Internet DRAFT - draft-ietf-oauth-v2-http-mac
draft-ietf-oauth-v2-http-mac
OAuth J. Richer
Internet-Draft The MITRE Corporation
Intended status: Standards Track W. Mills
Expires: July 19, 2014 Yahoo! Inc.
H. Tschofenig, Ed.
P. Hunt
Oracle Corporation
January 15, 2014
OAuth 2.0 Message Authentication Code (MAC) Tokens
draft-ietf-oauth-v2-http-mac-05.txt
Abstract
This specification describes how to use MAC Tokens in HTTP requests
to access OAuth 2.0 protected resources. An OAuth client willing to
access a protected resource needs to demonstrate possession of a
cryptographic key by using it with a keyed message digest function to
the request.
The document also defines a key distribution protocol for obtaining a
fresh session key.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 19, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Key Distribution . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Session Key Transport to Client . . . . . . . . . . . . . 6
4.2. Session Key Transport to Resource Server . . . . . . . . 8
5. The Authenticator . . . . . . . . . . . . . . . . . . . . . . 9
5.1. The Authenticator . . . . . . . . . . . . . . . . . . . . 9
5.2. MAC Input String . . . . . . . . . . . . . . . . . . . . 12
5.3. Keyed Message Digest Algorithms . . . . . . . . . . . . . 13
5.3.1. hmac-sha-1 . . . . . . . . . . . . . . . . . . . . . 13
5.3.2. hmac-sha-256 . . . . . . . . . . . . . . . . . . . . 14
6. Verifying the Authenticator . . . . . . . . . . . . . . . . . 14
6.1. Timestamp Verification . . . . . . . . . . . . . . . . . 15
6.2. Error Handling . . . . . . . . . . . . . . . . . . . . . 15
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8.1. Key Distribution . . . . . . . . . . . . . . . . . . . . 16
8.2. Offering Confidentiality Protection for Access to
Protected Resources . . . . . . . . . . . . . . . . . . . 16
8.3. Authentication of Resource Servers . . . . . . . . . . . 17
8.4. Plaintext Storage of Credentials . . . . . . . . . . . . 17
8.5. Entropy of Session Keys . . . . . . . . . . . . . . . . . 17
8.6. Denial of Service / Resource Exhaustion Attacks . . . . . 18
8.7. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 18
8.8. CSRF Attacks . . . . . . . . . . . . . . . . . . . . . . 19
8.9. Protecting HTTP Header Fields . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 19
9.2. MAC Token Algorithm Registry . . . . . . . . . . . . . . 20
9.2.1. Registration Template . . . . . . . . . . . . . . . . 20
9.2.2. Initial Registry Contents . . . . . . . . . . . . . . 21
9.3. OAuth Access Token Type Registration . . . . . . . . . . 21
9.3.1. The "mac" OAuth Access Token Type . . . . . . . . . . 21
9.4. OAuth Parameters Registration . . . . . . . . . . . . . . 22
9.4.1. The "mac_key" OAuth Parameter . . . . . . . . . . . . 22
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9.4.2. The "mac_algorithm" OAuth Parameter . . . . . . . . . 22
9.4.3. The "kid" OAuth Parameter . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 23
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Background Information . . . . . . . . . . . . . . . 26
A.1. Security and Privacy Threats . . . . . . . . . . . . . . 26
A.2. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 27
A.2.1. Confidentiality Protection . . . . . . . . . . . . . 28
A.2.2. Sender Constraint . . . . . . . . . . . . . . . . . . 28
A.2.3. Key Confirmation . . . . . . . . . . . . . . . . . . 29
A.2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . 30
A.3. Requirements . . . . . . . . . . . . . . . . . . . . . . 31
A.4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 35
A.4.1. Access to an 'Unprotected' Resource . . . . . . . . . 35
A.4.2. Offering Application Layer End-to-End Security . . . 36
A.4.3. Preventing Access Token Re-Use by the Resource Server 36
A.4.4. TLS Channel Binding Support . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
This specification describes how to use MAC Tokens in HTTP requests
and responses to access protected resources via the OAuth 2.0
protocol [RFC6749]. An OAuth client willing to access a protected
resource needs to demonstrate possession of a symmetric key by using
it with a keyed message digest function to the request. The keyed
message digest function is computed over a flexible set of parameters
from the HTTP message.
The MAC Token mechanism requires the establishment of a shared
symmetric key between the client and the resource server. This
specification defines a three party key distribution protocol to
dynamically distribute this session key from the authorization server
to the client and the resource server.
The design goal for this mechanism is to support the requirements
outlined in Appendix A. In particular, when a server uses this
mechanism, a passive attacker will be unable to use an eavesdropped
access token exchanged between the client and the resource server.
In addition, this mechanism helps secure the access token against
leakage when sent over a secure channel to the wrong resource server
if the client provided information about the resource server it wants
to interact with in the request to the authorization server.
Since a keyed message digest only provides integrity protection and
data-origin authentication confidentiality protection can only be
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added by the usage of Transport Layer Security (TLS). This
specification provides a mechanism for channel binding is included to
ensure that a TLS channel is not terminated prematurely and indeed
covers the entire end-to-end communication.
2. Terminology
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 [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [I-D.ietf-httpbis-p1-messaging]. Additionally, the
following rules are included from [RFC2617]: auth-param.
Session Key:
The terms mac key, session key, and symmetric key are used
interchangeably and refer to the cryptographic keying material
established between the client and the resource server. This
temporary key used between the client and the resource server,
with a lifetime limited to the lifetime of the access token. This
session key is generated by the authorization server.
Authenticator:
A record containing information that can be shown to have been
recently generated using the session key known only by the client
and the resource server.
Message Authentication Code (MAC):
Message authentication codes (MACs) are hash functions that take
two distinct inputs, a message and a secret key, and produce a
fixed-size output. The design goal is that it is
practically infeasible to produce the same output without
knowledge of the key. The terms keyed message digest functions
and MACs are used interchangeably.
