[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]
Versions: 00 01 02 03 04 05
Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Standards Track January 9, 2011
Expires: July 13, 2011
OAuth 2.0 MAC Token and Authentication
draft-hammer-oauth-v2-mac-token-00
Abstract
This document specifies the OAuth 2.0 MAC token type and
authentication scheme.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 13, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hammer-Lahav Expires July 13, 2011 [Page 1]
Internet-Draft OAuth 2.0 MAC Token January 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 5
2. Issuing MAC-Type Access Tokens . . . . . . . . . . . . . . . . 5
3. Making Requests . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Authorization Request Header . . . . . . . . . . . . . 6
3.2. Signature . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Normalized Request String . . . . . . . . . . . . . . 7
3.2.2. hmac-sha-1 . . . . . . . . . . . . . . . . . . . . . . 11
3.2.3. hmac-sha-256 . . . . . . . . . . . . . . . . . . . . . 12
4. Verifying Requests . . . . . . . . . . . . . . . . . . . . . . 12
5. Scheme Extensions . . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. Secrets Transmission . . . . . . . . . . . . . . . . . . . 13
6.2. Confidentiality of Requests . . . . . . . . . . . . . . . 13
6.3. Spoofing by Counterfeit Servers . . . . . . . . . . . . . 13
6.4. Plaintext Storage of Credentials . . . . . . . . . . . . . 13
6.5. Entropy of Secrets . . . . . . . . . . . . . . . . . . . . 14
6.6. Denial of Service / Resource Exhaustion Attacks . . . . . 14
6.7. Coverage Limitations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7.1. The "secret" OAuth Parameter . . . . . . . . . . . . . . . 15
7.2. The "secret" OAuth Parameter . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Document History . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Hammer-Lahav Expires July 13, 2011 [Page 2]
Internet-Draft OAuth 2.0 MAC Token January 2011
1. Introduction
OAuth 2.0 ([I-D.ietf-oauth-v2]) defines a token-based authentication
framework in which third-party applications (clients) access
protected resources using access tokens. Access tokens are obtained
via the resource owners' authorization from an authorization server.
This specification defines the MAC token type for use with the OAuth
2.0 framework. It defines type-specific token attributes and
provides a method for making authenticated HTTP requests with partial
cryptographic verification of the request - covering the HTTP method,
request URI, host, and in some cases the request body.
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 token types are supported by a resource server or
how the client may go about obtaining access tokens. This
specification assumes that the authorization server has issued the
client a MAC-type token and describes how the client authenticates
using that access token.
The MAC token type is not compatible with the "HMAC-SHA1" signature
method defined in OAuth 1.0 [RFC5849].
This specification is an extension of [I-D.ietf-oauth-v2] and uses
its terminology.
Please discuss this draft on the oauth@ietf.org [1] mailing list.
1.1. Example
The client attempts to access a protected resource without
authentication, making the following HTTP request to the resource
server:
GET /resource/1?b=1&a=2 HTTP/1.1
Host: example.com
The resource server returns the following authentication challenge:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: OAuth2
Date: Thu, 02 Dec 2010 21:39:45 GMT
Hammer-Lahav Expires July 13, 2011 [Page 3]
Internet-Draft OAuth 2.0 MAC Token January 2011
The client has previously obtained a set of token credentials for
accessing resources on the "http://example.com/" resource server.
The credentials issued to the client by the authorization server
included the following attributes:
Access token: h480djs93hd8
Token type: mac
MAC algorithm: hmac-sha-1
Token secret: 489dks293j39
The client attempts the HTTP request again, this time using the token
credentials issued by the authorization server earlier to
authenticate. To construct the authentication header, the client
calculates the current timestamp and a nonce. The nonce is unique to
the timestamp used, typically a random string:
Timestamp: 137131200
Nonce: dj83hs9s
The client normalizes the request and constructs the signature base
string (the new line separator character is represented by "\n" for
display purposes only):
h480djs93hd8\n
137131200\n
dj83hs9s\n
GET\n
example.com\n
80\n
/resource/1\n
a=2\n
b=1
The signature base string is signed using the specified MAC token
algorithm "hmac-sha-1" with the signature base string as text and the
token secret as key. The resulting digest is base64-encoded to
produce the request signature:
IdSrHQHTwCPWGrqzGGIR791ZJXE=
Hammer-Lahav Expires July 13, 2011 [Page 4]
Internet-Draft OAuth 2.0 MAC Token January 2011
The client includes the access token, timestamp, nonce, and signature
with the request using the "Authorization" request header field:
GET /resource/1 HTTP/1.1
Host: example.com
Authorization: MAC token='h480djs93hd8',
timestamp='137131200',
nonce='dj83hs9s',
signature='IdSrHQHTwCPWGrqzGGIR791ZJXE='
The resource server validates the request by calculating the
signature again based on the request received and verifies the
validity and scope of the access token. If valid, the resource
server responds with the requested protected resource representation.
