4 ms·

> What we're looking at there is a Base45-encoded, compressed, signed binary data structure. ?!?!?! QR codes support binary natively. What the hell even is ba

> What we're looking at there is a Base45-encoded, compressed, signed binary data structure. ?!?!?! QR codes support binary natively. What the hell even is base45?!

supermatt 5y ago
https://github.com/ehn-dcc-development/hcert-spec/blob/main/README.md#base45 https://github.com/ehn-dcc-development/hcert-spec/blob/main/...
- devit 5y ago
  The document says that the encoding is 11 bits for two characters, which means that Base45 plus that encoding is very efficient, since 45^2 / 2^11 = 0.988, so only 1.2% of the capacity is wasted.
RobinUS2 5y ago
https://datatracker.ietf.org/doc/draft-faltstrom-base45/ https://datatracker.ietf.org/doc/draft-faltstrom-base45/
dugmartin 5y ago
It is the first I've heard of it too but according to this draft spec QR codes can't support binary: https://datatracker.ietf.org/doc/draft-faltstrom-base45/ https://datatracker.ietf.org/doc/draft-faltstrom-base45/ "Even in Byte mode a typical QR-code reader tries to interpret a byte sequence as an UTF-8 or ISO/IEC 8859-1 encoded text. Thus QR-codes cannot be used to encode arbitrary binary data directly. Such data has to be converted into an appropriate text before that text could be encoded as a QR-code. Compared to already established Base64, Base32 and Base16 encoding schemes, that are described in RFC 4648 [RFC4648], the Base45 scheme described in this document offer a more compact QR-code encoding." Here is the output alphabet: Value Encoding Value Encoding Value Encoding Value Encoding 00 0 12 C 24 O 36 Space 01 1 13 D 25 P 37 $ 02 2 14 E 26 Q 38 % 03 3 15 F 27 R 39 * 04 4 16 G 28 S 40 + 05 5 17 H 29 T 41 - 06 6 18 I 30 U 42 . 07 7 19 J 31 V 43 / 08 8 20 K 32 W 44 : 09 9 21 L 33 X 10 A 22 M 34 Y 11 B 23 N 35 Z My initial thought is using "Space" as a valid encoded value seems like an enormous foot gun.
- justinmchase 5y ago
  To trim or not to trim, that is the question
- lymeswold 5y ago
  (2^16)^(1/3) < 41 So why base 45?
  - lesquivemeau 5y ago
    Because you encode using 45 different characters
    - KMag 5y ago
      The GP is saying 3 base41 characters can encode 2 bytes. Using base45 doesn't shorten the encoded message size unless they increase the encoding unit to more than 2 bytes.
  - kubanczyk 5y ago
    Yes, 41 characters would be sufficient. I would have omitted these four (the asterix being there only to have a consecutive subset): 36 Space 37 $ 38 % 39 * Especially % can be confusing, as %20AA seems like a valid base45 both before and after urldecode. The $ is a lesser footgun. I'd be keen to learn why they decided to use full 45 characters available in alphanumeric QR.
- jfrunyon 5y ago
  https://www.youtube.com/watch?v=0oBx7Jg4m-o https://www.youtube.com/watch?v=0oBx7Jg4m-o
kubanczyk 5y ago
Because the "binary" in QR codes (actually they call it "byte" mode) is supposed to be ISO-8859-1 per the ISO/IEC 18004:2005. Which means it is a text mode, and actual scanners do treat it as text (although usually UTF-8 which goes against that standard, meh).
deleted 5y ago
[deleted]
pyentropy 5y ago
Alphanumeric mode (which is 45 symbols: [A-Z0-9] and nine special symbols) is the only QR mode that's reliably supported among all scanner library implementations (latin1 is part of the extended mode, Unicode & raw binary get detected with implementation-dependent heuristics). The encoding is great, actually: 4n bytes will get encoded into 6n alnums (base45 symbols) which are 3n * 11 = 33n QR-bits. A loss of just 3% (33/32 - 1). This works because [ alnum1 alnum2 ] by spec must get packed into [ 11 bits ] in the QR message bitstream. Wrote an explanation here: https://news.ycombinator.com/item?id=27592936 https://news.ycombinator.com/item?id=27592936

