base92

A little library for encoding byte-strings into strings easily typeable on a standard US 101-key keyboard.

INSTALLATION

The C library is built with the GNU development tools (automake/autoconf), and can be built in the usual way with a make install.

The python library is available through pypi (http://pypi.python.org/pypi/base92/) with pip install base92, or with python setup.py install.

USAGE

C

#include <base92/base92.h>
...
strcmp(base92encode("hello world", 11), "Fc_$aOTdKnsM*k") == 0;
base92decode("Fc_$aOTdKnsM*k", &length);
length == 11;
...

Python

Fire up your favorite python:

>>> import base92
>>> base92.decode(base92.encode('hello world'))
'hello world'
>>> base92.b92encode('\x61\xf2\x05\x99\x42')
'DJ8gER!'

DESIGN DECISIONS

We are going to try and be clever, and leave out some printable ASCII characters from the general encoding scheme:

~, `, "

The ` and " are just too similar to a normal quote ' for comfort when typing out encoded strings. Hopefully you're using a good font when differentiating between l/1 and 0/O.

However, we'll use ~ for a special denotation (an empty string). There are 94 printable ascii characters, so we end up with 91 characters, or 6.5 bits per character. Once we include the ~, then we have 92 characters: hence, base92.

(and honestly, base91 was just too ugly a name to deal with)

Once we have 6.5 bits per characters, then we can take 13 bits at a time and produce two output characters with them, using a division and modulo scheme similar to base85. This might mean than base92 encoding is more resistant to corruption, because any corruption is more localized (one bit change affects only 2-3 bytes, not 4).

Note: the use of ~ as an empty string denoter might be needed in some cases that expect some output: however, passing an empty string to decode will not cause it to barf, so it's not a requirement to use ~.

Sidenote: previously base92 produced output that grew in length that was nonmonotonic with the length of the input. This is no longer the case.

Another sidenote: base64 and base85 are much more elegant, cleanly mapping a small integer of bytes onto another small integer of bytes. base92 maps 13 bytes to 16 characters, which is better than base85's 4 to 5 from a size perspective, but is fairly inelegant.

We also follow base85's convention of using the high divisor product as the first bytes.

BENCHMARKS

On average, characters saved:

For string lengths 1-32:

base64-base92 | base85-base92
-----------------------------
2.548           0.226

For string lengths 1-128:

base64-base92 | base85-base92
-----------------------------
7.441           1.142

Here, we see that base92 strictly wins in size over base64 and base85, as is expected with a higher bit density encoding.

There are no speed benchmarks, because this is a pure python implementation and I wouldn't want to benchmark anything that's not a native C library.

THANKS

tly1980: Filed issue #3
l31g: helped with resolving issue #3
seanyeh: helped with resolving issue #3

LICENSE

THE BEERWARE LICENSE (Revision 42):

@thenoviceoof wrote this file. As long as you retain this notice you can do whatever you want with this stuff. If we meet some day, and you think this stuff is worth it, you can buy me a beer in return

Nathan Hwang (thenoviceoof)

backbord/base92