The Base45 Data Encoding
datatracker.ietf.org
The Base45 Data Encoding
1–10 of 50 posts
Re: The Base45 Data Encoding
#2This could be useful for storing non-ascii armored crypto keys within a printed QR code format that can be stored in a fireproof safe and reasonably OCRed and decoded for use in disaster recovery or other applications.
Re: The Base45 Data Encoding
#3What are the benefits to base58?
Base45 uses chars like backslash. This is super annoying when the encoded string is used in an url.
Re: The Base45 Data Encoding
#4What are the benefits to base58? Base45 uses chars like backslash. This is super annoying when the encoded string is used in an url.
The draft specifically tells you not to do that:
"If the data is to be sent via some other transport, a transport encoding suitable for that transport should be used instead of Base45."
Base45 is mainly useful for binary information in QR codes.
Re: The Base45 Data Encoding
#5A nice tidbit: this RFC has its roots in the EU Covid Certificate project. The encoding was designed to cut the size of the QR payload (which for DCC is a CBOR - binary encoded - object) :)
The smaller the payload, the better and faster the scanning. Which is important for something that is designed to be used during border crossings and the like.
We have a number of implementations here:
Re: The Base45 Data Encoding
#6So, it’s using a 45-character alphabet which matches the QR code alphanumeric values table, which lets the QR code encoder switch to a more efficient mode that takes less space.
I just tried rendering a QR code of the 692 characters of the introductory paragraph (with lines joined appropriately), and compared it with a QR code of the same text, uppercased and with out-of-range characters `,`, `[` and `]` changed to %. This reduced an 89×89 code down to 77×77, a 24% reduction in area. If this is roughly the ratio, then Base45-encoding binary data by QR code will yield roughly 17% area savings compared with Base64. (Base45 gets 50% bloat, then 24% shrink = multiple of 1.14; Base64 gets 33% bloat = multiple of 1.33; Base45 / Base64 = 1.14 / 1.33 = 0.83.)
[Edit: edflsafoiewq’s figures on 40-L codes at https://news.ycombinator.com/item?id=27627915 come to about 23% savings, markedly more than my 17%.]
I can’t help but wonder if any of the other modes could be more efficient still—numeric mode, kanji mode and byte mode.
Of course, the specs for all this are ISO specs, so I can’t read them without coughing up the moolah.
In case it’s not clear, I am utterly inexpert in this domain.
Further thoughts:
Base45 encoding two octets in three characters is pretty wasteful: 45³ ÷ 256² ≈ 1.39, which is 39% waste. (By contrast, Base64 is 100% efficient with its alphabet: 64⁴ = 256³.) This means that if you were willing to do more complex encoding and decoding, you could shrink your QR code by roughly 39% more—to about 52% of the size of the Base64, rather than 83%. Leaving such a huge gap on the table puzzles me—I’d have thought that either you’d want something simple (where Base64 is well-understood) or want to minimise your QR codes, and Base45 sits in an awkward place in the middle.
For UTF-8, base-128 will be the most efficient you get. That’ll be ~14% inflation (7 bytes in 8 characters). Which… huh, that looks to be within ε of Base45’s 50% bloat and 24% shrinkage. Not sure if that’s a coincidence or not because I don’t know how alphanumeric mode versus byte mode works in QR codes. But this suggests that alphanumeric mode and Base45-but-not-wasteful would be markedly more efficient than byte mode and Base-122. Still leaves numeric and kanji modes open as possibilities. Again, I’m inexpert and don’t know how the encodings are actually done, and that’ll matter.
On edflsafoiewq’s 40-L figures: Base64 gets 2214 bytes, Base45 gets 2864 bytes, optimally-efficient base-45 would get log₂₅₆ 45⁴²⁹⁶ = 2949 bytes, only around 3% more. I think I must have made a mistake somewhere with some of my numbers.
Re: The Base45 Data Encoding
#7What are the benefits to base58? Base45 uses chars like backslash. This is super annoying when the encoded string is used in an url.
It's not for avoiding looks-similar-to-a-human chars like Base58. It's for QR codes. The alphabet is exactly the alphabet for Alphanumeric mode QR codes.
A 40-L code allows 4296 characters in Alphanumeric-mode = 2864 bytes after Base45 decoding.
The same code allows 2953 characters in Byte mode. If you use Byte mode to hold Base64, that's 2214 bytes after decoding, so Base45 is more efficient.
The reason it gives for why you can't use Byte mode to directly hold binary data is
> 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.
It also says you're not supposed to use it anywhere but QR codes (like URLs)
> If the data is to be sent via some other transport [not stored in a QR-code], a transport encoding suitable for that transport should be used instead of Base45. It is not recommended to first encode data in Base45 and then encode the resulting string in for example Base64 if the data is to be sent via email. Instead the Base45 encoding should be removed, and the data itself should be encoded in Base64.
Re: The Base45 Data Encoding
#8How does this compare with Data Matrix and C40 encoding?
Re: The Base45 Data Encoding
#9So instead of extending QR codes, which are inherently binary, to efficiently handle binary payloads, we invent yet another ASCII-based tunneling scheme. Why ever fix any problem when we can just pile workaround upon workaround upon workaround?
Re: The Base45 Data Encoding
#10So, it’s using a 45-character alphabet which matches the QR code alphanumeric values table, which lets the QR code encoder switch to a more efficient mode that takes less space. I just tried rendering a QR code of the 692 characters of the introductory paragraph (with lines joined appropriately), and compared it with a QR code of the same text, uppercased and with out-of-range characters `,`, `[` and `]` changed to %…
> I can’t help but wonder if any of the other modes could be more efficient still—numeric mode, kanji mode and byte mode.
Byte mode is obviously most efficient. Alphanumeric mode uses 5.5 bits per each character, so combined with base45 it uses 5.5 * 3 = 16.5 bits to pack two octets. Base45 is actually not that bad as it seems (3.1% overhead). [EDIT: I've since seen edflsafoiewq's mention that typical QR readers try to interpret binary data as UTF-8, so it is not pointless.]
[MORE EDIT: I've completely missed the last paragraph, so I was actually just confirming what chrismorgan said.]