HTTP response signing strawman

If we revise the abstract model for generating a Signature header along the lines suggested in my previous post, we get this:

1. Choose which key (security token) to use and create one or more identifiers for it.  One possible kind of key would be an X.509 certificate.
2. Choose which response headers to sign.  This would include at least Content-Type and probably Date and Expires.  It would not include hop-to-hop headers.
3. Compute the digest (cryptographic hash) of the full entity body of the requested URI. Base64-encode the digest.
4. Create a  Signature header template; this differs from the final Signature header only in that it has a blank string at the point where the final Signature header will have the base64-encoded signature value. It can specify the following information:
   • the type of key;
   • one or more identifiers for the key;
   • an identifier for the suite of cryptographic algorithms to be used;
   • an identifier for the header canonicalization algorithm to be used;
   • a list of the names of the response headers to be signed;
   • the request URI;
   • the base64 encoded digest (from step 4).
5. Combine the response headers that are to be signed with the Signature header template.
6. Canonicalize the headers from the previous step.  This ensures that the canonicalization of the headers as seen by the origin server are the same as the canonicalization of the headers as seen by the client, even if there are one or more HTTP/1.1 conforming proxies between the client and the origin server.
7. Compute the cryptographic hash of the canonicalized headers.
8. Sign the cryptographic hash created in the previous step.  Base64-encode this to create the signature value.
9. Create the final Signature header by inserting the base64-encoded signature value from the previous step into the Signature header template from step 5.

Note that when verifying the signature, as well as checking the signature value, you have to compute the digest of the entity body and check that it matches the digest specified in the Signature header.

The syntax could be something like this:

Signature = "Signature" ":" #signature-spec
signature-spec = key-type 1*( ";" signature-param )
key-type = "x509" | key-type-extension
signature-param =
   "value" = <"> <Base64 encoded signature> <">
   | "canon" = "basic" | canon-extension
   | "headers" = <"> 1#field-name <">
   | "request-uri" = quoted-string
   | "digest" = <"> <Base64 encoded digest> <">
   | "crypt" = ( "rsa-sha1" | crypt-extension )
   | "key-uri" = quoted-string
   | "key-uid" = sha1-fingerprint | uid-extension
   | signature-param-extension
sha1-fingerprint = <"> "sha1" 20(":" 2UHEX) <">
UHEX = DIGIT | "A" | "B" | "C" | "D" | "E" | "F"
uid-extension = <"> uid-type ":" 1*uid-char <">
uid-type = token
uid-char = <any CHAR except CTLs, <\> and <">>
key-type-extension = token
canon-extension = token
crypt-extension = token
hash-func-extension = token
signature-param-extension =
   token "=" (token | quoted-string)

There are several issues I'm not sure about.

• Should this be generalized to support signing of (some kinds of) HTTP request?
• What is the right way to canonicalize HTTP headers?
• Rather than having a digest parameter, would it be better to use the Digest header from RFC 3230 and then include that in the list of headers to be signed?
• Should the time period during which the signature is valid be specified explicitly by parameters in the Signature header rather than being inferred from other headers, such as Date and Expires (which would of course need to be included in the list of headers to sign)?
• Should support for security tokens other than X.509 certificates be specified?

HTTP: what to sign?

There's been quite a number of useful comments on my previous post, and even an implementation.  The main area where there seems to be disagreement is on the issue of what exactly to sign.

It seems to me that you can look at an HTTP interaction at two different levels:

• at a low level, it consist of request and response messages;
• at a slightly higher level, it consists of the transfer of the representations of resources.

With a simple GET, there's a one-to-one correspondence between a response message a representation transfer.  But with fancier HTTP features, like HEAD or conditional GET or ranges or the proposed PATCH method, these two levels start to diverge: the messages aren't independent entities in themselves, they are artifacts of the client attempting to efficiently synchronize the representation of the resource that it has with the current representation defined by the origin server.

The question then arises of whether, at an abstract level, the right thing to sign is messages or resource representations.  I think the right answer is resource representations: those are things whose integrity is important to applications.  For example, in the response to the HEAD message, the signature wouldn't simply sign the response to the HEAD message; rather it would cover the entity that would have been returned by a GET. The Signature header would thus be allowed in similar situations to the ETag header and would correspond to the same thing that a strong entity tag corresponds to.

It's important to remember that the representation of the resource doesn't consist of just the data in the entity body.  It also includes the metadata in the entity headers.  At the very least, I think you would want to sign the Content-Type header. Note that there are some headers that you definitely wouldn't want to sign, in particular hop-to-hop headers.  I don't think there's a single right answer as to which headers to sign, which means that the Signature header will need to explicitly identify which headers it is signing.

With this approach the signature doesn't need to cover the request.  However, it does need to relate the representation to a particular resource. Otherwise there's a nasty attack possible: the bad guy can replace the response to a request for one resource with the response to a request for another resource. (Suppose http://www.example.com/products/x/price returns the price of product x; an attacker could completely switch around the price list.)  I think the simplest way to solve this is for the Signature header in the response to include a uri="request_uri" parameter, where request_uri is the URI of the resource whose representation is being signed. This allows the signature verification process to work with just the response headers and body as input, which should simplify plugging this feature into implementations.

Although not including the request headers in the signature simplifies things, it must be recognized that it does lose some functionality. When there are multiple variants, the signature can't prove that you've got the right variant. However, I think that's a reasonable tradeoff.  Even if the request headers were signed, sometimes the response depends on things that aren't in the request, like the client's IP address (as indicated by Vary: *). The response can at least indicate that the response is one of several possible variants, by including Content-Location, Content-Language and/or Vary headers amongst the signed response headers.

The signature will also need to include information about the time during which the relationship between the representation and the resource applies.  I haven't figured out exactly how this should work.  It might be a matter of signing some combination of Date, Last-Modified, Expires and Cache-Control (specifically the s-maxage and maxage directives) headers, or it might involve adding timestamp parameters to the Signature header.

To summarize, the signature in the response should assert that a particular entity is a representation of a particular resource at a particular time.