Internet DRAFT - draft-ietf-oauth-signed-http-request
draft-ietf-oauth-signed-http-request
OAuth Working Group J. Richer, Ed.
Internet-Draft
Intended status: Standards Track J. Bradley
Expires: February 9, 2017 Ping Identity
H. Tschofenig
ARM Limited
August 08, 2016
A Method for Signing HTTP Requests for OAuth
draft-ietf-oauth-signed-http-request-03
Abstract
This document a method for offering data origin authentication and
integrity protection of HTTP requests. To convey the relevant data
items in the request a JSON-based encapsulation is used and the JSON
Web Signature (JWS) technique is re-used. JWS offers integrity
protection using symmetric as well as asymmetric cryptography.
Status of This Memo
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Generating a JSON Object from an HTTP Request . . . . . . . . 3
3.1. Calculating the query parameter list and hash . . . . . . 4
3.2. Calculating the header list and hash . . . . . . . . . . 5
4. Sending the signed object . . . . . . . . . . . . . . . . . . 6
4.1. HTTP Authorization header . . . . . . . . . . . . . . . . 6
4.2. HTTP Form body . . . . . . . . . . . . . . . . . . . . . 6
4.3. HTTP Query parameter . . . . . . . . . . . . . . . . . . 7
5. Validating the request . . . . . . . . . . . . . . . . . . . 7
5.1. Validating the query parameter list and hash . . . . . . 7
5.2. Validating the header list and hash . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. The 'pop' OAuth Access Token Type . . . . . . . . . . . . 9
6.2. JSON Web Signature and Encryption Type Values
Registration . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. Offering Confidentiality Protection for Access to
Protected Resources . . . . . . . . . . . . . . . . 9
7.2. Plaintext Storage of Credentials . . . . . . . . . . . . 10
7.3. Entropy of Keys . . . . . . . . . . . . . . . . . . . . . 10
7.4. Denial of Service . . . . . . . . . . . . . . . . . . . . 10
7.5. Validating the integrity of HTTP message . . . . . . . . 11
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
10. Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In order to prove possession of an access token and its associated
key, an OAuth 2.0 client needs to compute some cryptographic function
and present the results to the protected resource as a signature.
The protected resource then needs to verify the signature and compare
that to the expected keys associated with the access token. This is
in addition to the normal token protections provided by a bearer
token [RFC6750] and transport layer security (TLS).
Furthermore, it is desirable to bind the signature to the HTTP
request. Ideally, this should be done without replicating the
information already present in the HTTP request more than required.
However, many HTTP application frameworks insert extra headers, query
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parameters, and otherwise manipulate the HTTP request on its way from
the web server into the application code itself. It is the goal of
this draft to have a signature protection mechanism that is
sufficiently robust against such deployment constraints while still
providing sufficient security benefits.
The key required for this signature calculation is distributed via
mechanisms described in companion documents (see
[I-D.ietf-oauth-pop-key-distribution] and
[I-D.ietf-oauth-pop-architecture]). The JSON Web Signature (JWS)
specification [RFC7515] is used for computing a digital signature
(which uses asymmetric cryptography) or a keyed message digest (in
case of symmetric cryptography).
The mechanism described in this document assumes that a client is in
possession of an access token and asociated key. That client then
creates a JSON object including the access token, signs the JSON
object using JWS, and issues an request to a resource server for
access to a protected resource using the signed object as its
authorization. The protected resource validates the JWS signature
and parses the JSON object to obtain token information.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Other terms such as "client", "authorization server", "access token",
and "protected resource" are inherited from OAuth 2.0 [RFC6749].
We use the term 'sign' (or 'signature') to denote both a keyed
message digest and a digital signature operation.
3. Generating a JSON Object from an HTTP Request
This specification uses JSON Web Signatures [RFC7515] to protect the
access token and, optionally, parts of the request.
This section describes how to generate a JSON [RFC7159] object from
the HTTP request. Each value below is included as a member of the
JSON object at the top level.
at REQUIRED. The access token value. This string is assumed to have
no particular format or structure and remains opaque to the
client.
