OAuth Working Group J. Richer, Ed.
Internet-Draft The MITRE Corporation
Intended status: Experimental J. Bradley
Expires: October 26, 2014 Ping Identity
H. Tschofenig
ARM Limited
April 24, 2014
A Method for Signing an HTTP Requests for OAuth
draft-richer-oauth-signed-http-request-01
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 26, 2014.
Copyright Notice
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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. Selection of a hashing algorithm and size . . . . . . . . 4
3.2. Calculating the query parameter list and hash . . . . . . 4
3.3. Calculating the header list and hash . . . . . . . . . . 5
4. Verifying the Hashes . . . . . . . . . . . . . . . . . . . . 5
4.1. Validating the query parameter list and hash . . . . . . 6
4.2. Validating the header list and hash . . . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. The 'pop' OAuth Access Token Type . . . . . . . . . . . . 7
6.2. JSON Web Signature and Encryption Type Values
Registration . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7.1. Offering Confidentiality Protection for Access to
Protected Resources . . . . . . . . . . . . . . . . 8
7.2. Authentication of Resource Servers . . . . . . . . . . . 8
7.3. Plaintext Storage of Credentials . . . . . . . . . . . . 9
7.4. Entropy of Keys . . . . . . . . . . . . . . . . . . . . . 9
7.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 9
7.6. Protecting HTTP Header Fields . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In order to protect an HTTP request with a signature, a method for
conveying various parameters and to compute a signature is needed.
Ideally, this should be done without replicating the information
already present in the HTTP request. This version of the document
still replicates most of the headers though.
The keying material required for this signature calculation is
distributed via mechanisms described in companion documents (see
[I-D.bradley-oauth-pop-key-distribution] and
[I-D.hunt-oauth-pop-architecture]). The JSON Web Signature (JWS)
specification [I-D.ietf-jose-json-web-signature] is re-used for
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computing a digital signature (which uses asymmetric cryptography) or
a keyed message digest (in case of symmetric cryptography).
The scope of the mechanism described in this document is shown in
Figure 1 where a client in possession of keying material that is tied
to the access token creates a JSON object, signs it, and issues an
request to a resource server for access to a protected resource.
+-----------+ +------------+
| |--(1)- HTTP Request ->| Resource |
| Client | (+Signature, +Access Token)->| Server |
| | | |
| |<-(2)- HTTP Response ---------------| |
+-----------+ +------------+
Figure 1: Message Flow.
Many HTTP application frameworks insert extra headers, query
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 method of conveying the token and signed request to the protected
resource server is undefined by this document, but [RFC6750] could be
re-used.
The mechanism described in this document does not provide
authentication of the resource server to the client. This version of
the document does not provide a cryptographic binding to Transport
Layer Security (TLS) used underneath the an HTTPS request.
2. Terminology
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 RFC 2119 [RFC2119].
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 section describes how to generate a JSON object below is
included as a member of the JSON object. All members are OPTIONAL.
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m The HTTP Method used to make this request. This MUST be the
uppercase HTTP verb as a JSON string.
u 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 The HTTP URL path component of the request as an HTTP string.
q 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 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 The base64URL encoded hash of the HTTP Request body, calculated as
the HMAC of the byte array of the body.
ts The "ts" (timestamp) element provides replay protection of the
JSON object. Its value MUST be a number containing an IntDate
value representing number of whole integer seconds from midnight,
January 1, 1970 GMT.
3.1. Selection of a hashing algorithm and size
The hashes SHALL be calculated using the HMAC algorithm using a hash
size equal to the size of the surrounding JWT's alg header field.
That is, if the JWT uses HS256 or RS256, the HMAC here uses a 256-bit
HMAC. If the JWT uses RS512, the HMAC here uses 512-bit HMAC, and so
forth.
3.2. 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. Encodes the name and value of the query parameter as "name=value"
and appends it to the string buffer. [[Separated by an
ampersand? Alternatively we could have this also pulled into an
ordered list and post-process the concatenation, but that might
be too deep into the weeds. ]]
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 HMAC hash over the resulting string
buffer. The list and the hash result are added as the value of the
"p" member.
