[Python-Dev] Base-96 (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Aug 2 19:09:57 CEST 2008

Josiah Carlson wrote:

The standard high-bit-density encoding past base-64 is base-85 (http://en.wikipedia.org/wiki/Ascii85), which encodes 4 binary bytes as 5 ascii bytes, versus 3 binary bytes as 4 ascii bytes. It works, is an RFC somewhere,

RFC 1924, published on April 1, 1996, to shorten the representation of IPv6 addresses, so that you can write

ssh '4)+k&C#VzJ4br>0wv%Yp'

instead of having to write

ssh 1080:0:0:0:8:800:200C:417A

Most notably, section 7 (implementation issues) points out

Many current processors do not find 128 bit integer arithmetic, as required for this technique, a trivial operation. This is not considered a serious drawback in the representation, but a flaw of the processor designs.

For arbitrary-sized data, you'd have to give up 128-bit arithmetic, of course, and represent the input data to encode as a long integer.

Regards, Martin

P.S. Just in case it isn't clear: I would oppose any specific proposal to add this Ascii85 algorithm to the standard library. It would sound like we don't have any real problems to solve.

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