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ts RECOMMENDED. The timestamp. This integer provides replay
protection of the signed JSON object. Its value MUST be a number
containing an integer value representing number of whole integer
seconds from midnight, January 1, 1970 GMT.
m OPTIONAL. The HTTP Method used to make this request. This MUST
be the uppercase HTTP verb as a JSON string.
u OPTIONAL. The HTTP URL host component as a JSON string. This MAY
include the port separated from the host by a colon in host:port
format.
p OPTIONAL. The HTTP URL path component of the request as an HTTP
string.
q OPTIONAL. The hashed HTTP URL query parameter map of the request
as a two-part JSON array. The first part of this array is a JSON
array listing all query parameters that were used in the
calculation of the hash in the order that they were added to the
hashed value as described below. The second part of this array is
a JSON string containing the Base64URL encoded hash itself,
calculated as described below.
h OPTIONAL. The hashed HTTP request headers as a two-part JSON
array. The first part of this array is a JSON array listing all
headers that were used in the calculation of the hash in the order
that they were added to the hashed value as described below. The
second part of this array is a JSON string containing the
Base64URL encoded hash itself, calculated as described below.
b OPTIONAL. The base64URL encoded hash of the HTTP Request body,
calculated as the SHA256 of the byte array of the body
All hashes SHALL be calculated using the SHA256 algorithm. [[ Note to
WG: do we want crypto agility here? If so how do we signal this ]]
The JSON object is signed using the algorithm appropriate to the
associated access token key, usually communicated as part of key
distribution [I-D.ietf-oauth-pop-key-distribution].
3.1. Calculating the query parameter list and hash
To generate the query parameter list and hash, the client creates two
data objects: an ordered list of strings to hold the query parameter
names and a string buffer to hold the data to be hashed.
The client iterates through all query parameters in whatever order it
chooses and for each query parameter it does the following:
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1. Adds the name of the query parameter to the end of the list.
2. Percent-encodes the name and value of the parameter as specified
in [RFC3986]. Note that if the name and value have already been
percent-encoded for transit, they are not re-encoded for this
step.
3. Encodes the name and value of the query parameter as "name=value"
and appends it to the string buffer separated by the ampersand
"&" character.
Repeated parameter names are processed separately with no special
handling. Parameters MAY be skipped by the client if they are not
required (or desired) to be covered by the signature.
The client then calculates the hash over the resulting string buffer.
The list and the hash result are added to a list as the value of the
"q" member.
For example, the query parameter set of "b=bar", "a=foo", "c=duck" is
concatenated into the string:
b=bar&a=foo&c=duck
When added to the JSON structure using this process, the results are:
"q": [["b", "a", "c"], "u4LgkGUWhP9MsKrEjA4dizIllDXluDku6ZqCeyuR-JY"]
3.2. Calculating the header list and hash
To generate the header list and hash, the client creates two data
objects: an ordered list of strings to hold the header names and a
string buffer to hold the data to be hashed.
The client iterates through all query parameters in whatever order it
chooses and for each query parameter it does the following:
1. Lowercases the header name.
2. Adds the name of the header to the end of the list.
3. Encodes the name and value of the header as "name: value" and
appends it to the string buffer separated by a newline "\n"
character.
Repeated header names are processed separately with no special
handling. Headers MAY be skipped by the client if they are not
required (or desired) to be covered by the signature.
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The client then calculates the hash over the resulting string buffer.
The list and the hash result are added to a list as the value of the
"h" member.
For example, the headers "Content-Type: application/json" and "Etag:
742-3u8f34-3r2nvv3" are concatenated into the string:
content-type: application/json
etag: 742-3u8f34-3r2nvv3
"h": [["content-type", "etag"],
"bZA981YJBrPlIzOvplbu3e7ueREXXr38vSkxIBYOaxI"]
4. Sending the signed object
In order to send the signed object to the protected resource, the
client includes it in one of the following three places.
4.1. HTTP Authorization header
The client SHOULD send the signed object to the protected resource in
the Authorization header. The value of the signed object in JWS
compact form is appended to the Authorization header as a PoP value.
This is the preferred method. Note that if this method is used, the
Authorization header MUST NOT be included in the protected elements
of the signed object.
GET /resource/foo
Authorization: PoP eyJ....omitted for brevity...
4.2. HTTP Form body
If the client is sending the request as a form-encoded HTTP message
with parameters in the body, the client MAY send the signed object as
part of that form body. The value of the signed object in JWS
compact form is sent as the form parameter pop_access_token. Note
that if this method is used, the body hash cannot be included in the
protected elements of the signed object.
