A Method for Signing HTTP Requests
for OAuthietf@justin.richer.orgPing Identityve7jtb@ve7jtb.comhttp://www.thread-safe.com/ARM LimitedAustriaHannes.Tschofenig@gmx.nethttp://www.tschofenig.priv.at
Security
OAuth Working GroupThis 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.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 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
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 and ). The JSON Web Signature
(JWS) specification 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.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.Other terms such as "client", "authorization server", "access token",
and "protected resource" are inherited from OAuth
2.0.We use the term 'sign' (or 'signature') to denote both a keyed
message digest and a digital signature operation.This specification uses JSON Web Signatures
to protect the access token and, optionally, parts of the request.This section describes how to generate a JSON object from the HTTP request. Each value
below is included as a member of the JSON object at the top level.REQUIRED. The access token value. This string is
assumed to have no particular format or structure and remains opaque
to the client.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.OPTIONAL. The HTTP Method used to make this request.
This MUST be the uppercase HTTP verb as a JSON string.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.OPTIONAL. The HTTP URL path component of the request
as an HTTP string.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.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.OPTIONAL. The base64URL encoded hash of the HTTP
Request body, calculated as the SHA256 of the byte array of the
bodyAll 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.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:Adds the name of the query parameter to the end of the
list.Percent-encodes the name and value of the parameter as
specified in . Note that if the name and
value have already been percent-encoded for transit, they are not
re-encoded for this step.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:When added to the JSON structure using this process, the results
are: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:Lowercases the header name.Adds the name of the header to the end of the list.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.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:In order to send the signed object to the protected resource, the
client includes it in one of the following three places.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.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.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.Just like with a bearer token, 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), or it can use a service to look up token
information (such as introspection).
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.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: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.Percent-encodes the name and value of the parameter as
specified in . Note that if the name and
value have already been percent-encoded for transit, they are not
re-encoded for this step.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.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: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.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.Section 11.1 of defines the OAuth Access
Token Type Registry and this document adds another token type to this
registry.pop(none)Proof-of-possession
access token for use with OAuth 2.0IETF[[ this document ]]This specification registers the pop
type value in the IANA JSON Web Signature and Encryption Type Values
registry : "typ" Header Parameter Value: popAbbreviation for MIME Type: NoneChange Controller: IETFSpecification Document(s): [[ this document ]]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
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.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.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 . 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.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
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.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 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.This specification addresses machine to machine communications and
raises no privacy considerations beyond existing OAuth transactions.The authors thank the OAuth Working Group for input into this
work.