OAuth Request Body Hash 1.0 Draft 4

Abstract

This specification extends the OAuth signature to include integrity checks on HTTP request bodies with content types other than application/x-www-form-urlencoded.

Table of Contents

1.  Notation and Conventions
2.  Introduction
3.  The oauth_body_hash Parameter
    3.1.  Hash Algorithm
    3.2.  Signing the Request
    3.3.  Verifying the Request
4.  Example
5.  Security Considerations
Appendix A.  Obtaining the HTTP Entity Body
Appendix B.  Unkeyed vs Keyed Hash Algorithms
6.  References
§  Author's Address

1.  Notation and 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] (Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” .).

Unless otherwise noted, this specification is written as a direct continuation of [OAuth Core 1.0] (OAuth Core Workgroup, “OAuth Core 1.0,” .), inheriting the definitions and guidelines set by it.

2.  Introduction

The OAuth Core specification [OAuth Core 1.0] (OAuth Core Workgroup, “OAuth Core 1.0,” .) provides body integrity checking only for application/x-www-form-urlencoded request bodies. Other types of request bodies are left unsigned. An eavesdropper or man-in-the-middle who captures a signed request URL may be able to forward or replay that URL with a different HTTP request body. Nonce checking and the use of https can mitigate this risk, but may not be available or effective in some environments.

This specification describes a method to provide an integrity check on the request body without requiring signatures of arbitrary byte streams. An unkeyed hash of the request body is taken, the resulting hash value is added to the list of OAuth parameters, and the normal OAuth signature base string is signed.

This extension is forward compatible: Service Providers that have not implemented this extension can verify requests sent by Consumers that have implemented this extension. If the Service Provider implements this specification the integrity of the body is guaranteed. If the Service Provider does not check body signatures, the remainder of the request will still validate using the OAuth Core signature algorithm.

This specification is only useful when cryptographic signatures are used. The OAuth PLAINTEXT signature algorithm does not provide integrity checks for any portion of the request.

3.  The oauth_body_hash Parameter

The body hash parameter MUST NOT be sent on requests that use the application/x-www-form-urlencoded content-type. The body hash parameter MUST NOT be sent on HTTP GET or HEAD requests. The body hash parameter SHOULD be sent on all other requests.

3.1.  Hash Algorithm

The body hash algorithm is determined by the OAuth signature method used.

• If the OAuth signature method is HMAC-SHA1 or RSA-SHA1, SHA1 [RFC3174] (Eastlake, 3rd, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” .) MUST be used as the body hash algorithm.
• If the OAuth signature method is PLAINTEXT, use of this specification provides no security benefit and is NOT RECOMMENDED.

New OAuth signature methods SHOULD specify the hash algorithm used to generate the body hash.

3.2.  Signing the Request

To sign a request, the OAuth Consumer MUST:

1. Calculate the body hash value by executing the selected hash algorithm over the request body. The request body is the Entity Body as defined in [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .) section 7.2. If the request does not have an entity body, the hash should be taken over the empty string.
2. Base64 [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” .) encode the hash value.
3. Set the oauth_body_hash parameter to the Base64 encoded hash value.
4. Sign the request as per section 9 "Signing Request" of [OAuth Core 1.0] (OAuth Core Workgroup, “OAuth Core 1.0,” .). The oauth_body_hash parameter is included in the Signature Base String together with the other request parameters.
5. Include the oauth_body_hash parameter along with the other OAuth Protocol parameters in the signed OAuth request.

3.3.  Verifying the Request

Service Providers verify the integrity of request bodies by verifying the OAuth signature as described in [OAuth Core 1.0] (OAuth Core Workgroup, “OAuth Core 1.0,” .). and also verifying the value of the oauth_body_hash OAuth protocol parameter.

1. Verify the request as described in OAuth Core.
2. Determine whether the request should have an oauth_body_hash parameter according to the rules described in The oauth_body_hash Parameter (The oauth_body_hash Parameter).
3. If the request should not have an oauth_body_hash parameter, verify that no oauth_body_hash parameter is present. If an oauth_body_hash parameter is present on a request that should not contain an oauth_body_hash parameter, the request has been tampered with and MUST be rejected.
4. If the request should have an oauth_body_hash parameter but does not contain one, the request was sent by a Consumer that does not support this extension. Service Providers MAY accept such requests for interoperability, or they MAY reject such requests in favor of security.
5. The request contains an oauth_body_hash parameter and body that should be hashed. Calculate the expected body hash for the request as described in Signing the Request (Signing the Request).
6. Compare the calculated body hash with the value sent by the client in the oauth_body_hash parameter. If the values match, the body is intact and the request can proceed. (Rather than comparing text values, Service Providers MAY base64 decode the oauth_body_hash parameter and perform the comparison on the octets of the hash. This reduces the risk of minor differences in URL encoding or base64 encoding causing spurious integrity check failures.)