3. Architecture
The architecture of the proposal described in this document assumes
that the authorization server acts as a trusted third party that
provides session keys to clients and to resource servers. These
session keys are used by the client and the resource server as input
to a MAC. In order to obtain the session key the client interacts
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with the authorization server as part of the a normal grant exchange.
This is shown in an abstract way in Figure 1. Together with the
access token the authorization server returns a session key (in the
mac_key parameter) and several other parameters. The resource server
obtains the session key via the access token. Both of these two key
distribution steps are described in more detail in Section 4.
+---------------+
^| | AS-RS Key
// | Authorization |<*******
/ | Server | *
// | | *
/ | | *
(I) // /+---------------+ *
Access / // *
Token / / *
Request// // (II) Access Token *
/ / +Session Key (SK) *
// // *
/ v v
+-----------+ +------------+
| | | |
| | | Resource |
| Client | | Server |
| | | |
| | | |
+-----------+ +------------+
****: Out-of-Band Long-Term Key Establishment
----: Dynamic Session Key Distribution
Figure 1: Architecture: Interaction between the Client and the
Authorization Server.
Once the client has obtained the necessary access token and the
session key (including parameters) it can start to interact with the
resource server. To demonstrate possession of the session key it
computes a MAC and adds various fields to the outgoing request
message. We call this structure the "Authenticator". The server
evaluates the request, includes an Authenticator and returns a
response back to the client. Since the access token is valid for a
period of time the resource server may decide to cache it so that it
does not need to be provided in every request from the client. This
interaction is shown in Figure 2.
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+---------------+
| |
| Authorization |
| Server |
| |
| |
+---------------+
+-----------+ Authenticator (a) +------------+
| |---------------------->| |
| | [+Access Token] | Resource |
| Client | | Server |
| | Authenticator (b) | |
| |<----------------------| |
+-----------+ +------------+
^ ^
| |
| |
SK SK
+param +param
Figure 2: Architecture: Interaction between the Client and the
Resource Server.
4. Key Distribution
For this scheme to function a session key must be available to the
client and the resource server, which is then used as a parameter in
the keyed message digest function. This document describes the key
distribution mechanism that uses the authorization server as a
trusted third party, which ensures that the session key is
transported from the authorization server to the client and the
resource server.
4.1. Session Key Transport to Client
Authorization servers issue MAC Tokens based on requests from
clients. The request MUST include the audience parameter defined in
[I-D.tschofenig-oauth-audience], which indicates the resource server
the client wants to interact with. This specification assumes use of
the 'Authorization Code' grant. If the request is processed
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successfully by the authorization server it MUST return at least the
following parameters to the client:
kid
The name of the key (key id), which is an identifier generated
by the resource server. It is RECOMMENDED that the
authorization server generates this key id by computing a hash
over the access_token, for example using SHA-1, and to encode
it in a base64 format.
access_token
The OAuth 2.0 access token.
mac_key
The session key generated by the authorization server. Note
that the lifetime of the session key is equal to the lifetime
of the access token.
mac_algorithm
The MAC algorithm used to calculate the request MAC. The value
MUST be one of "hmac-sha-1", "hmac-sha-256", or a registered
extension algorithm name as described in Section 9.2. The
authorization server is assumed to know the set of algorithms
supported by the client and the resource server. It selects an
algorithm that meets the security policies and is supported by
both nodes.
For example:
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HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token":
"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDK0hTMjU2In0.
pwaFh7yJPivLjjPkzC-GeAyHuy7AinGcS51AZ7TXnwkC80Ow1aW47kcT_UV54ubo
nONbeArwOVuR7shveXnwPmucwrk_3OCcHrCbE1HR-Jfme2mF_WR3zUMcwqmU0RlH
kwx9txo_sKRasjlXc8RYP-evLCmT1XRXKjtY5l44Gnh0A84hGvVfMxMfCWXh38hi
2h8JMjQHGQ3mivVui5lbf-zzb3qXXxNO1ZYoWgs5tP1-T54QYc9Bi9wodFPWNPKB
kY-BgewG-Vmc59JqFeprk1O08qhKQeOGCWc0WPC_n_LIpGWH6spRm7KGuYdgDMkQ
bd4uuB0uPPLx_euVCdrVrA.
AxY8DCtDaGlsbGljb3RoZQ.
7MI2lRCaoyYx1HclVXkr8DhmDoikTmOp3IdEmm4qgBThFkqFqOs3ivXLJTku4M0f
laMAbGG_X6K8_B-0E-7ak-Olm_-_V03oBUUGTAc-F0A.
OwWNxnC-BMEie-GkFHzVWiNiaV3zUHf6fCOGTwbRckU",
"token_type":"mac",
"expires_in":3600,
"refresh_token":"8xLOxBtZp8",
"kid":"22BIjxU93h/IgwEb4zCRu5WF37s=",
"mac_key":"adijq39jdlaska9asud",
"mac_algorithm":"hmac-sha-256"
}
4.2. Session Key Transport to Resource Server
The transport of the mac_key from the authorization server to the
resource server is accomplished by conveying the encrypting mac_key
inside the access token. At the time of writing only one
standardized format for carrying the access token is defined: the
JSON Web Token (JWT) [I-D.ietf-oauth-json-web-token]. Note that the
header of the JSON Web Encryption (JWE) structure
[I-D.ietf-jose-json-web-encryption], which is a JWT with encrypted
content, MUST contain a key id (kid) in the header to allow the
resource server to select the appropriate keying material for
decryption. This keying material is a symmetric or an asymmetric
long-term key established between the resource server and the
authorization server, as shown in Figure 1 as AS-RS key. The
establishment of this long-term key is outside the scope of this
specification.
This document defines two new claims to be carried in the JWT:
mac_key, kid. These two parameters match the content of the mac_key
and the kid conveyed to the client, as shown in Section 4.1.
kid
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The name of the key (key id), which is an identifier generated
by the resource server.
mac_key
The session key generated by the authorization server.