POST /resource
Content-type: application/www-form-encoded
pop_access_token=eyJ....omitted for brevity...
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4.3. HTTP Query parameter
If neither the Authorization header nor the form-encoded body
parameter are available to the client, the client MAY send the signed
object as a query parameter. The value of the signed object in JWS
compact form is sent as the query parameter pop_access_token. Note
that if this method is used, the pop_access_token parameter MUST NOT
be included in the protected elements of the signed object.
GET /resource?pop_access_token=eyJ....
5. Validating the request
Just like with a bearer token [RFC6750], while the access token value
included in the signed object is opaque to the client, it MUST be
understood by the protected resource in order to fulfill the request.
Also like a bearer token, the protected resource traditionally has
several methods at its disposal for understanding the access token.
It can look up the token locally (such as in a database), it can
parse a structured token (such as JWT [RFC7519]), or it can use a
service to look up token information (such as introspection
[RFC7662]). Whatever method is used to look up token information,
the protected resource MUST have access to the key associated with
the access token, as this key is required to validate the signature
of the incoming request. Validation of the signature is done using
normal JWS validation for the signature and key type.
Additionally, in order to trust any of the hashed components of the
HTTP request, the protected resource MUST re-create and verify a hash
for each component as described below. This process is a mirror of
the process used to create the hashes in the first place, with a mind
toward the fact that order may have changed and that elements may
have been added or deleted. The protected resource MUST similarly
compare the replicated values included in various JSON fields with
the corresponding actual values from the request. Failure to do so
will allow an attacker to modify the underlying request while at the
same time having the application layer verify the signature
correctly.
5.1. Validating the query parameter list and hash
The client has at its disposal a map that indexes the query parameter
names to the values given. The client creates a string buffer for
calculating the hash. The client then iterates through the "list"
portion of the "p" parameter. For each item in the list (in the
order of the list) it does the following:
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1. Fetch the value of the parameter from the HTTP request query
parameter map. If a parameter is found in the list of signed
parameters but not in the map, the validation fails.
2. Percent-encodes the name and value of the parameter as specified
in [RFC3986]. Note that if the name and value have already been
percent-encoded for transit, they are not re-encoded for this
step.
3. Encode the parameter as "name=value" and concatenate it to the
end of the string buffer, separated by an ampersand character.
The client calculates the hash of the string buffer and base64url
encodes it. The protected resource compares that string to the
string passed in as the hash. If the two match, the hash validates,
and all named parameters and their values are considered covered by
the signature.
There MAY be additional query parameters that are not listed in the
list and are therefore not covered by the signature. The client MUST
decide whether or not to accept a request with these uncovered
parameters.
5.2. Validating the header list and hash
The client has at its disposal a map that indexes the header names to
the values given. The client creates a string buffer for calculating
the hash. The client then iterates through the "list" portion of the
"h" parameter. For each item in the list (in the order of the list)
it does the following:
1. Fetch the value of the header from the HTTP request header map.
If a header is found in the list of signed parameters but not in
the map, the validation fails.
2. Encode the parameter as "name: value" and concatenate it to the
end of the string buffer, separated by a newline character.
The client calculates the hash of the string buffer and base64url
encodes it. The protected resource compares that string to the
string passed in as the hash. If the two match, the hash validates,
and all named headers and their values are considered covered by the
signature.
There MAY be additional headers that are not listed in the list and
are therefore not covered by the signature. The client MUST decide
whether or not to accept a request with these uncovered headers.
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6. IANA Considerations
6.1. The 'pop' OAuth Access Token Type
Section 11.1 of [RFC6749] defines the OAuth Access Token Type
Registry and this document adds another token type to this registry.
Type name: pop
Additional Token Endpoint Response Parameters: (none)
HTTP Authentication Scheme(s): Proof-of-possession access token for
use with OAuth 2.0
Change controller: IETF
Specification document(s): [[ this document ]]
6.2. JSON Web Signature and Encryption Type Values Registration
This specification registers the "pop" type value in the IANA JSON
Web Signature and Encryption Type Values registry [RFC7515]:
o "typ" Header Parameter Value: "pop"
o Abbreviation for MIME Type: None
o Change Controller: IETF
o Specification Document(s): [[ this document ]]
7. Security Considerations
7.1. Offering Confidentiality Protection for Access to Protected
Resources
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, it provides no confidentiality protection.
