Re: [TLS] [EXTERNAL] Re: Narrowing allowed characters in ALPN ?

Viktor Dukhovni <> Thu, 20 May 2021 17:02 UTC

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Date: Thu, 20 May 2021 13:02:45 -0400
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Subject: Re: [TLS] [EXTERNAL] Re: Narrowing allowed characters in ALPN ?
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On Thu, May 20, 2021 at 04:45:23PM +0000, Andrei Popov wrote:

> ALPN IDs are byte strings; the fact that some of them can be displayed
> as ASCII character strings merely reflects the fact that those ALPN
> IDs were chosen by humans😊.

That's fine when they're just private signalling between a homebrew
client and homebrew server, but for ALNP values registered with IANA,
used in DNS HTTPS/SVCB records, ... the byte strings do need to be
broadly usable, so that any site operator can add them into their DNS
zone, type them into a web form, ...

If ALPN values are BIDI strings, with mixed left-to-right and
right-to-left fragments, comtain accented characters that may have
different NFKC vs. NFKD forms... usability and interoperability suffer.

It is fine for the TLS protocol to not care, but the *standard* ALPN
values in the IANA registry (that might then also appear in DNS
zone files, configuration files, ...) a more restricted character
set would actually be helpful.