This example shows a JWT claim set without header and without
encryption:
{"iss":"authorization-server.example.com",
"exp":1300819380,
"kid":"22BIjxU93h/IgwEb4zCRu5WF37s=",
"mac_key":"adijq39jdlaska9asud",
"aud":"apps.example.com"
}
QUESTIONS: An alternative to the use of a JWT to convey the access
token with the encrypted mac_key is use the token introspect
[I-D.richer-oauth-introspection]. What mechanism should be
described? What should be mandatory to implement?
QUESTIONS: The above description assumes that the entire access
token is encrypted but it would be possible to only encrypt the
session key and to only apply integrity protection to other
fields. Is this desirable?
5. The Authenticator
To access a protected resource the client must be in the possession
of a valid set of session key provided by the authorization server.
The client constructs the authenticator, as described in Section 5.1.
5.1. The Authenticator
The client constructs the authenticator and adds the resulting fields
to the HTTP request using the "Authorization" request header field.
The "Authorization" request header field uses the framework defined
by [RFC2617]. To include the authenticator in a subsequent response
from the authorization server to the client the WWW-Authenticate
header is used. For further exchanges a new, yet-to-be-defined
header will be used.
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authenticator = "MAC" 1*SP #params
params = id / ts / seq-nr / access_token / mac / h / cb
kid = "kid" "=" string-value
ts = "ts" "=" ( <"> timestamp <"> ) / timestamp
seq-nr = "seq-nr" "=" string-value
access_token = "access_token" "=" b64token
mac = "mac" "=" string-value
cb = "cb" "=" token
h = "h" "=" h-tag
h-tag = %x68 [FWS] "=" [FWS] hdr-name
*( [FWS] ":" [FWS] hdr-name )
hdr-name = token
timestamp = 1*DIGIT
string-value = ( <"> plain-string <"> ) / plain-string
plain-string = 1*( %x20-21 / %x23-5B / %x5D-7E )
b64token = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"="
The header attributes are set as follows:
kid
REQUIRED. The key identifier.
ts
REQUIRED. The timestamp. The value MUST be a positive integer
set by the client when making each request to the number of
milliseconds since 1 January 1970.
The JavaScript getTime() function or the Java
System.currentTimeMillis() function, for example, produce such
a timestamp.
seq-nr
OPTIONAL. This optional field includes the initial sequence
number to be used by the messages exchange between the client
and the server when the replay protection provided by the
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timestamp is not sufficient enough replay protection. This
field specifies the initial sequence number for messages from
the client to the server. When included in the response
message, the initial sequence number is that for messages from
the server to the client. Sequence numbers fall in the range 0
through 2^64 - 1 and wrap to zero following the value 2^64 - 1.
The initial sequence number SHOULD be random and uniformly
distributed across the full space of possible sequence numbers,
so that it cannot be guessed by an attacker and so that it and
the successive sequence numbers do not repeat other sequences.
In the event that more than 2^64 messages are to be generated
in a series of messages, rekeying MUST be performed before
sequence numbers are reused. Rekeying requires a new access
token to be requested.
access_token
CONDITIONAL. The access_token MUST be included in the first
request from the client to the server but MUST NOT be included
in a subsequent response and in a further protocol exchange.
mac
REQUIRED. The result of the keyed message digest computation,
as described in Section 5.3.
cb
OPTIONAL. This field carries the channel binding value from
RFC 5929 [RFC5929] in the following format: cb= channel-
binding-type ":" channel-binding-content. RFC 5929 offers two
types of channel bindings for TLS. First, there is the 'tls-
server-end-point' channel binding, which uses a hash of the TLS
server's certificate as it appears, octet for octet, in the
server's Certificate message. The second channel binding is
'tls-unique', which uses the first TLS Finished message sent
(note: the Finished struct, not the TLS record layer message
containing it) in the most recent TLS handshake of the TLS
connection being bound to. As an example, the cb field may
contain cb=tls-unique:9382c93673d814579ed1610d3
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h
OPTIONAL. This field contains a colon-separated list of header
field names that identify the header fields presented to the
keyed message digest algorithm. If the 'h' header field is
absent then the following value is set by default: h="host".
The field MUST contain the complete list of header fields in
the order presented to the keyed message digest algorithm. The
field MAY contain names of header fields that do not exist at
the time of computing the keyed message digest; nonexistent
header fields do not contribute to the keyed message digest
computation (that is, they are treated as the null input,
including the header field name, the separating colon, the
header field value, and any CRLF terminator). By including
header fields that do not actually exist in the keyed message
digest computation, the client can allow the resource server to
detect insertion of those header fields by intermediaries.
However, since the client cannot possibly know what header
fields might be defined in the future, this mechanism cannot be
used to prevent the addition of any possible unknown header
fields. The field MAY contain multiple instances of a header
field name, meaning multiple occurrences of the corresponding
header field are included in the header hash. The field MUST
NOT include the mac header field. Folding whitespace (FWS) MAY
be included on either side of the colon separator. Header
field names MUST be compared against actual header field names
in a case-insensitive manner. This list MUST NOT be empty.
See Section 8 for a discussion of choosing header fields.
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.
5.2. MAC Input String
An HTTP message can either be a request from client to server or a
response from server to client. Syntactically, the two types of
message differ only in the start-line, which is either a request-line
(for requests) or a status-line (for responses).
Two parameters serve as input to a keyed message digest function: a
key and an input string. Depending on the communication direction
either the request-line or the status-line is used as the first value
followed by the HTTP header fields listed in the 'h' parameter.
Then, the timestamp field and the seq-nr field (if present) is
concatenated.
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As an example, consider the HTTP request with the new line separator
character represented by "\n" for editorial purposes only. The h
parameter is set to h=host, the kid is 314906b0-7c55, and the
timstamp is 1361471629.
POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1
Host: example.com
Hello World!
The resulting string is:
POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1\n
1361471629\n
example.com\n
5.3. Keyed Message Digest Algorithms
The client uses a cryptographic algorithm together with a session key
to calculate a keyed message digest. This specification defines two
algorithms: "hmac-sha-1" and "hmac-sha-256", and provides an
extension registry for additional algorithms.