1.2. Notational Conventions
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
document are to be interpreted as described in [RFC2119].
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[I-D.ietf-httpbis-p1-messaging].
2. Issuing MAC-Type Access Tokens
Authorization servers issuing MAC-type access tokens MUST include the
following parameters whenever a response includes the "access_token"
parameter:
secret
REQUIRED. The token shared secret used as the MAC algorithm
key.
algorithm
REQUIRED. The MAC algorithm used to calculate the request
signature. Value MUST be one of "hmac-sha-1", "hmac-sha-256",
or a registered extension algorithm name as described in
Section 5.
3. Making Requests
To make authenticated requests, the client must be in possession of a
valid MAC-type access token, issued by an authorization server
Hammer-Lahav Expires July 13, 2011 [Page 5]
Internet-Draft OAuth 2.0 MAC Token January 2011
accepted by the resource server. The client constructs the request
by calculating of a set of attributes, and adding them to the HTTP
request using the Authorization header field (Section 3.1).
Authenticated request can be sent in response to an authentication
challenge or directly.
3.1. The Authorization Request Header
The "Authorization" request header field uses the framework defined
by [RFC2617] as follows:
credentials = 'MAC' [ RWS 1#param ]
param = access-token /
timestamp /
nonce /
signature
access-token = 'token' '=' quoted-string
timestamp = 'timestamp' '=' <"> 1*DIGIT <">
nonce = 'nonce' '=' quoted-string
signature = 'signature' '=' quoted-string
The "token" attribute value is set to the access token received from
the authorization server.
The "timestamp" attribute value is set to the current time expressed
in the number of seconds since January 1, 1970 00:00:00 GMT, and MUST
be a positive integer.
The "nonce" attribute value is set to a random string, uniquely
generated by the client to allow the resource server to verify that a
request has never been made before and helps prevent replay attacks
when requests are made over an insecure channel. The nonce value
MUST be unique across all requests with the same timestamp and access
token combination.
To avoid the need to retain an infinite number of nonce values for
future checks, servers MAY choose to restrict the time period after
which a request with an old timestamp is rejected. Such a
restriction implies a level of synchronization between the client's
and server's clocks. The client MAY use the "Date" response header
field to synchronize its clock after a failed request.
The "signature" attribute value is set as described in Section 3.2.
Hammer-Lahav Expires July 13, 2011 [Page 6]
Internet-Draft OAuth 2.0 MAC Token January 2011
Each of the four attributes MUST appear once, and only once.
3.2. Signature
The client uses the MAC token algorithm and the access token secret -
both provided by the authorization server - to calculate the request
signature. This specification defines two algorithms: "hmac-sha-1"
and "hmac-sha-256", and provides an extension registry for additional
algorithms.
3.2.1. Normalized Request String
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 resource server can both sign the same string. The string
is constructed by concatenating together, in order, the following
HTTP request elements:
1. The access token.
2. A new line character (ASCII code 10).
3. The timestamp value calculated for the request.
4. A new line character (ASCII code 10).
5. The nonce value generated for the request.
6. A new line character (ASCII code 10).
7. The HTTP request method in upper case. For example: "HEAD",
"GET", "POST", etc.
8. A new line character (ASCII code 10).
9. The hostname included in the HTTP request using the "Host"
request header field in lower case.
10. A new line character (ASCII code 10).
11. The port as included in the HTTP request using the "Host"
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).
12. A new line character (ASCII code 10).
Hammer-Lahav Expires July 13, 2011 [Page 7]
Internet-Draft OAuth 2.0 MAC Token January 2011
13. The path component of the HTTP request URI as defined by
[RFC3986] section 3.3.
14. A new line character (ASCII code 10).
15. The query component of the HTTP request URI as defined by
[RFC3986] section 3.4, normalized as described in
Section 3.2.1.1.
For example, the HTTP request:
GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1
Host: example.com
using access token "kkk9d7dh3k39sjv7", timestamp "137131201", and
nonce "7d8f3e4a" is normalized into the following string (the new
line Separator character is represented by "\n" for display purposes
only):
kkk9d7dh3k39sjv7\n
137131201\n
7d8f3e4a\n
GET\n
example.com\n
80\n
/request\n
a2=r%20b\n
a3=2%20q\n
a3=a\n
b5=%3D%253D\n
c%40=\n
c2=
3.2.1.1. Parameters Normalization
The query component is parsed into a list of name/value parameter
pairs by treating it as an "application/x-www-form-urlencoded"
string, separating the names and values and decoding them as defined
by [W3C.REC-html401-19991224] section 17.13.4.
