Re: [TLS] [Technical Errata Reported] RFC8446 (5874)

Benjamin Kaduk <kaduk@mit.edu> Sat, 26 October 2019 05:53 UTC

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Date: Fri, 25 Oct 2019 22:52:47 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Andrei Popov <Andrei.Popov@microsoft.com>
Cc: RFC Errata System <rfc-editor@rfc-editor.org>, "ekr@rtfm.com" <ekr@rtfm.com>, "rdd@cert.org" <rdd@cert.org>, "caw@heapingbits.net" <caw@heapingbits.net>, "joe@salowey.net" <joe@salowey.net>, "sean+ietf@sn3rd.com" <sean+ietf@sn3rd.com>, "lperrin@bellaliant.net" <lperrin@bellaliant.net>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] [Technical Errata Reported] RFC8446 (5874)
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Thanks for letting us know, Andrei.
That's rather depressing, but is fairly indicative that the errata report
should be verified.  (I think I will edit the "Notes" slightly when doing
so, but am too tired to do a proper job tonight, so I'm holding off on
actually marking the report as verified in the database for now.)

-Ben

On Fri, Oct 25, 2019 at 06:13:18PM +0000, Andrei Popov wrote:
> My reading of the TLS 1.2 and 1.3 RFCs is that zero-length application_data records must still be encrypted and authenticated. Otherwise, MITM can inject arbitrary numbers of these.
> 
> However, the current language is vague enough that I've seen major SW vendors send (and accept) 0x17 0x03 0x03 0x00 0x00 and insist that this is RFC-compliant, because " Zero-length fragments of Application Data MAY be sent".
> 
> Therefore, I support clarifications in this area.
> 
> Cheers,
> 
> Andrei
> 
> -----Original Message-----
> From: TLS <tls-bounces@ietf.org>; On Behalf Of RFC Errata System
> Sent: Friday, October 11, 2019 9:22 PM
> To: ekr@rtfm.com; rdd@cert.org; kaduk@mit.edu; caw@heapingbits.net; joe@salowey.net; sean+ietf@sn3rd.com
> Cc: lperrin@bellaliant.net; tls@ietf.org; rfc-editor@rfc-editor.org
> Subject: [TLS] [Technical Errata Reported] RFC8446 (5874)
> 
> The following errata report has been submitted for RFC8446, "The Transport Layer Security (TLS) Protocol Version 1.3".
> 
> --------------------------------------
> You may review the report below and at:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.rfc-editor.org%2Ferrata%2Feid5874&amp;data=02%7C01%7CAndrei.Popov%40microsoft.com%7C079c32a628aa4a46749e08d74ecbc7c7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637064509493396889&amp;sdata=61wnX96xJGd6FSLbxEQAJNvwERSVAMGoxxvjgb7DuRo%3D&amp;reserved=0
> 
> --------------------------------------
> Type: Technical
> Reported by: Mr Laurie Perrin <lperrin@bellaliant.net>;
> 
> Section: 5.1
> 
> Original Text
> -------------
> ....
> 
>    Application Data messages contain data that is opaque to TLS.
>    Application Data messages are always protected.  Zero-length
>    fragments of Application Data MAY be sent, as they are potentially
>    useful as a traffic analysis countermeasure.  Application Data
>    fragments MAY be split across multiple records or coalesced into a
>    single record.
> 
> Corrected Text
> --------------
> ....
> 
>    Application Data messages contain data that is opaque to TLS.
>    Application Data messages are always protected.  Zero-length
>    fragments of Application Data (i.e. those encapsulating an
>    TLSInnerPlaintext record having a content field of length zero)
>    MAY be sent, as they are potentially useful as a traffic analysis
>    countermeasure. Application Data fragments MAY be split across
>    multiple records or coalesced into a single record.
> 
> Notes
> -----
> In the interest of clarity, it may be prudent to specify the type of record for which a fragment of length zero is being considered - it cannot be that of the TLSCiphertext itself, for "Application Data messages are always protected,"
> therefore I infer this relates to the TLSInnerPlaintext content field (of length "TLSPlaintext.length") - i.e. to the TLSPlaintext fragment.
> 
> Note: This comment also applies to previous versions of the TLS specification, in particular with the introduction of the respective text concerning zero-length fragments in RFC 5246. In TLS 1.2, this would be the GenericXXCipher content field of length "TLSCompressed.length" - i.e. to the TLSCompressed fragment.
> 
> Note: The implications of zero-length records must be considered with respect to potential vectors for denial of service.
> 
> Instructions:
> -------------
> This erratum is currently posted as "Reported". If necessary, please use "Reply All" to discuss whether it should be verified or rejected. When a decision is reached, the verifying party can log in to change the status and edit the report, if necessary. 
> 
> --------------------------------------
> RFC8446 (draft-ietf-tls-tls13-28)
> --------------------------------------
> Title               : The Transport Layer Security (TLS) Protocol Version 1.3
> Publication Date    : August 2018
> Author(s)           : E. Rescorla
> Category            : PROPOSED STANDARD
> Source              : Transport Layer Security
> Area                : Security
> Stream              : IETF
> Verifying Party     : IESG
> 
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