5.3.1. hmac-sha-1
"hmac-sha-1" uses the HMAC-SHA1 algorithm, as defined in [RFC2104]:
mac = HMAC-SHA1 (key, text)
Where:
text
is set to the value of the input string as described in
Section 5.2,
key
is set to the session key provided by the authorization server,
and
mac
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is used to set the value of the "mac" attribute, after the
result string is base64-encoded per Section 6.8 of [RFC2045].
5.3.2. hmac-sha-256
"hmac-sha-256" uses the HMAC algorithm, as defined in [RFC2104], with
the SHA-256 hash function, defined in [NIST-FIPS-180-3]:
mac = HMAC-SHA256 (key, text)
Where:
text
is set to the value of the input string as described in
Section 5.2,
key
is set to the session key provided by the authorization server,
and
mac
is used to set the value of the "mac" attribute, after the
result string is base64-encoded per Section 6.8 of [RFC2045].
6. Verifying the Authenticator
When receiving a message with an authenticator the following steps
are performed:
1. When the authorization server receives a message with a new
access token (and consequently a new session key) then it obtains
the session key by retrieving the content of the access token
(which requires decryption of the session key contained inside
the token). The content of the access token, in particular the
audience field and the scope, MUST be verified as described in
Alternatively, the kid parameter is used to look-up a cached
session key from a previous exchange.
2. Recalculate the keyed message digest, as described in
Section 5.3, and compare the request MAC to the value received
from the client via the "mac" attribute.
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3. Verify that no replay took place by comparing the value of the ts
(timestamp) header with the local time. The processing of
authenticators with stale timestamps is described in Section 6.1.
Error handling is described in Section 6.2.
6.1. Timestamp Verification
The timestamp field enables the server to detect replay attacks.
Without replay protection, an attacker can use an eavesdropped
request to gain access to a protected resource. The following
procedure is used to detect replays:
o At the time the first request is received from the client for each
key identifier, calculate the difference (in seconds) between the
request timestamp and the local clock. The difference is stored
locally for later use.
o For each subsequent request, apply the request time delta to the
timestamp included in the message to calculate the adjusted
request time.
o Verify that the adjusted request time is within the allowed time
period defined by the authorization server. If the local time and
the calculated time based in the request differ by more than the
allowable clock skew (e.g., 5 minutes) a replay has to be assumed.
6.2. Error Handling
If the protected resource request does not include an access token,
lacks the keyed message digest, contains an invalid key identifier,
or is malformed, the server SHOULD return a 401 (Unauthorized) HTTP
status code.
For example:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: MAC
The "WWW-Authenticate" request header field uses the framework
defined by [RFC2617] as follows:
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challenge = "MAC" [ 1*SP #param ]
param = error / auth-param
error = "error" "=" ( token / quoted-string)
Each attribute MUST NOT appear more than once.
If the protected resource request included a MAC "Authorization"
request header field and failed authentication, the server MAY
include the "error" 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.
For example:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: MAC error="The MAC credentials expired"
7. Example
[Editor's Note: Full example goes in here.]
8. 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 and the
security threats they want to mitigate.
8.1. Key Distribution
This specification describes a key distribution mechanism for
providing the session key (and parameters) from the authorization
server to the client. The interaction between the client and the
authorization server requires Transport Layer Security (TLS) with a
ciphersuite offering confidentiality protection. The session key
MUST NOT be transmitted in clear since this would completely destroy
the security benefits of the proposed scheme. Furthermore, the
obtained session key MUST be stored so that only the client instance
has access to it. Storing the session key, for example, in a cookie
allows other parties to gain access to this confidential information
and compromises the security of the protocol.
8.2. Offering Confidentiality Protection for Access to Protected
Resources
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This specification can be used with and without Transport Layer
Security (TLS).
Without TLS this protocol provides a mechanism for verifying the
integrity of requests and responses, it provides no confidentiality
protection. Consequently, eavesdroppers will have full access to
request content and further messages exchanged between the client and
the resource server. This could be problematic when data is
exchanged that requires care, such as personal data.
When TLS is used then confidentiality can be ensured and with the use
of the TLS channel binding feature it ensures that the TLS channel is
cryptographically bound to the used MAC token. TLS in combination
with channel bindings bound to the MAC token provide security
superior to the OAuth Bearer Token.
The use of TLS in combination with the MAC token is highly
recommended to ensure the confidentiality of the user's data.
8.3. Authentication of Resource Servers
This protocol allows clients to verify the authenticity of resource
servers in two ways:
1. The resource server demonstrates possession of the session key by
computing a keyed message digest function over a number of HTTP
fields in the response to the request from the client.
2. When TLS is used the resource server is authenticated as part of
the TLS handshake.
8.4. Plaintext Storage of Credentials
The MAC key works in the same way passwords do in traditional
authentication systems. In order to compute the keyed message
digest, the client and the resource server must have access to the
MAC key in plaintext form.
If an attacker were to gain access to these MAC keys - or worse, to
the resource server's or the authorization server's database of all
such MAC keys - he or she would be able to perform any action on
behalf of any client.
It is therefore paramount to the security of the protocol that these
session keys are protected from unauthorized access.
8.5. Entropy of Session Keys
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Unless TLS is used between the client and the resource server,
eavesdroppers will have full access to requests sent by the client.
They will thus be able to mount off-line brute-force attacks to
recover the session key used to compute the keyed message digest.
Authorization servers should be careful to generate fresh and unique
session keys with sufficient entropy to resist such attacks for at
least the length of time that the session keys are valid.
For example, if a session key is valid for one day, authorization
servers must ensure that it is not possible to mount a brute force
attack that recovers the session key in less than one day. Of
course, servers are urged to err on the side of caution, and use the
longest session key reasonable.
It is equally important that the pseudo-random number generator
(PRNG) used to generate these session keys be of sufficiently high
quality. Many PRNG implementations generate number sequences that
may appear to be random, but which nevertheless exhibit patterns,
which make cryptanalysis easier. Implementers are advised to follow
the guidance on random number generation in [RFC4086].