Once separated and decoded, the parameters are concatenated back
together as follows:
Hammer-Lahav Expires July 13, 2011 [Page 8]
Internet-Draft OAuth 2.0 MAC Token January 2011
1. First, the name and value of each parameter are escaped using the
[RFC3986] percent-encoding (%XX) mechanism. Characters in the
unreserved character set as defined by [RFC3986] section 2.3
(ALPHA, DIGIT, "-", ".", "_", "~") MUST NOT be encoded. All
other characters MUST be encoded. The two hexadecimal characters
used to represent encoded characters MUST be upper case.
2. The parameters are sorted by name, using ascending byte value
ordering. If two or more parameters share the same name, they
are sorted by their value.
3. The name of each parameter is concatenated to its corresponding
value using an "=" character (ASCII code 61) as separator, even
if the value is empty.
4. The sorted name/value pairs are concatenated together into a
single string by using an new line character (ASCII code 10) as
separator.
Note that the percent-encoding method described is different from the
encoding scheme used by the "application/x-www-form-urlencoded"
content-type (for example, it encodes space characters as "%20"
instead of the "+" character). It MAY be different from the percent-
encoding functions provided by web development frameworks (e.g.
encode different characters, use lower case hexadecimal characters).
For example, the HTTP request URI:
/request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q
Contains the following (fully decoded) parameters used in the
signature base sting:
+------+-------+
| Name | Value |
+------+-------+
| b5 | =%3D |
| a3 | a |
| c@ | |
| a2 | r b |
| c2 | |
| a3 | 2 q |
+------+-------+
Note that the value of "b5" is "=%3D" and not "==". Both "c@" and
"c2" have empty values. While the encoding rules specified in this
Hammer-Lahav Expires July 13, 2011 [Page 9]
Internet-Draft OAuth 2.0 MAC Token January 2011
specification for the purpose of constructing the signature base
string exclude the use of a "+" character (ASCII code 43) to
represent an encoded space character (ASCII code 32), this practice
is widely used in "application/x-www-form-urlencoded" encoded values,
and MUST be properly decoded, as demonstrated by one of the "a3"
parameter instances (the "a3" parameter is used twice in this
request).
The parsed parameters are normalized as follows:
Encoded:
+------+----------+
| Name | Value |
+------+----------+
| b5 | %3D%253D |
| a3 | a |
| c%40 | |
| a2 | r%20b |
| c2 | |
| a3 | 2%20q |
+------+----------+
Sorted:
+------+----------+
| Name | Value |
+------+----------+
| a2 | r%20b |
| a3 | 2%20q |
| a3 | a |
| b5 | %3D%253D |
| c%40 | |
| c2 | |
+------+----------+
Hammer-Lahav Expires July 13, 2011 [Page 10]
Internet-Draft OAuth 2.0 MAC Token January 2011
Concatenated Pairs:
+-------------+
| Name=Value |
+-------------+
| a2=r%20b |
| a3=2%20q |
| a3=a |
| b5=%3D%253D |
| c%40= |
| c2= |
+-------------+
And concatenated together into a single string (the new line
separator character is represented by "\n" for display purposes
only):
a2=r%20b\n
a3=2%20q\n
a3=a\n
b5=%3D%253D\n
c%40=\n
c2=
3.2.2. hmac-sha-1
"hmac-sha-1" uses the HMAC-SHA1 algorithm as defined in [RFC2104]:
digest = HMAC-SHA1 (key, text)
Where:
text
is set to the value of the normalize request string as
described in Section 3.2.1.
key
is set to the access token shared-secret provided by the
authorization server.
digest
is used to set the value of the "signature" attribute, after
the result octet string is base64-encoded per [RFC2045] section
6.8.
Hammer-Lahav Expires July 13, 2011 [Page 11]
Internet-Draft OAuth 2.0 MAC Token January 2011
3.2.3. hmac-sha-256
"hmac-sha-1" uses the HMAC algorithm as defined in [RFC2104] together
with the SHA-256 hash function defined in [NIST FIPS-180-3]:
digest = HMAC-SHA256 (key, text)
Where:
text
is set to the value of the normalize request string as
described in Section 3.2.1.
key
is set to the access token shared-secret provided by the
authorization server.
digest
is used to set the value of the "signature" attribute, after
the result octet string is base64-encoded per [RFC2045] section
6.8.
4. Verifying Requests
A servers receiving an authenticated request validates it by
performing the following REQUIRED steps:
1. Recalculate the request signature as described in Section 3.2 and
compare it to the value received from the client via the
"signature" attribute.
2. Ensure that the combination of nonce, timestamp, and access token
received from the client has not been used before in a previous
request (the server MAY reject requests with stale timestamps;
the determination of staleness is left up to the server to
define).
