Re: [websec] Strict-Transport-Security syntax redux

Julian Reschke <julian.reschke@gmx.de> Thu, 27 October 2011 19:27 UTC

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Date: Thu, 27 Oct 2011 21:27:26 +0200
From: Julian Reschke <julian.reschke@gmx.de>
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To: =JeffH <Jeff.Hodges@KingsMountain.com>
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Cc: IETF WebSec WG <websec@ietf.org>
Subject: Re: [websec] Strict-Transport-Security syntax redux
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On 2011-10-27 20:03, =JeffH wrote:
> I've been working with Julian on simplifying the STS header field
> syntax. Here's where it's presently at -- thoughts?
>
> thanks again to Julian and Ryan for their earlier feedback.
>
> =JeffH

That looks much better to me. More inline.

> ###
>
> 5.1. Strict-Transport-Security HTTP Response Header Field
>
> The Strict-Transport-Security HTTP response header field indicates to
> a UA that it MUST enforce the HSTS Policy in regards to the host
> emitting the response message containing this header field.
>
> Note: this specification uses the augmented BNF (ABNF) notation from
> Section 2 of [RFC2616], including its rules for "implied linear
> whitespace (LWS)", and case-insensitivity of quoted-string literals.
>
> The ABNF syntax for the Strict-Transport-Security (STS) HTTP Response
> Header field is:
>
>
> Strict-Transport-Security = "Strict-Transport-Security" ":"
> directive *( ";" [ directive ] )
>
> STS directives:
>
> directive = max-age | includeSubDomains | STS-d-ext
>
> max-age = "max-age" "=" delta-seconds

What happens with

   max-age="1"

?

Do you expect all recipients to reject this? Depending on the parsing 
API they use they might not even know that the value was quoted on the wire.

> includeSubDomains = "includeSubDomains"

There's a tiny risk that some implementations will handle value-less 
parameters the same way as parameters with empty values, such as

   includeSubDomains=

or

   includeSubDomains=""

but maybe I'm over-engineering here. (To get this right an API will need 
to distinguish between "parameter missing", "parameter present and 
valueless" and "parameter present and having a zero-length value".

Also, identifiers "max-age" and "includeSubDomains" are 
case-insensitive, right? This follows from the ABNF, but might be worth 
saying again in prose; in particular because it also needs to be the 
case for all future extensions.

> The max-age directive MUST appear once in the Strict-Transport-Security
> header field value. The includeSubDomains directive MAY appear once.
> The order of appearance of directives in the Strict-Transport-Security
> header field value is not significant.
>
> Additional directives extending the the semantic functionality of
> the Strict-Transport-Security header field may be defined in other
> specifications, using the STS directive extension point (STS-d-ext)
> syntax:
>
> STS-d-ext = token [ "=" ( token | quoted-string ) ]
>
>
> Defined in [RFC2616]:
>
> delta-seconds = <1*DIGIT, defined in [RFC2616], Section 3.3.2>
> token = <token, defined in [RFC2616], Section 2.2>
> quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
>
>
> ###

Best regards, Julian