8.6. Denial of Service / Resource Exhaustion Attacks
This specification includes a number of features which may make
resource exhaustion attacks against resource servers possible. For
example, a resource server may need to need to consult back-end
databases and the authorization server to verify an incoming request
including an access token before granting access to the protected
resource.
An attacker may exploit this to perform a denial of service attack by
sending a large number of invalid requests to the server. The
computational overhead of verifying the keyed message digest alone
is, however, not sufficient to mount a denial of service attack since
keyed message digest functions belong to the computationally fastest
cryptographic algorithms. The usage of TLS does, however, require
additional computational capabity to perform the asymmetric
cryptographic operations. For a brief discussion about denial of
service vulnerabilities of TLS please consult Appendix F.5 of RFC
5246 [RFC5246].
8.7. Timing Attacks
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
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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.
Implementers are encouraged to use fixed-time string comparators for
MAC verification. This means that the comparison operation is not
terminated once a mismatch is found.
8.8. CSRF Attacks
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.
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.
To keep this specification simple, HTTP headers are not part of the
string to be MACed. 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.
8.9. Protecting HTTP Header Fields
This specification provides flexibility for selectively protecting
header fields and even the body of the message. At a minimum the
following fields are included in the keyed message digest.
9. IANA Considerations
9.1. JSON Web Token Claims
This document adds the following claims to the JSON Web Token Claims
registry established with [I-D.ietf-oauth-json-web-token]:
o Claim Name: "kid"
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o Change Controller: IETF
o Specification Document(s): [[ this document ]]
o Claim Name: "mac_key"
o Change Controller: IETF
o Specification Document(s): [[ this document ]]
9.2. MAC Token Algorithm Registry
This specification establishes the MAC Token Algorithm registry.
Additional keyed message digest 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 [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.
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. ]]
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.
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).
IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.
9.2.1. Registration Template
Algorithm name:
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The name requested (e.g., "example").
Change controller:
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.
Specification document(s):
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.
9.2.2. Initial Registry Contents
The HTTP MAC authentication scheme algorithm registry's initial
contents are:
o Algorithm name: hmac-sha-1
o Change controller: IETF
o Specification document(s): [[ this document ]]
o Algorithm name: hmac-sha-256
o Change controller: IETF
o Specification document(s): [[ this document ]]
9.3. OAuth Access Token Type Registration
This specification registers the following access token type in the
OAuth Access Token Type Registry.
9.3.1. The "mac" OAuth Access Token Type
Type name:
mac
Additional Token Endpoint Response Parameters:
secret, algorithm
HTTP Authentication Scheme(s):
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MAC
Change controller:
IETF
Specification document(s):
[[ this document ]]
9.4. OAuth Parameters Registration
This specification registers the following parameters in the OAuth
Parameters Registry established by [RFC6749].
9.4.1. The "mac_key" OAuth Parameter
Parameter name: mac_key
Parameter usage location: authorization response, token response
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
9.4.2. The "mac_algorithm" OAuth Parameter
Parameter name: mac_algorithm
Parameter usage location: authorization response, token response
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
9.4.3. The "kid" OAuth Parameter
Parameter name: kid
Parameter usage location: authorization response, token response
Change controller: IETF
Specification document(s): [[ this document ]]
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Related information: None
10. Acknowledgments
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.
As part of this initial work the following persons provided feedback:
Ben Adida, Adam Barth, Rasmus Lerdorf, James Manger, William Mills,
Scott Renfro, Justin Richer, Toby White, Peter Wolanin, and Skylar
Woodward
Further work in this document was done as part of OAuth working group
conference calls late 2012/early 2013 and in design team conference
calls February 2013. The following persons (in addition to the OAuth
WG chairs, Hannes Tschofenig, and Derek Atkins) provided their input
during these calls: Bill Mills, Justin Richer, Phil Hunt, Prateek
Mishra, Mike Jones, George Fletcher, Leif Johansson, Lucy Lynch, John
Bradley, Tony Nadalin, Klaas Wierenga, Thomas Hardjono, Brian
Campbell
In the appendix of this document we re-use content from [RFC4962] and
the authors would like thank Russ Housely and Bernard Aboba for their
work on RFC 4962.
11. References
11.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", draft-ietf-
httpbis-p1-messaging-25 (work in progress), November 2013.
[I-D.ietf-jose-json-web-encryption]
Jones, M., Rescorla, E., and J. Hildebrand, "JSON Web
Encryption (JWE)", draft-ietf-jose-json-web-encryption-19
(work in progress), December 2013.
[I-D.ietf-oauth-json-web-token]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token-14 (work in
progress), December 2013.
[I-D.richer-oauth-introspection]
Richer, J., "OAuth Token Introspection", draft-richer-
oauth-introspection-04 (work in progress), May 2013.
[I-D.tschofenig-oauth-audience]
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Tschofenig, H., "OAuth 2.0: Audience Information", draft-
tschofenig-oauth-audience-00 (work in progress), February
2013.
[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-
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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[W3C.REC-html401-19991224]
Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Recommendation
REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
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11.2. Informative References
[I-D.hardjono-oauth-kerberos]
Hardjono, T., "OAuth 2.0 support for the Kerberos V5
Authentication Protocol", draft-hardjono-oauth-kerberos-01
(work in progress), December 2010.
[I-D.tschofenig-oauth-hotk]
Bradley, J., Hunt, P., Nadalin, A., and H. Tschofenig,
"The OAuth 2.0 Authorization Framework: Holder-of-the-Key
Token Usage", draft-tschofenig-oauth-hotk-02 (work in
progress), February 2013.
[NIST-FIPS-180-3]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS). FIPS PUB 180-3, October 2008",
October 2008.