Consequently, eavesdroppers will have full access to communication
content and any 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 of the transmission can be
ensured between endpoints, including both the request and the
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response. The use of TLS in combination with the signed HTTP request
mechanism is highly recommended to ensure the confidentiality of the
data returned from the protected resource.
7.2. Plaintext Storage of Credentials
The mechanism described in this document works in a similar way to
many three-party authentication and key exchange mechanisms. In
order to compute the signature over the HTTP request, the client must
have access to a key bound to the access token in plaintext form. If
an attacker were to gain access to these stored secrets at the client
or (in case of symmetric keys) at the resource server they would be
able to perform any action on behalf of any client just as if they
had stolen a bearer token.
It is therefore paramount to the security of the protocol that the
private keys associated with the access tokens are protected from
unauthorized access.
7.3. Entropy of Keys
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
attempt recovery of the session key or private key used to compute
the keyed message digest or digital signature, respectively.
This specification assumes that the key used herein has been
distributed via other mechanisms, such as
[I-D.ietf-oauth-pop-key-distribution]. Hence, it is the
responsibility of the authorization server and or the client to be
careful when generating fresh and unique keys with sufficient entropy
to resist such attacks for at least the length of time that the
session keys (and the access tokens) are valid.
For example, if the key bound to the access token is valid for one
day, authorization servers must ensure that it is not possible to
mount a brute force attack that recovers that key in less than one
day. Of course, servers are urged to err on the side of caution, and
use the longest key length possible within reason.
7.4. Denial of Service
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 process the incoming request,
verify the access token, perform signature verification, and might
(in certain circumstances) have to consult back-end databases or the
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authorization server before granting access to the protected
resource. Many of these actions are shared with bearer tokens, but
the additional cryptographic overhead of validating the signed
request needs to be taken into consideration with deployment of this
specification.
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
not likely sufficient to mount a denial of service attack. To help
combat this, it is RECOMMENDED that the protected resource validate
the access token (contained in the "at" member of the signed
structure) before performing any cryptographic verification
calculations.
7.5. Validating the integrity of HTTP message
This specification provides flexibility for selectively validating
the integrity of the HTTP request, including header fields, query
parameters, and message bodies. Since all components of the HTTP
request are only optionally validated by this method, and even some
components may be validated only in part (e.g., some headers but not
others) it is up to protected resource developers to verify that any
vital parameters in a request are actually covered by the signature.
Failure to do so could allow an attacker to inject vital parameters
or headers into the request, ouside of the protection of the
signature.
The application verifying this signature MUST NOT assume that any
particular parameter is appropriately covered by the signature unless
it is included in the signed structure and the hash is verified. Any
applications that are sensitive of header or query parameter order
MUST verify the order of the parameters on their own. The
application MUST also compare the values in the JSON container with
the actual parameters received with the HTTP request (using a direct
comparison or a hash calculation, as appropriate). Failure to make
this comparison will render the signature mechanism useless for
protecting these elements.
The behavior of repeated query parameters or repeated HTTP headers is
undefined by this specification. If a header or query parameter is
repeated on either the outgoing request from the client or the
incoming request to the protected resource, that query parameter or
header name MUST NOT be covered by the hash and signature.
This specification records the order in which query parameters and
headers are hashed, but it does not guarantee that order is preserved
between the client and protected resource. If the order of
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parameters or headers are significant to the underlying application,
it MUST confirm their order on its own, apart from the signature and
HTTP message validation.
8. Privacy Considerations
This specification addresses machine to machine communications and
raises no privacy considerations beyond existing OAuth transactions.
9. Acknowledgements
The authors thank the OAuth Working Group for input into this work.
10. Normative References
[I-D.ietf-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-ietf-oauth-pop-architecture-08 (work
in progress), July 2016.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-02 (work in progress), October 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<http://www.rfc-editor.org/info/rfc6750>.
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[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<http://www.rfc-editor.org/info/rfc7662>.
Authors' Addresses
Justin Richer (editor)
Email: ietf@justin.richer.org
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Hannes Tschofenig
ARM Limited
Austria
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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