
</artwork>
</figure>
<t>
The <spanx style='verb'>WWW-Authenticate</spanx> request header field uses the framework
defined by <xref target='RFC2617' /> as follows:
</t>
<figure>
<artwork>
<![CDATA[
challenge = "MAC" [ 1*SP #param ]
param = error / auth-param
error = "error" "=" ( token / quoted-string)
]]>
</artwork>
</figure>
<t>
Each attribute MUST NOT appear more than once.
</t>
<t>
If the protected resource request included a MAC
<spanx style='verb'>Authorization</spanx> request header field and failed authentication,
the server MAY include the <spanx style='verb'>error</spanx> attribute to provide the
client with a human-readable explanation why the access request was declined to assist
the client developer in identifying the problem.
</t>
<figure>
<preamble>
For example:
</preamble>
<artwork>
<![CDATA[
HTTP/1.1 401 Unauthorized
WWW-Authenticate: MAC error="The MAC credentials expired"
]]>
</artwork>
</figure>
</section>
</section>
<section title='Use with OAuth 2.0' anchor='oauth2'>
<t>
OAuth 2.0 (<xref target='RFC6749' />) defines an extensible token-based
authentication framework. The MAC authentication scheme can be used to make OAuth-based
requests by issuing MAC-type access tokens.
</t>
<t>
This specification does not define methods for the client to specifically request a
MAC-type token from the authorization server. Additionally, it does not include any
discovery facilities for identifying which HMAC algorithms are supported by a resource
server, or how the client may go about obtaining MAC access tokens for any given protected
resource.
</t>
<section title='Issuing MAC-Type Access Tokens'>
<t>
Authorization servers issuing MAC-type access tokens MUST include the following
parameters whenever a response includes the <spanx style='verb'>access_token</spanx>
parameter:
</t>
<t>
<list style='hanging' hangIndent='6'>
<t hangText='access_token'>
<vspace />
REQUIRED. The MAC key identifier.
</t>
<t hangText='mac_key'>
<vspace />
REQUIRED. The MAC key.
</t>
<t hangText='mac_algorithm'>
<vspace />
REQUIRED. The MAC algorithm used to calculate the request MAC. Value MUST be one
of <spanx style='verb'>hmac-sha-1</spanx>, <spanx style='verb'>hmac-sha-256</spanx>, or
a registered extension algorithm name as described in <xref target='registry' />.
</t>
</list>
</t>
<figure>
<preamble>
For example:
</preamble>
<artwork>
<![CDATA[
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token":"SlAV32hkKG",
"token_type":"mac",
"expires_in":3600,
"refresh_token":"8xLOxBtZp8",
"mac_key":"adijq39jdlaska9asud",
"mac_algorithm":"hmac-sha-256"
}
]]>
</artwork>
</figure>
</section>
</section>
<section title='Security Considerations' anchor='Security'>
<t>
As stated in <xref target='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.
</t>
<section title='MAC Keys Transmission'>
<t>
This specification describes two mechanism for obtaining or transmitting MAC keys, both
require the use of a transport-layer security mechanism when sending MAC keys to the
client. Additional methods used to obtain MAC credentials must ensure that these
transmissions are protected using transport-layer mechanisms such as TLS or SSL.
</t>
</section>
<section title='Confidentiality of Requests'>
<t>
While this protocol 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.
</t>
</section>
<section title='Spoofing by Counterfeit Servers'>
<t>
This protocol 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 using this protocol, and should require transport-layer
security for any requests where the authenticity of the resource server or of request
responses is an issue.
</t>
</section>
<section title='Plaintext Storage of Credentials'>
<t>
The MAC key functions the same way passwords do in traditional authentication systems. In
order to compute the request MAC, the server must have access to the MAC key in plaintext
form. This is in contrast, for example, to modern operating systems, which store only a
one-way hash of user credentials.
</t>
<t>
If an attacker were to gain access to these MAC keys - or worse, to the server's database
of all such MAC keys - he or she would be able to perform any action on behalf of any
resource owner. Accordingly, it is critical that servers protect these MAC keys from
unauthorized access.
</t>
</section>
<section title='Entropy of MAC Keys'>
<t>
Unless a transport-layer security protocol is used, eavesdroppers will have full access
to authenticated requests and request MAC values, and will thus be able to mount offline
brute-force attacks to recover the MAC key used. Servers should be careful to assign MAC
keys which are long enough, and random enough, to resist such attacks for at least the
length of time that the MAC credentials are valid.