[NIST800-63]
Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
and E. Nabbus, "NIST Special Publication 800-63-1,
INFORMATION SECURITY", December 2008.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management", BCP
132, RFC 4962, July 2007.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
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[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
Appendix A. Background Information
With the desire to define a security mechanism in addition to bearer
tokens a design team was formed to collect threats, explore different
threat mitigation techniques, describe use cases, and to derive
requirements for the MAC token based security mechanism defined in
the body of this document. This appendix provides information about
this thought process that should help to motivate design decision.
A.1. Security and Privacy Threats
The following list presents several common threats against protocols
utilizing some form of tokens. This list of threats is based on NIST
Special Publication 800-63 [NIST800-63]. We exclude a discussion of
threats related to any form of identity proofing and authentication
of the Resource Owner to the Authorization Server since these
procedures are not part of the OAuth 2.0 protocol specification
itself.
Token manufacture/modification:
An attacker may generate a bogus tokens or modify the token
content (such as authentication or attribute statements) of an
existing token, causing Resource Server to grant inappropriate
access to the Client. For example, an attacker may modify the
token to extend the validity period. A Client may modify the
token to have access to information that they should not be able
to view.
Token disclosure: Tokens may contain personal data, such as real
name, age or birthday, payment information, etc.
Token redirect:
An attacker uses the token generated for consumption by the
Resource Server to obtain access to another Resource Server.
Token reuse:
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An attacker attempts to use a token that has already been used
once with a Resource Server. The attacker may be an eavesdropper
who observes the communication exchange or, worse, one of the
communication end points. A Client may, for example, leak access
tokens because it cannot keep secrets confidential. A Client may
also re-use access tokens for some other Resource Servers.
Finally, a Resource Server may use a token it had obtained from a
Client and use it with another Resource Server that the Client
interacts with. A Resource Server, offering relatively
unimportant application services, may attempt to use an access
token obtained from a Client to access a high-value service, such
as a payment service, on behalf of the Client using the same
access token.
We excluded one threat from the list, namely 'token repudiation'.
Token repudiation refers to a property whereby a Resource Server is
given an assurance that the Authorization Server cannot deny to have
created a token for the Client. We believe that such a property is
interesting but most deployments prefer to deal with the violation of
this security property through business actions rather than by using
cryptography.
A.2. Threat Mitigation
A large range of threats can be mitigated by protecting the content
of the token, using a digital signature or a keyed message digest.
Alternatively, the content of the token could be passed by reference
rather than by value (requiring a separate message exchange to
resolve the reference to the token content). To simplify the
subsequent description we assume that the token itself is digitally
signed by the Authorization Server and therefore cannot be modified.
To deal with token redirect it is important for the Authorization
Server to include the identifier of the intended recipient - the
Resource Server. A Resource Server must not be allowed to accept
access tokens that are not meant for its consumption.
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To provide protection against token disclosure two approaches are
possible, namely (a) not to include sensitive information inside the
token or (b) to ensure confidentiality protection. The latter
approach requires at least the communication interaction between the
Client and the Authorization Server as well as the interaction
between the Client and the Resource Server to experience
confidentiality protection. As an example, Transport Layer Security
with a ciphersuite that offers confidentiality protection has to be
applied. Encrypting the token content itself is another alternative.
In our scenario the Authorization Server would, for example, encrypt
the token content with a symmetric key shared with the Resource
Server.
To deal with token reuse more choices are available.
A.2.1. Confidentiality Protection
In this approach confidentiality protection of the exchange is
provided on the communication interfaces between the Client and the
Resource Server, and between the Client and the Authorization Server.
No eavesdropper on the wire is able to observe the token exchange.
Consequently, a replay by a third party is not possible. An
Authorization Server wants to ensure that it only hands out tokens to
Clients it has authenticated first and who are authorized. For this
purpose, authentication of the Client to the Authorization Server
will be a requirement to ensure adequate protection against a range
of attacks. This is, however, true for the description in
Appendix A.2.2 and Appendix A.2.3 as well. Furthermore, the Client
has to make sure it does not distribute the access token to entities
other than the intended the Resource Server. For that purpose the
Client will have to authenticate the Resource Server before
transmitting the access token.
A.2.2. Sender Constraint
Instead of providing confidentiality protection the Authorization
Server could also put the identifier of the Client into the protected
token with the following semantic: 'This token is only valid when
presented by a Client with the following identifier.' When the
access token is then presented to the Resource Server how does it
know that it was provided by the Client? It has to authenticate the
Client! There are many choices for authenticating the Client to the
Resource Server, for example by using client certificates in TLS
[RFC5246], or pre-shared secrets within TLS [RFC4279]. The choice of
the preferred authentication mechanism and credential type may depend
on a number of factors, including
o security properties
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o available infrastructure
o library support
o credential cost (financial)
o performance
o integration into the existing IT infrastructure
o operational overhead for configuration and distribution of
credentials
This long list hints to the challenge of selecting at least one
mandatory-to-implement Client authentication mechanism.
A.2.3. Key Confirmation
A variation of the mechanism of sender authentication described in
Appendix A.2.2 is to replace authentication with the proof-of-
possession of a specific (session) key, i.e., key confirmation. In
this model the Resource Server would not authenticate the Client
itself but would rather verify whether the Client knows the session
key associated with a specific access token. Examples of this
approach can be found with the OAuth 1.0 MAC token [RFC5849],
Kerberos [RFC4120] when utilizing the AP_REQ/AP_REP exchange (see
also [I-D.hardjono-oauth-kerberos] for a comparison between Kerberos
and OAuth), the Holder-of-the-Key approach
[I-D.tschofenig-oauth-hotk], and also the MAC token approach defined
in this document.
To illustrate key confirmation the first examples borrow from
Kerberos and use symmetric key cryptography. Assume that the
Authorization Server shares a long-term secret with the Resource
Server, called K(Authorization Server-Resource Server). This secret
would be established between them in an initial registration phase.