> What we're looking at there is a Base45-encoded, compressed, signed binary data structure. ?!?!?! QR codes support binary natively. What the hell even is base45?!

supermatt 5y ago

https://github.com/ehn-dcc-development/hcert-spec/blob/main/README.md#base45 https://github.com/ehn-dcc-development/hcert-spec/blob/main/...

devit 5y ago
The document says that the encoding is 11 bits for two characters, which means that Base45 plus that encoding is very efficient, since 45^2 / 2^11 = 0.988, so only 1.2% of the capacity is wasted.

RobinUS2 5y ago

https://datatracker.ietf.org/doc/draft-faltstrom-base45/ https://datatracker.ietf.org/doc/draft-faltstrom-base45/

dugmartin 5y ago

It is the first I've heard of it too but according to this draft spec QR codes can't support binary: https://datatracker.ietf.org/doc/draft-faltstrom-base45/ https://datatracker.ietf.org/doc/draft-faltstrom-base45/ "Even in Byte mode a typical QR-code reader tries to interpret a byte sequence as an UTF-8 or ISO/IEC 8859-1 encoded text. Thus QR-codes cannot be used to encode arbitrary binary data directly. Such data has to be converted into an appropriate text before that text could be encoded as a QR-code. Compared to already established Base64, Base32 and Base16 encoding schemes, that are described in RFC 4648 [RFC4648], the Base45 scheme described in this document offer a more compact QR-code encoding." Here is the output alphabet: Value Encoding Value Encoding Value Encoding Value Encoding 00 0 12 C 24 O 36 Space 01 1 13 D 25 P 37 $ 02 2 14 E 26 Q 38 % 03 3 15 F 27 R 39 * 04 4 16 G 28 S 40 + 05 5 17 H 29 T 41 - 06 6 18 I 30 U 42 . 07 7 19 J 31 V 43 / 08 8 20 K 32 W 44 : 09 9 21 L 33 X 10 A 22 M 34 Y 11 B 23 N 35 Z My initial thought is using "Space" as a valid encoded value seems like an enormous foot gun.

justinmchase 5y ago
To trim or not to trim, that is the question
lymeswold 5y ago
(2^16)^(1/3) < 41 So why base 45?
- lesquivemeau 5y ago
  Because you encode using 45 different characters
  - KMag 5y ago
    The GP is saying 3 base41 characters can encode 2 bytes. Using base45 doesn't shorten the encoded message size unless they increase the encoding unit to more than 2 bytes.
- kubanczyk 5y ago
  Yes, 41 characters would be sufficient. I would have omitted these four (the asterix being there only to have a consecutive subset): 36 Space 37 $ 38 % 39 * Especially % can be confusing, as %20AA seems like a valid base45 both before and after urldecode. The $ is a lesser footgun. I'd be keen to learn why they decided to use full 45 characters available in alphanumeric QR.
jfrunyon 5y ago
https://www.youtube.com/watch?v=0oBx7Jg4m-o https://www.youtube.com/watch?v=0oBx7Jg4m-o

kubanczyk 5y ago

Because the "binary" in QR codes (actually they call it "byte" mode) is supposed to be ISO-8859-1 per the ISO/IEC 18004:2005. Which means it is a text mode, and actual scanners do treat it as text (although usually UTF-8 which goes against that standard, meh).

deleted 5y ago

[deleted]

pyentropy 5y ago

Alphanumeric mode (which is 45 symbols: [A-Z0-9] and nine special symbols) is the only QR mode that's reliably supported among all scanner library implementations (latin1 is part of the extended mode, Unicode & raw binary get detected with implementation-dependent heuristics). The encoding is great, actually: 4n bytes will get encoded into 6n alnums (base45 symbols) which are 3n * 11 = 33n QR-bits. A loss of just 3% (33/32 - 1). This works because [ alnum1 alnum2 ] by spec must get packed into [ 11 bits ] in the QR message bitstream. Wrote an explanation here: https://news.ycombinator.com/item?id=27592936 https://news.ycombinator.com/item?id=27592936