</t>
<t>
For example, if the MAC credentials are valid for two weeks, servers should ensure that
it is not possible to mount a brute force attack that recovers the MAC key in less than
two weeks. Of course, servers are urged to err on the side of caution, and use the
longest MAC key reasonable.
</t>
<t>
It is equally important that the pseudo-random number generator (PRNG) used to generate
these MAC keys 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.
</t>
</section>
<section title='Denial of Service / Resource Exhaustion Attacks'>
<t>
This specification includes a number of features which may make resource exhaustion
attacks against servers possible. For example, this protocol 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, this protocol can require servers
to perform potentially expensive computations in order to verify the request MAC 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.
</t>
<t>
Resource Exhaustion attacks are by no means specific to this specification. However,
implementers should be careful to consider the additional avenues of attack that this
protocol 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) MAC keys. When implementing this
protocol, servers should consider which of these presents a more serious risk for their
application and design accordingly.
</t>
</section>
<section title='Timing Attacks'>
<t>
This specification makes use of HMACs, for which a signature verification involves
comparing the received MAC string to the expected one. If the string comparison operator
operates in observably different times depending on inputs, e.g. because it compares the
strings character by character and returns a negative result as soon as two characters
fail to match, then it may be possible to use this timing information to determine the
expected MAC, character by character.
</t>
<t>
Service implementers are encouraged to use fixed-time string comparators for MAC
verification.
</t>
</section>
<section title='CSRF Attacks'>
<t>
A Cross-Site Request Forgery attack occurs when a site, evil.com, initiates within the
victim's browser the loading of a URL from or the posting of a form to a web site where a
side-effect will occur, e.g. transfer of money, change of status message, etc. To prevent
this kind of attack, web sites may use various techniques to determine that the
originator of the request is indeed the site itself, rather than a third party. The
classic approach is to include, in the set of URL parameters or form content, a nonce
generated by the server and tied to the user's session, which indicates that only the
server could have triggered the action.
</t>
<t>
Recently, the Origin HTTP header has been proposed and deployed in some browsers. This
header indicates the scheme, host, and port of the originator of a request. Some web
applications may use this Origin header as a defense against CSRF.
</t>
<t>
To keep this specification simple, HTTP headers are not part of the string to be MAC'ed.
As a result, MAC authentication cannot defend against header spoofing, and a web site
that uses the Host header to defend against CSRF attacks cannot use MAC authentication to
defend against active network attackers. Sites that want the full protection of MAC
Authentication should use traditional, cookie-tied CSRF defenses.
</t>
</section>
<section title='Coverage Limitations'>
<t>
The normalized request string has been designed to support the authentication methods
defined in this specification. Those designing additional methods, should evaluated the
compatibility of the normalized request string with their security requirements.
Since the normalized request string does not cover the entire HTTP request, servers
should employ additional mechanisms to protect such elements.
</t>
<t>
The request MAC does not cover entity-header fields which can often affect how the
request body is interpreted by the server (i.e. Content-Type). If the server behavior is
influenced by the presence or value of such header fields, an attacker can manipulate the
request header without being detected.
</t>
</section>
</section>
<section title='IANA Considerations' anchor='IANA'>
<section title='The HTTP MAC Authentication Scheme Algorithm Registry' anchor='registry'>
<t>
This specification establishes the HTTP MAC authentication scheme algorithm registry.
</t>
<t>
Additional MAC algorithms are registered on the advice of one or more Designated Experts
(appointed by the IESG or their delegate), with a Specification Required (using
terminology from <xref target='RFC5226' />). However, to allow for the allocation of
values prior to publication, the Designated Expert(s) may approve registration once they
are satisfied that such a specification will be published.
</t>
<t>
Registration requests should be sent to the [TBD]@ietf.org mailing list for review and
comment, with an appropriate subject (e.g., "Request for MAC Algorithm: example").
[[ Note to RFC-EDITOR: The name of the mailing list should be determined in consultation
with the IESG and IANA. Suggested name: http-mac-ext-review. ]]
</t>
<t>
Within at most 14 days of the request, the Designated Expert(s) will either approve or
deny the registration request, communicating this decision to the review list and IANA.
Denials should include an explanation and, if applicable, suggestions as to how to make
the request successful.
</t>
<t>
Decisions (or lack thereof) made by the Designated Expert can be first appealed to
Application Area Directors (contactable using app-ads@tools.ietf.org email address or
directly by looking up their email addresses on http://www.iesg.org/ website) and, if the
appellant is not satisfied with the response, to the full IESG (using the iesg@iesg.org
mailing list).