When the Client requests an access token the Authorization Server
creates a fresh and unique session key Ks and places it into the
token encrypted with the long term key K(Authorization Server-
Resource Server). Additionally, the Authorization Server attaches Ks
to the response message to the Client (in addition to the access
token itself) over a confidentiality protected channel. When the
Client sends a request to the Resource Server it has to use Ks to
compute a keyed message digest for the request (in whatever form or
whatever layer). The Resource Server, when receiving the message,
retrieves the access token, verifies it and extracts K(Authorization
Server-Resource Server) to obtain Ks. This key Ks is then used to
verify the keyed message digest of the request message.
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Note that in this example one could imagine that the mechanism to
protect the token itself is based on a symmetric key based mechanism
to avoid any form of public key infrastructure but this aspect is not
further elaborated in the scenario.
A similar mechanism can also be designed using asymmetric
cryptography. When the Client requests an access token the
Authorization Server creates an ephemeral public / privacy key pair
(PK/SK) and places the public key PK into the protected token. When
the Authorization Server returns the access token to the Client it
also provides the PK/SK key pair over a confidentiality protected
channel. When the Client sends a request to the Resource Server it
has to use the privacy key SK to sign the request. The Resource
Server, when receiving the message, retrieves the access token,
verifies it and extracts the public key PK. It uses this ephemeral
public key to verify the attached signature.
A.2.4. Summary
As a high level message, there are various ways how the threats can
be mitigated and while the details of each solution is somewhat
different they all ultimately accomplish the goal.
The three approaches are:
Confidentiality Protection:
The weak point with this approach, which is briefly described in
Appendix A.2.1, is that the Client has to be careful to whom it
discloses the access token. What can be done with the token
entirely depends on what rights the token entitles the presenter
and what constraints it contains. A token could encode the
identifier of the Client but there are scenarios where the Client
is not authenticated to the Resource Server or where the
identifier of the Client rather represents an application class
rather than a single application instance. As such, it is
possible that certain deployments choose a rather liberal approach
to security and that everyone who is in possession of the access
token is granted access to the data.
Sender Constraint:
The weak point with this approach, which is briefly described in
Appendix A.2.2, is to setup the authentication infrastructure such
that Clients can be authenticated towards Resource Servers.
Additionally, Authorization Server must encode the identifier of
the Client in the token for later verification by the Resource
Server. Depending on the chosen layer for providing Client-side
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authentication there may be additional challenges due Web server
load balancing, lack of API access to identity information, etc.
Key Confirmation:
The weak point with this approach, see Appendix A.2.3, is the
increased complexity: a complete key distribution protocol has to
be defined.
In all cases above it has to be ensured that the Client is able to
keep the credentials secret.
A.3. Requirements
In an attempt to address the threats described in Appendix A.1 the
Bearer Token, which corresponds to the description in Appendix A.2.1,
was standardized and the work on a JSON-based token format has been
started [I-D.ietf-oauth-json-web-token]. The required capability to
protected the content of a JSON token using integrity and
confidentiality mechanisms is work in progress at the time of
writing.
Consequently, the purpose of the remaining document is to provide
security that goes beyond the Bearer Token offered security
protection.
RFC 4962 [RFC4962] gives useful guidelines for designers of
authentication and key management protocols. While RFC 4962 was
written with the AAA framework used for network access authentication
in mind the offered suggestions are useful for the design of other
key management systems as well. The following requirements list
applies OAuth 2.0 terminology to the requirements outlined in RFC
4962.
These requirements include
Cryptographic Algorithm Independent:
The key management protocol MUST be cryptographic algorithm
independent.
Strong, fresh session keys:
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Session keys MUST be strong and fresh. Each session deserves an
independent session key, i.e., one that is generated specifically
for the intended use. In context of OAuth this means that keying
material is created in such a way that can only be used by the
combination of a Client instance, protected resource, and
authorization scope.
Limit Key Scope:
Following the principle of least privilege, parties MUST NOT have
access to keying material that is not needed to perform their
role. Any protocol that is used to establish session keys MUST
specify the scope for session keys, clearly identifying the
parties to whom the session key is available.
Replay Detection Mechanism:
The key management protocol exchanges MUST be replay protected.
Replay protection allows a protocol message recipient to discard
any message that was recorded during a previous legitimate
dialogue and presented as though it belonged to the current
dialogue.
Authenticate All Parties:
Each party in the key management protocol MUST be authenticated to
the other parties with whom they communicate. Authentication
mechanisms MUST maintain the confidentiality of any secret values
used in the authentication process. Secrets MUST NOT be sent to
another party without confidentiality protection.
Authorization:
Client and Resource Server authorization MUST be performed. These
entities MUST demonstrate possession of the appropriate keying
material, without disclosing it. Authorization is REQUIRED
whenever a Client interacts with an Authorization Server. The
authorization checking prevents an elevation of privilege attack,
and it ensures that an unauthorized authorized is detected.
Keying Material Confidentiality and Integrity:
While preserving algorithm independence, confidentiality and
integrity of all keying material MUST be maintained.
Confirm Cryptographic Algorithm Selection:
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The selection of the "best" cryptographic algorithms SHOULD be
securely confirmed. The mechanism SHOULD detect attempted roll-
back attacks.
Uniquely Named Keys:
Key management proposals require a robust key naming scheme,
particularly where key caching is supported. The key name
provides a way to refer to a key in a protocol so that it is clear
to all parties which key is being referenced. Objects that cannot
be named cannot be managed. All keys MUST be uniquely named, and
the key name MUST NOT directly or indirectly disclose the keying
material.
Prevent the Domino Effect:
Compromise of a single Client MUST NOT compromise keying material
held by any other Client within the system, including session keys
and long-term keys. Likewise, compromise of a single Resource
Server MUST NOT compromise keying material held by any other
Resource Server within the system. In the context of a key
hierarchy, this means that the compromise of one node in the key
hierarchy must not disclose the information necessary to
compromise other branches in the key hierarchy. Obviously, the
compromise of the root of the key hierarchy will compromise all of
the keys; however, a compromise in one branch MUST NOT result in
the compromise of other branches. There are many implications of
this requirement; however, two implications deserve highlighting.