To summarize, Service Providers may encounter any of the following situations:

• Request should not have oauth_body_hash, no oauth_body_hash is present: no additional checks are necessary.
• Request should not have oauth_body_hash, oauth_body_hash is present: reject request.
• Request should have oauth_body_hash, no oauth_body_hash is present: Consumer does not support extension. Service Provider may or may not reject request, according to local security policy.
• Request should have oauth_body_hash, oauth_body_hash is present: Verify the body hash.

4.  Example

Sample HTTP request:

  PUT /resource HTTP/1.1
  Host: www.example.com
  Content-Type: text/plain
  Content-Length: 12

  Hello World!

Base64 encoded SHA-1 hash of the body:

  Lve95gjOVATpfV8EL5X4nxwjKHE=

Signature Base String (with some line breaks added for readability):

  PUT&http%3A%2F%2Fwww.example.com%2Fresource&oauth_body_hash%3D
  Lve95gjOVATpfV8EL5X4nxwjKHE%253D%26oauth_consumer_key%3Dconsum
  er%26oauth_nonce%3D10369470270925%26oauth_signature_method%3DH
  MAC-SHA1%26oauth_timestamp%3D1236874236%26oauth_version%3D1.0

Signed request with body hash (with some line breaks added for readability):

  PUT /resource HTTP/1.1
  Host: www.example.com
  Authorization: OAuth realm="http%3A%2F%2Fwww.example.com",
      oauth_body_hash="Lve95gjOVATpfV8EL5X4nxwjKHE%3D",
      oauth_consumer_key="consumer",
      oauth_signature_method="HMAC-SHA1",
      oauth_timestamp="1236874155", oauth_nonce="10288510250934",
      oauth_version="1.0",
      oauth_signature="08bUFF%2Fjmp59mWB7cSgCYBUpJ0U%3D"
  Content-Type: text/plain
  Content-Length: 12

  Hello World!

5.  Security Considerations

Many factors besides the bytes of the request body can influence the interpretation of the body of the HTTP request. For example, a content-type or content-encoding header can change the way a server handles an HTTP request. This specification does not include an integrity check on the HTTP request headers. OAuth deployments whose security could be impacted by an attacker who tampers with HTTP request headers should use other mechanisms (such as HTTPS) to protect the confidentiality and integrity of the entire HTTP request.

Appendix A.  Obtaining the HTTP Entity Body

Not all programming platforms provide an easy mechanism to obtain the raw octets of the HTTP entity body. Reading the entity body as raw octets may have side effects, such as inhibiting automatic character set conversion. Transfer-encodings such as gzip also complicate implementation. On the other hand, reading the entity body as text may perform lossy conversions that prevent recovery of the original octet stream in some situtations. Character set conversions are not always one-to-one mappings, so solutions that rely on converting text back to the original octet stream are likely to fail in environments with multibyte characters.

All of these factors, and others, can make it difficult to drop in a "verify-the-signature" filtering layer prior to other request processing. The verification layer must consider the assumptions of downstream processing code on the state of the request body.

Implementers of this specification should be aware of these difficulties and consider the best way to address them in their programming frameworks.

Appendix B.  Unkeyed vs Keyed Hash Algorithms

This specification deliberately uses an unkeyed hash algorithm (SHA-1) to provide an integrity check on the body instead of a keyed hash algorithm such as HMAC-SHA1. This decision was made because signing arbitrary octet streams is poor cryptographic hygiene. It can lead to unexpected problems with cryptographic protocols.

For example, consider a proxy that uses OAuth to add authentication information to requests sent by an untrusted third-party. If the proxy signs arbitrary octet streams, the third-party can use the proxy as an oracle to forge authentication messages.

Including the result of an unkeyed hash in the normal signature base string allows the proxy to add an integrity check on the original message without creating a signing oracle.

6. References

Author's Address