3. Verify the scope and status of the access token.
If the request fails verification, the server SHOULD respond with an
HTTP 401 (unauthorized) status code, and SHOULD include a token
scheme authentication challenge using the WWW-Authenticate header
field. The server MAY include further details about why the request
was rejected using the error attribute.
Hammer-Lahav Expires July 13, 2011 [Page 12]
Internet-Draft OAuth 2.0 MAC Token January 2011
5. Scheme Extensions
[[ TBD ]]
6. Security Considerations
As stated in [RFC2617], the greatest sources of risks are usually
found not in the core protocol itself but in policies and procedures
surrounding its use. Implementers are strongly encouraged to assess
how this protocol addresses their security requirements.
6.1. Secrets Transmission
This specification does not describe any mechanism for obtaining or
transmitting access token secrets. Methods used to obtain tokens
should ensure that these transmissions are protected using transport-
layer mechanisms such as TLS or SSL.
6.2. Confidentiality of Requests
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.
6.3. Spoofing by Counterfeit Servers
This protocol makes no attempt to verify the authenticity of the
resource 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.
6.4. Plaintext Storage of Credentials
The access token shared-secret functions the same way passwords do in
traditional authentication systems. In order to compute the
signature, the server must have access to the secret in plaintext
form. This is in contrast, for example, to modern operating systems,
which store only a one-way hash of user credentials.
Hammer-Lahav Expires July 13, 2011 [Page 13]
Internet-Draft OAuth 2.0 MAC Token January 2011
If an attacker were to gain access to these secrets - or worse, to
the server's database of all such secrets - he or she would be able
to perform any action on behalf of any resource owner. Accordingly,
it is critical that servers protect these secrets from unauthorized
access.
6.5. Entropy of Secrets
Unless a transport-layer security protocol is used, eavesdroppers
will have full access to authenticated requests and signatures, and
will thus be able to mount offline brute-force attacks to recover the
secret used. Authorization servers should be careful to assign
shared-secrets which are long enough, and random enough, to resist
such attacks for at least the length of time that the shared-secrets
are valid.
For example, if shared-secrets are valid for two weeks, authorization
servers should ensure that it is not possible to mount a brute force
attack that recovers the shared-secret in less than two weeks. Of
course, authorization servers are urged to err on the side of
caution, and use the longest secrets reasonable.
It is equally important that the pseudo-random number generator
(PRNG) used to generate these secrets be of sufficiently high
quality. Many PRNG implementations generate number sequences that
may appear to be random, but which nevertheless exhibit patterns or
other weaknesses which make cryptanalysis or brute force attacks
easier. Implementers should be careful to use cryptographically
secure PRNGs to avoid these problems.
6.6. Denial of Service / Resource Exhaustion Attacks
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 signature on incoming requests. An attacker may
exploit this to perform a denial of service attack by sending a large
number of invalid requests to the server.
Resource Exhaustion attacks are by no means specific to 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
Hammer-Lahav Expires July 13, 2011 [Page 14]
Internet-Draft OAuth 2.0 MAC Token January 2011
guessable) secrets. When implementing this protocol, servers should
consider which of these presents a more serious risk for their
application and design accordingly.
6.7. Coverage Limitations
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.
7. IANA Considerations
7.1. The "secret" OAuth Parameter
Parameter name: secret
Parameter usage location: The end-user authorization endpoint
response and the token endpoint response.
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
7.2. The "secret" OAuth Parameter
Parameter name: secret
Parameter usage location: The end-user authorization endpoint
response and the token endpoint response.
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
8. Acknowledgments
The author would like to thank [[ some people ]] for their
suggestions, feedback, and continued support.
Hammer-Lahav Expires July 13, 2011 [Page 15]
Internet-Draft OAuth 2.0 MAC Token January 2011
Appendix A. Document History
[[ To be removed by the RFC editor before publication as an RFC. ]]
-00
o Initial draft.
9. References
9.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-08 (work in
progress), October 2009.
[I-D.ietf-oauth-v2]
Hammer-Lahav, E., Recordon, D., and D. Hardt, "The OAuth
2.0 Protocol Framework", draft-ietf-oauth-v2-11 (work in
progress), November 2010.
[NIST FIPS-180-3]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS). FIPS PUB 180-3, October 2008".
[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.
[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.
Hammer-Lahav Expires July 13, 2011 [Page 16]
Internet-Draft OAuth 2.0 MAC Token January 2011
[W3C.REC-html401-19991224]
Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
9.2. Informative References
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
URIs
[1] <https://www.ietf.org/mailman/listinfo/oauth>
Author's Address
Eran Hammer-Lahav
Yahoo!
Email: eran@hueniverse.com
URI: http://hueniverse.com
Hammer-Lahav Expires July 13, 2011 [Page 17]
Html markup produced by rfcmarkup 1.129c, available from
https://tools.ietf.org/tools/rfcmarkup/