First, the scope of the keying material must be defined and
understood by all parties that communicate with a party that holds
that keying material. Second, a party that holds keying material
in a key hierarchy must not share that keying material with
parties that are associated with other branches in the key
hierarchy.
Bind Key to its Context:
Keying material MUST be bound to the appropriate context. The
context includes the following.
* The manner in which the keying material is expected to be used.
* The other parties that are expected to have access to the
keying material.
* The expected lifetime of the keying material. Lifetime of a
child key SHOULD NOT be greater than the lifetime of its parent
in the key hierarchy.
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Any party with legitimate access to keying material can determine
its context. In addition, the protocol MUST ensure that all
parties with legitimate access to keying material have the same
context for the keying material. This requires that the parties
are properly identified and authenticated, so that all of the
parties that have access to the keying material can be determined.
The context will include the Client and the Resource Server
identities in more than one form.
Authorization Restriction:
If Client authorization is restricted, then the Client SHOULD be
made aware of the restriction.
Client Identity Confidentiality:
A Client has identity confidentiality when any party other than
the Resource Server and the Authorization Server cannot
sufficiently identify the Client within the anonymity set. In
comparison to anonymity and pseudonymity, identity confidentiality
is concerned with eavesdroppers and intermediaries. A key
management protocol SHOULD provide this property.
Resource Owner Identity Confidentiality:
Resource servers SHOULD be prevented from knowing the real or
pseudonymous identity of the Resource Owner, since the
Authorization Server is the only entity involved in verifying the
Resource Owner's identity.
Collusion:
Resource Servers that collude can be prevented from using
information related to the Resource Owner to track the individual.
That is, two different Resource Servers can be prevented from
determining that the same Resource Owner has authenticated to both
of them. This requires that each Authorization Server obtains
different keying material as well as different access tokens with
content that does not allow identification of the Resource Owner.
AS-to-RS Relationship Anonymity:
This MAC Token security does not provide AAS-to-RS Relationship
Anonymity since the Client has to inform the resource server about
the Resource Server it wants to talk to. The Authorization Server
needs to know how to encrypt the session key the Client and the
Resource Server will be using.
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As an additional requirement a solution MUST enable support for
channel bindings. The concept of channel binding, as defined in
[RFC5056], allows applications to establish that the two end-points
of a secure channel at one network layer are the same as at a higher
layer by binding authentication at the higher layer to the channel at
the lower layer.
Furthermore, there are performance concerns specifically with the
usage of asymmetric cryptography. As such, the requirement can be
phrases as 'faster is better'. [QUESTION: How are we trading the
benefits of asymmetric cryptography against the performance impact?]
Finally, there are threats that relate to the experience of the
software developer as well as operational policies. Verifying the
servers identity in TLS is discussed at length in [RFC6125].
A.4. Use Cases
This section lists use cases that provide additional requirements and
constrain the solution space.
A.4.1. Access to an 'Unprotected' Resource
This use case is for a web client that needs to access a resource
where no integrity and confidentiality protection is provided for the
exchange of data using TLS following the OAuth-based request. In
accessing the resource, the request, which includes the access token,
must be protected against replay, and modification.
While it is possible to utilize bearer tokens in this scenario, as
described in [RFC6750], with TLS protection when the request to the
protected resource is made there may be the desire to avoid using TLS
between the client and the resource server at all. In such a case
the bearer token approach is not possible since it relies on TLS for
ensuring integrity and confidentiality protection of the access token
exchange since otherwise replay attacks are possible: First, an
eavesdropper may steal an access token and represent it at a
different resource server. Second, an eavesdropper may steal an
access token and replay it against the same resource server at a
later point in time. In both cases, if the attack is successful, the
adversary gets access to the resource owners data or may perform an
operation selected by the adversary (e.g., sending a message). Note
that the adversary may obtain the access token (if the
recommendations in [RFC6749] and [RFC6750] are not followed) using a
number of ways, including eavesdropping the communication on the
wireless link.
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Consequently, the important assumption in this use case is that a
resource server does not have TLS support and the security solution
should work in such a scenario. Furthermore, it may not be necessary
to provide authentication of the resource server towards the client.
A.4.2. Offering Application Layer End-to-End Security
In Web deployments resource servers are often placed behind load
balancers. Note that the load balancers are deployed by the same
organization that operates the resource servers. These load
balancers may terminate Transport Layer Security (TLS) and the
resulting HTTP traffic may be transmitted in clear from the load
balancer to the resource server. With application layer security
independent of the underlying TLS security it is possible to allow
application servers to perform cryptographic verification on an end-
to-end basis.
The key aspect in this use case is therefore to offer end-to-end
security in the presence of load balancers via application layer
security.
A.4.3. Preventing Access Token Re-Use by the Resource Server
Imagine a scenario where a resource server that receives a valid
access token re-uses it with other resource server. The reason for
re-use may be malicious or may well be legitimate. In a legitimate
use case consider a case where the resource server needs to consult
third party resource servers to complete the requested operation. In
both cases it may be assumed that the scope of the access token is
sufficiently large that it allows such a re-use. For example,
imagine a case where a company operates email services as well as
picture sharing services and that company had decided to issue access
tokens with a scope that allows access to both services.
With this use case the desire is to prevent such access token re-use.
This also implies that the legitimate use cases require additional
enhancements for request chaining.
A.4.4. TLS Channel Binding Support
In this use case we consider the scenario where an OAuth 2.0 request
to a protected resource is secured using TLS but the client and the
resource server demand that the underlying TLS exchange is bound to
additional application layer security to prevent cases where the TLS
connection is terminated at a load balancer or a TLS proxy is used
that splits the TLS connection into two separate connections.
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In this use case additional information is conveyed to the resource
server to ensure that no entity entity has tampered with the TLS
connection.
Authors' Addresses
Justin Richer
The MITRE Corporation
Email: jricher@mitre.org
William Mills
Yahoo! Inc.
Email: wmills@yahoo-inc.com
Hannes Tschofenig (editor)
Austria
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Phil Hunt
Oracle Corporation
Email: phil.hunt@yahoo.com
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