3.3. 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. Adds the name of the header to the end of the list.
2. Encodes the name and value of the header as "name: value" and
appends it to the string buffer. [[Separated by a newline?
Alternatively we could have this also pulled into an ordered list
and post-process the concatenation, but that might be too deep
into the weeds. ]]
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.
The client then calculates the HMAC hash over the resulting string
buffer. The list and the hash result are added as the value of the
"h" member.
4. Verifying the Hashes
Validation of the overall signature is done using the standard JWS
mechanisms for JSON structures. However, in order to trust any of
the hashed mechanisms above, an application MUST re-create and verify
a hash for each component. Additionally, an application MUST compare
the replicated values included in various JSON fields with the actual
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header fields of the request. Failure to-do so will allow an
attacker to modify the underlying request, connect do different
resources while at the same time having the application layer verify
the signature correctly.
4.1. 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:
1. Fetch the value of the parameter from the HTTP request parameter
map. If a parameter 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. [[same separator issue as above.]]
The client calculates the hash of the string buffer and base64url
encodes it. The client 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.
4.2. 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. [[same separator issue as above.]]
The client calculates the hash of the string buffer and base64url
encodes it. The client compares that string to the string passed in
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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.
5. Example
Example goes in here but will look like something like this
(symmetric key case).
1) HTTP Request (plain)
POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2 HTTP/1.1
Host: example.com
2) JWS protected JSON object
{"typ":"pop",
"alg":"HS256",
"kid":"client12345@example.com"}
.
{"m":"POST",
"u":"example.com",
"p":"request",
"q":[["a3", "b5", "a2"], "m2398f32i2o3roiu2313aa"],
"ts":1300819380
}
.
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
Figure 2: Message Flow.
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
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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
[I-D.ietf-jose-json-web-signature]:
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 can be ensured; this version of
the specification does, however, not provide the TLS channel binding
feature, which ensures that the TLS channel is cryptographically
bound to the application layer protocol authentication defined in
this document.
The use of TLS in combination with the signed HTTP request mechanism
is highly recommended to ensure the confidentiality of the user's
data.
7.2. Authentication of Resource Servers
This protocol allows clients to verify the authenticity of resource
servers only when TLS is used. With TLS the resource server is
authenticated as part of the TLS handshake. The mechanism described
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in this document does not provide any mechanism for the client to
authenticate the resource server at the application layer.
7.3. Plaintext Storage of Credentials
The mechanism described in this document works similar 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 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 the
private keys associated with the access tokens are protected from
unauthorized access.
7.4. 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
recover the session key or private key used to compute the keyed
message digest or digital signature, respectively.
This specification assumes that the keying material for use with the
described HTTP signing mechanism has been distributed via other
mechanisms, such as [I-D.bradley-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 reasonable.
7.5. 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 need to the resource server
has to process the incoming request, verify the access token, perform
signature verification, and might have (in certain circumstances)
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consult back-end databases or the authorization server 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 situation may, however, be different
when using asymmetric cryptography, which is also supported by the
JWS.
7.6. Protecting HTTP Header Fields
This specification provides flexibility for selectively protecting
header fields and even the body of the message. Since all components
of the HTTP request are only optionally protected by this method, and
even some components may be protected only in part (e.g., some
headers but not others) it is up to application developers to verify
that any parameters in a request are actually covered by the
signature.
The application verifying this signature MUST NOT assume that any
particular parameter is appropriately covered by the signature. 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. Failure to
make this comparison will render the signature mechanism useless.
8. Acknowledgements
The authors acknowledge the OAuth Working Group and submit this draft
for feedback and input into the ongoing work of signed HTTP requests
for the interaction between clients and resource servers.
9. References
9.1. Normative References
[I-D.ietf-jose-json-web-signature]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", draft-ietf-jose-json-web-signature-25
(work in progress), March 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
9.2. Informative References
[I-D.bradley-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-bradley-oauth-pop-key-
distribution-00 (work in progress), April 2014.
[I-D.hunt-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-hunt-oauth-pop-architecture-00 (work
in progress), April 2014.
Authors' Addresses
Justin Richer (editor)
The MITRE Corporation
Email: jricher@mitre.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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