Re: [Cbor] Use and development of draft-faltstrom-base45 (was: Re: CBOR in QRcodes)

Carsten Bormann <cabo@tzi.org> Fri, 25 June 2021 16:06 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Fri, 25 Jun 2021 18:06:49 +0200
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Subject: Re: [Cbor] Use and development of draft-faltstrom-base45 (was: Re: CBOR in QRcodes)
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On 2021-06-25, at 17:51, Michael Richardson <mcr+ietf@sandelman.ca> wrote:
> 
> Signed PGP part
> 
> I don't have enough caffeine in me to figure out how much more efficient
> base45 is over base32.  I think it's log(45)/log(32), right?

Base 32 is ld(32) = 5 bits per character.
Base 4[12345] is 16 bits per three characters, i.e. 5.33 bits per character.
(Base 45 wastes most of another half bit that it might have.)


> In places where base64 or base85 won't work, it seems to me that base32 is
> probably safer than base45.   As I wrote, I think that the limitation for
> QRcode entry has to do with forms on web pages or 3270 terminals, where the
> scanner is a bump-in-the-cord on the keyboard interface.  Whether it's a
> PS2/PS2 or a second USB input.
> 
> I'm surprised that base45 includes space, + and / :-)

QR code has an alphabet.  That *happens* to have 45 characters, because it uses half an 11-bit code (2**5.5 = 45.25), i.e., 5.5 bits off the raw encoding.

Of course you want to be able to have spaces etc. in the human-readable messages that QR-Code might convey.  But using all these for a basenn encoding is not needed, just because you have them.

That 45 is not a meaningful basexx number, as shown above, as Math.log2(45) = 5.49; you can’t do a lot with that.  
(That 5.49 is Math.log2((2**5.5).floor) if you want to know why).
Including the weird characters (“ “ , ”%”, choose another two) in a basenn encoding is, er, not so bright, as the poor DGC people had to find out.

Grüße, Carsten