</t>
<t>
IANA should only accept registry updates from the Designated Expert(s), and should direct
all requests for registration to the review mailing list.
</t>
<section title='Registration Template'>
<t>
<list style='hanging'>
<t hangText='Algorithm name:'>
<vspace />
The name requested (e.g., "example").
</t>
<t hangText='Change controller:'>
<vspace />
For standards-track RFCs, state "IETF". For others, give the name of the
responsible party. Other details (e.g., postal address, e-mail address, home page
URI) may also be included.
</t>
<t hangText='Specification document(s):'>
<vspace />
Reference to document that specifies the algorithm, preferably including a URI that
can be used to retrieve a copy of the document. An indication of the relevant
sections may also be included, but is not required.
</t>
</list>
</t>
</section>
<section title='Initial Registry Contents'>
<t>
The HTTP MAC authentication scheme algorithm registry's initial contents are:
</t>
<t>
<list style='symbols'>
<t>
Algorithm name: hmac-sha-1
</t>
<t>
Change controller: IETF
</t>
<t>
Specification document(s): [[ this document ]]
</t>
</list>
</t>
<t>
<list style='symbols'>
<t>
Algorithm name: hmac-sha-256
</t>
<t>
Change controller: IETF
</t>
<t>
Specification document(s): [[ this document ]]
</t>
</list>
</t>
</section>
</section>
<section title='OAuth Access Token Type Registration'>
<t>
This specification registers the following access token type in the OAuth Access Token
Type Registry.
</t>
<section title='The "mac" OAuth Access Token Type'>
<t>
<list style='hanging'>
<t hangText='Type name:'>
<vspace />
mac
</t>
<t hangText='Additional Token Endpoint Response Parameters:'>
<vspace />
secret, algorithm
</t>
<t hangText='HTTP Authentication Scheme(s):'>
<vspace />
MAC
</t>
<t hangText='Change controller:'>
<vspace />
IETF
</t>
<t hangText='Specification document(s):'>
<vspace />
[[ this document ]]
</t>
</list>
</t>
</section>
</section>
<section title='OAuth Parameters Registration'>
<t>
This specification registers the following parameters in the OAuth Parameters Registry
established by <xref target='RFC6749' />.
</t>
<section title='The "mac_key" OAuth Parameter'>
<t>
<list style='hanging'>
<t hangText='Parameter name:'>
mac_key
</t>
<t hangText='Parameter usage location:'>
authorization response, token response
</t>
<t hangText='Change controller:'>
IETF
</t>
<t hangText='Specification document(s):'>
[[ this document ]]
</t>
<t hangText='Related information:'>
None
</t>
</list>
</t>
</section>
<section title='The "mac_algorithm" OAuth Parameter'>
<t>
<list style='hanging'>
<t hangText='Parameter name:'>
mac_algorithm
</t>
<t hangText='Parameter usage location:'>
authorization response, token response
</t>
<t hangText='Change controller:'>
IETF
</t>
<t hangText='Specification document(s):'>
[[ this document ]]
</t>
<t hangText='Related information:'>
None
</t>
</list>
</t>
</section>
</section>
</section>
<section title='Contributors'>
<t>This document is based on OAuth 1.0 and we would like to thank Eran Hammer-Lahav for his work
on incorporating the ideas into OAuth 2.0.</t>
</section>
<section title='Acknowledgments'>
<t>
The author would like to thank Ben Adida, Adam Barth, Phil Hunt, Rasmus Lerdorf,
James Manger, William Mills, Scott Renfro, Justin Richer, Toby White, Peter Wolanin,
and Skylar Woodward for their contributions, suggestions, and feedback.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
&RFC2119;
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2045.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2104.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2616.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2617.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.3986.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5226.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5246.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.6265.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-httpbis-p1-messaging.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.6749.xml' ?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml4/reference.W3C.REC-html401-19991224.xml' ?>
<reference anchor='NIST-FIPS-180-3'>
<front>
<title>Secure Hash Standard (SHS). FIPS PUB 180-3, October 2008</title>
<author>
<organization>National Institute of Standards and Technology</organization>
</author>
</front>
<format type='pdf' target='http://www.nist.gov/itl/upload/fips180-3_final.pdf' />
</reference>
</references>
<references title='Informative References'>
&I-D.tschofenig-oauth-security;
&I-D.tschofenig-oauth-hotk;
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5849.xml' ?>
</references>
</back>
</rfc>
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